1. 1. CAD/CAM Module 2 AM/JA 1 Dept. of Mechanical Engg, AJCE MODULE II Syllabus Numerical control: Need advantages and disadvantages classifications point to point,straight cut and contouring positioning incremental and absolute systems open loop andclosed loop systems DDA integrator and interpolars resolution CNC and DNC.Programmable logic controllers (PLC): need relays logic ladder program timers Simple exercises only. Devices in N. C. Systems: Driving devices feed back devices:encoders, moire fringes, digitizer, resolver, inductosyn, tachometer. DISCLAIMER These notes are not the ultimate look-up for Model and University exams. Students are advised to read the references mentioned at the end thoroughly for the exams Numerical Control Introduction The word NC which stands for numerical control refer to control of a machine or a process using symbolic codes consisting of characters and numerals. Numerical Control (NC) refers to the method of controlling the manufacturing operation by means of directly inserted coded numerical instructions into the machine tool. It is important to realize that NC is not a machining method, rather, it is a concept of machine control. Although the most popular applications of NC are in machining, NC can be applied to many other operations, including welding, sheet metalworking, riveting, etc. The word CNC came into existence in seventies when microprocessors and microcomputers replaced integrated circuit IC based controls used for NC machines. A CNC machine is an numerical control machine with the added feature of an on board computer. The computer is referred to as the machine control unit (MCU). Computer numerical control (CNC) is the numerical control system in which a dedicated computer is built into the control to perform basic and advanced NC functions. CNC controls are also referred to as soft-wired NC systems because most of their control functions are implemented by the control software programs. CNC is a computer assisted process to control general purpose machines from instructions generated by a processor and stored in a memory system. It is a specific form of control system where position is the principal controlled variable. All numerical control machines manufactured since the seventies are of CNC type. The computer allows for the following: storage of additional programs, program editing, running of program from memory, machine and control diagnostics, special routines, inch/metric, incremental/absolute switchability. CNC machines can be used as standalone units or in a network of machines such as flexible machine centers. The controller uses a permanent resident program called an executive
2. 2. CAD/CAM Module 2 AM/JA 2 Dept. of Mechanical Engg, AJCE program to process the codes into the electrical pulses that control the machine. In any CNC machine, executive program resides in ROM and all the NC codes in RAM. The information in ROM is written into the electronic chips and cannot be erased and they become active whenever the machine is on. The contents in RAM are lost when the controller is turned off. Some use special type of RAM called CMOS memory, which retains its contents even when the power is turned off. History The development of numerical control owes much to the United States air force. The concept of NC was proposed in the late 1940s by John Parsons who recommended a method of automatic machine control that would guide a milling cutter to produce a curvilinear motion in order to generate smooth profiles on the work-pieces. In 1949, the U.S Air Force awarded Parsons a contract to develop new type of machine tool that would be able to speed up production methods. Parsons sub-contracted the Massachusetts Institute of Technology (MIT) to develop a practical implementation of his concept. Scientists and engineers at M.I.T built a control system for a two axis milling machine that used a perforated paper tape as the input media. This prototype was produced by retrofitting a conventional tracer mill with numerical control servomechanisms for the three axes of the machine. By 1955, these machines were available to industries with some small modifications. The machine tool builders gradually began developing their own projects to introduce commercial NC units. Also, certain industry users, especially airframe builders, worked to devise numerical control machines to satisfy their own particular production needs. The Air force continued its encouragement of NC development by sponsoring additional research at MIT to design a part programming language that could be used in controlling N.C. machines. In a short period of time, all the major machine tool manufacturers were producing some machines with NC, but it was not until late 1970s that computer-based NC became widely used. NC matured as an automation technology when electronics industry developed new products. At first, miniature electronic tubes were developed, but the controls were big, bulky, and not very reliable. Then solid-state circuitry and eventually modular or integrated circuits were developed. The control unit became smaller, more reliable, and less expensive. Need of Numerical Controls CNC machines are widely used in the metal cutting industry and are best used to produce the following types of product: Parts with complicated contours Parts requiring close tolerance and/or good repeatability Parts requiring expensive jigs and fixtures if produced on conventional machines Parts that may have several engineering changes, such as during the development stage of a prototype In cases where human errors could be extremely costly Parts that are needed in a hurry
3. 3. CAD/CAM Module 2 AM/JA 3 Dept. of Mechanical Engg, AJCE Small batch lots or short production runs Advantages of Numerical control Some of the dominant advantages of the CNC machines are: CNC machines can be used continuously and only need to be switched off for occasional maintenance. These machines require less skilled people to operate unlike manual lathes / milling machines etc. CNC machines can be updated by improving the software used to drive the machines. Training for the use of CNC machines can be done through the use of 'virtual software'. The manufacturing process can be simulated virtually and no need to make a prototype or a model. This saves time and money. Once programmed, these machines can be left and do not require any human intervention, except for work loading and unloading. These machines can manufacture several components to the required accuracy without any fatigue as in the case of manually operated machines, so less scrap is produced. Savings in time that could be achieved with the CNC machines are quite significant. Time for inspection is reduced as high accuracy is obtained Non-productive time is reduced More complex geometric parts are possible Engineering changes can be accommodated more gracefully Operator skill level requirements are reduced Less floor space required. Disadvantages of the NC machines CNC machines are generally more expensive than manually operated machines. The CNC machine operator only needs basic training and skills, enough to supervise several machines. Increase in electrical maintenance, high initial investment and high per hour operating costs than the traditional systems. Fewer workers are required to operate CNC machines compared to manually operated machines. Investment in CNC machines can lead to unemployment. Part programing is reqired. Classification of NC machines CNC machine tool systems can be classified in various ways such as : 1. Point-to-point or contouring : depending on whether the machine cuts metal while the workpiece moves relative to the tool 2. Incremental or absolute : depending on the type of coordinate system adopted to parameterise the motion commands 3. Open-loop or closed-loop : depending on the control system adopted for axis motion control Relative Tool Motion type:
4. 4. CAD/CAM Module 2 AM/JA 4 Dept. of Mechanical Engg, AJCE Point-to-Point Positioning Point-to-point positioning is used when it is necessary to accurately locate the spindle, or the work piece mounted on the machine table, at one or more specific Locations to perform such operations as drilling, reaming, boring, tapping, and punching (Fig. 9). Point-to-point positioning is the process of positioning from one coordinate (XY) position or location to another, performing the machining operation, and continuing this pattern until all the operations have been completed at all programmed locations. Some machine tools for example drilling, boring and tapping machines etc, require the cutter and the work piece to be placed at a certain fixed relative positions at which they must remain while the cutter does its work. These machines are known as point-to-point machines as shown in figure 22.1 (a) and the control equipment for use with them are known as point-to-point control equipment. Feed rates need not to be programmed. In theses machine tools, each axis is driven separately. In a point-to-point control system, the dimensional information that must be given to the machine tool will be a series of required position of the two slides. Servo systems can be used to move the slides and no attempt is made to move the slide until the cutter has been retracted back. In Fig. 9 point 1 to point 2 is a straight line, and the machine moves only along the X axis; but points 2 and 3 require that motion along both the X and Y axes takes place. As the distance in the X direction is greater than in the Y direction, Y will reach its position first, leaving X to travel in a straight line for the remaining distance. A similar motion takes place between points 3 and 4. Fig. 9 The path followed by point-to-point positioning to reach various programmed points (machining locations) on the XY axis. Contouring or Continuous path positioning Contouring, or continuous path machining, involves work such as that produced on a lathe or milling machine, where the cutting tool is in contact with the work piece as it travels from one programmed point to the next. Continuous path positioning is the ability to control motions on two or more machine axes simultaneously to keep a constant cutter-work piece relationship. The programmed information in the CNC program must accurately position the cutting tool from one point to the next and follow a predefined accurate path at a programmed feed rate in order to produce the form or contour required (Fig. 10). Other type of machine tools involves motion of work piece with respect to the cutter while cutting operation is taking place. These machine tools include milling, routing machines etc. and are known as contouring machines as shown in figure and the controls required for their control are known as contouring control.
5. 5. CAD/CAM Module 2 AM/JA 5 Dept. of Mechanical Engg, AJCE Contouring machines can also be used as point-to-point machines, but it will be uneconomical to use them unless the work piece also requires having a contouring operation to be performed on it. These machines require simultaneous control of axes. In contouring machines, relative positions of the work piece and the tool should be continuously controlled. The control system must be able to accept information regarding velocities and positions of the machines slides. Feed rates should be programmed. Fig. 10 Types of contour machining (A) Simple contour; (B) complex contour Control Loops: Open loop systems Open loop systems have no access to the real time data about the performance of the system and therefore no immediate corrective action can be taken in case of system disturbance. Programmed instructions are fed into the controller through an input device. These instructions are then converted to electrical pulses (signals) by the controller and sent to the servo amplifier to energize the servo motors. The primary drawback of the open-loop system is that there is no feedback system to check whether the program position and velocity has been achieved. If the system performance is affected by load, temperature, humidity, or lubrication then the actual output could deviate from the desired output. For these reasons the open -loop system is generally used in point-to-point systems where the accuracy requirements are not critical. Very few continuous-path systems utilize open-loop control.This system is normally applied only to the case where the output is almost constant and predictable. Therefore, an open loop system is unlikely to be used to control machine tools since the cutting force and loading of a machine tool is never a constant The only exception is the wire cut machine for which some machine tool builders still prefer to use an open loop system because there is virtually no cuttingforce in wire cut machining.
6. 6. CAD/CAM Module 2 AM/JA 6 Dept. of Mechanical Engg, AJCE Close Loop Systems In a close loop system, feed back devices closely monitor the output and any disturbance will be corrected in the first instance. Therefore high system accuracy is achievable. This system is more powerful than the open loop system and can be applied to the case where the output is subjected to frequent change. Nowadays, almost all CNC machines use this control system. The closed-loop system has a feedback subsystem to monitor the actual output and correct any discrepancy from the programmed input. These systems use position and velocity feed back. The feedback system could be either analog or digital. The analog systems measure the variation of physical variables such as position and velocity in terms of voltage levels. Digital systems monitor output variations by means of electrical pulses. To control the dynamic behavior and the final position of the machine slides, a variety of position transducers are employed. Majority of CNC systems operate on servo mechanism, a closed loop principle. If a discrepancy is revealed between where the machine element should be and where it actually is, the sensing device signals the driving unit to make an adjustment, bringing the movable component to the required location. Closed-loop systems are very powerful and accurate because they are capable of monitoring operating conditions through feedback subsystems and automatically compensating for any variations in real- time. Dimension Systems: Incremental Systems: Incremental program locations are always given as the distance and direction from the immediately preceding point (Fig. 6). Command codes which tell the machine to move the table, spindle, and knee are explained here using a vertical milling machine as an example:
7. 7. CAD/CAM Module 2 AM/JA 7 Dept. of Mechanical Engg, AJCE Fig. 6 A work piece dimensioned in the incremental system mode. (Icon Corporation) A X plus (X+) command will cause the cutting tool to be located to the right of the last point. A X minus (X-) command will cause the cutting tool to be located to the left of the last point. A Y plus (Y+) command will cause the cutting tool to be located toward the column. A Y minus (Y-) will cause the cutting tool to be located away from the column. A Z plus (Z+) command will cause the cutting tool or spindle to move up or away from the work piece. A Z minus (Z-) moves the cutting tool down or into the work piece. In incremental programming, the G91 command indicates to the computer and MCU (Machine Control Unit) that programming is in the incremental mode. Absolute systems Absolute program locations are always given from a single fixed zero or origin point (Fig. 7). The zero or origin point may be a position on the machine table, such as the corner of the worktable or at any specific point on the work piece. In absolute dimensioning and programming, each point or location on the work piece is given as a certain distance from the zero or reference point.
8. 8. CAD/CAM Module 2 AM/JA 8 Dept. of Mechanical Engg, AJCE Fig. 7 A work piece dimensioned in the absolute system mode. Note: All dimensions are given from a known point of reference. A X plus (X+) command will cause the cutting tool to be located to the right of the zero or origin point. A X minus (X-) command will cause the cutting tool to be located to the left of the zero or origin point. A Y plus (Y+) command will cause the cutting tool to be located toward the column. A Y minus (Y-) command will cause the cutting tool to be located away from the column. In absolute programming, the G90 command indicates to the computer and MCU that the programming is in the absolute mode. Interpolation The method by which contouring machine tools move from one programmed point to the next is called interpolation. This ability to merge individual axis points into a predefined tool path is built into most of todays MCUs. There are five methods of interpolation: linear, circular, helical, parabolic, and cubic. All contouring controls provide linear interpolation, and most controls are capable of both linear and circular interpolation. Helical, parabolic, and cubic interpolation are used by industries that manufacture parts which have complex shapes, such as aerospace parts and dies for car bodies. DDA integrator & Interpolators DDA is essentially an algorithm for digital integration and generates a pulse train varying in frequency. Digital integration is performed by successive additions using an Euler approximation method shown in Fig. 24.1 Digital differential analyzer (DDA) integration is based upon an iterative technique controlled by an interrupt clock. At each interrupt, a single iteration of a routine is executed, which in turn can provide an output pulse. The DDA integrator consists of two n-bit registers, Q register is an n-bit binary adder and the V register is an n-bit up/down counter.
9. 9. CAD/CAM Module 2 AM/JA 9 Dept. of Mechanical Engg, AJCE Overflow and a reference pulse. Thus the rate of generartion of the reference pulse would be proportional to the value of p. A schematic diagram of a DDA integrator is shown in Fig. 24.2. It consists of two n-bit registers, p and q, and one adder.
10. 10. CAD/CAM Module 2 AM/JA 10 Dept. of Mechanical Engg, AJCE Linear Interpolation Linear Interpolation consists of any programmed points linked together by straight lines, whether the points are close together or far apart (Fig. 11). Curves can be produced with linear interpolation by breaking them into short, straight-line segments. This method has limitations, because a very large number of points would have to be programmed to describe the curve in order to produce a contour shape. A contour programmed in linear interpolation requires the
11. 11. CAD/CAM Module 2 AM/JA 11 Dept. of Mechanical Engg, AJCE coordinate positions (XY positions in two-axis work) for the start and finish of each line segment. Therefore, the end point of one line or segment becomes the start point for the next segment, and so on, throughout the entire program. Circular Interpolation The development of MCUs capable of circular interpolation has greatly simplified the process of programming arcs and circles. To program an arc (Fig. 12), the MCU requires only the coordinatepositions (the XY axes) of the circle center, the radius of the circle, the start point and end point of the arc being cut, and the direction in which the arc is to be cut (clockwise or counterclockwise) See Fig. 12. The information required may vary with different MCUs.
12. 12. CAD/CAM Module 2 AM/JA 12 Dept. of Mechanical Engg, AJCE CNC System A CNC system consists of the following 6 major elements:- a. Input Device b. Machine Control Unit c. Machine Tool d. Driving Device e. Feed Back System f. Display Unit
13. 13. CAD/CAM Module 2 AM/JA 13 Dept. of Mechanical Engg, AJCE Working principle of CNC Machine Input Devices a. Floppy Disk Drive Floppy disk is a small magnetic storage device for CNC data input. It has been the most common storage media up to the 1970s, in terms of data transfer speed, reliability, storage size, data handling and the ability to read and write. Furthermore, the data within a floppy could be easily edited at any point as long as you have the proper program to read it. However, this method has proven to be quite problematic in the long run as floppies have a tendency to degrade alarmingly fast and are sensitive to large magnetic fields and as well as the dust and scratches that usually existed on the shop floor. b. USB Flash Drive A USB flash drive is a removable and rewritable portable hard drive with compact size and bigger storage size than a floppy disk. Data stored inside the flash drive are impervious to dust and scratches that enable flash drives to transfer data from place to place. In recent years, all computers support USB flash drives to read and write data that make it become more and more popular in CNC machine control unit. c. Serial communication
14. 14. CAD/CAM Module 2 AM/JA 14 Dept. of Mechanical Engg, AJCE The data transfer between a computer and a CNC machine tool is often accomplished through a serial communication port. International standards for serial communications are established so that information can be exchanged in an orderly way. The most common interface between computers and CNC machine tools is referred to the EIA Standard RS-232. Most of the personal computers and CNC machine tools have built in R5232 port and a standard RS-232 cable is used to connect a CNC machine to a computer which enables the data transfer in reliable way. Part programs can be downloaded into the memory of a machine tool or uploaded to the computer for temporary storage by running a communication program on the computer and setting up the machine control to interact with the communication software. d. Ethernet communication Due to the advancement of the computer technology and the drastic reduction of the cost of the computer, it is becoming more practical and economic to transfer part programmes between computers and CNC machines via an Ethernet communication cable. This media provides a more efficient and reliable means in part programme transmission and storage. Most companies now built a Local Area Network (LAN) as their infrastructure. More and more CNC machine tools provide an option of the Ethernet Card for direct communication within the LAN. e. Conversational Programming Part programmes can be input to the controller via the keyboard. Built-in intelligent software inside the controller enables the operator to enter the required data step by step. This is a very efficient way for preparing programmes for relatively simple workpieces involving up to 21/2 axis machining. Machine Control Unit (MCU) The machine control unit is the heart of the CNC system. There are two sub-units in the machine control unit: the Data Processing Unit (DPU) and the Control Loop Unit (CLU). a. Data Processing Unit On receiving a part programme, the DPU firstly interprets and encodes the part programme into internal machine codes. The interpolator of the DPU then calculate the intermediate positions of the motion in terms of BLU (basic length unit) which is the smallest unit length that can be handled by the controller. The calculated data are passed to CLU for further action. b. Control Loop Unit The data from the DPU are converted into electrical signals in the CLU to control the driving system to perform the required motions. Other functions such as machine spindle ON/OFF, coolant ON/OFF, tool clamp ON/OFF are also controlled by this unit according to the internal machine codes. Machine Tool
15. 15. CAD/CAM Module 2 AM/JA 15 Dept. of Mechanical Engg, AJCE This can be any type of machine tool or equipment. In order to obtain high accuracy and repeatability, the design and make of the machine slide and the driving leadscrew of a CNC machine is of vital importance. The slides are usually machined to high accuracy and coated with anti-friction material such as PTFE and Turcite in order to reduce the stick and slip phenomenon. Large diameter recirculating ball screws are employed to eliminate the backlash and lost motion. Other design features such as rigid and heavy machine structure; short machine table overhang, quick change tooling system, etc also contribute to the high accuracy and high repeatability of CNC machines. Driving System The driving system is an important component of a CNC machine as the accuracy and repeatability depend very much on the characteristics and performance of the driving system. The requirement is that the driving system has to response accurately according to the programmed instructions. This system usually uses electric motors although hydraulic motors are sometimes used for large machine tools. The motor is coupled either directly or through a gear box to the machine leadscrew to moves the machine slide or the spindle. Three types of electrical motors are commonly used. The Display Unit serves as an interactive device between the machine and the operator. When the machine is running, the Display Unit displays the present status such as the position of the machine slide, the spindle RPM, the feed rate, the part programmes, etc. In an advanced CNC machine, the Display Unit can show the graphics simulation of the tool path so that part programmes can be verified before the actually machining. Much other important information about the CNC system can also displayed for maintenance and installation work such as machine parameters, logic diagram of the programmer controller, error massages and diagnostic data. Applications of CNC Machines CNC machines are widely used in the metal cutting industry and are best used to produce the following types of product Parts with complicated contours Parts requiring close tolerance and/or good repeatability Parts requiring expensive jigs and fixtures if produced on conventional machines Parts that may have several engineering changes, such as during the development stage of a prototype In cases where human errors could be extremely costly Parts that are needed in a hurry
16. 16. CAD/CAM Module 2 AM/JA 16 Dept. of Mechanical Engg, AJCE Small batch lots or short production runs Some common types of CNC machines and instruments used in industry are as following: Drilling Machine Lathe / Turning Centre Milling / Machining Centre Turret Press and Punching Machine Wirecut Electro Discharge Machine (EDM) Grinding Machine Laser Cutting Machine Water Jet Cutting Machine Electro Discharge Machine Coordinate Measuring Machine Industrial Robot DIRECT NUMERICAL CONTROL (DNC) Early NC machines used a tape reader for storing and inputting the program into the memory of the NC machine tool. Because of the unreliability of the tape reader as well as the low speed of operation NC engineers were searching for a suitable alternative. The advent of CNC in mid-sixtees opened up the possibility of improving the performance of NC machines by interfacing them with minicomputers. Yet another significant technological development was the interfacing of several NC machines with a computer, which can store the part programs and transfer them to the NC machine concerned as and when needed. The computer is connected between the tape reader and the NC machine thereby bypassing the tape reader. This system was therefore called as behind the tape reader system (BTR). This development became very popular with NC machine users because of a number of significant advantages. i. A number of NC machines can be connected to a single computer. In many cases a single computer can manage all the machines on a shop floor. ii. Programs in full or in segments can be transferred to the NC machines in a multiplexing mode. iii. The computer can be conveniently used for program editing. iv. Since the computer has large memories there is no limitation on the number or size of programs stored. v. The computer can be used for other tasks like program creation using computer aided part program generation software as well as for operation management tasks like production planning, scheduling etc. With the development of CNC, DNC concept was extended to CNC machines also mainly for part program management. The DNC computer (sometimes referred to as host computer) could serve a number of CNC machines in shop floor. Figure 12.19 shows a typical DNC network. The DNC computer stores all the part programs and transfers the part programs to the CNC machines in response to the requests of the operators. DNC systems are generally designed for 4, 8, or 16 CNC machines. However, with the wide spread acceptance of the local area network concept, the possibility of connecting more CNC machine in a DNC network has become a reality. The concepts of Internet, Intranet and Extranet have further enlarged the scope of distributed numerical control.
17. 17. CAD/CAM Module 2 AM/JA 17 Dept. of Mechanical Engg, AJCE Direct Numerical Control(DNC) There are number of problems inherent in conventional machiningwhich are motivated machine tool builders to seek imprivements in the basic NC system. Among the difficulties encountered in using conventional numerical control machine are the following Part programming mistakes,Punched tape, Tape reader, Controller, Management information. DNC defined It can be defined as a manufacturing system in which a number of machines are controlled by a computer through direct connection and in real time. The tape reader is omitted in DNC thus releiving the system of its least reliable component. Instead of using tape reader the part program is tranmitted to the machine tool directly from the computer memory. In principle,one computer can be used to control more than 100 seperate machines. Figure below illustrates the general DNC configuration The system consists of four components. 1. Central computer 2. Bulk memory which stores the NC part programs. 3. Telecommunication lines. 4. Machine Tools. The computer calls the part program instructions from bulk storage and sends data back from the machines. This two way information flow occurs in real time,which means that machine's requests for instructions must be satisfied almost
18. 18. CAD/CAM Module 2 AM/JA 18 Dept. of Mechanical Engg, AJCE instantaneously. Similarly, the computer must always be ready to receive information from the machines and respond accordingly. The remarkable feature of the DNC system is that the computer is ervicing a large number of seperate machine tools,all in real time. Depending upon the number of machines and computational requirements that are imposed on the computer it is sometimes necessary to make use of satellite computersas shown in figure. 12.6.1 OBJECTIVES OF DNC DNC serves many purposes and is now considered as essential for the efficient management of CNC machine tools in the shop floor. The main objectives of implementing DNC are given below: 1. Upload and download CNC programs to and from machine tools simultaneously and directly from the CNC systems. 2. Easy editing of the existing programs. 3. Eliminating the use of manual switch boxes to multiplex CNC machines. 4. Organizing and cataloguing of all programs for instant access. 5. Eliminating the need for manually punching the program at the keyboard thereby saving considerable costly machine time. 6. Eliminating the need for paper tape in the old generation of NC machines. 7. Copy programs to and from the floppy discs and other media to the DNC computer. 8. Compare files edited at the CNC to the original program. 9. Rename or delete or update programs or create new programs. 10. Show pictures of set ups for graphic catalogue of set up and machining operations.
19. 19. CAD/CAM Module 2 AM/JA 19 Dept. of Mechanical Engg, AJCE 11. Providing system transaction files of all activity on the DNC computer. 12. Pass word protection at different points of the CNC system wherever the operator could cause damage to the NC code by overwriting. 13. Tool length offsets from tool pre-setters can be transferred directly to machine tool controls. It is also possible to connect co-ordinate measuring machines to DNC networks. PLC-Programmable Logic Controllers modern controller device used extensively for sequence control today in transfer lines, robotics, process control, and many other automated systems is the Programmable Logic Controller (PLC). In essence, a PLC is a special purpose industrial microprocessor based real-time computing system, which performs the following functions in the context of industrial operations Monitor Input/Sensors Execute logic, sequencing, timing, counting functions for Control/Diagnostics Drives Actuators/Indicators Communicates with other computers Some of the following are advantages of PLCs due to standardized hardware technology, modular design of the PLCs, communication capabilities and improved development program development environment: Easy to use to simple modular assembly and connection; Modular expansion capacity of the input, outputs and memory; Simple programming environments and the use of standardized task libraries and debugging aids; Communication capability with other programmable controllers and computers Reliability Flexibility Advanced Functions Communications Speed Diagnostics
20. 20. CAD/CAM Module 2 AM/JA 20 Dept. of Mechanical Engg, AJCE Ease of programming Ease of maintenance Designed for industrial environment Quick installation Adaptable to change A tabulated data of advanced advantages of PLC is given below
21. 21. CAD/CAM Module 2 AM/JA 21 Dept. of Mechanical Engg, AJCE
22. 22. CAD/CAM Module 2 AM/JA 22 Dept. of Mechanical Engg, AJCE Evolution of the PLC Before the advent of microprocessors, industrial logic and sequence control used to be performed using elaborate control panels containing electromechanical or solid-state relays, contactors and switches, indicator lamps, mechanical or electronic timers and counters etc., all hardwired by complex and elaborate wiring. In fact, for many applications such control panels are used even today. However, the development of microprocessors in the early 1980s quickly led to the development of the PLCs, which had significant advantages over conventional control panels. Some of these are: Programming the PLC is easier than wiring physical components; the only wiring required is that of connecting the I/O terminals. The PLC can be reprogrammed using user-friendly programming devices. Controls must be physically rewired. PLCs take up much less space. Installation and maintenance of PLCs is easier, and with present day solid-state technology, reliability is grater. The PLC can be connected to a distributed plant automation system, supervised and monitored. Beyond a certain size and complexity of the process, a PLC-based system compare favorably with control panels. Ability of PLCs to accept digital data in serial, parallel and network modes imply a drastic reduction in plant sensor and actuator wirings, since single cable runs to remote terminal I/O units can be made. Wiring only need to be made locally from that point. Special diagnostic and maintenance modes for quick troubleshooting and servicing, without disrupting plant operations. However, since it evolved out of relay control panels the PLCs adopted legacy concepts, which were applicable to such panels. To facilitate maintenance and modification of the physically wired control logic, the control panel was systematically organized so that each control formed a rung much like a rung on a ladder. The development of PLCs retained the ladder logic concept where control circuits are defined like rungs on a ladder where each rung begins with one or more inputs and each rung usually ends with only one output. A typical PLC ladder structure is shown below.
23. 23. CAD/CAM Module 2 AM/JA 23 Dept. of Mechanical Engg, AJCE Architecture of PLCs The PLC is essentially a microprocessor-based real-time computing system that often has to handle significant I/O and Communication activities, bit oriented computing, as well as normal floating point arithmetic. A typical set of components that make a PLC System is shown in Fig. below.
24. 24. CAD/CAM Module 2 AM/JA 24 Dept. of Mechanical Engg, AJCE Central controller The central controller (CC) contains the modules necessary for the main computing operation of the Programmable controller (PC). The central controller can be equipped with the following: Memory modules with RAM or EPROM (in the memory sub modules) for the program (main memory); Interface modules for programmers, expansion units, standard peripherals etc; Communications processors for operator communication and visualization, communication with other systems and configuring of local area networks. A bus connects the CPUs with the other modules. Central Processing units The CPUs are generally microprogrammed processors sometimes capable of handling multiple data width of either 8, 16 or 24 bits. In addition some times additional circuitry, such as for bit processing is provided, since much of the computing involves logical operations involving digital inputs and auxiliary quantities. Memory with battery backup is also provided for the following: Flags ( internal relays), timers and counters; Operating system data Process image for the signal states of binary inputs and outputs. The user program is stored in memory modules. During each program scan, the processor reads the statement in the program memory, executes the corresponding operations. The bit processor, if it exists, executes binary operations. Often multiple central controllers can be configured in hot standby
25. 25. CAD/CAM Module 2 AM/JA 25 Dept. of Mechanical Engg, AJCE mode, such that if one processor fails the other can immediately pick up the computing tasks without any failure in plant operations. Communications processors Communications processors autonomously handle data communication with the following: Standard peripherals such as printers, keyboards and CRTs, Supervisory Computer Systems, Other Programmable controllers, The data required for each communications processors is stored in a RAM or EPROM sub module so that they do not load the processor memories. A local area network can also be configured using communications processors. This enables the connection of various PLCs over a wide distance in various configurations. The network protocols are often proprietary. However, over the last decade, interoperable network protocol standards are also supported in modern PLCs. Program and Data memory The program and data needed for execution are stored in RAM or EPROM sub modules. These sub modules are plugged into the processors. Additional RAM memory modules can also be connected. Expansion units Modules for the input and output of signals are plugged into expansion units. The latter are connected to the central controller via interface modules. Expansion units can be connected in two configurations. A. Centralized configuration The expansion units (EU) are located in the same cabinet as the central controllers or in an adjacent cabinet in the centralized configuration, several expansion units can be connected to one central controller. The length of the cable from the central controller to the most distant expansion unit is often limited based on data transfer speeds. B. Distributed configuration The expansion units can be located at a distance of up to 1000 m from the central controller. In the distributed configuration, up to 16 expansion units can be connected to one central controller. Four additional expansion units can be connected in the centralized configuration to each distributed expansion unit and to the central controller. Input/Output Units A host of input and output modules are connected to the PLC bus to exchange data with the processor unit. These can be broadly categorized into Digital Input Modules, Digital Output Modules, Analog Input Modules, Analog Output Modules and Special Purpose Modules. Digital Input Modules The digital inputs modules convert the external binary signals from the process to the internal digital signal level of programmable controllers.
26. 26. CAD/CAM Module 2 AM/JA 26 Dept. of Mechanical Engg, AJCE Digital Output Modules The digital output modules convert the internal signal levels of the programmable controllers into the binary signal levels required externally by the process. Analog Input Modules The analog input modules convert the analog signals from the process into digital values which are then processed by the programmable controller. Analog Output Modules The analog output modules convert digital values from the programmable controller into the analog signals required by the process. Special Purpose Modules These may include special units for: High speed counting High accuracy positioning On-line self-optimizing control Multi axis synchronisation, interpolation These modules contain additional processors, and are used to relieve the main CPU from the high computational loads involved in the corresponding tasks Programmers External programming units can be used to download programs into the program memory of the CPU. The external field programmers provide several software features that facilitate program entry in graphical form. The programmers also provide comprehensive aids for debugging and execution monitoring support logic and sequence control systems. Printer can be connected to the programmers for the purpose of documenting the program. In some cases, special programming packages that run on Personal Computers, can also be used as programming units. There are two ways of entering the program: A. Direct program entry to the program memory (RAM) plugged into the central controller. For this purpose, the programmer is connected to the processor or to the programmer interface modules. B. Programming the EPROM sub modules in the programmer without connecting it to the PC (off-line). The memory sub modules are then plugged into the central controller. Other Miscellaneous Units Other units such as Power Supply Units, Bus Units etc. can also be connected to the PLC system
27. 27. CAD/CAM Module 2 AM/JA 27 Dept. of Mechanical Engg, AJCE PLC Power supplies are typically designed to meet normal operation of +10 to -15%. Fluctuation in voltage. Converts the incoming voltage to a useable form for the internal electronics Protects the PLC s components from voltage spikes .For voltage condition that are unstable insist on a CVT between PLC & primary power source. Operates either on 120V AC/ 240 V AC/ 24VDC. PLCs Power Supply is designed to meet short power losses without affecting the operation of the system. PLC can operate for several ms without line power before the PS signals the processor that it can no longer provide adequate DC Power to the system. The CPU executes a controlled shut down which saves the users program & data in memory The other factor affecting the function of the PLC is EMI or electrical noise.Use an isolation transformer, take care of shielding from Drives, ensure proper earthing & cabling practices. Program Execution There are different ways and means of executing a user program. Normally a cyclic execution program is preferred and this cyclic operators are given due priorities. Program processing in a PLC happens cyclically with the following execution: 1. After the PLC is initialised, the processor reads the individual inputs. This status of the input is stored in the process- image input table (PII). 2. This processor processes the program stored in the program memory. This consists of a list of logic functions and instructions, which are successively processed, so that the required input information will already be accessed before the read in PII and the matching results are written into a process-image output table (PIQ). Also other storage areas for counters, timers and memory bits will be accessed during program processing by the processor if necessary. 3. In the third step after the processing of the user program, the status from the PIQ will transfer to the outputs and then be switched on and/or off. Afterwards it begins the execution of the next cycle from step 1. The same cyclic process also acts upon an RLL program. The time required by the microprocessor to complete one cycle is known as the scan time. After all rungs have been tested, the PLC then starts over again with the first rung. Of course the scan time for a particular processor is a function of the processor speed, the number of rungs, and the complexity of each rung.
28. 28. CAD/CAM Module 2 AM/JA 28 Dept. of Mechanical Engg, AJCE Programming Languages PLC programs can be constructed using various methods of representation. Some of the common ones are, described below. The Relay Ladder Logic (RLL) Diagram A Relay Ladder Logic (RLL) diagram, also referred to as a Ladder diagram is a visual and logical method of displaying the control logic which, based on the inputs determine the outputs of the program. The ladder is made up of a series of rungs of logical expressions expressed graphically as series and parallel circuits of relay logic elements such as contacts, timers etc. Each rung consist of a set of inputs on the left end of the rung and a single output at the right end of each rung. The structure of a rung is shown below in Fig. 19.1(a) & (b). Fig. 19.1 shows the internal structure of a simple rung in terms its element contacts connected in a series parallel circuit.
29. 29. CAD/CAM Module 2 AM/JA 29 Dept. of Mechanical Engg, AJCE RLL Programming Paradigms: Merits and Demerits For the programs of small PLC systems, RLL programming technique has been regarded as the best choice because a programmer can understand the relations of the contacts and coils intuitively. Additionally, a maintenance engineer can easily monitor the operation of the RLL program on its graphical representation because most PLC manufacturers provide an animated display that clearly identifies the states of the contacts and coils. Although RLL is still an important language of IEC 1131-3, as the memory size of today's PLC systems increases, a large-sized RLL program brings some significant problems because RLL is not particularly suitable for the well-structured programming: It is difficult to structure an RLL program hierarchically.
30. 30. CAD/CAM Module 2 AM/JA 30 Dept. of Mechanical Engg, AJCE Example: Forward Reverse Control This example explains the control process of moving a motor either in the forward direction or in the reverse direction. The direction of the motor depends on the polarity of the supply. So in order to control the motor, either in the forward direction or in the reverse direction, we have to provide the supply with the corresponding polarity. The Fig 19.2 depicts the procedure to achieve this using Relay Ladder Logic. Here, the Ladder consists of two rungs corresponding to forward and reverse motions. The rung corresponding to forward motion consists of 1. A normally closed stop push-button (IN001), 2. A normally opened forward run push-button (IN002) in parallel with a normally opened auxillary contact(OP001), 3. A normally closed auxillary contact(OP002) and 4. The contacter for coil(OP001). Similarly, the rung corresponding to reverse motion consists of 1. A normally closed stop push-button (IN001), 2. A normally opened forward run push-button (IN003) in parallel with a normally opened auxillary contact(OP002), 3. A normally closed auxillary contact(OP001) and 4. The contacter for coil(OP002). Operation: The push-buttons(PB) represented by IN--- are real input push-buttons, which are to be manually operated. The auxillary contacts are operated through program. Initially the machine is at standstill, no voltage supply is present in the coils, and the PBs are as shown in the fig. The stop PB is intially closed, the motor will not move until the forward run PB/reverse run PB is closed. Suppose we want to run the motor in the forward direction from standstill, the outputs of the coils contacters have logic 0 and hence both the auxillary contacts are turned on. If we press and release the forward
31. 31. CAD/CAM Module 2 AM/JA 31 Dept. of Mechanical Engg, AJCE run PB, the positive voltage from the +ve voltage rail is passed to thecoil. Once the coil contacter gives the logic 1, the following consequences takes place simultaneously A. The auxillary contact OP001 in the second rung becomes opened,which stops the voltage for reverse motion of the motor. At this stage, the second rung is not turned on even the reverse run PB is pressed by mistake. B. The auxillary contact OP001 is the first rung is on, which provides the path for the positive voltage until the stop PB is pressed. Here the auxillary contact OP001 acts as a latch, which facilitates even to remove the PB IN002 once the coil OP001 is on. If we want to rotate the motor in the reverse direction, the stop PB is to be pressed sothat no voltage in the coil is present, then we can turn on the PB corresponding to reverse run. This is a simple example of interlocking, where each rung locks the operation of the other rung. There are several other programming paradigms for PLCs. Two of them are mentioned here for briefly. The Function Chart (IEC) Depicts the logic control task symbols in terms of functional blocks connected symbolically in a graphic format. The Statement List (STL) Is made up of series of assembly language like statements each one of which represents a logic control statement executable by the processor of the programmable controller. The statement list is the most unrestricted of all the methods of representation. Individual statements are made up of mnemonics, which represent the function to be executed. This method of representation is favoured by those who have already had experience in programming microprocessors or computers. Programmable controller contacts and electromechanical relay contacts operate in a very similar fashion. For example, lets take relay A (see Figure 3-14a) which has two sets of contacts, one normally open contact (A-1) and one normally closed contact (A-2). If relay coil A is not energized (i.e., it is OFF), contact A-1 will remain open and contact A-2 will remain closed (see Figure 3-14b). Conversely, if coil A is energized, or turned ON, contact A-1 will close and contact A-2 will open (see Figure 3-14c). The blue lines highlighting the coil and contacts denote an ON, or closed, condition.
32. 32. CAD/CAM Module 2 AM/JA 32 Dept. of Mechanical Engg, AJCE The following seven points describe guidelines for translating from hardwired logic to programmed logic using PLC contact symbols:
33. 33. CAD/CAM Module 2 AM/JA 33 Dept. of Mechanical Engg, AJCE Normally open contact. When evaluated by the program, this symbol is examined for a 1 to close the contact; therefore, the signal referenced by the symbol must be ON, CLOSED, activated, etc. Normally closed contact. When evaluated by the program, this symbol is examined for a 0 to keep the contact closed; thus, the signal referenced by the symbol must be OFF, OPEN, deactivated, etc. ` Output. An output on a given rung will be energized if any left-to right path has all contacts closed, with the exception of power flow going in reverse before continuing to the right. An output can control either a connected device (if the reference address is also a termination point) or an internal output used exclusively within the program. An internal output does not control a field device. Rather, it provides interlocking functions within the PLC. Input. This contact symbol can represent input signals sent from connected inputs, contacts from internal outputs, or contacts from connected outputs. Contact addresses. Each program symbol is referenced by an address. If the symbol references a connected input/output device, then the address is determined by the point where the device is connected. Repeated use of contacts. A given input, output, or internal output can be used throughout the program as many times as required. Logic format. Contacts can be programmed in series or in parallel, depending on the output control logic required. The number of series contacts or parallel branches allowed in a rung depends on the PLC. Table 3-6a show how simple hardwired series and parallel circuits can be translated into programmed logic. A series circuit is equivalent to the Boolean AND operation; therefore, all inputs must be ON to activate the output. A parallel circuit is equivalent to the Boolean OR operation; therefore, any one of the inputs must be ON to activate the output. The STR and OUT Boolean statements stand for START (of a new rung) and OUTPUT (of a rung), respectively. Table 3-6b further explains Table 3-6a.
34. 34. CAD/CAM Module 2 AM/JA 34 Dept. of Mechanical Engg, AJCE
35. 35. CAD/CAM Module 2 AM/JA 35 Dept. of Mechanical Engg, AJCE MEMORY TYPES * The storage and retrieval requirements for the executive and application memory sections are not the same; therefore, they are not always stored in the same type of memory. For example, the executive requires a memory that permanently stores its contents and cannot be erased or altered either by loss of electrical power or by the user. This type of memory is often unsuitable for the application program. Memory can be separated into two categories: volatile and nonvolatile. Volatile memory loses its programmed contents if all operating power is lost or removed, whether it is normal power or some form of backup power. Volatile memory is easily altered and quite suitable for most applications when supported by battery backup and possibly a disk copy of the program. Nonvolatile memory retains its programmed contents, even during a complete loss of operating power, without requiring a backup source. Nonvolatile memory generally is unalterable, yet there are special nonvolatile memory types that are alterable. Todays PLCs include those that use nonvolatile memory, those that use volatile memory with battery backup, as well as those that offer both. There are two major concerns regarding the type of memory where the application program is stored. Since this memory is responsible for retaining the control program that will run each day, volatility should be the prime concern. Without the application program, production may be delayed or forfeited, and the outcome is usually unpleasant. A second concern should be the ease with which the program stored in memory can be altered. Ease in altering the application memory is important, since this memory is ultimately involved in any interaction between the user and the controller. This interaction begins with program entry and continues with program changes made during program generation and system start-up, along with on-line changes, such as changing timer or counter preset values. The following discussion describes six types of memory and how their characteristics affect the manner in which programmed instructions are retained or altered within a programmable controller. READ-ONLY MEMORY Read-only memory (ROM) is designed to permanently store a fixed program that is not alterable under ordinary circumstances. It gets its name from the fact that its contents can be examined, or read, but not altered once information has been stored. This contrasts with memory types that can be read from and written to (discussed in the next section). By nature, ROMs are generally immune to alteration due to electrical noise or loss of power. Executive programs are often stored in ROM. Programmable controllers rarely use read-only memory for their application memory. However, in applications that require fixed data, read- only memory offers advantages when speed, cost, and reliability are factors. Generally, the manufacturer creates ROM-based PLC programs at the factory. Once the manufacturer programs the original set of instructions, the user can never alter it. This typical approach to the programming of ROM-based controllers assumes that the program has already been debugged
36. 36. CAD/CAM Module 2 AM/JA 36 Dept. of Mechanical Engg, AJCE and will never be changed. This debugging is accomplished using a random-access memory based PLC or possibly a computer. The final program is then entered into ROM. ROM application memory is typically found only in very small, dedicated PLCs. RANDOM-ACCESS MEMORY Random-access memory (RAM), often referred to as read/write memory (R/W), is designed so that information can be written into or read from the memory storage area. Random-access memory does not retain its contents if power is lost; therefore, it is a volatile type of memory. Random-access memory normally uses a battery backup to sustain its contents in the event of a power outage. For the most part, todays programmable controllers use RAM with battery support for application memory. Random-access memory provides an excellent means for easily creating and altering a program, as well as allowing data entry. In comparison to other memory types, RAM is a relatively fast memory. The only noticeable disadvantage of battery-supported RAM is that the battery may eventually fail, although the processor constantly monitors the status of the battery. Battery-supported RAM has proven to be sufficient for most programmable controller applications. If a battery backup is not feasible, a controller with a nonvolatile memory option (e.g., EPROM) can be used in combination with the RAM. This type of memory arrangement provides the advantages of both volatile and nonvolatile memory. Figure 5-2 shows a RAM chip. PROGRAMMABLE READ-ONLY MEMORY Programmable read-only memory (PROM) is a special type of ROM because it can be programmed. Very few of todays programmable controllers use PROM for application memory. When it is used, this type of memory is most likely a permanent storage backup for some type of RAM. Although a PROM is programmable and, like any other ROM, has the advantage of nonvolatility, it has the disadvantage of requiring special programming equipment. Also, once programmed, it cannot be easily erased or altered; any program change requires a new set of PROM chips. A PROM memory is suitable for storing a program that has been thoroughly checked while residing in RAM and will not require further changes or on-line data entry. ERASABLE PROGRAMMABLE READ-ONLY MEMORY Erasable programmable read-only memory (EPROM) is a specially designed PROM that can be reprogrammed after being entirely erased by an ultraviolet (UV) light source. Complete erasure of the contents of the chip requires that the window of the chip (see Figure 5-3) be exposed to a UV light source for approximately twenty minutes. EPROM can be considered a semipermanent storage device, because it permanently stores a program until it is ready to be altered. EPROM provides an excellent storage medium for application programs that require nonvolatility, but that do not require program changes or on-line data entry. Many OEMs use controllers with EPROM-type memories to provide permanent storage of the machine program after it has been debugged and is fully operational. OEMs use EPROM because most of their machines will not require changes or data entry by the user. An application memory composed of EPROM alone is unsuitable if on-line changes or data entry are required. However, many controllers offer EPROM application memory as an optional
37. 37. CAD/CAM Module 2 AM/JA 37 Dept. of Mechanical Engg, AJCE backup to battery-supported RAM. EPROM, with its permanent storage capability, combined with RAM, which is easily altered, makes a suitable memory system for many applications. ELECTRICALLY ALTERABLE READ-ONLY MEMORY Electrically alterable read-only memory (EAROM) is similar to EPROM, but instead of requiring an ultraviolet light source to erase it, an erasing voltage on the proper pin of an EAROM chip can wipe the chip clean. Very few controllers use EAROM as application memory, but like EPROM, it provides a nonvolatile means of program storage and can be used as a backup to RAM-type memories. ELECTRICALLY ERASABLE PROGRAMMABLE READ-ONLY MEMORY Electrically erasable programmable read-only memory (EEPROM) is an integrated circuit memory storage device that was developed in the mid- 1970s. Like ROMs and EPROMs, it is a nonvolatile memory, yet it offers the same programming flexibility as RAM does. Several of todays small and medium-sized controllers use EEPROM as the only memory within the system. It provides permanent storage for the program and can be easily changed with the use of a programming device (e.g., a PC) or a manual programming unit. These two features help to eliminate downtime and delays associated with programming changes. They also lessen the disadvantages of electrically erasable programmable read only memory. One of the disadvantages of EEPROM is that a byte of memory can be written to only after it has been erased, thus creating a delay. This delay period is noticeable when on-line program changes are being made. Another disadvantage of EEPROM is a limitation on the number of times that a single byte of memory can undergo the erase/write operation (approximately 10,000). These disadvantages are negligible, however, when compared to the remarkable advantages that EEPROM offers. MEMORY STRUCTURE AND CAPACITY BASIC STRUCTURAL UNITS PLC memories can be thought of as large, two-dimensional arrays of singleunit storage cells, each storing a single piece of information in the form of 1 or 0 (i.e., the binary numbering format). Since each cell can store only one binary digit and bit is the acronym for binary digit, each cell is called a bit. A bit, then, is the smallest structural unit of memory. Although each bit stores information as either a 1 or a 0, the memory cells do not actually contain the numbers 1 and 0 per se. Rather, the cells use voltage charges to represent 1 and 0the presence of a voltage charge represents a 1, the absence of a charge represents a 0. A bit is considered to be ON if the stored information is 1 (voltage present) and OFF if the stored information is 0 (voltage absent). The ON/OFF information stored in a single bit is referred to as the bit status. Sometimes, a processor must handle more than a single bit of data at a time. For example, it is more efficient for a processor to work with a group of bits when transferring data to and from memory. Also, storing numbers and codes requires a grouping of bits. A group of bits handled simultaneously is called a byte. More accurately, a byte is the smallest group of bits that can be handled by the processor at one time. Although byte size is normally eight bits,
38. 38. CAD/CAM Module 2 AM/JA 38 Dept. of Mechanical Engg, AJCE this size can vary depending on the specific controller. The third and final structural information unit used within a PLC is a word. In general, a word is the unit that the processor uses when data is to be operated on or instructions are to be performed. Like a byte, a word is also a fixed group of bits that varies according to the controller; however, words are usually one byte or more in length. For example, a 16-bit word consists of two bytes. Typical word lengths used in PLCs are 8, 16, and 32 bits. Figure 5-4 illustrates the structural units of a typical programmable controller memory.
39. 39. CAD/CAM Module 2 AM/JA 39 Dept. of Mechanical Engg, AJCE Timers and Counters
40. 40. CAD/CAM Module 2 AM/JA 40 Dept. of Mechanical Engg, AJCE
41. 41. CAD/CAM Module 2 AM/JA 41 Dept. of Mechanical Engg, AJCE EXAMPLE: The first version of sequence control program for the industrial stamping process A hastily constructed RLL program for the above process may look like the one given in Figure 20.2. The above program logic indicates that the Up solenoid output becomes activated when the Master Switch is on and the bottom Limit Switch is on. Also there is interlock provided, so that when the Down solenoid is on, the Up solenoid cannot be on. Further, once the Up solenoid is on, the output is latched by an auxiliary contact, so that it remains on till the bottom LS is made on, when it turns off. A similar logic has been implemented for the activation of the Down solenoid. However, on closer examination, several problems may be discovered with the above program. Some of these are discussed below. For example, there is no provision for a Master stop switch to stop the press from stopping in an emergency, except by turning the Master Switch off. This would indeed stop the process, however, if the press stops midway, both bottom and top limit switches would be off. Now the process would not start, even if the Master switch is turned on again. Therefore, either a manual jogging control needs to be provided, so that the operator can return the piston to the up position by manually operating the hydraulics, or special auto mode logic should be designed to perform this. As a second example, note that this process does not have a part detect sensor. This implies that the moment the Master switch is on the press would start going up and down at its own travel speed, regardless of whether a part has been placed for pressing or not. Apart from wastage
42. 42. CAD/CAM Module 2 AM/JA 42 Dept. of Mechanical Engg, AJCE of energy, this could be safety hazard for an operator who has to place the part on the machine between the interval of a cycle of operation. The above discussion clearly indicates the need for a systematic approach towards the development of RLL programs for industrial logic control problems. This is all the more true since industrial process control is critical application domain where control errors can lead to loss of production or operator safety. Therefore, in this chapter, we discuss a systematic approach towards the design of RLL programs. Timer These are special operands of a PLC, which represent a time delay relay in a relay logic system. The time functions are a fixed component of the central processing unit. The number of these varies from manufacturer to manufacturer and from product to product. It is possible to achieve time delays in the range of few milliseconds to few hours. Representation for timers is shown in Fig. 19.4. Timers have a preset register value, which represent the maximum count it can hold and can be set using software/program. The figure shown below has a enable reset logic and run logic in connection with the timer. The counter doesnot work and the register consists of zero until the enable reset logic is on. Once the enable reset logic is on, the counter starts counting when the run logic is on. The output is on only when the counter reaches the maximum count. Various kinds of timers are explained as follows
43. 43. CAD/CAM Module 2 AM/JA 43 Dept. of Mechanical Engg, AJCE On delay timer: The input and output signals of the on delay timer are as shown in the Fig. 19.6. When the input signal becomes on, the output signal becomes on with certain delay. But when the input signal becomes off, the output signal also becomes off at the same instant. If the input becomes on and off with the time which less than the delay time, there is no change in the output and remains in the off condition even the input is turned on and off i.e., output is not observed until the input pulse width is greater than the delay time. Realization of on-delay timer: The realization of on-delay timer using the basic timer shown in the previous fig is explained here. The realization is as shown in the Fig. 19.6, which shows a real input switch(IN001), coil1(OP002), two normally opened auxillary contacts(OP002), coil2(OP002). When the real input switch is on the coil(OP002) is on and hence both the auxillary switches are on. Now the counter value starts increasing and the output of the timer is on only after it reaches the maximum preset count. The behaviour of this timer is shown in figure, which shows the on-delay timer. The value in the counter is reset when the input switch(IN001) is off as the enable reset logic is off. This is a non- retentive timer. Off delay timer: The input and output signals of the off delay timer are as shown in the Fig. 19.7. When the input signal becomes on, the output signal becomes on at the same time. But when the input signal becomes off, the output signal becomes off with certain delay. If the input becomes on and off with the time which less than the delay time, there is no change in the output and remains in the on condition even the ipnut is turned on and off i.e., the delay in the output is not observed until the input pulse width is greater than the delay time.
44. 44. CAD/CAM Module 2 AM/JA 44 Dept. of Mechanical Engg, AJCE Retentive timer: The input and output signals of the retentive timer are as shown in the 19.11. This is also implemented internally in a register as in the previous case. When the input is on , the internal counter starts counting until the input is off and at this time, the counter holds the value till next input pulse is applied and then starts counting starting with the value existing in the register. Hence it is named as retentive timer. The output is on only when the counter reaches its terminal count. Non-retentive timer: The input and output signals of the non-retentive timer are as shown in the Fig. 19.11. This is implemented internally in a register. When the input is on, the internal counter starts counting until the input is off and at this time the value in the counter is reset to zero. Hence it is named as non-retentive timer. The output is on only when the counter reaches its terminal count.
45. 45. CAD/CAM Module 2 AM/JA 45 Dept. of Mechanical Engg, AJCE Counter The counting functions (C) operate as hardware counters, but are a fixed component of the central processing unit. The number of these varies for each of the programmable controllers. It is possible to count up as well as to count down. The counting range is from 0 to 999. The count is either dual or BCD coded for further processing. User defined Data If the memory capacity of the flag area is not sufficient to memorize the signal status and data, the operand area data (D) is applied. In general, in the flag area, primarily binary conditions apply, whereas in the data area digital values prevail and are committed to memory. The data is organized into data blocks (DB). 256 data words with 16 bit each can be addressed to each data block. The data is stored in the user memory sub module. The available capacity within the module has to be shared with the user program. Addressing The designation of a certain input or output within the program is referred to as addressing. Different PLC manufacturers adopt different conventions for specifying the address of a specific input or output signal. A typical addressing scheme adopted in PLCs manufacturers by Siemens is illustrated in the sequel. The inputs and outputs of the PLCs are mostly defined in groups of eight on digital input and/or digital output devices. This eight unit is called a byte. Every such group receives a number as a byte address. Each in/output byte is divided into 8 individual bits, through which it can respond with. These bits are numbered from bit 0 to bit 7. Thus one receives a bit address. For example, in the address I0.4, I denotes that the address type is specified as Input, 0 is the byte address and 4 the bit address. Similarly in the address Q5.7, Q denotes that the address type is specified as Output, 5 is the byte address and 7 is the bit address.
46. 46. CAD/CAM Module 2 AM/JA 46 Dept. of Mechanical Engg, AJCE Driving devices- Electrical and Mechanical Components A drive system consists of amplifier circuits, drive motors, and ball lead-screws. The MCU feeds the control signals (position and speed) of each axis to the amplifier circuits. The control signals are augmented to actuate drive motors which in turn rotate the ball lead-screws to position the machine table. The driving system is an important component of a CNC machine as the accuracy and repeatability depend very much on the characteristics and performance of the driving system. The requirement is that the driving system has to response accurately according to the programmed instructions. This system usually uses electric motors although hydraulic motors are sometimes used for large machine tools. The motor is coupled either directly or through a gear box to the machine leadscrew to moves the machine slide or the spindle. Three types of electrical motors are commonly used. Power units In machine tools, power is generally required for For driving the main spindle For driving the saddles and carriages. For providing power for some ancillary units. The motors used for CNC system are of two kinds Electrical - AC , DC or Stepper motors Electric motors are by far the most common component to supply mechanical input to a linear motion system. Stepper motors and servo motors are the popular choices in linear motion machinery due to their accuracy and controllability. They exhibit favourable torque-speed characteristics and are relatively inexpensive.
47. 47. CAD/CAM Module 2 AM/JA 47 Dept. of Mechanical Engg, AJCE Electrical Components of Driving System a. DC Servo Motor This is the most common type of feed motors used in CNC machines. The principle of operation is based on the rotation of an armature winding in a permanently energised magnetic field. The armature winding is connected to a commutator, which is a cylinder of insulated copper segments mounted on the shaft. DC current is passed to the commutator through carbon brushes, which are connected to the machine terminals. The change of the motor speed is by varying the armature voltage and the control of motor torque is achieved by controlling the motor's armature current. In order to achieve the necessary dynamic behaviour it is operated in a closed loop system equipped with sensors to obtain the velocity and position feedback signals.
48. 48. CAD/CAM Module 2 AM/JA 48 Dept. of Mechanical Engg, AJCE The digital servo motor controller directs operation of the servo motor by sending velocity command signals to the amplifier, which drives the servo motor. An integral feedback device (resolver) or devices (encoder and tachometer) are either incorporated within the servo motor or are remotely mounted, often on the load itself. These provide the servo motor's position and velocity feedback that the controller compares to its programmed motion profile and uses to alter its velocity signal. Servo motors feature a motion profile, which is a set of instructions programmed into the controller that defines the servo motor operation in terms of time, position, and velocity. The ability of the servo motor to adjust to differences between the motion profile and feedback signals depends greatly upon the type of controls and servo motors used. b. AC Servo Motor In an AC servomotor, the rotor is a permanent magnet while the stator is equipped with 3-phase windings. The speed of the rotor is equal to the rotational frequency of the magnetic field of the stator, which is regulated by the frequency converter. AC motors are gradually replacing DC servomotors. The main reason is that there is no commutator or brushes in AC servomotor so that maintenance is virtually not required. Furthermore, AC servos have a smaller power-to-weight ratio and faster response.
49. 49. CAD/CAM Module 2 AM/JA 49 Dept. of Mechanical Engg, AJCE c. Stepping Motor A stepping motor is a device that converts the electrical pulses into discrete mechanical rotational motions of the motor shaft. This is the simplest device that can be applied to CNC machines since it can convert digital data into actual mechanical displacement. It is not necessary to have any analog-to-digital converter nor feedback device for the control system. They are ideally suited to open loop systems. However, stepping motors are not commonly used in machine tools due to the following drawbacks: slow speed, low torque, low resolution and easy to slip in case of overload. Examples of stepping motor application are the magnetic head of floppy-disc drive and hard disc drive of computer, daisy-wheel type printer, X-Y tape control, and CNC EDM Wire-cut machine. d. Linear Motor A linear electric motor is an AC rotary motor laid out flat. The same principle used to produce torque in rotary motors is used to produce force in linear motors. Through the electromagnetic interaction between a coil assembly and a permanent magnet assembly, the electrical energy is converted to linear mechanical energy to generate a linear motion. As the motion of the motor is linear instead of rotational, therefore it is called linear motor. Linear motors have the advantages of high speeds, high precision and fast response. In the 1980s, machine tool builders started using linear motors with the common motion control
50. 50. CAD/CAM Module 2 AM/JA 50 Dept. of Mechanical Engg, AJCE servo drives in the machine tool design. Among different designs of linear motors, permanent magnet brushless motors demonstrate a high force density, high maximum speed, and stable force constant. The lack of a brushed commutator assembly has the advantages of fewer maintenance, higher reliability and better smoothness. An iron core brushless linear motor is similar to a conventional brushless rotary motor slit axially and then rolled out flat. The unrolled rotor is a stationary plate consisting of magnets tiled on an iron back plate and the unrolled stator is a moving coil assembly consisting of coils wound around a laminated steel core. Coil windings are typically connected in conventional 3 phase arrangement and commutation is often performed by Hall-effect sensors or sinusoidal. It has high efficiency and good for continuous force. An ironless linear motor consists of a stationary U shaped channel filled with permanent magnets tiled along both interior walls. A moving coil
51. 51. CAD/CAM Module 2 AM/JA 51 Dept. of Mechanical Engg, AJCE assembly traverses between two opposing rows of magnets. Commutation is done electronically either by Hall-effect sensors or sinusoidal. The ironless linear motor has the advantages of lower core mass, lower inductance and no cogging for smooth motion as the ironless motors have no attractive force between the frameless components. Mechanical Components of Driving System The drive units of the carriages in NC machine tools are generally the screw & the nut mechanism. There are different types of screws and nuts used on NC machine tools which provide low wear, higher efficiency, low friction and better reliability. (1) Recirculating ball screw The recirculating ball screw assembly shown in figure 25.1 has the flanged nut attached to the moving chamber and the screw to the fixed casting. Thus the moving member will move during rotational movement of the screw. These ball screw designs can have ball gages of internal or external return, but all of them are based upon the "Ogival" or "Gothic arc". In these types of screws, balls rotate between the screw and nut and convert the sliding friction (as in conventional nut & screw) to the rolling friction. As a consequence wear will be reduced and reliability of the system will be increased. The traditional ACME thread used in conventional machine tool has efficiency ranging from 20% to 30% whereas the efficiency of ball screws may reach up to 90%.
52. 52. CAD/CAM Module 2 AM/JA 52 Dept. of Mechanical Engg, AJCE Figure 25.1: Recirculating ball screw assembly There are two types of ball screws. In the first type, balls are returned through an external tube after few threads. In another type, the balls are returned to the start through a channel inside the nut after only one thread. To make the carriage movement bidirectional, backlash between the screw and nut should be minimum. One of the methods to achieve zero backlash is by fitting two nuts. The nuts are preloaded by an amount which exceeds the maximum operating load. These nuts are either forced apart or squeezed together, so that the balls in one of the nuts contact the opposite side of the threads.
53. 53. CAD/CAM Module 2 AM/JA 53 Dept. of Mechanical Engg, AJCE These ball screws have the problem that minimum diameter of the ball (60 to 70% of the lead screw) must be used, limiting the rate of movement of the screw. Roller screw Roller screw These types of screws provide backlash-free movement and their efficiency is same as that of ball screws. These are capable of providing more accurate position control. Cost of the roller screws are more compared to ball screws. The thread form is triangular with an included angle of 90 degrees. There are two types of roller screws: planetary and recirculating screws. Planetary roller screws: Planetary roller screws are shown in figure 25.3. The rollers are threaded with a single start thread. Teeth are cut at the ends of the roller, which meshes with the internal tooth cut inside the nut. The rollers are equally spaced around and are retained in their positions by spigots or spacer rings. There is no axial movement of the rollers relative to the nut and they are capable of transmitting high load at fast speed.
54. 54. CAD/CAM Module 2 AM/JA 54 Dept. of Mechanical Engg, AJCE Recirculating roller screws: The rollers in this case are not threaded and are provided with a circular groove and are positioned circumferentially by a cage. There is some axial movement of the rollers relative to the nut. Each roller moves by a distance equal to the pitch of the screw for each rotation of the screw or nut and moves into an axial recess cut inside the nut and disengage from the threads on the screw and the nut and the other roller provides the driving power. Rollers in the recess are moved back by an edge cam in the nut. Recirculating roller screws are slower in operation, but are capable of transmitting high loads with greater accuracy. Feedback Devices The feedback system is also referred to as the measuring system. It uses position and speed transducers to continuously monitor the position at which the cutting tool is located at any particular instant. The MCU uses the difference between reference signals and feedback signals to generate the control signals for correcting position and speed errors. In order to have a CNC machine operating accurately, the position values and speed of axels need to constantly updated. Two type of feedback devices are generally used, position feed back and velocity feed back devices Encoders An encoder is a device that converts information from one format or code to another, for the purposes of standardization. The encoder is a transducer that is connected directly to the rotor or the lead screw and hence is the simplest arrangement requiring no additional gearing. An optical rotary encoder converts the rotary motion of the motor into a sequence of digital pulses. The pulses counted to convert to the position measurement. The optical encoder consists of a disc with a number of accurately etched equidistant lines or slots along the periphery. The encoder disc is attached to the shaft of the machine whose rotary position needs to be measured. The disc is placed between a light source and a light-measuring device. When the disc rotates the lines are interrupted and the light-measuring device counts the number of times the light is interrupted. By a careful counting and appropriate calculations it is possible to know the position traversed by the shaft.
55. 55. CAD/CAM Module 2 AM/JA 55 Dept. of Mechanical Engg, AJCE An incremental optical encoder (left-hand diagram in Figure 6.12) is a disc divided into sectors that are alternately transparent and opaque. A light source is positioned on one side of the disc, and a light sensor on the other side. As the disc rotates, the output from the detector switches alternately on and off, depending on whether the sector appearing between the light source and the detector is transparent or opaque. Thus, the encoder produces a stream of square wave pulses which, when counted, indicate the angular position of the shaft. Available encoder resolutions (the number of opaque and transparent sectors per disc) range from 100 to 65,000, with absolute accuracies approaching 30 arc-seconds (1/43,200 per rotation). Most incremental encoders feature a second light source and sensor at an angle to the main source and sensor, to indicate the direction of rotation. Many encoders also have a third light source and detector to sense a once-per-revolution marker. Without some form of revolution marker, absolute angles are difficult to determine. A potentially serious disadvantage is that incremental encoders require external counters to determine absolute angles within a given rotation. If the power is momentarily shut off, or if the encoder misses a pulse due to noise or a dirty disc, the resulting angular information will be in error.
56. 56. CAD/CAM Module 2 AM/JA 56 Dept. of Mechanical Engg, AJCE The absolute optical encoder (right-hand diagram in Figure 6.12) overcomes these disadvantages but is more expensive. An absolute optical encoder's disc is divided up into N sectors (N = 5 for example shown), and each sector is further divided radially along its length into opaque and transparent sections, forming a unique N-bit digital word with a maximum count of 2N 1. The digital word formed radially by each sector increments in value from one sector to the next, usually employing Gray code. Binary coding could be used, but can produce large errors if a single bit is incorrectly interpreted by the sensors. Gray code overcomes this defect: the maximum error produced by an error in any single bit of the Gray code is only 1 LSB after the Gray code is converted into binary code. A set of N light sensors responds to the N-bit digital word which corresponds to the disc's absolute angular position. Industrial optical encoders achieve up to 16-bit resolution, with absolute accuracies that approach the resolution (20 arc seconds). Both absolute and incremental optical encoders, however, may suffer damage in harsh industrial environments. Morie Fringes High precision linear displacement measurements can be made with Moire Fringes, as shown in See The Moire Fringe Effect. Both of the strips are transparent (or reflective), with black lines at measured intervals. The spacing of the lines determines the accuracy of the position measurements. The stationary strip is offset at an angle so that the strips interfere to give irregular patterns. As the moving strip travels by a stationary strip the patterns will move up, or down, depending upon the speed and direction of motion.
57. 57. CAD/CAM Module 2 AM/JA 57 Dept. of Mechanical Engg, AJCE The Moire Fringe Effect A device to measure the motion of the moire fringes is shown in See Measuring Motion with Moire Fringes. A light source is collimated by passing it through a narrow slit to make it one slit width. This is then passed through the fringes to be detected by light sensors. At least two light sensors are needed to detect the bright and dark locations. Two sensors, close enough, can act as a quadrature pair, and the same method used for quadrature encoders can be used to determine direction and distance of motion.
58. 58. CAD/CAM Module 2 AM/JA 58 Dept. of Mechanical Engg, AJCE Digitizer A digitizer is the most widely used input medium by the CAD designer. It is used for converting the physical locations into coordinate values so that accurate transfer of data can be achieved. A digitizing tablet is considered as a pointing and locating device. It is a small, low- resolution digitising board often used in conjunction with a graphics display. The tablet is a flat surface over which a stylus or a puck can be moved by the user. The close resemblance of the tablet and stylus to paper and pencil contributes to its popularity as an input device. The puck contains a rectile and at least one pushbutton. The engraved cross-hairs of rectile help locate a point for digitising. Pressing the pushbutton sends the coordinates at the cross-hairs to the computer. The sizes of digitising tablets range from 11 x 11 to 36 x 36 inches. The resolution of a tablet is 0.005 inch or 200 dots per inch. The tablet operation is based on sensitising its surface area to be able to track the pointing element (stylus or puck) motion on the surface. The surface of the tablet is magnetised and is embedded with wires in the x and v directions. The physical motion of the stylus is converted by the wires into a digital location signal, which is then routed to the computer and displayed on the graphics terminal. Measuring Motion with Moire Fringes These are used in high precision applications over long distances, often meters. They can be purchased from a number of suppliers, but the cost will be high. Typical applications include Coordinate Measuring Machines (CMMs).
59. 59. CAD/CAM Module 2 AM/JA 59 Dept. of Mechanical Engg, AJCE With Digitizer, you can read in a bitmap, define axes and click on data points. The program will convert the pixel coordinates to plot coordinates and can save them to a tab-delimited text list. Copy and Paste ar