Mechatronics

Module 1

• Fundamentals of NC• Advantages of NC• Classification of NC

– PTP- Contour– NC-CNC– Incremental – absolute– Open loop – closed loop

• Punched Card

Manufacturing Systems Evolution

• Construction of simple production machine started in 1770• Fixed automatic mechanisms and transfer lines for mass

production came along at this century• Introduction to Numerical control NC in 1952• The logical extension of NC – CNC• Industrial robots were developed simultaneously with CNC• First commercial robot in 1961• 1969 Mechatronics – • FMS with CAD / CAM

Fundamentals of Numerical Control

• Fundamentals of Numerical Control

Definition – EIAElectronics Industries Assoc.

• Numerical Control is a system in which actions are controlled by the direct insertion of numerical data at some point. The system must automatically interpret at least some portion of the data.

MACHINE UNIT

NUMERICAL CONTROLLER

NUMERICAL DATA

(NC CODE)

MANUFACTURINGOPERATOR

PROCESSED PART

Drive Control

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Types of Numerical Control

• Conventional Numerical Control (NC)– Data is sent to the machine tool by means of

punch cards or tapes. The reader at the machine performs no calculations or interpolations.

• Computer Numerical Control (CNC)– The idea of computer numerical control is to

position a computer right at the machine tool.

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Computer Numerical Control (CNC)

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• Part Program- Numerical data on a punched tape.

• The block contains , in coded form , all information for processing a segment of work piece like Segment length, cutting speed,

• NC replaces the manual actions of the skilled operator.

• Part Programmer V/s Skilled operator.• APT – Automatically Programmed Tool• BLU- Each unit corresponds to the position

resolution of axes of motion referred to Basic Length Unit.

• BLU is also known as ‘increment size’ or ‘bit weight’.

•

• The part dimensions are expressed in part programs by integers

• To calculate position command in NC, the actual length is divided by the BLU value.

• To move 0.7 inch in positive X direction , with BLU =0.001, the position command is X+700

• Axis of motion – An axis in which the cutting tool moves relative to the work piece.XYZ

• Machine tools- drilling,milling,lathe• Rotary motions also possible.

NC machine parts

• MCU– Data Processing Unit

• To decode the information received from tape, process it, and provide data to CLU

– Control Loops Unit.• Operated Drive attached.

• Machine tool

Advantages of NC

• First NC- 1952.• Accuracy- Measuring Time• 70-80 % Time wasted on measurements• Tolerance improved.• A Further saving of time is achieved while

passing from one operation to another during the machining of the work space.

• Rate of production decreases in conventional method

• Contour cutting in 3 D or 2D can not performed by manual operation

• Two Hand wheels required for this operation

CNC - Disadvantages

• High initial investment• A long preparation time for each production

series.• Inflexibility of the process. Machine is planned

to make a certain fixed cycle of operation. If the part configuration changed , The machine adjustment must be rebuilt or altered.

• A big stock of parts is required for the process.

• NC in Flexible Manufacturing System• Production in low quantity• Eg . Aircraft• Machines should be economical.• When a new product required , only the part

program has to b chgange.

Advantages of NC

1. A Full flexibility ; a part pgm is needed for producing a new part

2. Accuracy is maintained through the full range of speeds and feeds.

3. A shorter production time4. Easy adjustment of the machine. Less time than other

manufacturing methods5. The possibility of manufacturing a part of complicated

contour.6. Need for highly skilled & Experienced labor is avoided7. The operator has free time

Disadvantages of NC

• A relatively high cost• More complicated maintenance- Special

maintenance crew is desirable.• Highly Skilled & properly Traned part

programmer is needed

•

Classification of NC

• According to type– Point to point / contouring

• According to Structure– NC/ CNC

• According to programmed method– Incremental / absolute

• According to type of control loops– Open loop / closed loop

Point to point

• Drilling machine• Work piece is moved along axis of motion until

center of hole is drilled is exactly beneath the drill

• Drill is automatically moved to the work piece, hole is drilled , and the drill moves out in a rapid traverse speed.

Contouring systems

• In countering or continuous-path systems, the tool is cutting while the axis of motion are moving.

• Eg. milling machine•

Interpolator

• Interpolator contained in the DPU of contouring systems provide the proper velocity commands for each axis.

Straight Cut Systems

• PTP machine equipped with milling tool.

NC & CNC

• NC- Hard ware based• CNC- software based• The digital control in NC employs voltage

pulse.• 1 pulse = BLU (NC)• Pulses to actuate motors.

– Stepper- open loop– DC motor – closed loop

• 1 bit = BLU (CNC)• Simplest hardware configurations – emulates

hardware based NC and transmit output pulse.• 1 bit =1 pulse =BLU• The main difference in the operation between NC

and CNC is in the way that the punched tape is read.– NC – read 1 block and cut– CNC – store complete data , stored in memory before

cutting starts.

Incremental / Absolute

Absolute - Advantages

• With an incremental system , each time the work is interrupted the operator must restart the part pgm and repeat the entire operation

• Dimensional data can change at any time.• One reference point

Incremental - Advantages

• Inspection of the part pgm is easy. Sum of position commands must be zero

• The performance of the incremental system can be checked by a closed-loop tape. This is a diagnostic punched tape which test the various operations and performance of the NC Machine

• Mirror image programming is facilitated with incremental systems. The sign of the corresponding position commands is changed from + to - . No new calculation required for the position

Open Loop / Closed loop