Novel Petrinet and Labview based Approaches for Automation of Small Scale Soap Industry with

FPGA and Comparative EvaluationC. Sreeshaa, V.M.Vaidyanb,*, and M. V. Vaidyanc

a,cDepartment of Electrical Engineering, National Institute of Technology, Calicut, India

bDepartment of Computer Engineering, King Khalid University, Abha, Kingdom of Saudi Arabia

*varghese86@gmail.com

Abstract – This paper proposes novel approach for automation of small scale soap industry using dedicated Field Programmable Gate Array (FPGA). Usually Programmable logic controllers (PLCs) are used in automation of soap industries. But FPGA’s fast processing ability, synchronized signal generation, parallel reactive architecture, compactness and cost effective nature makes this logic device a better candidate than PLC. With small parameter variations in reconfigurable FPGA, different soaps of different qualities like color, transparency, softness, etc. can be manufactured. Two new approaches of automation of soap industry using FPGA with Petri net and LabVIEW is proposed. In addition, a comparative evaluation of the two proposed approaches is done based on the results.

Keywords- Field Programmable Gate Arrays; Petri Net; Process Control; Manufacturing Processes; Automation

I. INTRODUCTION

AUTOMATION of industrial processes leads to success in competitive industries and viability of the manufacturing plants in addition to improvement in productivity, product quality and profitability. Novel approaches in automating soap industry are crucial in generating high throughput and profit in soap production. Industries with batch processing uses Programmable logic controllers (PLCs) in such process control applications ([1]-[5]). Most of the small scale soap industries adopt batch processing and hence PLCs. Performance of PLC is highly constrained by speed of the microprocessor and real-time firmware of the PLC logic control used in manufacturing systems. Hence for small or limited I/Os PLCs are preferred. Performance of PLCs is limited by the cyclic scan period which depends on the program length and complexity [6]. For overcoming drawbacks in PLCs for manufacturing industry, Field Programmable Gate Array (FPGA) is a good candidate. FPGA’s have been widely been used successfully in many industrial control applications ([8]-[14]). To make the approach user-friendly and to have good graphical interface LabVIEW is used for programming the FPGA [15]. LabVIEW is well suited for FPGA programming

because it clearly represents parallelism and data flow and this simplifies the designing of FPGA. In addition to monitoring and control of field devices it also provides MMI (man machine interface) through which we can change the process parameters online. Petri net (PN) is a well-established mechanism for system modeling. Petri net based models have been widely used due to their ease of understanding, declarative, logic based and modular modeling principles, and finally because they can be represented graphically ([16]-[27]). The use of Petri nets for the specification, analysis and

synthesis of digital system has proved great advantage. It provides model concurrency, conflict, sequential operation ability and synchronization of events in a digital system. PN based models also have the advantages like easiness and ability to adjust modelling power to various types of behaviour at different abstraction levels, formal operational semantics, verification of correctness properties and possibility of mechanical synthesis of circuits from net models. It is possible to translate Petri nets to HDL (Hardware Description Language), and vice versa, making it possible to integrate Petri nets tools into existing design environments of FPGA. The rest of the paper is organized as follows: Section II briefly introduces the processes in the small scale soap industry. Petri net preliminaries are introduced in Section III. Section IV describes the novel approaches developed in automation of the processes using FPGA. Section V discusses the results and gives comparative analysis of the novel approaches developed. Section VI concludes the paper.

II. PROCESS DESCRIPTION Automation of soap industry includes automation of

manufacturing and packing process. In small scale industries soap manufacturing is done through batch processing. The whole process is divided into two parts.

1) Manufacturing: The soap is composed of two liquids (A-caustic soda and B-process oil). The components A and B are

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inside stored in two separate tanks. The amount of liquid being added is done based on time. The process starts with addition of liquid B to the mixing tank which is then heated to a particular temperature. Next step is addition of liquid A to the mixing tank. The compound of liquid A and Liquid B in the mixing tank is stirred well. An alarm gets triggered when the manufacturing process is completed. Sometimes occasional heating is also required.

2) Filling: This step starts after end of manufacturing step. On arrival of can in the filling zone, a proximity sensor detects the presence of can and starts filling it. The valve is closed after r a predetermined time. Until the low level sensor of mixing tank is activated the process is repeated.

III. PETRINET PRELIMINARIES The definitions used are mainly from ([28], [29], and [30]) Definition 3.1. Consider a set of places, P = {P1, P2, P3, . . . . . Pn} and a set of transitions, T = {t 1, t 2, t 3 . . . . . t m}, then a Petri net G is a four-tuple (P, T, IN, OUT) . With P ∩ T = Φ and P U T ≠ Φ, Where, OUT: (P×T) N is an output function which defines directed arcs from transitions to places. IN: (P×T) N is an input function which defines directed arcs from places to transitions, and

Fig. 1. (a) A Petri net model that represents the processing a task (b) Initial marking M0 of the Petri net Model and (c) Marking M1 reached after firing t1 in (b). Definition 3.2. Functions, IP an OP can be defined as, IP: T 2P and OP: T 2P as below: IP (tj) = {Pi∈ P: IN (Pi,tj) ≠ 0} ∀ tj ∈ T (1) OP (tj)= {P∈ P: OUT (Pi,tj) ≠ 0} ∀ tj∈T (2) if 2P be the power set of P. Where, IP (tj) = Input places of tj, OP (tj) = Output places of tj. For the Petri net model in fig l (a), OP (tl) = IP (t2) = {P3}. (3) IP (t1) = OP (t2) = {pl, p2} (4) Definition 3.3. Consider a set of non-negative integers, N and a marking, M then G is a Petri net function, M: P N A marked Petri net (MPN) is denoted by (G,M). Generally an initial marking M0 is associated with a given Petri net model. Initial state of the system is represented by M0.

A Petri net with n places is an (n × 1) vector and relates each place with a certain amount of tokens which are denoted by dots inside the places. In Fig, 1(b) a marked Petri net with marking, M0 is given. Where,

M0= (M0 (pl), M0 (p2), M0 (p3))T = (1, 1, 0 )T For Fig. l (c) marking, M1 is

M1 = (M1 (pl), M1 (p2), M1 (p3))T = (0, 0, 1)T Definition 3.4. Transition ti is enabled in a marking, M when

M(Pi)≥ IN(pi, tj) ∀ pi ∈P. A transition tj which is enabled can fire at any time. A new marking M' reaches when a transition tj fires.

M' (pi) = M(pi) + OUT(pi, tj) - IN(pi, tj)) ∀ pi ∈P. In Fig. l(b), enabling of transition t1 in marking Mo is shown. Definition 3.5. On set of all markings, reach ability of markings is a relation which is transitive and reflexive. The reach ability set of Mo, R [Mo] can be defined as a set of all markings reachable from an initial marking Mo. From Figures l(b) and l(c) we can infer that R[Mo] = R[M1] = {Mo, M1} . Definition 3.6. A Petri net G = (P, T, IN, OUT) can be considered to be self-loop free or pure if (pi,tj) ∈ P× T and IN(pi, tj) ≠ 0 and OUT(pi, tj) ≠ 0 . No place in a pure Petri net is an output place and an input place of the same transition. Always an equivalent pure Petri net can be obtained from an impure Petri net by introducing dummy transition and places. It is shown in Figs.2 (a) and 2(b).

Fig. 2. (a) Impure Petri net. (b) Pure Petri net equivalent of the above impure Petri net where t1 is a dummy transition and pl'' is a dummy place. Definition 3.7. If m is the number of transitions and n is the number of places, incidence matrix of a pure Petri net, C is an (n × m) matrix can be given as C(i,j) with (i,j)th element of C, where i is set of numbers i = 1, 2, 3,...n and j set of numbers, j= 1,2,3,.....m. C (i,j) = - IN(pi, tj) when IN (pi, tj )≠ 0 = OUT (pi, tj) when OUT (pi, tj) ≠ 0 = 0 otherwise. Definition 3.8. Consider two pure Petri nets, G1 = (P1, T1, IN1, OUT1) and G2 = (P2, T2, IN2, OUT2) without pairs, t ∈ Tl∩T2 and p ∈ P1∩P2 , satisfying

2013 IEEE Conference on Systems, Process & Control (ICSPC2013), 13 - 15 December 2013, Kuala Lumpur, Malaysia

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either OUT1(p,t)≠0 and OUT2(p,t )≠0 or IN1(p,t) ≠ 0 and IN2(p,t) ≠ 0 Their union can be defined as G = (P, T, IN, OUT), where t = Tl U T2, p= P1 U P2 , OUT=OUT1 U OUT2 and IN = IN1 U IN2. The union of the Petri nets of Figures 3(a) and 3(b) is given in Fig. 3 (c).

Fig. 3 (a) Petri net 1, (b) Petri net 2, (c) Union of Petri net 1

and Petri net 2

IV. NEW APPROACHES FOR AUTOMATION OF SMALL SCALE SOAP INDUSTRY WITH FPGA

A. Programming With LabVIEW graphical programming Sequential execution of PLC programs was implemented in LabVIEW using flat sequence structure in structure palette. Different controls were developed for automation process.

1) Level control of tanks. Level control of tank is initiated by a start button .Two

pumps (pump A and pump B) are turned on. The two tanks A and B are filled up to their high level. Two magnetic type float switches are used to detect the high level in these tanks. The low levels in the tanks are also detected through float type switches. After reaching high level both pumps are turned off.

2) Mixing control, Stirring and filling Process Control. Mixing control of liquid components is ratio control.

Ratio control is implemented through timers. Timer A will decide the control valve A opening. Timer B will decide the control valve B opening. Both are on delay timer set after a delay time. Equal time period is set for the two timers as the ratio is 1:1.

Heating is required in the batch process at various stages. A heater control is implemented through timing constraint. The resulting mixture is stirred using a stirrer. These are all digital controls. Filling process is implemented through a counter. A digital circuit was made to generate the stepping sequence 0000,0010,0100,1000 under the following conditions

• If mixing tank low level is met stop the conveyor • If vessel sensor is still on after some time period the

process will stop • If vessel sensor is off within a period the conveyor

moves until it detect the next sensor. A Virtual Instrument (VI) is developed which generate the

stepping sequence in the order. The individual VIs are then merged together using a flat sequence structure as the process flow is sequential. The FPGA inputs and outputs are 3.3V

compatible. FPGA have push button switches, slide switches and rotary switches for giving inputs in addition to low voltage I/O pins. It has also got 8 LEDs to indicate the results.

B. Programming FPGA using Petri Net The soap manufacturing process is a sequential batch process and the sequential flow is developed. Simulation gives the state of each place at different time periods. The Fig.4 shows the Petri net diagram developed for soap industry automation.

Fig.4 Petri net representation of soap industry automation

Here p1,p2,p3,p4,p5,p6,p7 are the places and t1,t2,t3,t4,t5,t6,t7 are the transitions. Initially a single token is placed at p1 to indicate a start. This token will move from one place to other sequentially if the corresponding transition fires. A set of input signals associated with each transition controls the firing of that transition, when the corresponding signals become high the transition fires automatically. The outputs are associated with place holders. Whenever a place holder contains a token the corresponding output becomes active. Here pump, VAL A, VAL B, VAL C, heater and stirrer are the output variables associated with real world outputs. Timer1, timer 2, timer3, start, stop, lev 1 and lev 2 are the input variables associated with actual real world inputs. PNs are well suited for modeling and formal analysis of complex discrete systems while VHDL is a standard hardware description language which exploits concurrency in the specification of digital systems, allowing their simulation and synthesis. PNs and VHDL may complement each other and they also provide a proof subsystem that accepts the same user interface descriptions for all design tasks. A VHDL textual PN description can be automatically generated by a translational tool. The CONPAR specification format was developed as a bridge between the text logic description of a PN and its VHDL model. It supports the translation of a hierarchical symbolic representation of PNs directly to VHDL format. A software frame work is there which consist of CONPAR compiler to which we give CONPAR specified PN and which generates the VHDL code for the FPGA target. As CONPAR language is proprietary a method similar to it was used to convert P net modeling to corresponding RT level VHDL code. The textual representation of PN was done and a set of Boolean equation was written for each place, transition and output. Then these set of place holder, transition and output equations were used for generation of VHDL code for the above program. The above program is then converted to bit files that are loaded to FPGA.

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V. RESULTS AND DISCUSSION

A. LabVIEW Programs Developed and simulation results VIs were created for controlling each process in the industry. The individual VIs were then merged together using a flat sequence structure for ensuring sequential execution. The programs developed are given in Figures 5-7. Then the graphical program was simulated on the developmental computer and verified the control flow.

1) Level control

Fig. 5 Level control of tanks

2) Control of mixing, Stirring and filling process

Fig.6. Mixing, Stirring and filling Process control

3) Front Panel of Main Program for automation of soap

industry using LabVIEW-FPGA

Fig.7. Front panel of Soap Industry Automation

B. Petri Net Approach and results Petri net modeling of soap industry is done in VISUAL PETRI NET+ software. Simulation studies are carried out by placing token at the first place. State of each place at different time instants are indicated in the simulation window given in Fig. 8.

Fig.8. Simulation studies on Petri net model of soap industry After the simulation studies the VHDL code for the Petri net model is generated from the Boolean equations of states and transitions. This code is verified for the functionality. The VHDL code simulation results are shown in Fig. 9. The verified program is then converted to bit files and implemented on FPGA. After implementation the design is analyzed for performance against constraints, device resource utilization, timing performance, and power utilization. The static report file obtained is shown in Fig. 10 gives resources utilized for the given program.

Fig.9 Simulation result of VHDL code

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Fig.10 Device utilization summary

C. Comparison of LabVIEW and Petri Net Based Approaches

The graphical user interface (GUI) in LabVIEW provides gives detailed overview of the process under operation. The LabVIEW based FPGA programming using the LabVIEW FPGA module has impact because of its GUI and MMI (man machine interface). The MMI provides operator console through this even an unskilled operator can monitor and control the process. It also enables predictive fault detection and maintenance. The graphical programming language in LabVIEW makes programming easy and it also makes debugging of a program simple and faster. The parallel programming architecture of LabVIEW makes instruction execution faster and thereby increasing processing speed. But lack of instruction set in LabVIEW FPGA module limits its application. Lack in mathematical tools like formula node for implementing mathematical equations and elapsed time node for timing which are available in LabVIEW are not supported in FPGA module. LabVIEW FPGA module only supports constants. Lack of information about design optimization in LabVIEW FPGA programming is another concern. It does not give information about the resources utilized like how many CLBs, memory and wiring resources are used for a particular program. LabVIEW gives a vague idea of developmental time optimization. Lack of information about design optimization like resource and time optimization, lack of performance evaluation tools and limited instruction set are major drawbacks of LabVIEW programming. Petri nets (PN) models on the other hand are the most widely used tools to model and evaluate the behavior of discrete complex industrial and dynamic systems. Petri net models help in understanding the interactions and relations of stochastic events, to visualize conflicts and problems with buffers and to detect deadlocks. They also provide quantitative analysis methods for resource utilization, consequences of system failures, system throughput rates and more. It’s a well-developed tool based on mathematical formulation. PN modeling is ideal to implement real-time control for discrete industrial systems. So for the particular soap manufacturing automation (discrete batch process) problem is well solved in PN. PN based models have been widely used due to their ease of understanding as the graphical programming is easy to understand, declarative, logic based and modular modeling

principles. It’s well suited to model concurrent and asynchronous systems. These models are also self regulating. Compilation is optimized in VHDL. The very deterministic and strongly typed nature of programming also favors VHDL. The XILINX tools provide an implementation analysis, where the design is analyzed for performance against constraints, device resource utilization, timing performance, and power utilization. By viewing results in static report files and by looking at actual device implementation in graphical layout tools, such as the Plan Ahead software and FPGA Editor gives an absolute idea of the design and design optimization for various resources are also possible. Drawbacks are that there is no user interface as compared with LabVIEW and program debugging is tougher. The user can choose from the two new approaches based on the requirements in their industry.

VI. CONCLUSION Two new approaches for automation of small scale

soap industry using Field Programmable Gate Array is presented in this paper. It’s a low cost and highly effective solution to the automation of small scale soap industry problem and has high processing speed. All the processes in the soap industry were implemented through FPGA by two new approaches, LabVEW-FPGA and Petri net. LabVIEW-FPGA based approach has the advantage that it gives user friendly graphical user interface and an easy way of graphical programming and debugging. On the other hand, the Petri net based approach in VHDL offers options for design optimization and comprehensive simulation. The user can choose between the two new approaches based on the requirements. A comparative study between the two new approaches are also given which gives the user more understanding on which approach to choose from, based on their requirements.

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