Control of a Large Process Using a Small PLC
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1. are used for addressing the eight states and the last output ENB is used for buffering purposes Each state consists of three consecutive sub states all red green and green yellow The flashing mode has two sub states all red for 5 seconds then flashing Whenever one of these states needs to be activated the PLC program will output the required sequence of bits Figure 4 shows the design of the decoding circuit used First an octal buffer is used to latch and synchronize the eight PLC outputs Then a 3 to 8 decoder is used to decode the 3 bit state code into one of the eight basic states The combinational logic circuit will then turn on only those lights corresponding to the active state Finally the timer is used to provide the decoding circuit with its flashing capability Several assumptions were made in order to simplify the design We assumed that there are no separate right turn signals in either road Also the minor road has no separate left turn signals In effect states 6 and 8 of Figure 2 were not used because they require that the minor road have separate left turn signals One limitation of our decoding circuit is that some transitions are not possible for example between states 1 and 5 During this transition the straightforward signal should stay green while the left signal changes from green to green yellow to red The way the circuit is built requires that all green lights go through the intermediate s2. 2 State 7 a Figure 5 State diagram for a dumb control b intelligent control Figure 5 a shows the state diagram for the dumb control mode The system has to go through a sequence of four consecutive states 5 2 7 4 Each consisting of three sub states all red green and green yellow The outputs that should be generated and their timing sequence are shown in Table 1 Notice that we gave state 5 the longest total duration since it corresponds to the major road Table 1 also shows the timers that are associated with each output in the PLC program Whenever an output needs to be turned on for the corresponding duration a combination of internal timers and latches are used For example the VG output is to be kept high in state 5 for the duration of 20 secs 15 5 and in states 2 7 and 4 for an equal duration of 15 secs 10 5 Hence for the VG output alone four timers are used TIMO TIM1 TIM2 and TIM3 each of which is kept on for the prescribed duration by its own latch Figure 5 b shows the state diagram for intelligent control In the absence of any interrupts from the push buttons the intelligent control mode is the same as the dumb control mode This is indicated by the circular transition lines which are similar to those of the dumb control However if an interrupt occurs the control will activate the corresponding state This is indicated on the state diagram by the added transitions in the middle
3. The timing of each state remains the same but the sequence of states may be different A situation may arise when several interrupts occur simultaneously The ideal solution is to service these interrupts based on a first come first served basis or based on the relative importance of these requests In this project however there are no priorities defined The program simply activates whichever state it scans first in the ladder diagram START PB1 PB5 PB6 PB10 PB3 PB4 PB1 PB5 PB6 PB10 PB3 PB4 ws PB8 PB9 PB3 PB4 PB8 PB9 PB2 PB7 PB2 PB7 PB8 PB9 y PB2 PB7 b If the flashing mode is selected then the system will first stay in the all red state for 5 seconds for a safe transition from a currently active state then starts to flash If the master controller PC suddenly fails the PLC will automatically turn the flashing signal on because the SO and S1 inputs will be low The required output sequence is shown below in Table 2 3 2 Ladder Implementation The implementation of the control logic described above is accomplished using ladder logic The first part of the ladder sets the control in the desired control mode This is done by scanning the PLC inputs 000 and 001 If the dumb or intelligent modes are chosen the control will start from state 5 as indicated on the state diagrams of Figure 5 The program then continues to scan the PLC inputs during the activated mode in order to det
4. Control of a Large Process Using a Small PLC RAMADAN A FAN Systems Engineer Saudi Aramco Dhahran Saudi Arabia L CHEDED Assistant Professor Systems Engineering Department KFUPM Dhahran Saudi Arabia ABSTRACT The objective of this paper is to demonstrate how a programmable logic controller PLC with a limited input output capability such as the 12 input 8 output Omron PLC coupled with a low complexity interface circuit can be used to control a large process The test bed for this process is a traffic light system whose number of outputs 26 far exceeds the 8 outputs available from the PLC The key and novel idea behind this successful PLC Interface combination resides in the clever coding of the various states that the process under control has to go through An illustrative low cost demonstration system was recently successfully tested at the Systems Engineering Department Finally the coding technique used here can also be extended to other similar large processes 1 Introduction Programmable logic controllers PLCs are becoming more and more popular in industry due to their flexibility reliability and cost effectiveness Their architecture is similar to that of a conventional digital computer consisting of a central processing unit a memory unit and a number of input output I O terminals The strength of a PLC however lies in its optimized manipulation of one bit logic operations The program is scanned continuously
5. e 1 is used for activating the output ENB for a very small duration 0 5 seconds in order to buffer the PLC outputs to the decoding circuit whenever there is a change in the state The procedure for encoding state 5 is duplicated for the remaining possible states of the system with the exception of the startup events for both the dumb and intelligent modes The complete ladder diagram was documented as part of the project report 4 Conclusions In this paper the control of a traffic light system requiring more outputs than can be provided for by the small PLC used was illustrated It was shown that the solution to this control problem hinged upon the key idea of using the limited PLC outputs for the sequencing of the various states of the process to be controlled rather than for the direct control of the process outputs themselves This entailed the use of a software encoding scheme followed by a hardware decoding circuit of modest complexity We believe that this encoding decoding approach offers a good compromise between complexity and low cost on the one hand and design flexibility on the other and as such should therefore be attractive in situations where the cost of a larger PLC is prohibitive Finally not only can this approach be used to increase the PLC s fan in capability as pointed earlier but it is also general in nature and hence can be applied to other large processes with suitable modifications in the i
6. e feedback loop connects the ten push buttons of the traffic light circuit back to the PLC input section The remaining two PLC inputs are used by the PC for mode selection according to the following codes 00 Flashing Used for emergency repairs at the intersection or for signaling PC breakdowns 01 Dumb Control Control is done by outputting pretimed sequences 10 Intelligent Control Control senses requests from pedestrians and vehicles which will interrupt the normal sequence 1 1 All Red Used as an intermediate state for safe transitions between consecutive modes A RED LIGHT YELLOW LIGHT GREEN LIGHT X PUSH BUTTON Figure 1 To o 1 2 3 4 lt gt ee s gt JIN lt gt v hd 5 6 7 8 Figure 2 The eight basic states for traffic light control arbitrarily numbered 2 Hardware Design 2 1 PLC Output Interface The eight PLC outputs numbered here by their addresses 100 107 are assigned as follows Address Label Meaning 100 VG Vehicle Green Light 101 VY Vehicle Yellow Light 102 PG Pedestrian Green Light 103 E Emergency Signal 104 A 105 B 3 bit State Code 106 C 107 ENB Buffer Enable Signal The first four outputs are used as control signals to differentiate between the intermediate states The red lights are not included since they are always the complement of their corresponding green lights The next three outputs A B C
7. ect any change in the desired mode or in the case of intelligent control detect requests from push buttons Based on these inputs a certain state can be entered and the corresponding outputs can be activated For example in order to enter state 5 the program has to sense either a dumb mode start up an intelligent mode start up a push button depression or simply the end of the previous state state 4 if it is in normal operation The first output to be activated is C as indicated in Table 1 initially for a duration of 5 seconds TIM18 then for 20 seconds TIM19 The reason for using two timers here is that the end of TIM18 is used to send a signal to two other timers namely TIM8 for activating PG and TIMO for activating VG Similarly the end of TIM8 sends a signal to TIM4 for activating VY Duration State C B A PG VY VG sec azl Li z o0 0 0 0 0 5 5 AE 0 0 mJ o HE 15 i s o0 0 0 G2 Leos z 0 w E o 9 0 5 2 0 0 AE l2 o HE 10 z x 0 0 ija O E 2 ile a g 0 0 0 0 5 7 ha IE ES 6 i s 10 z a So z z ifs u 0 o bz Lh 5 0 if ie Og 0 0 5 4 0 ilS H Hre AE 10 0 1 1 0 Ela i 5 Table 1 Timing sequence for the PLC outputs in the four states used in the dumb and intelligent modes Duration C B A E PG VY VG sec 0 0 0 0 0 5 0 0 0 1 0 0 Indefinite Table 2 Timing sequence for the PLC outputs in the flashing mode An additional timer TIM23 not shown on Tabl
8. it The extension of the PLC fan in capability was not included in this project 2 3 PC PLC Interface The role of the PC in this project is limited only to selecting between the three different control modes It has no influence whatsoever on the traffic light control itself In fact Figure 3 shows that the PC is indeed outside the control loop The PC merely simulates a remote master controller which uses the PLC as a slave 3 Software Design 3 1 Control Logic The coding of the control logic is accomplished on two different platforms The first is the PC which uses a BASIC program to provide the user interface and mode selection capability The other is the PLC which uses ladder logic to activate the desired control mode The communication between the two programs is done through the SO and S1 signals 26 lines DECODING TRAFFIC LIGHT CIRCUIT CIRCUIT 10 lines Figure 3 Block diagram of the traffic light control system used in the project 555 ELR ELY ELG ESR ESY TIMER ESG EPR EPG SSR SSY SSG SPR COMBINATIONAL SPG E 74LS373 CIRCUIT WLR A BUFFER 74LS138 DECODER Sf a Oa _ L_isv WLY WLG WSR WsY WSG WPR WPG NSR NSY NSG NPR NPG Figure 4 PLC output interface using a decoding circuit START State 5 State 4 State
9. n and operation of this PLC based controller 1 1 PLC Process Interface Usually the interface between a PLC and a process begins with an analysis of the number of I O connections required This will determine the size type and cost of the controller But what if we were restricted to a PLC that has only a limited number of I O connections can we still use it to accomplish the desired control task One solution to this problem as presented in this paper would be the introduction of an intermediate stage which will interface the PLC to the complex process We propose here an encoding decoding approach that would expand the fan in and fan out capabilities of the small PLC used The argument behind this is that although PLCs were originally developed to supplant hard wired controllers a hybrid solution involving a PLC and some hard wired interface may entail an increase in complexity but would in return offer a cost saving Considering the time frame of our project only one part of the problem was investigated i e extending the fan out capability of the PLC used A digital decoding circuit was designed to interface the PLC outputs to a traffic light system We assume that the same approach can be used to extend the fan in capability of the PLC leading to the design of an encoding interface circuit between the traffic lights and the PLC inputs 1 2 Design of the Experiment Figure 1 shows the four way isolated intersection u
10. nterface circuit References 1 John W Webb Programmable Logic Controllers Principles and Applications New York Macmillan 1988 2 Ryan G Rosandich What to Know About PLC Ladder Diagram Programming EC amp M Jun 1996 20 B Kelvin T Erickson Programmable Logic Controllers IEEE Potentials Feb Mar 1996 14 4 Omron User Manual and Installation Guide 1989 5 Charles V Zegeer and Sharon F Zegeer Pedestrians and Traffic Control Measures No 139 Washington DC National Research Council Nov 1988 6 James H Kell and Iris J Fullerton Manual of Traffic Signal Design New Jersey Prentice Hall 1982
11. sed in this project The major road has separate signals for straight forward and left turning traffic whereas the minor road has a common signal for both The push buttons represent pedestrian and vehicle sensors The result is a total of 26 lights and 10 push buttons The PLC used is the Omron Sysmac C20K with 12 inputs and 8 outputs 4 Although the inputs are sufficient for direct connection with the push buttons the outputs are not enough for the traffic lights Therefore we decided to consider the 8 PLC outputs as an 8 bit code string that will be decoded by a digital decoder into the required number of outputs But the question is how to encode the various traffic light states A literature survey on traffic control schemes 5 6 showed that there are eight basic states which the controller has to switch between Figure 2 shows these states arbitrarily numbered from 1 to 8 Based on the fact that there are extra intermediate states such as the green yellow and the all red states between the above mentioned eight states it was decided to use some additional control signals for a proper sequencing of the traffic lights The project was designed to carry out the control in three different modes dumb control intelligent control and flashing mode These three modes are programmed in the PLC and their selection is made from a PC which acts as a master controller The complete setup is shown in Figure 3 where th
12. to see if certain conditions on the inputs are met so that appropriate actions on the outputs are taken One of the major concerns when selecting a PLC for an application besides speed is the number of input output I O connections it can provide PLCs come in different sizes and with varying capabilities ranging from micro PLCs that have up to 32 I O connections and 2K words of memory to large ones with an expandable I O capability of up to 8192 I O connections and 4M words memory 3 The choice of a PLC appropriate for a particular control task is usually determined by analyzing the process to be controlled and finding out the number of I O connections required for the interface As expected the required number of I O connections will increase with the complexity of the control task at hand and a bigger PLC will have to be used hence raising the cost of the overall system A question that naturally arises at this stage is the following Can a PLC with a modest number of VO terminals be used to control a relatively sophisticated process An affirmative answer to such question would certainly be of great educational and practical benefit The feasibility of this idea was tested in a senior project course in the Systems Engineering Department at KFUPM where the process chosen was a traffic light system providing both dumb and intelligent control This paper presents both the hardware and software aspects of the successful desig
13. ub states before entering the next state The solution is to either add these green yellow sub states to the eight basic states and use a larger decoder or simply avoid transitions such as 1 5 3 5 1 7 and 3 7 in the control The latter option was chosen to keep the overall design simple as it was meant for illustration purposes 2 2 PLC Input Interface The PLC inputs are connected as follows Address Label Meaning 000 SO Mode Selection Bit 001 S1 Mode Selection Bit 002 PB1 East Straight Vehicle Push Button 003 PB2 East Left Vehicle Push Button 004 PB3 East Pedestrian Push Button 005 PB4 South Vehicle Push Button 006 PBS South Pedestrian Push Button 007 PB6 West Straight Vehicle Push Button 008 PB7 West Left Vehicle Push Button 009 PB8 West Pedestrian Push Button 010 PB9 North Vehicle Push Button 011 PB10 North Pedestrian Push Button PC with PCL812 card L_ AL 3 8 lines 2 lines PLC The twelve PLC inputs were sufficient for our application Recall that two of these were used for mode selection and the remaining ten for push button connections But had their been more push buttons a hardware solution could be found by analyzing the requests and seeing which ones could be ORed together or multiplexed by hardware and then demultiplexed by software which is the reverse of what we have done in the output decoding circu
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