PLC Experiment: Teaching Logic Controllers & Diagrams for Students, Study notes of Programmable Logic Controllers

Download PLC Experiment: Teaching Logic Controllers & Diagrams for Students and more Study notes Programmable Logic Controllers in PDF only on Docsity! Page 1 of 14 Measurements and Control Lab Exp.4: Introduction to PLC OBJECTIVES: The aim of this experiment is to familiarize the student with Programmable Logic Controllers (PLC) as hardware and the software used to program it, also to familiarize him with programming PLC using ladder logic diagrams in order to control an electro-pneumatic system. EXPERIMENT EQUIPMENTS: 1. Programmable Logic Controller Trainer, mod. PLC-8/EV, see figure 1. 2. Electro-pneumatic system elements (cylinders, directional control valves,sensors…) 3. Personal computer. Figure 1. Programmable Logic Controller Trainer Pre-lab Assignments:
Write (draw) the ladder diagrams that do all the following logical functions: o Q0.1 = NOT I0.1 o Q0.2 = I0.2 (OR) I0.3 o Q0.3 = I0.4 (AND) I0.5 o Q0.4 = I1.2 (XOR) I1.3
Draw a ladder diagram for each of the following: 1. Turning the output Q0.0 ON when you activate the start pushbutton I0.0, and stop when you activate the pushbutton I0.1 2. When you press the pushbutton (I0.0), (Q0.0) activated immediately, wait 5 seconds, and then turn Q0.1 ON. Page 2 of 14 INTRODUCTION TO PLC: A computer system for automation has to satisfy many requirements that we take more or less for granted. It has to run all around the day and night since a production break may cost huge amounts of money. It has to work in real time taking various time requirements and process disturbances into consideration. Whatever happens, the system has to behave in a predictable way. It has to be safe, both for the process and for humans. In most automation systems there are events that will bring the process into another state of operation. Furthermore, there are a lot of applications in both the process and manufacturing industries where control involves primarily switching and sequencing. In both the process and manufacturing industries there is a wealth of applications of switching circuits for combinatorial and sequencing control. Switching theory, which provides the foundation for binary control, is not only used in automation technology but is also of fundamental importance in many other fields. This theory provides the very principle on which the function of digital computers is based. In general, binary combinatorial and sequencing control is simpler than conventional feedback (analog and digital) control, because both the measurement values and the control signals are binary. However, binary control also has specific properties that have to be considered in more detail. Programmable logical controllers (PLCs) have been in use since the 1960s and are still the basis for the low level control in many automation systems. Today PLCs can handle not only the lowest levels of control but also advanced control of hybrid systems, where time-driven continuous controllers have to be integrated with event- driven controllers. The Development of PLCs The modern computer control system of today is the result of two parallel developments, one from relay technology to implement logical circuits and the other from continuous instrumentation and pneumatic proportional integral derivative (PID) controllers developing into software realizations of continuous controllers. Logical circuits have traditionally been implemented with different techniques. The primary reason for designing a PLC was to eliminate the large cost involved in replacing the complicated relay-based machine control systems. When production requirements changed, so did the control system. This becomes very expensive when the changes are frequent. Since relays are mechanical devices they also have a limited lifetime, which required strict adhesion to maintenance schedules. Troubleshooting was also quite tedious when so many relays were involved. Usually, the machine control panel includes many, possibly hundreds or thousands of, individual relays. Then it is easily recognized that alternative solutions were sought. PLC brands Siemens, Allen-Bradley, IDEC, ABB, Mitsubishi, Omron, Honeywell, Schneider Electric, Saia-Burgess Controls, and General Electric. In this experiment we will use: I. PLC SIEMENS SIMANTIC S7-200. PLC SIEMENS S7-200 has the following characteristics: Program 4096 words User data 2560 words User program storage type EEPROM Local I/O 14 In/10 Out • Digital Inputs: I0.0-I0.7 and I1.0-I1.5 Page 5 of 14 Digital logic functions Using multiple contacts, simple and complex logic functions can be constructed easily. It is useful to use standard binary notation for the status of the switches and lamp (0 for un-actuated or de-energized; 1 for actuated or energized), a truth table can be made to show how the logic works: OR Gate Now, the output will come on if either contact A or contact B is actuated, because all it takes for the lamp to be energized is to have at least one path for current from wire L1 to the wire L2. What we have is a simple OR logic function, implemented with nothing more than contacts and an output. Page 6 of 14 AND Gate Now, the output energizes only if contact A and contact B are simultaneously actuated. A path exists for current from wire L1 to the wire L2, if and only if both switch contacts are closed. A pattern quickly reveals itself when ladder circuits are compared with their logic gate counterparts: • Parallel contacts are equivalent to an OR gate. • Series contacts are equivalent to an AND gate. • Normally-closed contacts are equivalent to a NOT gate (inverter). Latching If we have an electrical motor and a pushbutton for starting the motor, we can simply connect the pushbutton to one of the PLC's inputs and the motor to an output relay. The ladder will be as shown in figure 9. Figure 9. Pushbutton operates motor. But the problem here is that the output relay (motor) will turn off once you remove your finger from the pushbutton, but this is not the purpose of the pushbutton. To solve this problem, we need to latch the output in parallel to the push button, as shown in figure 7. Page 7 of 14 Figure 7. Latching. Now, when you activate the push button, the output relay will be energized, so the electrical motor will turn ON, at the same moment the normally open contact (Output Relay) will be activated. As a result, even if you hold the pushbutton the output relay will stay ON due to the other contact. But here the motor will stay ON forever; to stop it you need another pushbutton, let say Stop PB as shown in figure 8. Figure 8. Latching the start with stop pushbutton. Now, to stop the motor, all what you need to is pressing the stop pushbutton, so the output relay will disconnect the power of the motor. TIMERS/COUNTERS On-Delay Timer The On-Delay Timer (TON) instruction counts time when the enabling input is ON. When the current value (Txxx) is greater than or equal to the present time (PT), the timer bit is ON. The On-Delay timer current value is cleared when the enabling input is OFF. This timer continues counting after the Preset is reached, and it stops counting at the maximum value of 32767. Page 10 of 14 Count Up Counter The Count Up (CTU) instruction counts up from the current value each time the count-up input CU makes the transition from off to on. When the current value (Cxxx) is greater than or equal to the Preset Value (PV), the counter bit (Cxxx) turns on. The counter is reset when the Reset (R) input turns on, or when the Reset instruction is executed. The counter stops counting when it reaches the maximum value (32,767). Counter ranges: Cxxx=C0 through C255 Count Down Counter The Count Down (CTD) instruction counts down from the current value of that counter each time the count down input CD makes the transition from off to on. When the current value Cxxx is equal to zero, the counter bit (Cxxx) turns on. The counter resets the counter bit (Cxxx) and loads the current value with the preset value (PV) when the load input (LD) turns on. The Down Counter stops counting when it reaches zero, and the counter bit Cxxx turns on. Counter ranges: Cxxx=C0 through C255 Figure 13. (a) Ladder logic diagram for Count Down Counter. (b) The timing diagram. Page 11 of 14 HOW TO PROGRAM THE PLC: To program the PLC, we will use the STEP7-Micro/WIN software. 1. Turn the PC ON. 2. Turn the PLC ON. 3. Start V4.0 STEP 7 Micro WIN SP5. 4. Select View>>Ladder. 5. Form Bit Logic double, click the normally open contact. Page 12 of 14 6. Click on the red question marks, and rename the normally open contactor with a physical input name, I0.0- I0.7 or I1.0-I1.5, let us say I0.0. 7. Now from the Bit Logic, double click the output, and name it with a physical output name, Q0.0-Q0.7 or Q1.0-Q1.1, let us say Q0.0, the result rung will be as shown in the next figure.