LAB 10 INTRODUCTION TO PLCs. Rajesh Rajamani. ME 4231 Department of Mechanical Engineering University Of Minnesota. User Entered Program

Transcription

1 LAB 10 INTRODUCTION TO PLCs Rajesh Rajamani ME 4231 Department of Mechanical Engineering University Of Minnesota PLCs Inputs User Entered Program Outputs A PLC (Programmable Logic Controller) works by looking at its inputs, and depending upon their state, turning ON/OFF its outputs The logic that determines the outputs is entered by the user, usually through a software program. 1

2 PLCs PLCs were invented to replace the sequential relay circuits used in machine control. What is a relay? A relay is an electrically operated switch. INTRODUCTION TO RELAYS Consider the following starter circuit - + Starter motor This circuit would not work too well for starting your car 2

3 INTRODUCTION TO RELAYS - + Problem: A massive amount of current is required to turn the starter motor and crank your engine That much current would burn out the contacts of an ordinary switch Also You want shortest path from battery to starter You don t want that many amps near you INTRODUCTION TO RELAYS - + switch solenoid Starter motor Solution: Use a relay The contacting plate is big enough to carry the amps. 3

4 INTRODUCTION TO RELAYS A relay is an electrically operated switch It is made up of a coil that can be energized with just a little power a moving armature(plunger) of some sort that pulls in when the coil is energized. one (or more) pairs of contacts that the plunger makes an electrical connection with. These contacts usually carry the high power. PLCs - HISTORY PLCs were introduced in the late 1960s Primary Motivation Eliminating the large cost involved in replacing the complicated relay based machine control systems In machine control systems, when production requirements changed, so did the control system. Changing the control system was time consuming and expensive when relay circuits were being used 4

5 PLCs Other problems with relays Limited lifetime, require strict maintenance (since they are mechanical devices) Troubleshooting is tedious when many relays are involved When hundreds of relays are involved, the initial wiring is extremely complicated. PLCs Logic controllers that can survive harsh industrial environments and use a programming technique that people using relays were familiar with. WHAT IS INSIDE A PLC? A PLC consists of a CPU, memory areas, appropriate circuits to receive/ send input/output data and a program terminal Program Terminal CPU/ Memory I/O Modules 5

6 WHAT IS INSIDE A PLC? Schematically, a PLC can be considered to be a box full of hundreds or thousands of relays counters timers and data storage locations Do these counters, timers, etc. really exist? No, they may not physically exist but rather they are simulated and can be considered software counters, timers, etc. PLC COMPONENTS Input Relays Counters Output Relays Internal Utility Relays Timers Data Storage 6

7 PLC COMPONENTS Input relays Physically exist and connected to the outside world Typically they are not relays but are transistors Internal utility relays Simulated relays Counters Count pulses. Could be simulated (slower) or physical (faster). Timers Typically of the ON-delay type. Simulated. Output relays Physically exist. Could be relays, triacs or transistors. Data storage Registers used to store data for math or data manipulation. PLC OPERATION A PLC works by continually scanning a program Check input states One scan step is defined as the time taken to execute the 3 steps Execute program Update output states 7

8 PROGRAMMING Ladder logic diagram Programming elements Normally open (NO) contact Normally closed (NC) contact Coil Connectors (shorts) Timer Counter PROGRAMMING AND A AND B A B OR A A OR B B 8

9 SIMPLE EXERCISE For the starter motor, suppose I want to wire in an anti-theft device such that There is a switch in the glove box that must be ON for the car to start After turning the secret switch ON, I can start my car normally. Otherwise, nothing happens SIMPLE EXERCISE SOLUTION START PROT MOTOR rungs of the ladder 9

10 TIMERS If the enable input is ON, then the timer measures the elapsed time. The output T1 goes ON once the elapsed time reaches the set point. Traditional representation of timer In 1 Enable/ reset preset Tx.xxx accum out If the enable input is ON and the input In 1 also ON, the timer keeps increasing accum (an integer) until accum becomes equal to preset as soon as accum becomes equal to preset, the output of the timer goes ON accum is reset to zero when enable/reset is OFF ALTERNATE (FLASHING) LIGHTS A controller is needed for an intersection where both sides should alternately flash red lights (the equivalent of a 4-way stop) The timing diagram is as shown below Red 1 Red

11 FLASHING LIGHTS PROBLEM Red 1 Red An event happens every 2 seconds Need a timer The ON-OFF cycle repeats indefinitely Do not need a counter FLASHING LIGHTS PROBLEM Red 1 Red sec 1 OFF 2 ON 2 sec 1 ON 2 OFF 4 sec 1 OFF 2 ON 11

12 FLASHING LIGHTS PROBLEM Red 1 Red How many timers are needed? An event happens every 2 seconds. Is one timer enough? Needs a little thought. Easier to use 2 timers Use a sampling period of 0.1 seconds and a preset of 20. FLASHING LIGHTS PROBLEM 20 T T 0.1 Initially both and are OFF. Timer 1 starts accumulating. After 2 secs, goes ON. This immediately makes Timer 2 start accumulating After 4 secs, Timer 2 accumulates to 2 secs and goes ON. Immediately goes OFF. goes OFF. 12

13 FLASHING LIGHTS PROBLEM 20 T T FLASHING LIGHTS PROBLEM 20 T T 0.1 Red 1 Red 2 13

14 FLASHING LIGHTS PROBLEM CLICK SOFTWARE LADDER LOGIC ELEMENTS Element Description Ladder Logic Representation Normally Open Contact Is ON when the defined address (eg. X001, Y001, C1, or T1) is ON Normally Closed Contact Is ON when the defined address (eg. X001, Y001, C1, or T1) is OFF Out Coil Turns ON/OFF a given address (eg. Y001 or C1) when the rung is true/false. Timer When enabled, measures the elapsed time. Turns on output address once it reaches the set point. Counter When enabled, counts up or down until it reaches the set point End Instruction Marks the termination point of a program TRAFFIC SIGNAL PROBLEM A controller is needed to control the traffic lights of an intersection of two streets Street number 1 (Washington Avenue) is a main street Street number 2 (Union Street) is a side street The timing diagram for the controller is shown on the next slide 14

15 TRAFFIC SIGNAL PROBLEM Green Yellow Red 1 Green 2 Yellow Red TRAFFIC SIGNAL PROBLEM How many timing events? Green 1 OFF, Yellow 1 ON 35 seconds Yellow 1 OFF, Red 1 ON, Green 2 ON 40 seconds Green 2 OFF, Yellow 2 ON 55 seconds Yellow 2 OFF, Red 2 ON, Green 1 ON 60 seconds Need 4 timers 15

16 TRAFFIC SIGNAL PROBLEM 35 T 1 ON 35 OFF T 0.1 ON 40 OFF T 1 Y3 ON 55 OFF 60 Y3 50 T 0.1 ON 60 OFF 60 Y3 TRAFFIC SIGNAL PROBLEM Green 1 Etc 16

17 TRAFFIC SIGNAL PROBLEM Simpler but less robust solution 35 T 1 ON 35 OFF T 1 ON 40 OFF T 1 Y3 ON 55 OFF T 1 ON 60 OFF 60 TASKS IN LAB Task 1 Introduction to the lab equipment CLICK Series PLCs CLICK software for Relay Ladder Logic programming Task 2 Implementation of two simple examples The oscillating lamp The real-time clock Task 3 Implementation of the Flashing lights solution Task 4 Implementation of the traffic lights solution 17

18 POST LAB PROBLEM Analyze the ladder logic diagram below and predict the signals and for time between 0 and 10 seconds. 20 T