• The fundamental laboratories are designed to cover basic understanding of the technology aspects, and some basic theory of practical control systems. Some of the laboratories will be conducted before the material has been covered in class. When this is the case, all efforts will be made to ensure that the level of knowledge is sufficient.
15.1.1 Lab 1: Introduction to Micrologix Controllers
15.1.2 Lab 2: Electrical Wiring
4. Develop ladder logic for the process below, and develop a set of test inputs to verify the operation. The ladder logic can be developed before the laboratory and brought in on disk. All ladder logic must be commented.
5. Select a process and develop a description of an interesting control function (use Boolean equations, flowcharts, or some other structured design technique). After this you may write your own individual ladder logic program for the PLC. The most creative programs will receive top marks. Develop a table of test values to verify operation.
There are four sensors on a large stamping press. One of the sensors (I/2) is on when a blank is present and ready for stamping. A second sensor is off when there are hands inside the press (I/3). A third sensor (I/4) is a start push-button. It is on momentarily when the press is to go into an active state (O/1). The fourth push-button (I/5) is a stop button, and all should stop when it is pushed.
There are also two outputs available. One output (a status light) is the press-on (O/0). It is active after the start button is pushed, this can be shut off if the stop button is pushed. The press will begin stamping cycles if the press-on output (O/1) is active. This will occur when the press-on output is active, and a part is present, and there are not hands inside.
2. Make sure the ground is connected to all devices.These are typically color coded as green, or have a ground symbol. Note: power should not flow through the ground, it is only for emergencies to draw current out of the cases of electrical equipment, and into the ground beneath the building. In other words the ground and common are not the same thing.
3. Normally the AC has 2 wires (for a single phase). For consumer applications we need to make sure the polarity is correct, so that the ‘hot’ (black) wire is switched off, making electrical shocks less likely. Note: in reality, even if these wires are backwards the power will still be delivered to the load.
6. Do not leave loose, or exposed wires. These will only lead to short circuits, electric shocks, or other problems. Tighten the wires. If doing this for permanent jobs, the wire should also wrap around the screw. Note: leads with banana plugs or aligator clips are particularly prone to creating short circuits.
1. A PLC rarely has an internal power supply for inputs or outputs. You must always connect an external power supply for inputs or outputs. Note that the Micrologix does have a small 24Vdc power supply, but it is not connected to any of the inputs or outputs, and it will not drive a large load.
2. The ground and common are terms that are badly confused. A true ground is an electrical connection to the ground beneath a building that will draw away current if there is an electrical fault. A common is a reference voltage for all parts of a circuit, typically 0V. When connecting devices such as sensors and actuators we want to connect them to a common. This problem is normally overlooked, but when we have systems with mixed power sources (eg. 115Vac, low voltage DC) we must separate these. DO NOT CONNECT the common to the ground. BE WARNED, many low voltage devices (such as power supplies, sensors, etc.) show the common as a ground.
4. On the Micrologix PLC there is a small power supply that can be used to power a limited number of inputs or low power outputs. The figure below shows an example of three NO pushbuttons connected using this power supply. Normally a PLC will NOT have a power supply and you should recognize that this configuration is an exception.
The laboratory focus will be on building the system shown in the wiring diagram below. The wiring diagram loosely follows industrial wiring diagram standards. In the diagram the AC power is connected across the ‘L1’ and ‘N’ rails (vertical lines). The connections to the devices are indicated as shown on the devices. The inputs and outputs to be connected to the system include:
2. As a group, enter and test a ladder logic program as developed in step 3. of the pre-lab. The program will be checked by monitoring on-line, and by completing the test table. The professor must check that the program is operational and assign a grade.
15.1.3 Lab 3: Simple Motor Control
The motor will be driven with 12V, switched by an output relay in the PLC. This will cause rotation at approximately 100rpm. A simple encoder will be made by using a clear disk with blacked-out areas. The disk will rotate with the shaft of the motor. An optical sensor will be used to detect the blacked-out areas. The result will be input pulses that go into the PLC. By counting the pulses we can tell how many times the shaft has rotated. The figure below shows the photodetector circuit.
15.1.4 Lab 4: Introduction to PLC-5 Controllers
15.1.5 Lab 5- Sequential Logic Control
Develop a PLC program that will control a miniature set of traffic lights. These lights will go through a normal sequence, but will have pedestrian cross walk buttons that will activate a cross walk signal when pressed. When done the student should understand the design and implementation of time dependent control circuits.
4. Develop a creative description of a process for the PLC that is time dependent. Create the state transition/petri net/etc model for the problem. Then create the ladder logic to support the design. Note: the boards for these labs also contain relays, switches, buzzers, lights, a motor, optical sensors, etc. A test method must be developed for the results.
We want to develop a controller for a set of traffic lights that is at the cross of Main St. and a less used Cross Rd. The lights under two possible sequences as shown below. In the normal sequence the green for cross is shorter with no cross walk light. If a cross walk button is pushed while the Main light is green or yellow the Cross green light will be on longer with a walk sign.
1. The instructor will describe how to connect the PLC, power supply, buttons, etc at the beginning of the laboratory period. As a group you will connect the circuits. Components used will include push buttons and red/yellow/green LEDs for lights.
3. Individually wire, enter and test the pre-lab step 4. The results must be demonstrated to the professor. Marks will be deducted for excessive debugging time. (1% per minute past 30 minutes up to 100%)
15.1.6 Lab 6a: Analog Input/Output
Analog inputs and outputs are done with multipurpose cards in the PLC rack. To control these cards there is some overhead required to set voltage ranges, scales, values, etc. We do this by putting values in the PLCs integer memory, and then the contents are moved to the analog I/O card where values are read or set.
To write voltages to the PLC we set up a block of memory, the function shows this starting at N9:0, and it is 13 words long. The contents are described in the analog card manual. The block transfer function also needs a control block of memory, this is BT10:1
To read voltages we use a similar method. In the example below the input value will be read when the analog input is on. When done the result will be stored in N7:10+4 = N7:14. The value will range from -4095 for -10V to 4095 for 10V.
15.1.7 Lab 6b: PID Control
The PID calculation is effectively a calculation in the PLC. One basic method of PID control is i) read voltage, ii) do PID calculation, iii) set output voltage. (Note: it is also common to get a self contained PID card for the PLC that deals with all inputs and outputs). The ladder logic below shows a PID control function.
15.1.8 Lab 7: Communications
1. Write two programs to run on separate PLCs. When a button is pushed on one PLC (node #1), it should send data to another PLC to turn on an output. The message should be passed using the DH+ network. Write a second program that uses DH+ to run on a PLC so that when a button is pushed it requests data to set an output.
When PLCs communicate, one PLC must write contents of its memory to a second, or one PLC must request contents of memory from a second. The program below shows the basic steps involved in communication.
Serial communications can be done using an RS-232 interface. This is the most common interface available (most personal computers have 2). On the PLC-5 CPU there is one RS-232 interface that we have been using for programming, we can also use this for normal communication. (Note: it is very common to purchase a separate card that provides a serial port. This keeps the port on the CPU available for programming.)
1. Join your team with another team to test the DH+ program. Wire the PLC DH+ on each CPU together. Pick one as node 1 and the other as node 2 and set the switches on the back of the CPU cards. Test the prelab programs.
2. (single teams) Have two computers available. One will be used for programming the PLC, and the other will be used as an ASCII terminal. Enter and download the program to the PLC as normal. Disconnect the serial cable, and connect it to the other PC. Run ‘hyperterm’ and test the program.
15.1.9 Lab 8: Pneumatics
15.1.10 Lab 9: DVT Vision Systems
• These laboratories will be used to pull together 4 individual control systems into a complete manufacturing control system. Although each group will solve a different control problem, each laboratory will end with all stations in a fully functioning control system.
• The descriptions below will be used to develop a design and ladder logic before arriving at the laboratory. All laboratories are to be done on design sheets like those found in the course notes, or equivalent.
• NOTE: In this lab three of the stations use the hole detect to start an operation. Even when the operation is done the hole in the keytag will remain. You must write your program so that after the press has retracted the process will not start immediately. Only after the hole is gone will the program start looking for a new hole. You might want to add another state that waits until the hole is gone.
15.2.1 Lab 10a: Shear Press
The shear press will detect when the material is in place for shearing when a hole is detected by an optical sensor mounted. When sensed it will set a bit true in memory (B3:0/0) that will cause the material feeder to stop. A pneumatic cylinder will be actuated to clamp the strip. At this point shearing will begin by advancing the hydraulic cylinder until a hydraulic cylinder advanced limit switch is actuated. At this point the advance solenoid will be turned off, and the return cylinder solenoid will be actuated. This will continue until the retracted limit switch is actuated. At this point both the hydraulic solenoids are turned off. Finally the pneumatic solenoid is released, and the material feeder is allowed to continue.
15.2.2 Lab 10b: Feeder Positioning
The feeder uses a stepper motor to advance the material strip. The feeder will continue to advance the material until one of the other machines orders the feeding to stop by setting flags true in memory locations (B3:0/00, B3:0/01, B3:0/02). The stepper motor is driven by specifying direction (we will always go forwards), and each time an output is pulsed it will step forward one pulse. It will take a large number of pulses to move the material one inch. Immediate outputs may make it possible to generate pulses faster that the ladder logic scan rate.
15.2.3 Lab 11a: Stamping Press Control
The stamping press will detect when the material is in place for stamping (embossing) when a hole is detected by an optical sensor mounted. When sensed it will set a bit true in memory (B3:0/01) that will cause the material feeder to stop. A pneumatic cylinder will be actuated to clamp the strip. At this point stamping will begin by advancing the hydraulic cylinder until a hydraulic cylinder advanced limit switch is actuated. At this point the advance solenoid will be turned off, a two (or more) delay (0.5s) is required to allow the embossing to occur. After this the return cylinder solenoid will be actuated. This will continue until the retracted limit switch is actuated. At this point both the hydraulic solenoids are turned off. The pneumatic solenoid is released, and the material feeder is allowed to continue.
15.2.4 Lab 11b: Variable Feed Drill
The drill press will detect when the material is in place for drilling when a hole is detected by an optical sensor mounted. When sensed it will set a bit true in memory (B3:0/02) that will cause the material feeder to stop. A pneumatic cylinder will be actuated to clamp the strip. At this point drilling will begin. There are three modes for controlling the drill described below (We will use mode ii). When the drill is done the pneumatic solenoid is released and the material feeder allowed to continue.
i) Velocity Control: the drill may also be controlled using analog output card for feed velocity, and using digital inputs to measure position. The limit switches are used as in mode i). For drilling the analog output card should produce a voltage about +2V, -10V is used for retracting, and 0V for no motion. The ladder logic below will make the analog output card drive the drill to advance when a bit is set, and retract when a second bit is set, otherwise the drill will be idle.
2. Set up the I/O modules. You will need to make sure the analog output card is also set up. When setting it up use the ‘autoset’ values option. This will pick memory locations for the card to use: take note of these. (You will need to do this again when all of the programs are combined.) The use the ‘insert ladder rungs’ program to put the functions you need in the ladder diagram.