LADDER LOGIC PROGRAMMING EXAMPLES
1. This example provide a practical example of software coding of motors for use in Programmable Logic Controllers
(PLC).
This example shows the states a motor can be detected, it control and PLC Software Programming.
DIGITAL INPUTS
For this example we have considered the following Digital Inputs to the PLC. Inputs may be excluded where the signal is not available or not-required by the application.
| Symbol |
DESCRIPTION |
ON State |
OFF State |
|
AUTO |
Automatic Selection |
AUTO |
- |
|
MAN |
Manual Selection |
MANUAL |
- |
|
START_PB |
Manual Start Push-Button |
START |
- |
| STOP _PB |
Manual Stop Push-Button |
STOP |
- |
|
RESET |
Alarm Reset Push-Button |
- |
- |
|
RUN |
Motor Running, contactor
feedback |
RUNNING |
STOPPED |
|
ESTOP |
Emergency Stop, aux contact from
emergency stop circuit. |
STOP |
HEALTHY |
|
TRIP |
Motor Tripped Signal, aux contact
from motor overload. |
TRIPPED |
HEALTHY |
Notes: For this example we have assumed that the auxiliary contacts for the emergency stop and the motor overload will be ON for the trip condition.
The automatic controls have not been shown in this example, as these signals will be derived from the sequencing software.
DIGITAL OUTPUTS
For this example we have considered the following Digital Inputs to the PLC, Inputs may be excluded where the signal is not available or not required by your application.
Note: For this example we have used a single output for the control of the motor.
Motor States
From the inputs available the following states of the motor can be derived. We have shown the automatic states in the following table, each state exists for manual mode and the auto text replaced by manual.
CONTROL LOGIC
Detecting, Latching and Reporting Alarms
The first alarms that will be detected within the logic are Emergency stop and Tripped. This is simple digital logic. These signals must be hardwired into the motor control circuit, but the software will duplicate the hardwired logic to report the alarm to the operator.
SOLVED PROBLEMS
1. Give an example of where a PLC could be used.
2. Give a concise description of a PLC.
3. List the advantages of a PLC over relays.
4. A PLC can effectively replace a number of components. Give examples and discuss
some good and bad applications of PLCs.
5. Explain the trade-offs between relays and PLCs for control applications.
6. Explain why ladder logic outputs are coils?
ANSWERS
1. To control a conveyor system, level control etc.
2. A PLC is a computer based controller that uses inputs to monitor a process, and. uses outputs to control a process. A simple program is used to set the controller behavior.
3. Less expensive for complex processes, debugging tools, reliable, flexible, easy to expend, etc.
4. A PLC could replace a few relays. In this case the relays might be easier to install and less expensive. To control a more complex system the controller might need timing, counting and other mathematical calculations. In this case a PLC would be a better choice.
5. Trade-off's include: cost, complexity, easy of debugging, etc.
6. The ladder logic outputs were modelled on relay logic diagrams. The output in a relay ladder diagram is a relay coil.
UNSOLVED PROBLEMS
1. Describe PLCs software functions in detail.
2. Write short notes on following PLC components:
-
CPU
-
Memory
-
Input Interface
-
Output Interface
-
Input/Output section
3. Explain PLC operation and show how scan time is related to timer.
4. Write a short note on the ladder diagram like programming language used for PLC. Why is such language popular in factory automation?
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