PLC (Programmable Logic Controller), possessing capabilities of logic control, timing control, analog control, and multi-machine communication.
When repairing PLC circuits, one does not necessarily need to understand the PLC's programming, but it is imperative to understand the PLC's external wiring.
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| FX3SA-20MT |
PLC Input Wirring
Taking FX3SA-20MT as an example, the input terminals L, N, and PE are connected to the power supply and ground. The S/S and X1-X12 terminals are for switching signal inputs.
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| FX3SA-20MT Input |
PLC Input Terminal Wiring
PLC Input Terminal Wiring
The relationship between S/S and X0-X13 in PLC input terminal wiring. When S/S is connected to a +24V signal, a 0V signal is applied to the X0 terminal through a switch, generating a DC24V voltage between S/S and X0. The X0 indicator light turns on, transmitting an input signal to the PLC program.
The relationship between S/S and X0-X13 resembles the wiring of an intermediate relay coil.
When the S/S terminal is connected to a +24V signal, the switch must input a 0V signal to the X0-X13 terminals. When using three-wire inductive switches or proximity switches, it is necessary to use NPN type switches that output a 0V signal.
When the S/S terminal is connected to a 0V signal, the switch must input a +24V signal to the X0-X13 terminals. When using three-wire inductive switches or proximity switches, it is necessary to use PNP type switches that output a +24V signal.
PLC Output Wiring
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| FX3SA-20MT |
FX3SA-20MT Output: The 24V and 0V terminals represent the output of a 24V DC power supply, which can be used as the power supply for the input terminals.
There will be a clear dividing line on the PLC's output terminals, dividing them into groups. The FX3SA-20MT output terminals are divided into five groups: com1 with Y0, com2 with Y1, com3 with Y2, com4 with Y3, and com5 with Y4-Y7, where com5 is the common terminal for Y4-Y7.
The relationship between com1 and Y0 is similar to the normally open contacts of an intermediate relay. When com1 is connected to a +24V signal and the PLC outputs signal Y0, the Y0 indicator light turns on, and com1 and Y0 are conducting, with Y0 outputting a +24V signal.
The FX3SA-20MT has transistor outputs, with an output control voltage typically of 24V DC. When there is a need to output a 220V AC voltage, a PLC with relay or thyristor outputs can be selected.
Transistor PLCs have a faster response speed and can output pulse signals, making them suitable for controlling stepper motors and servo motors.
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| Stepper Motor Control Wiring Diagram |
Routine Maintenance of PLC Circuits
Routine Maintenance of PLC Circuits
Once you understand how to wire a PLC, when maintaining a PLC circuit, start by understanding the process flow of the equipment. In the initial state of the equipment, observe the indicator lights on the PLC controller to see the status of the PLC's inputs and outputs. When the PLC circuit is equipped with a touchscreen, you can observe the I/O points on the touchscreen to understand the PLC's input and output status. Determine how many input signals are required for the machine to start normally and which input switch signal needs to be closed when the machine needs to perform the next action.
The most common fault in PLC circuits is input signal failure. Damage to switches or mechanical failure to move to a fixed position can prevent the switch from transmitting signals to the PLC program, causing a fault.
Output terminal faults: The output terminals of a PLC are generally connected to relays and contactors. When the PLC sends an output signal and the output indicator light turns on but there is no output, you can measure the output voltage and the voltage value of the contactor or relay coil. Common faults include damage to contactors or relays, broken wires, and damage to PLC contacts.
Damage to PLC contacts is also a common fault. When there are spare contacts on the PLC, you can change the damaged contact to a spare one in the PLC program (there are many tutorials on this online). Connect the wire under the damaged contact to the spare contact. Pay attention to the voltage of each contact. If the voltage values are inconsistent, you can use an intermediate relay for conversion. Of course, the PLC can also be removed and sent out for maintenance, with the fault phenomenon clearly indicated.
- PLC program faults are not common in routine maintenance and rarely occur. Older PLCs may easily lose their programs due to a dead battery. In this case, you can replace the battery and then export the program from the same equipment and import it into the PLC with the lost program.
In routine maintenance of PLC circuits, there is generally no need to understand the content of the PLC program.