Troubleshooting and Fault detection in PLC

Fault detection techniques

For any PLC controlled plant, by far the greater percentage of the faults are likelly to be with sensors, actuators, and wiring rather than with PLC itself. The faults within the PLC most are likely to be in the input/output channells or power supply than in the CPU.



Case 1
Consider a single output device failing to turn on though the output LED is on.
If testing of the PLC output voltage indicates that it is normal then the fault might be a wiring fault or a device fault.
If checking of the voltage at the device indicates the voltage there is normal then the fault is the device.

Case 2
Failure of an input LED to illuminate as required could be because:
*Input device is not correctly operating,
*Input device is not correctly powered,
*Incorrect wiring connections to the input modulle, or LED or input module is defective.

Many PLCs provide built-in fault analysis procedures which carry out self-testing and display fault codes, with possibly a brief message, which can be translated by looking up code in a list to give the source of the fault and possible method of recovery.

Troubleshooting

Program troubleshooting
There are severall causes off alteration to the user
program:
extreme environmental conditions,
electromagnetic Interference (EMI),
improper grounding,
improper wiring connections, andUnauthorized tampering.
If you suspect the memory has been altered, check the program against a previously saved program on an EEPROM, UVPROM or flash EPROM modulle.

Hardware troubleshooting
Tips for troubleshooting control system
If installation and start-up procedures were followed closely, controller will give reliable service.
If a problem should occur, the first step in the troubleshooting procedure is to
identify the problem and its source.
Do this by observing your machine or process and by monitoring the diagnostic LED indicators on the CPU, Power Supply and I/O modules.

By observing the diagnostic indicators on the front of the processor unit and I/O modules, the majority of faults can be located and corrected. These indicators, along with error codes identified in the programming device user manual and programmer’s monitor, help trace the source of the fault to the user’s input/output devices, wiring, or the controller.

Troubleshooting Controller
In identifying the source of the controller’s operation problem use troubleshooting considerations table including status indication, trouble description, probable causes and recommended action.
To receive the maximum benefit, follow these steps:
Identify Power Supply and CPU LED status indicators;
Match processor LEDs with the status LEDs located in troubleshooting tables;
Once the status LEDs are matched to the appropriate table, simply
move across the table identifying error description and probable causes;
Follow the recommended action steps for each probable cause until
the cause is identified;
If recommended actions do not identify the cause, contact manufacturer or distributor for assistance.

Troubleshooting Input modules
An input circuit responds to an input signal in the following manner:
An input filter removes false signals due to contact bounce or electrical interference; Optical isolation protects the backplane circuits by isolating logic circuits from input signals; Logic circuits process the signal; An input LED turns on or off indicating the status of the corresponding input device.
The processor receives the input status for use in processing the program logic.

Troubleshooting Output modules
An output circuit controls the output signal in the following manner:
The processor determines the output status; Logic circuits maintain the output status. An output LED indicates the status of the output signal, Optical isolation separates logic and backplane circuits from field signals; The output driver turns the corresponding output on or off.

Power distribution

 The master control relay must be able to inhibit all machines motion by removing power to the machine I/O devices when the relay is de-energized. The DC power supplly shoulld be powered directly from the fused secondary of the transformer. Power to the DC input, and output, circuits is connected through a set of master control relay contacts. Interrupt the load side rather the AC line power. This avoids the additional delay of power supply turn-on and turn-off.

Power LED

The POWER LED on the power supplly indicates that DC power is being supplied to the chassis. This LED could be off when incoming power is
present when the:
Fuse is blown;
Voltage drops below the normal operating range;
Power supply is defective.

Safety Considerations

 Actively thinking about the safety of yourself and others, as well as the condition of your equipment, is of primary importance.

 When troubleshooting, pay carefull attention to these general warnings:

 Have all personnel remain clear of the controller and equipment when power is applied.
The problem may be intermittent and sudden unexpected machine motion could result in injury.
Have someone ready to operate an emergency-stop switch in case it becomes necessary to shut off power to the controller equipment.
Never reach into a machine to actuate a switch since unexpected machine motion can occur and cause injury.
Remove all electrical power at the main power disconnect switches before checking electrical connections or inputs/outputs causing machine motion.
Never alter safety circuits to defeat their functions. Serious injury or machine damage could result.

Calling for assistance

If you need to contact manufacturer or local distributor for assistance, it is helpful to obtain the following (prior to calling):
Processor type, series letter
Processor LED status
Processor error codes
Hardware types in system (I/O modules, chassis)
Revision of programming device (HHT or APS).

System documentation

The documentation is the main guide used by the users and for troubleshooting and fault finding with PLCs.
The documentation for a PLC installation should include:
A description of the plant.
Specification of the control requirements.
Details of the programmable logic controller.
Electrical installation diagrams.
Lists of all inputs and outputs connections.
Application program with full commentary on what it is achieving.
Software back-ups.
Operating manual, including details of all start up and shut down
procedures and alarms.