PLC (Programmable logic Controllers ) Trouble Shooting Manual


Check List

  1. Check the integrity of the PLC’s power and ground.

a) Visually inspect the power and ground wiring, looking for loose, corroded, or otherwise questionable connections.

b) The integrity of the ground can be electrically checked by measuring the voltage between the PLC ground terminal and a known ground. Using a digital meter set on the lowest scale, both the AC and DC voltages should be zero.
2. The power supply also can be tested electrically.

a) If the PLC processor has an AC power source, check the input voltage; it should be within the manufacturer’s recommended range. PLC processors actually operate on DC power, so that also must be checked. Measure each of the outputs of the DC power supply and check if the voltages are within the recommended ranges.

b) Check the DC supplies for AC ripple. This can be done using a digital meter set on a low AC range, and the value measured should be well below the manufacturer’s specifications. Excess ripple has drastic effects on the operation of the microprocessors and memory devices typically found in PLC processors.
Ripple Factor Calculation.
Ripple Factor (%) =E max. – E min. / E mean.
E min. = Mean Value of Pulsating Component
E max = Maximum Value of pulsating Component
E mean = Average value DC Component
c) Measure the voltage of any batteries in the system. Battery power is often used to prevent a PLC from losing its program during power outages, and battery voltages should be within recommended values.

  1. Other causes for erratic processor behavior are electro-magnetic interference (EMI) or radio frequency interference (RFI).

a) Try to correlate the erratic behavior with an external EMI or RFI event like a large motor starting, arc welding in the area, lightning strikes, or even the use of handheld radio transmitters. Although they may seem harmless, handheld radios commonly used by maintenance personnel emit powerful RF radiation and can seriously disturb the operation of unprotected electronic equipment.

  1. Verify the program with the backup, and reload the program if problems are encountered.

  2. Troubleshooting inputs and outputs.

a) The function of a digital input module is to determine the ON/OFF status of a signal or signals in the external field and communicate that information to the PLC processor.
b) Most digital input modules detect changes in voltage levels, and they are available with various AC, DC, or universal ratings, with universal modules typically accepting a fairly wide range of either AC or DC signals.
c) A typical AC input channel has indicator lamps on both the power and logic sides of the circuit. If only one indicator is present, it’s important for troubleshooting purposes to determine where it is connected. If the threshold unit on an active input has failed, for example, a power-side indicator would be ON while a logic side indicator would be OFF.
d) Faults in field wiring and devices may, blow a fuse, trip a breaker, or cause some other power disruption. If input power is not present, determine and rectify the cause of the failure before proceeding.
e) If input power is present, connect a voltmeter across the input actuate and measure the voltage at the PLC input, to determine if it changes adequately when the field device changes state. If it does not, the field devices or wiring are most likely at fault. If a proper voltage change is observed, the power and/or logic indicators on the module should change when the voltage does, and the addressed location in the PLC, when monitored with the programming device, also should change state. If the indicators do not properly reflect the state of the input, replace the input module.
f) If the input module is working properly but the PLC still is not registering the input internally, the problem lies in the system used to communicate input information from the module to the processor. Consult the manufacturer’s documentation to determine how to troubleshoot this equipment, which may include an I/O rack, back plane, communication module, and cabling.
g) Analog inputs measure the actual value of a voltage or current and communicate it to the processor. Analog input modules are available in many DC voltage and current ranges. First determine if the input is isolated or non-isolated, and determine the source of power and verify that it is present.
h) Next, change the voltage or current level generated by the field device, verify that the change is reflected at the input module terminals, and verify that the content of the address associated with the input reflects the voltage or current change.
i) Field wiring can be tested by temporarily replacing the field device with a signal transmitter and observing the reaction at the PLC to signal changes.
6. Troubleshooting output modules.

a) Output modules are designed to cause some change in the field in response to an instruction in the PLC processor. Digital outputs will often be used to perform tasks like starting motors, turning on indicating lights, and energizing solenoid valves. Many different digital output module types are available, with the most common varieties being DC outputs that rely on transistors as switching devices, AC outputs that rely on triacs, and universal outputs that use relay switching.
b) The power to drive PLC outputs, like inputs, is usually not supplied by the module, so it’s important to find out where that power comes from.
c) Check the field Conditions. Faults in field wiring and devices can blow that fuse.
d) Analog outputs are used to generate a variable voltage or current typically used to perform tasks like throttling the speed of a variable speed drive, adjusting the position of a control valve, or driving a panel indicator. As with input modules, analog output modules are available in many DC voltage and current ranges. Usually, there is no indication on the module to reflect the level of the input. As such, you must determine what range of voltage or current the module is designed to produce and what numerical scale is associated with that range in the PLC. An output with a 4-20 mA DC range may be expected to react to a change from 0 to 1000 in a PLC register, for example.

A programmable logic controller (PLC) is a digital computer used for automation of processes. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in PLC processor
An industrial PLC PROCESSOR
Typically PLC processors are coming with control panel board with other accessories like MCBs Lighting devices; Relays and Power supply for the processors.

		             Typical Industrial PLC control Panel 

As PLC is the heart of the automation system any fault may cause to the shut down of entire operations,
Major Fault issues are:

a) PLC not sensing the inputs.
b) PLC not giving output
c) Partial Inputs/ outputs.
d) No display on HMI panel.
Service notes
A. PLC not sensing the inputs.

Check on
• Power supply to the processor
• Out put supply on SMPS
• The lamp indication on the processor
• Field wirings by continuity checking,
• Cabling problems.
• Input Relay is activating or not.

B. PLC not giving output.
Check on:
• Out relays.
• Indication on the out put module of processor.
• Field wiring connections to field instruments.
• Add on out put modules sittings.

C. Partial Inputs/ outputs.

Check on:
Field wirings
Errors on programming

D. No display on HMI panel.

Check on:
• Connection port
• Cabling
• HMI power supply