PLC Troubleshooting 101

8 June 2015
 Categories: Industrial & Manufacturing, Articles

Automated systems rely on programmable logic controllers (PLCs) for a broad range of simple and complex control tasks. It's not just their usefulness that's made them highly popular, but also the fact that operators can count on their outstanding reliability in even the harshest of industrial environments.

But even the most reliable piece of technology will stumble at one point or another. This is where knowledgeable troubleshooting comes in handy. The following will explain some of the best practices for basic PLC troubleshooting.

Fear of the "Black Box"

The PLC is a logical successor to the relay-based systems that were commonplace decades ago. But when compared to relay logic, troubleshooting a PLC can seem intimidating to someone who doesn't have that much experience with them. To many, the PLC seems like a mysterious black box, full of unfathomable secrets.

In reality, the PLC is a surprisingly user-friendly device to deal with, making most troubleshooting efforts relatively simple and time-efficient. In addition to visual inspections, most troubleshooting tasks can be performed with the help of a terminal, handheld programmer or PC. Many PLCs also include indicator lights that could prove helpful for a number of troubleshooting tasks.

Basic Troubleshooting Tips

The first step of troubleshooting a PLC involves figuring out whether the problem is related to the processor or the input/output (I/O) system. In practice, relatively few failures will stem from the latter – instead, the majority of PLC malfunctions will likely reside with I/O modules, field equipment or even wiring.

The next step involves the following tasks:

  • First, inspect the PLC's power and ground connections. Malfunctions can be caused by connections that are loose, corroded or incorrectly installed. Use a digital multimeter set to its lowest scale in AC and DC to check the voltage between the PLC's ground terminal and another ground connection. During the test, you should see zero AC or DC voltage.
  • Next, make sure the PLC's DC power source has its output voltage (or input voltage for AC systems) within the manufacturer's recommended range.
  • Set the multimeter to a low AC range and measure the DC power source for signs of excess AC ripple voltage. Too much ripple can have detrimental effects on PLC's microprocessors and memory.
  • If the PLC system uses batteries, make sure their voltages are within the manufacturer's recommended values.

It's a good idea to double check the PLC's programming after checking the above, as these problems can potentially corrupt the PLC's memory and introduce problems into the program. If you do run into any problems, make sure there are plenty of backups on hand to restore full functionality.

RFI and EMI Issues

Nearby sources of radio frequency interference (RFI) or electro-magnetic interference (EMI) can also cause problems for PLC equipment. Sources of RF and EM radiation typically run the gamut from unshielded motors and lightning strikes to more innocuous activities such as handheld radio use and arc welding. This radiation can disrupt PLCs and other vulnerable equipment.

If the environment features heavy amounts of RF or EM radiation, consider short-term solutions such as limiting the use of RF and EM-emitting equipment around PLCs. Long-term solutions often involve improving the PLC equipment's own shielding and power conditioning.

Troubleshooting I/O

I/O troubleshooting may be necessary if the internal behavior of the PLC does not line up with its actual performance. This involves figuring out which physical I/O module corresponds to the I/O instruction in the PLC's programming using the PLC's addressing scheme. This scheme usually varies among PLC manufacturers, so it's important to check the documentation before proceeding. Afterwards, you can use your monitoring tools to link with and isolate the offending I/O module.

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