The programmable logic controller, more commonly known as the PLC, plays a role in almost every automated manufacturing process. The PLC is an electronically programmable device capable of almost infinite combinations of relays, contacts, and timing circuits used to control industrial machinery of all types and sizes. It is essentially a computer designed to withstand a harsh manufacturing environment. It was designed to replace complex system of relays, cam switches, and other electromechanical circuits. The first PLC’s were introduced in the late 1970’s and they continued to grow in popularity since the day of their release.
PLC’s are produced today by numerous major manufacturers including Siemens, Allen Bradley, Koyo, Fuji, Telemecanique, and Mitsubishi as well as a host of others. The programming of the PLC is performed through proprietary software produced by the specific manufacturer. Software among PLC’s is not compatible and the logic programming, although it is very similar, has many subtle differences across the different brands. The program stored in the PLC is called the ladder logic program. Problems in the ladder program can become quite involved and that is an entirely different subject altogether. Hardware problems however, are fairly similar and can be easy to diagnose if you know a little about the particular model and it’s input and output configuration.
Hardware failure of the brain, or main processor, is rare. It can be easily diagnosed visually most often by an illuminated fault indicator on the face of the PLC. An internal power supply failure would be a possibility if the PLC screen or power indicator were not lit and the technician verified that the correct voltage is applied to the power terminals. Some PLC’s are equipped with an internal fuse which can also fail. These problems are not quite as common as a failure of an input or output terminal.
Inputs and outputs are available in multiple configurations and voltages. Both AC and DC are available separately and sometimes in the same unit. These input/outputs can be both digital and analog and can be configured as a transistor output or as a dry relay contact. On both the input and output side it is critical to determine the specifications of the PLC first. Having the appropriate PLC software available at the time of troubleshooting is also necessary.
The first recommendation would be to compare the illuminated inputs and outputs against the software in real time. An illuminated input not displaying properly as an energized input in the software is an indication of a failed or open input terminal. On the output side, an illuminated terminal not conducting the proper output voltage can mean one of two things, the output has failed in an open position, or an improper voltage is being applied to the common output terminal. The only way to verify this would be through the use of a multimeter set on the proper voltage scale.
Overall, PLC hardware troubleshooting is not complex, yet it needs to be approached in an orderly fashion because both hardware and software are required to work in harmony for a PLC to perform its intended tasks.
Chet is an longtime associate of the Obsolete Industrial Parts, an informative site dedicated to locating obsolete industrial parts including obsolete plc hardware, servo motors, prox switches, and a host of other parts. Their official website obsoleteindustrial.com is owned and operated by Val Marketing of Carbondale, Pennsylvania.