Friday, August 31, 2018

Machine Failure Analysis

Machine Failure Analysis:

PLC Troubleshooting

Most of the time when people refer to PLC Troubleshooting, they are actually referring to using the PLC to troubleshoot machine failure more quickly. In this post, we are applying the machine failure analysis to troubleshooting the PLC itself.

Most reading this, already know how the laws of physics are applied to machine failure analysis and used for machinery failure probability assessments. So a company can assess what parts to stock to minimize downtime cost-effectively. But we have found many neglect to take into account those basic laws when troubleshooting the PLC itself (and other machine automation equipment). So below is the list again, in order of most likely to least likely just as a reference.

Failure probability is categorized like this …
(In order of most likely to least likely)

High Current (because of associated high heat)
Low Current
Solid State

Environment can be an exception to the general rules.
(Relay card most likely, and MTBF is useless unless solid-state, as frequency PLC program activates, dictates failure rate as well as environment.)

The above probability guide translates to PLCs as ...

External to PLC
Output Cards
Input Cards
Power Supply
CPU (processor Card)

The environment can change basic order above. A couple examples, in a foundry, the metal dust causes backplane to fail more frequently. In the machining industry, penetrating fluid getting into sensors causes input cards to fail more than output cards because the liquid causes an input card to sink more current than designed to. In wire manufacturing were a lot of high voltage insulation testing is going on, CPUs fail more often because control voltage for PLC does not have a line filter in line with it. etc. etc. Other variables that affect the failure rate are improper design, like not allowing the 20% extra when calculating relay card or power supply current, etc, etc.

To dig deeper into the analysis, let's take a PLC thermal couple input card as an example. You want to decide, should I stock spares of this type of PLC input card? It is an interesting one, as it has no moving parts, but in industrial applications, commonly used in a high heat environment. So its probability of failure could be classified in the more likely categories of mechanical and high current, even though it is not literally mechanical or high current. (The environment in which it commonly operates places it up there with those high probabilities of failure.) Technically an input card, yet risk at the level of output card due to environment. Point being, while the two priority list above is a general guide, you need to consider your particular environment too. (There are exceptions to every rule. 😊 )  

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Don (Follow me on Industrial Skills Training Blog and on Twitter @IndTraining .) Be sure to to stay on top!