Many industrial operations are plagued with unscheduled downtime associated with failed sensors. When this occurs, you have to take the time to troubleshoot, physically locate the failed sensors, find replacement sensors and repair the problem before restarting the operation. How much time did you lose? How much in-process material did you scrap? What did this issue cost you in dollars, time-to-market, energy and man hours? How many of these occur in your operation per week, month or year?
When I was a maintenance superintendent in a large integrated steel mill over 25 years ago, I faced this serious and costly dilemma. As radical as it sounds, when we replaced an entire seamless pipe finishing operation (including its control system) I instructed that we install redundant sensors or limit switches to electrically and physically back up the first ones.
The advent of the PLC was the key to executing this plan, and the expense associated with these redundancies was more than recouped when the first failure was avoided. I can’t tell you what the historic savings have been due to this redundancy, but I guarantee that it has been astronomical!
How It Works
Two opposed beam optical presence sensors (or any other type of sensor of limit switch) input to a PLC. Interfacing through interposing plug-in relays protects the PLC from external voltage or noise introductions and allows for easier at-a-glance visual condition monitoring. An algorithm written in the PLC compares the conditions of both sensors and looks for the one that is operating inconsistently with how it should in given conditions. The PLC then disables the faulty sensor from inputting further information into the PLC and sends a message via the established network to the maintenance department. During this time, the backup sensor continues to operate normally. The faulty sensor is then replaced at the next scheduled down period and primary/backup operation is restored for the sensor application.
Try It Before Implementing Site-wide
You might want to try this on one bothersome sensor location in your operation, or test it in an off-line configuration before implementing this solution across your entire operation. Once you are satisfied with your new sensor backup configuration, install it into your operating environment and everywhere you experience difficulties with your sensors. What do you have to lose? What do you have to gain?
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