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Guest jimtech67

Slc pid

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Has any tried using a SLC500 PID to contoll web tension on a winder or unwinder. Closed loop with load cell feeback. The shaft pressure is controlled by an E/P from an anlog output. I found this difficult because when the set point and process are equal the Control Variable is zero. This not good for tension. I need the control output maintain a changing level output to maintain tension on the web. I have tried scaling the output many different ways but this did not work because the output I need from the PID when the error is zero is constantly different depening on setpoint that input. Also as the roll builds up or down the air pressure needed to to keep the same set point changes. Am I missing something or is this just the way the loop works in a SLC500? I have writen my own loop that works nicely but is cumbersome.

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Yes, an SLC500 PID instruction can be used to control web tension in a winder or unwind.  The key I believe you need is that the PID loop must be set up to be bipolar (i.e. capable of forward and reverse control).  A friend of mine implemented just what you are looking to do.  He had to call AB tech support to find out how to configure the PID instruction for bipolar operation.  If you only implement a unidirectional PID loop, it will regulate tension in one direction only (i.e. your error signal won't be able to swing negative and won't allow the drive you are controlling to brake or regen when it needs to).  AB has a knowledge base article that explains how to configure the PID loop, the link is http://domino.automation.rockwell.com/applica....ocument  Hope this helps.

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Your best bet is to scale the incoming value to the required level of tension needed. the bridge for the load cell will keep the value constance. as the roll increases on decreases the tension will adjust. you do not need a pid

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