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davidtorati

PID Controller

7 posts in this topic

Hello I have to make foodstuff weighing putted on a conveyor belt that is hanged on a weigh-scale with a transducer output of 4-20ma according to weight. I have to keep a constant avarage weight. There is a second conveyor belt that is feeding the first one with the foodstuff according the velocity controlled by a VFD using also 4-20ma input equivalent to 25-80 Hz. Notice that input signal fluctuates . I have user a process pid controller with a filter on the input signal but it dosen't help and the controller change the speed of the VFD very rapidly Any suggestion what to do? Attached a schematic diagram Thanks Edited by davidtorati

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Sounds like you need to tune your PID controller. Without knowing anything else, sounds like your gain may be a little to high.

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I have already tune my controller and it also has Auto tuning option.

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To keep it simple there are a number of things that you can look into 1) Make sure that the product on the feed conveyor is evenly spread - I usually test this by running both conveyors at a fixed speed and capturing a trace of the loadcell. 2) Add Feedforward control - If the product spread on the feed conveyor is always even then the feedforward controller can predict the VFD speed for the loadcell setting. 3) use a smarter filter on the feedback - e.g. rolling moving average mg

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Most stand alone PID controller's use an autotune algorithm for optimized for relatively slow thermal processes. I suspect this loop is fairly fast responding, more like a flow control loop. 1) Try filtering the input signal so minor input fluctuations get averaged out over time. 2) some controllers use gain, some use proportional band (PB). Some can be configured for either. Each is the reciprocal of the other when it comes to the action achieved by changing the values for either. To dampen the gain response, use a smaller value. To dampen the PB response, use a larger value. Do you know whether your controller uses PB or gain? 3) Can you turn the derivative term off or set it to minimum? Dan

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Why would one want to do that? The derivative term may be necessary to tune the response hot enough to follow the changes in the weight feedback. BTW, Michael G had pretty good answer. The feed forward term is a good idea but it gain would need to change as a function of the feed conveyor belt loading. Moving average? Why not use a low simple low pass filter? If the feed conveyor was evenly loaded then much of the problem would go away.

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Not sure if you have solved your problem, but this info might help you smooth out your process. I have encountered erratic feedback signals that have caused undesireable fluctuations or contributes to oscillations. As mentioned in the other replies it would be best to try to filter the feedback signal, but if this is not available or proves in-capable why not filter the 4-20mA signal going to the VFD. I have used the the acceleration and deceleration times in a VFD to sort of prolong or damp out erratic speed reference signals. Increasing the accel and decel times in the VFD will add more time constant to the system but may give a more average weight. (Most VFD's have the ability to filter the 4-20mA speed signal as well) Hope this helps.

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