Have a level control application that consists of a small chamber relative to the size of the incoming and outgoing pumps. I've done this type of control in the past with success using a straight PID on the return pump. However in this application the level control feedback supplied is noisy so I'm having to run a first-order low-pass filter on the signal and coupled with the Ramp rates on the pumps I'm finding too much lag in the system when the supply pump varies in speed. These pumps are positive displacement and the ratios can be determined so my question is would I be better to run ratio control on the return pump and use the PID to correct for the error in the ratio based on level. In other words the PID setpoint will be 0 error and whatever error occurs will be applied on top of the ratio to the return pump speed.
PID level control with ratio of pumps.
- Thread starter robertmee
- Start date
kamenges
Member
I don't necessarily think ratio control wold be any better in this situation but an element of that would help. I feel you aren't using the available information as well as you could. If you are even considering doing ratio control that means you have access to the speed of the supply pump. Just pass that straight through to the exit pump, appropriately scaled for any difference in pump volume of course. You know that a change in supply pump flow will result in an error so why wait for it to happen? You can add your PID correction to this value to account for things you can't model. Just make sure you allow the PID to go bipolar and limit the sum of the baseline speed and the pump correction between your outfeed pump limits.
The thing I don't like about ratio control is the correction responds differently at different speeds. You need to account for that.
Keith
The thing I don't like about ratio control is the correction responds differently at different speeds. You need to account for that.
Keith
Thanks Keith...my thoughts too on the supply to return speed passed as a ratio. Almost as a feed forward. The error comes from a dip station in this chamber so some amount of product is taken away by the web. It acts as a reduction in supply so that's what I'm hoping the pid takes care of. The error produced by removal of product before it hits the return. There's no way to predict the amount as it changes based on manual operator settings but I believe I can tune the PID to compensate for it.
Last edited:
kamenges
Member
If you have access to web speed another compensation you could make would be to modify the return pump speed based on web speed, which would be related to your rate of take-away. But if what you have now works you could just stick with that too.
Keith
Keith
Typically I tie Web speed to supply pump speed and thereby return pump speed would take care of itself in this mode. However the line runs at a very small speed range so in this particular app the plant opted out of varying the supply pump speed.
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