Re: To PID or not to PID
- From: Tim Wescott <tim@xxxxxxxxxxxxxxxx>
- Date: Thu, 30 Jun 2005 08:36:25 -0700
Yep, I'm here. Didn't mean to cause such a ruckus.
Don't be -- it means you asked a good question.
Yes, open-loop control is always stable given a stable plant. It sounds like if there are any variations that need to be guarded against at all it's going to either be density changes in the meat or wear in the pump.
Yes, the PD means positive displacement. The product is meat. The auger is to feed the pump. The density should be consistent.
Actually, I thought about the same thing because it is a positive displacement pump. Could I just eliminate the PID and the weigh transmitter (maybe just use it as verfication). After all, open loop control is always more stable than closed loop control, right?
For a given speed, I would know the volume per revolution being pumped. Knowing the specific gravity of the product, I should be able to figure out mass, right?
To answer an earlier question, one of the reasons that I was looking at doing a moving average was because I expect that there will be a lot of noise on the signal. I wasn't sure that filtering alone would do the trick. There are a couple of other auger motors on the hopper, near the top, to force the meat down into the bottom auger that then feeds the pump. Of course the problem with this is that I then have to have enough samples before I can close the loop. Also, how to start up from a stop.
If you set up your PLC to record the weight of the hopper at the start of the batch, the weight close to the end, and the running time, then you'll be able to get a good estimate of the actual mass flow rate. If you really want to get into programming the PLC you could collect a few statistics at each step and get a linear fit of the weight vs. time -- that would give you the closest estimate of the rate of change of mass, without ever using a differentiator.
You could then use this actual mass flow estimate in one of three ways: you could just have the PLC publish it as a statistic, you could use it as an alarm to indicate equipment problems or quality problems with the meat, or you could update a PI loop once each hopper load. This latter would do nothing to protect you against minute-to-minute changes in meat density or pump characteristics, but it would be outstanding at compensating for any issues with pump wear -- and you could still raise an alert if the mass flow vs. pump speed ratio indicated something seriously wrong.
-- ------------------------------------------- Tim Wescott Wescott Design Services http://www.wescottdesign.com .
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