Re: To PID or not to PID

Tim Wescott wrote: 
cbolwerk wrote:

Yep, I'm here. Didn't mean to cause such a ruckus. 


Don't be -- it means you asked a good question.


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.

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.

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.

You took the words out of my mouth. (Actually, I got sidetracked and swallowed them.) The way you put it is specific. I'll be general.

If a system's components are almost good enough to be run open loop, than using a background supervisory controller usually works well. In navigation, dead reckoning works well in the short turn, but an occasional position fix is a Very Good Thing. Controlling this way doesn't eliminate the need for transducers that can sense the result of an action, but it simplifies the plan of action and nearly always guarantees stability.

Jerry
--
Engineering is the art of making what you want from things you can get.
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