Control Rate of Change logic

[Pete.S.]

Yes, I know how "real" PID work but the integrator still winds up until the limit is reached.

I think you are misinterpreting the problem. You are talking about limiting the output. I think the OP wants to limit the output rate of change.

I think you might have misread my reply. I was talking about the output rate-of-change. The integrator stops both when the output limit is reached of course but also when the output RATE limit is reached.

The integrator doesn't wind up because it's stops integrating the second the output becomes rate limited.

Since the output is already responding at it's max allowed when it's rate limited then there is no point in any further integration (for the moment).

Output rate-of-change limits are typically set in % per minute. Recommendation when using output rate limiting is to do the tuning of the loop with the rate limiting activated.

I'm actually not sure what the OP wants or needs so I consider this more of an academic discussion at this point. :nodi:

 

[industrial 951]

The level change in the wet well is based off plant flow, it's a waster water treatment plant so there's no way to limit the incoming flow unless you get people to stop flushing toilets. The idea is however to SMOOTH out the flow changes from the pumps by limiting the amount that the speed can change for a given time so as not to upset process to much downstream.

 

[Peter Nachtwey]

The level change in the wet well is based off plant flow, it's a waster water treatment plant so there's no way to limit the incoming flow unless you get people to stop flushing toilets. The idea is however to SMOOTH out the flow changes from the pumps by limiting the amount that the speed can change for a given time so as not to upset process to much downstream.

.
That is easy but you haven't provide enough information and that is the problem with many threads. If you don't understand the problem then how can you expect to come up with an answer?

It is easy to model a tank level problem if given the right data.

If everyone flushes their toilets at the same time and you limit the the speed of the outflow pump or even the rate of change in your pump outflow speed then how do you guarantee the the ***** won't overflow your tank?

You need to be able to simulate your problem so you can understand it.

The only reason I can see to limit the acceleration or speed it to ensure the NPSH requirements are met for the pump.

Again, Mickey's suggestion would probably work well enough. You just need to make sure there is enough stuff in the tank so the NPSH requirements are met before ramping up the pump. That means 10% level may be no pump speed and 95% level is 100% pump speed. It is that simple.

 

[rupej]

I really can't see the rate of level change being enough to require the pump to ramp up/down fast enough to hurt it. I would just keep your proportional control and use the VFD accel/decel parameters to limit it as an extra precaution. Seriously, this is way too simple of problem to require any setpoint ramping/etc.

 

[Peter Nachtwey]

I really can't see the rate of level change being enough to require the pump to ramp up/down fast enough to hurt it.

The only problem I see is when the stuff level is low there many not be enough NPSH for a fast acceleration of the pump.

I would just keep your proportional control and use the VFD accel/decel parameters to limit it as an extra precaution. Seriously, this is way too simple of problem to require any setpoint ramping/etc.

Yes, that is what Mickey said at the beginning. Basically scale the pump speed to the level.
My objection is to the output limiting. That reduces the pumps capacity so it may not be able to keep up the the inflow.