hi all,

i have tested my intuition.

now has anybody used negative i and d parameters?
if yes, for what application?
if no, then what did your intuition yield?

 

How about you tell us YOUR results first?

 

My intuition yielded that a negative time would be an imaginary thing.

Rob
www[.]lymac.co.nz

 

rob,

yes that's correct, negative time as such has no meaning. but if we look at the i term contribution in the total o/p, 1/ti (integral of i dt), so if ti is negative, the i term's contribution gets subtracted from the overall, not a big deal, so it has got perfect meaning.

this is what my COUNTER INTUITION yields. my intuition yielded exactly as rob's(@ bruce)

 

What you are talking about is a negative Gain on the integral term. If your implementation of the PID algorithm has either a Beta Value or a separate integral gain you *may* be able to achieve this effect. Many PID algorithms will not let you do this.

In any case why would you want the operation of the Integral action to work opposite to the Proportional Action - In almost any case I can think of you would be creating an unstable controller.

Rob

 

A long time ago the Moore Products company included in their range of pneumatic stack Controllers/Relays a device that could perform a derivative function. There were two versions: the 59D that gave an output with a derivative term added to the input, and the 59R that subtracted a derivative term from the input. This was described as "Inverse Derivative" and marketed as a "Noise Filter". In today's language it was derivative with a negative parameter.

I had some experience with this device where I needed to control the level in a small tank with two input flows. One flow was controlling level and the other flow was ratioed to the primary flow. The paper industry has many such loops, with large mixing chests. I wanted to control a "Stuff Box" that had only about 1 cu metre capacity. The "Inverse Derivative" function stabilised this system.

I guess today we could use a "Lead/Lag" functions in our DCSs, but I only had pneumatic instruments in 1962.

Peter Green

 

Oh yes ... That @#$%^& stuff box.

Thirty years later we were still trying to control it with a PID loop. A classic example of false economy - where if we spent the money wasted on PID controllers and man hours over the years we could easily have replaced it 10 times over with a properly sized tank and feed pump.

Last time I visited that paper machine it was still there. And still as badly controlled as ever.


Rob

 

headbox controls shouldn't be a problem these days