Auto-Preact Trim Algorithm

I currently have a PLC based scale controlled packaging machine that fills large 1,000-2,000lb containers. Depending on the material's density final scale weights can be off by significant margins.
My warehouse techs don't quite understand the preact function within my program, and ultimately just adjust their final weight setpoint to account for over-fill or under-fill.
I am looking into incorporating a automatic trim on the preact function, and am curious of any other's who may have done the same. My initial thoughts are to take 10% of the error, and adjust the preact according to +/- final weight.
Any ideas on how to maybe implement this smoothly.

Micrologix1500 Processor
Hardy 1769-WS Scale Controller

I'm also looking into porting this to 5K, to add in more functionality at a later date.

Rockwell has an AOI called Weigh Scale Dosing and is part of the Plant PAx collection. They have a preact programmed in there.

I'm interested in this too. I've thought about doing something like that before, but I've always been afraid to implement it because if something screws up during one of the batch operations, the next batch operation would be screwed up too.

But your idea of only using 10% of the error would seem to minimize that, maybe...

Just use an exponential average
preact = preact * (1 - alpha ) + error * Alpha

where alpha is your weighting (eg 10% = 0.1)

It is worth putting an initialisation that sets the preact to the error if preact=0

I've done this before, it turned into a pretty tedious job because if you get bad readings the error just multiplies. Just make sure you add some logic for outliers or bad scale readings. It ended up working pretty well once i spent some time on it. I'll dig into my old programs and see if i can find it.

I have a decent beginning to this, that only adjusts the preact if the error is within +/- 50lbs. Still sort of scared to implement it though. I still would like something to zero the preact when initializing to load a certain grade of product. The HMI to this is an older PVc. I can add to it, but it is very limited.