Loop gain in aluminum extrusion processes

Béla Lipták

MY FIRST question is how tight a control is needed, i.e. does the ram speed directly impact the quality of the extrusion product? If control doesn’t need to be very tight, then the proposed scenario might work, assuming that the total resistance for each pump is similar, and each pump performs in the same way. In mentioning total resistance, my concern is that identical pumps can certainly be manifolded together in a way that is balanced or in more challenging designs that are very unbalanced, with widely different distances and resistance barriers (piping size differences or changes, elbows, etc.). Different types of pumps are more or less prone to issues with the resistance.

Assuming that the speed is critical, I have a couple general concerns: namely, 1) the relative performance of each pump; 2) levers for the operator to adjust limits; 3) situations that will credibly occur where one lead pump is not sufficient; and 4) the inferred stroke from the valve position.

When I’ve run across multiple-pump systems in the past, I had systems with pumps of widely different capacity and response curves. For tighter control, I generally linearized the output for each pump. In cases of widely different capacity, I’ve run plant experiments to set different gains on the controller, depending on which combination of pumps was running. I also generally had issues where either chronically or during abnormal situations, a given pump might need to be derated (or limited to a different capacity). I generally put in an Automan station, where the operator can manage a manual bias to handle this derating. I often put in low-pressure or flow overrides that will put a second and at times multiple pumps into CAS-to-controller to respond to larger upsets (including trip of the lead pump).

What would the proposed controller do when one of the base pumps tripped offline, and the capacity of the lead wasn’t sufficient? What would happen if the lead pump tripped offline? Can the process respond fast enough to move other pumps into the PID control and respond before the process excursion warrants a trip? Does the amount-to-base-load get continually recalculated? Finally, I have generally used a direct flow measurement in these cases to have some process feedback (rather than relying on the inferred flow from the source valves). What happens to that inferred flow as the valves wear or as leaks develop in the system? Direct measurement of the stroke would likely improve reliability.

In terms of gain, this might be a place to use a gap gain, leaving slow response close to set point, and then use a stronger gain outside the gap. It also might be interesting to continually recalculate the base-load set point and use the lone lead pump as the trim controller.

In addition, in the control scheme that you mention, it seems to me you’re just measuring the gain of one pump at a time (the lead pump). If all the base pumps deliver the base load perfectly, and all the pumps individually respond the same, and the pump curve ends up being linear across the span that you operate (so 1% of output change results in the same stroke response across the curve), then the gain should effectively be about the same. I have rarely found that type of perfect balance in practice.

When you talk about sluggishness or oscillation, the issue that immediately comes to mind is dead time. Are the pumps close to the extruder, etc.? Beyond that, it might be something inherent to the mechanical specifics, perhaps some sort of mechanical backlash or valve stiction.

Bridget Fitzpatrick, principal technical consultant, bridget.fitzpatrick@mustangeng.com