Why are PID loops with feed forward not more common?

Hi guys!

It seems to me that PID loops with feed forward are underutilized. Maybe it's just me but I think almost every loop would work better with some kind of feed forward component but I seldom see them in automation and process control. How come?

Right now I'm working on making some changes to a gas burner application. Standard PI loops are used to set control valves to get a certain gas flow. In this case the control valves are the linear type and the pressure is almost constant which means that there is a linear relationship between the flow you want and the control valve opening. To me that seems like a PI loop with feed forward calculating the approximate control valve output and the PI feedback fine tuning it would be a perfect match.

Do you guys use a lot of feed forward in your applications?

Use feedforward a lot in my field. Programming CFB and BFB type boilers (~75 MW and upwards). Use feedforward for for super heater control and main steam pressure controller. Also used for air pressure (air to combustion) controller, furnace pressure controller, fuel silo level controller, reduction station for low and medium pressure steam.

On fuel side it's not used, in the ones I've worked on. Have to wait for air before adding fuel basically.

To use feedforward you need to have a process model. Many folks don't understand the process and equipment well enough to do that.

Agreed. Most people either don't understand it (even though it's generally quite simple) or just don't take the time to figure out how to apply it.

While a process model would be ideal, even just trending the existing PID Output at various set points can be very useful for generating an effective FF profile.

One of the problems with FF is it's effectiveness can often be dependent on parameters beyond the PLC's knowledge. What works great today may really foul things up a year from now (or next week?) when the process/conditions change. I have seen many end-users (and even some integrators) who can't even effectively tune a PID loop, applying FF likely isn't on their radar.

I use whatever PI the PLC model have included for simplicity. Had I worked with rockets or airplanes it would probably not be sufficient... And for any process control I've seen also the PI is overkill, but if it's included in the software it will be the least effort.

To my PI I then add whatever needed... for instance a dryer room must not overheat, but in general a PI could be the base.

Feedforwards are a must for most position motion control. When the feedforward gains are set correctly the integrator will contribute very little to the output. This avoids overshoot due to integrator windup.

FF is great for well defined and repeatable systems. In my world of engine testing, we will run engines on very transient cycles. We utilize feedforward very heavily for these cycles. The feedforward must be very well mapped through.