How to adapt to speed of response changes
I have periodically come across a problem which must be common enough for someone to have put some thought into. We have an off-gas system, which takes waste gases and processes them to remove contaminants.
As the quantity of material going in upstream varies, the speed of gas flow changes, and consequently the response of the PV to the CV changes. In essence there is a variable time delay in the feedback loop. With a fast gas speed we can use higher integral gains with good control and still get a stable loop, but under slower conditions the parameters can be unstable, so we are forced to use cautious lower gains.
We can get an idea of what this time delay might be (from the quantities of material added), but it isn't so straight-forward to use that information. I have tried an element of feed-forward, with varying success, and have toyed with the idea of using dependent gains (which means the integral gain and derivative gain are essentially times), and adjusting these times to match the speed, but am concerned in case the system becomes unstable when I'm not watching it.
Has anyone got any relevant pointers or experience?
I have periodically come across a problem which must be common enough for someone to have put some thought into. We have an off-gas system, which takes waste gases and processes them to remove contaminants.
As the quantity of material going in upstream varies, the speed of gas flow changes, and consequently the response of the PV to the CV changes. In essence there is a variable time delay in the feedback loop. With a fast gas speed we can use higher integral gains with good control and still get a stable loop, but under slower conditions the parameters can be unstable, so we are forced to use cautious lower gains.
We can get an idea of what this time delay might be (from the quantities of material added), but it isn't so straight-forward to use that information. I have tried an element of feed-forward, with varying success, and have toyed with the idea of using dependent gains (which means the integral gain and derivative gain are essentially times), and adjusting these times to match the speed, but am concerned in case the system becomes unstable when I'm not watching it.
Has anyone got any relevant pointers or experience?