PID question

[ratcliffe_ic 2]

Can anyone point me in the right direction with this one: I need to use a PID loop within the PLC to monitor pressure in pipework and control a pump accordingly to maintain a set value. That part's not too bad. However, it is required that if demand is high and the pump is reaching its capacity or some threshold to be determined then I am to start a 2nd pump to supplement the 1st pump. Any ideas how this could work, how would you factor this in with the PID loop?

[Steve Bailey 118]

There are several ways to do it. Does the second pump have the ability to modulate or must it run at a fixed output? If it must run at a fixed output, and that output is less than the first pump, then you may be able to simply turn it on and let the PID ramp the first pump down to a lower output. If the second pump is larger than the first, that approach won't work. If the second pump can modulate, then you can lock the first pump at some output level and let the PID control the second pump in response to changes in pressure. You could also simply turn on the second pump and let the PID control both simultaneously. Issues you might have to deal with include whether a single set of PID tuning parameters will work for both pumps if only one is allowed to modulate at a time. The closer the two pumps are in capacity, the better your chances. If both pumps can modulate simultaneously, will the PID tuning parameters that work for a single pump also work when both pumps are contributing.

[ratcliffe_ic 2]

Does the second pump have the ability to modulate or must it run at a fixed output? If it must run at a fixed output, and that output is less than the first pump, then you may be able to simply turn it on and let the PID ramp the first pump down to a lower output. If the second pump is larger than the first, that approach won't work. The 2nd pump will be the same size as the first, I had thought I could ramp this one up to a speed and the 1st will lower its speed via the PID to compensate. Say demand now drops I will need the 2nd pump to ramp down to standstill and the 1st pump to speed-up via the PID loop. I suppose I could apply some threshold parameters to perform this. If the second pump can modulate, then you can lock the first pump at some output level and let the PID control the second pump in response to changes in pressure. I must control the pressure with one pump and use the 2nd to supplement it. You could also simply turn on the second pump and let the PID control both simultaneously. I must control the pressure with one pump and use the 2nd to supplement it. Issues you might have to deal with include whether a single set of PID tuning parameters will work for both pumps if only one is allowed to modulate at a time. The closer the two pumps are in capacity, the better your chances. If both pumps can modulate simultaneously, will the PID tuning parameters that work for a single pump also work when both pumps are contributing. Another thought has just occurred, control the pressure with pump 1 PID control, when demand increases allow a certain frequency to be acheived, then lock that pump at that frequency. As frequency demand increases, start-up the 2nd pump and use a 2nd PID loop to control the pressure using pump 2. Anyone see any holes in that?

[Peter Nachtwey 15]

I like Steve Bailey's suggestion of just turning on pump 2 and letting pump 1 do all the control while pump 2 is in manual or some semi-manual mode. The only catch is that pump 2 should not ramp up faster than one pump 1 can respond. This is determined by the integrator time constant of pump 1. However, if there is a disturbance in the pressure one could use pump two to estimate a correction value so pump 1 does not need to change its operating point too much. The gains would need to change if there was a switch from 1 to 2 pumps controlling.

[kaiser_will 65]

Consider setting the pumps up in a lead-lag configuration with the lead pump to run from the PID output then call on the lag pump (with another PID loop) should the lead pump not produce enough output (which would be switched into full output mode). Put limits around PID output change (as a bandwidth that is sampled in time from the PID output with a +/- allowable band). If the pumps are high-HP (i.e., costly motor/pump repairs), the lead-lag configuration is optimal if programmed to swap lead-lag configuration at some interval. I have found this to come in very handy in balancing large-HP motor/pump runtime hours.

[IamJon 0]

the problem with this is, if you lock the P1 at a frequency and use P2 to control, your demand for P2 could be small, meaning it will want to run at, say 5% speed, which could cause it to pull too much current. the best thing i think would be to turn on P2 at a specified output (75%) and continue to allow P1 to control. and as the other person said, have P2 ramp up slowly so you don't get a huge pressure surge. what kaiser said is a good idea. that way you wont have too much wear and tear on 1 motor with the other just sitting there. another option is to use P2 is steps. say, once P1 gets to 75%, turn P2 on at 20%. if P1 reaches 75% again, turn P2 on to 40%, etc. that will keep both motors running, but only 1 controlling. and it prevents P1 from reaching max capacity while P2 is off most of the time. but maybe you don't have speed control on P2??? Edited 8 Feb 2008 by IamJon