Flow Control via VFD Controlled Pump

[RAJH 0]

I have an application requiring me to control water flow using a VFD controlled pump. My customer wants me to initially operate at a manual % of pump speed for a short duration before automatically ramping pump speed to the desired flow rate. My question concerns automatically ramping pump speed to the desired flow rate. The way I see it I have two options. I could operate the PID in manual operation while ramping the manual % pump speed set-point until close to the desired flow rate and then switch to automatic PID operation or I could operate in automatic PID operation all the way while ramping the automatic flow rate set point. I believe my customer wants me to use automatic PID control all the way. My concern with using automatic PID operation all the way is PID tuning and how the PID will respond to the ramped flow rate set point. Any suggestions?

[Shiner 2]

what platform, drive, analog inputs are you using?

[RAJH 0]

The processor is a CompactLogix 1769-L32E. I am using the PIDE instruction for control. I don't have information on the drive (hardware is by others) but all analog is 4-20ma.

[Michael Lloyd 85]

There is a Hand mode in the PIDE block that I believe will let you ramp the output and not disturb the function of the loop. Enable Hand Ramp the output. I use a compare, timer, and add or subtract block to ramp but I'm sure there is another way. Disable Hand I think thats the name of the function. I haven't used it in a while. Interestingly enough I have an upcoming project that will require this very thing

[RAJH 0]

I typically use the ProgAuto and ProgManual PIDE inputs to switch between auto PID operation and manual PID operation. When operating in auto operation the setpoint is taken from the SPProg PIDE input and when operating in manual operation the setpoint is taken from the CVProg PIDE input. The auto input is flow rate. The manual input is % of CV. I don't believe that Hand mode will work. Hand mode is used when something else is in control of the device - CV is set to 0.

[Guided_monkey 0]

Try this..... on VFD flow control bit active... and not timer.dn MOVE an initial value into the VFD.SPProg, at the same time start the timer. Once the timer.dn is complete MOVE the required flow rate value into the VFD.SPProg. The initial value can be a % of total flow by using a simple calculation. Hopefully this helps.