Remote Sump and Tank Level Control, AB 5/04

[DiaperWolf]

Hey,

Ran into this issue while commissioning on customer site. Background info first:
Hardware Setup as follows
5/04 CPU
Tank Pump 1100 gpm (desgn. flow to sump) via Pwflx 700 VFD
Sump Pump 1300 gpm (desgn. flow to tank) via Pwflx 700 VFD on remote i/o via DH+

I'm currently using an SCP to scale my "actual" tank level (0-10V) transducer....and the same thing on the sump side.

so the SCP for both the tank and sump would automatically increase the 4-20ma output (using a 1746-NI04I) for the VFD run command. higher tank/sump level=faster run signal.

My level control worked perfectly (+- .5") until we had to adjust a throttling valve on the tank side, just closed it down 1/8 of range...

now I'm into +- 1.5/2" of water. the tank vfd speeds up, level drops, then the vfd slows down, level rises. I can't smooth it out. Any ideas?

 

[Mickey]

If the control was working perfectly ( your words) why did you make the adjustment?

You may have to reduce the pump's output. Rescale to slow it down a bit.

 

[daba]

If what I read into what your "level control" system does is correct, then it isn't a "level control" system, it was just lucky that it achieved what you wanted in the original setup.

By lucky, I mean that the scaling of your inputs to your output was just about right to achieve the level balance without "closing the loop".

Now that you have modified a system parameter, your "open loop" control is now "hunting", because the corrective action provided by the arbitrary scaling to the output is wrong.

What you need is a "closed loop" controller, namely a PID.

Investigate using a PID instruction in your 5/04 to control the tank level.

With a PID, you will provide a level setpoint, SP, feedback from your tank level will be your process variable, PV, and the output, OP will drive your pump.

Once you've got the PLC code sorted, you can "tune" the PID by adjusting its Proportional Gain (P), Integral gain (I), and Derivative Gain (D) (if needed).

Properly setup, a PID control of the level will achieve much, much better than +- .5" and will automatically compensate for any changes to the system, whether man-made, or by mechanical deterioration of pumps, filters, etc.