PID question

[Knight]

Hi all, I'm new with Unitronics PID and struggling a bit so please help. I'm having some trouble with PID auto tune for a pumping application trying to achieve a set point pressure. I have V700 and 2x IO-AI4-AO2, a 0-10bar (4-20mA) pressure transducer as my process feedback and 4-20mA output to a VFD scaled to 0-50Hz, using 4x pumps of varying size so introduce the next size up if duty pump cant reach SP, therefore using  4x PID loops writing to 4x analogue output channels. When starting auto tune I'm getting -4 status fairly quickly and also -11 often so auto tune not completing. Being verticle multistage  pumps, once the VFD is given run enable, the VFD is internally set up to ramp quickly to 30Hz min speed before following the mA speed ref, which appears to be 50Hz anyway from the PID control output. Is this integral error -4 due to forcing the VFD to 30Hz outside of the PID control on start up?. Can anyone suggest why I'm getting this -4, do I need to force the integral error to zero?

Your help will be greatly appreciated.

[kratmel]

In standard multi-pump control used one PID loop for one pump. All other pumps works in start-stop mode.

Some info about multi-pump configuration

ATV610_Multi-pump_en.pdf

Schneider-VW3A3502-Manual.pdf

you can see standard multi-pump application setup.

 

[Knight]

Thanks Kratmel, the issue with this set up is I have 4kw, 7.5kw, 18.5kw and 55kw pumps so as to provide a flow rate range depending on draw off pressure line. My thinking there is each pump will need its own PID which I have running continuously, however only one pump will be enabled to respond to pressure drop depending on rate of fall from SP pressure. Using this theory, I am probably best stopping the disabled pumps respective PID loops until the VFD is called?.

[Ausman]

Knight, to correctly set all of this you need to have empirical data on pressures vs different pump speeds for all 4 pumps.  Only then will you know the exact setting for the particular pump's initial speed to exactly be whatever combination it is taking over from.  Pumps are not a linear relationship to revs which you likely know.  You're reference to 30Hz minimum is the thing I'm talking about.  It will be different for all the pumps.  Cascading pumps needs careful planning of sequences involved and empirical data measurements.    The same PID control can be used but needs to be matched to what pump is being called in the sequencing, which is what is explained in Multi-Pump2 page 4 in Kratmels ATV610 pdf.   

You're able to further compound the complexity of this by doing pump operation hours calculations as well, to even out wear.  The permutations for this sort of thing can be large.  If you have all 3 lower capacity pumps in use, and the biggest of them on the VFD at the time then reaches 100% and more is needed, the system changes all 3 of them to DOL and the largest of your 4 comes on and ramps to whatever % actually makes a difference to the pressure.  But it can also take over the entire operation on it's own at some stage at higher speeds, but likely with all the others off, it's input % will have to change as well.  And each transition like this means a pressure bump of some sort as the DOLs drop off or restart etc..  What you're wanting to do can be actually quite complex, but effective, if you can put in the time.  However, you have an innate issue because your pumps are not identical, which means for each of them you need a different PID set.  I'm wondering whether you need PID at all, and simply have the VFD follow the pressure directly according to a set of rules for each "what is running vs lead lag vs need" scenario in operation at the time.  For some things PID is not the fantastic solution that people think it is.

I've used this sort of thing a few times in refrigeration compressors cascading according to loads.  The same principles apply, and it can be fairly simple, or exceedingly complex, in structure.

cheers, Aus

[Knight]

Thanks Aus, I do agree with you re my innate issue of different size pumps. The pump set is providing water for irrigation reticulation and has a fairly conservative pressure SP re maximum bar for the pipework, with PRV etc where appropriate, so pressure bumps are not a concern. I had hoped with careful management of pump and PID enables, I could manage this but its proving to be a mare. As you suggest it may be more effective to reroute the feedback into the VFD's, then use one only VFD's for PID, enabled via the PLC when conditions are met.   Can you expand on the reason for the  -4 PID Status on start up of Autotune?, I don't really follow what can be done to mitigate this.

[Gabriel Franco]

For this type of application, I use a single PID; then compare CV with some limits to decide when to start the booster pumps and control their speeds based on PID CV, mini speed,  main pump max speed, CV to start booster pump and some other parameters.

The integral error must be reset under some circumstances such as at the start of regulations.

Before you perform autotune, be sure you set all PID parameters to initial values, specially input and output ranges and sample time. I also set initial Kp, Ki and Kd values.

I recommend that you to use PID server to tune your system.

More than knowledge of Unitronics, you need process control background to achieve this goal successfully.

[Ausman]

21 hours ago, Knight said:

expand on the reason for the  -4 PID Status

I don't use PID any more and can't answer your question. I haven't used it for years as I conclusively proved that my own methods resulted in far less wear and tear on physical components, which significantly reduced overall system costs, as well as giving a nicer environment for the humans using the process. Wondering if the Error Integral controls might help in fixing it.

Hoping others will chime in.

cheers, Aus

[Knight]

Thanks everyone that responded, I've used the VFD on board PID for each pump, and the V700 for pump specific enable conditions. Working a treat. Cheers.