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Old January 27th, 2014, 04:46 PM   #1
shiftedbits
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PowerFlex 525 – Open Loop Metering Pump Control Help

I have an application coming up that requires a metering pump to be controlled via a PowerFlex 525 VFD.
Commands to the VFD are to be sent via the Ethernet interface from a Logix CPU.

Metering Pump Characteristics
------------------------------------------
- 2000 l/hr max Flow
-No feedback from Pump (No built in pump controls. Basic pump with Motor and mechanical drive)
-PD Type metering pump (diaphragm actuator is driven by a worm gear)

Customer requirements: (operating modes)
-----------------------------------
Open loop pump control (without any external feedback). Customer does not want to add a flowmeter.
1. Ability to dose (dispense) a given volume (i.e. Command pump to dispense 200ml and stop)
2. Ability to Dispense at full flow rate until commanded to stop.
3. Switch between 1 and 2 above on the fly.

Question:
Can powerflex 525 be used? Or is the above is even possible without a feed back mechanism
How would I go about configuring the PowerFlex 525 VFD for above (open Loop) operation.
Read thru the PowerFlex manual and a bit confused as to how to get this VFD configured to work in open loop mode.
Any help would be greatly appreciated.
Thx.
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Old January 31st, 2014, 12:37 PM   #2
shiftedbits
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Anyone?
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Old January 31st, 2014, 02:03 PM   #3
jraef
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Quote:
Originally Posted by shiftedbits View Post
I have an application coming up that requires a metering pump to be controlled via a PowerFlex 525 VFD.
Commands to the VFD are to be sent via the Ethernet interface from a Logix CPU.

Metering Pump Characteristics
------------------------------------------
- 2000 l/hr max Flow
-No feedback from Pump (No built in pump controls. Basic pump with Motor and mechanical drive)
-PD Type metering pump (diaphragm actuator is driven by a worm gear)

Customer requirements: (operating modes)
-----------------------------------
Open loop pump control (without any external feedback). Customer does not want to add a flowmeter.
1. Ability to dose (dispense) a given volume (i.e. Command pump to dispense 200ml and stop)
2. Ability to Dispense at full flow rate until commanded to stop.
3. Switch between 1 and 2 above on the fly.

Question:
Can powerflex 525 be used? Or is the above is even possible without a feed back mechanism
How would I go about configuring the PowerFlex 525 VFD for above (open Loop) operation.
Read thru the PowerFlex manual and a bit confused as to how to get this VFD configured to work in open loop mode.
Any help would be greatly appreciated.
Thx.
First off, a VFD, regardless of how it is configured, will not be capable of dosing all by itself without a feedback loop of some sort somewhere. It is possible though, depending on the desired accuracy, to use empirical data in understanding a desired running speed that APPROXIMATES a dose rate, and you can very accurately keep the pump running at that speed with the 525 drive.

Now, if by "open loop" mode all they mean is no encoder feedback from the motor but accurate speed control from a command initiated by the Logix processor, which IS providing a PID control loop of the dosing process, then that is very attainable. The 525 is capable of what we call "Sensorless Vector Control" and other people sometimes erroneously still refer to that as "Open Loop Vector Control", so that may be what is being referred to. However if, for some unimaginable reason, they are attempting to do dosing control with no PI or PID loop control, they are going to be unhappy with the results, unless reasonable accuracy is essentially unimportant to them.

So first define your process requirements and what is meant by "open loop".

By the way if you are in Northern CA, send me a PM and I will help you directly, I am the "drives guy" for Rockwell here. If you are south of Kern County, I can connect you with my counterpart down in "the desolation of smog"...
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Old January 31st, 2014, 10:20 PM   #4
Lemming
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What speed is the shaft to the pump running ?? You maybe able to add a prox to count x volume increments with a standard DI. Cheap is an easier sell to the customer
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Old February 1st, 2014, 11:45 AM   #5
Tom Jenkins
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Actually, this will work OK without feedback of any sort. The assumption is that the pump is a positive displacement type, so a given rpm will produce a predictable flow rate, satisfying the second requirement. The total volume delivered is also a multiple of the number of revolutions, so keeping track of Hz and time will provide a dosage figure.

Note, however, that the customer has to understand there will be a lack of precision. The slip in the motor and the internal leakage of the pump will have to be ignored or assumed, and the effect of acceleration time on the motor will be ignored. If the result is close enough for the purpose of the process then it will be OK.

Note that "open loop" to the drive means no tachometer feedback - essentially Volts/Hz operation. In the control loop it means no feedback.
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Old February 1st, 2014, 03:15 PM   #6
robertmee
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What Tom said.....I do open loop volumetric flow with PD pumps quite frequently, however, you must understand a few things about PD pumps:

They are NOT linear across the flow curve. So pick a narrow operating range in speed and stay there, It will be fairly linear within that narrow range, but non-linear on the upper and lower ends of the pump. You can build a flow curve look up table for the non-linear band, but it will take a bit of trial and error at various speeds to accomplish.

You will have to insure that the inlet of the PD pump is saturated....meaning any introduction of air, the PD pump will happily pump it a long with the material and that will of course affect your accuracy. So I don't know if you have a static head of pressure from a vessel or are feeding from a loop or what, but keep that in mind.

PD pumps wear. Most of my applications are with bitumen which is very aggressive in breaking down the inner vanes of a PD pump. Over time, material slippage WILL occur. I usually have a fudge constant just for this purpose that's varied over the lifetime of the pump. So, don't know what you're pumping but keep the wear factor in mind.

You have a 2000 L/Hr pump but want to dose 200 mL???? That's a pump of 555 mL/s, so at 200 ml your pump is on 1/2 second. Your math is way out of whack for this to work. Your pump/VFD won't be able to ramp up/down in that time.
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