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Old February 3rd, 2017, 07:15 AM   #1
freju
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PowerFlex 525 - Lobe Pump Flow PID Control

Hello,

I take part in commissioning of process station with lobe pump. Speed of the pump is controlled by PowerFlex 525. A customer requirement is to set a desired setpoint of flow for a pump. But there is no flow meter, there is only an encoder mounted on a shaft of a drive. Depending on a shaft gear ratio and pump efficiency (assumed that a dose per revolution is constant) I compute flow. To control this flow I want to use embeded PID control. Everything is cotrolled with PLC over Ethernet network so I set parameter A459 [PID 1 Ref Sel] to "Ethernet/IP" and parameter A460 [PID 1 Fdback Sel] as well. There is no problem with PID Reference signal - I can add parameter A464 [PID 1 Setpoint] to data structure and set it from PLC over Ethernet. But how can I connect actual value of flow computed in PLC to PID Feedback signal? I can't find this option anywhere.

I will be very thankful for any remarks.
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Old February 3rd, 2017, 07:54 AM   #2
rdrast
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This is for fluid?
A lobe pump for fluid generally has a fixed displacement per revolution, depending on the design and medium, it can be absolute or within a few percent so:

Flow = Rev's * Displacement
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Old February 3rd, 2017, 07:58 AM   #3
freju
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Thanks for your reply but you missedunderstood me. I know how to calculate flow and I do it in PLC so I have an actual value of flow. The problem is how to control this flow to obtain desired setpoint using embeded PID control in PowerFlex over Ethernet network.
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Old February 3rd, 2017, 08:04 AM   #4
Tom Jenkins
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You do not need or want a PID loop to control the flow of the pump. Simply back calculate the required rpm for the set flow rate, calculate the corresponding Hz, and send the command to the VFD. Done.
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Old February 3rd, 2017, 08:34 AM   #5
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^ Agree....The 525 has it's own built in PID speed control. All you care about is that the 525 runs the speed (flow) you request. Since flow on a PD pump is proportional to speed, and you've already done the math, it's as simple as setting the speed SP for a given flow requirement.
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Old February 3rd, 2017, 09:47 AM   #6
rdrast
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Right. It is a constant displacement pump, so there is no need for a PID. Even if a mixing application, you just slave it to another component at a ratio, but your customer just wants to set X Gallons Per Minute or some such, no PID needed.
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Old February 7th, 2017, 05:38 AM   #7
freju
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Thank you for your remarks, you are right. For this purpose I don't need closed loop control - based on dependency between frequency and drive speed I will calculate appropriate frequency from flow setpoint.

But for future purposes - how can I connect value in PLC to PID feedback signal after setting parameter A460 [PID 1 Fdback Sel] to Ethernet/IP ?
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Old February 7th, 2017, 09:55 AM   #8
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^If you have ethernet communications to a PLC, it would make more sense to do the PID in the PLC itself. That way you have much more control over everything.
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