VFD or Servo Control

dginbuffalo · Jun 27, 2019

I have a project where I have to drive a linear screw which is attached to a slide valve. The slide valve will modulate flow and will most likely be controlled by a PID loop. I'm not sure if I can get open close limit switches mounted so I think I will use a absolute encoder and will store the limits in the PLC. I don't really need to "position" the valve, I just need to modulate it open or closed based on flow. I'm wondering if it would be best to go with a servo or if I can just pull this off with a VFD and gearbox? Thoughts?

Dhitzfelder · Jun 27, 2019

VFD and gearbox should work just fine. The advantage of a servo in this application would be (depending on your control system and requirements) that it would take care of some of the logic for you, and ease integration into a PID loop.

James Mcquade · Jun 27, 2019

what is the material you want to control?

we used leslie control valves for steam at one plant using pid loops, worked great.

james

GaryS · Jun 27, 2019

A good VFD would work fine in this application
if the gate is horizontal you may not need an encoder if you don't need to hold the weight of the gate. Either way you should always install travel limits to prevent crashes.
the VFD shuld be able to ramp all the way down to zero speed. some of them will only allow you a minimum speed of about 6 HZ this would always have the loop hunting it would be unstable
i would recommend a Yaskawa A1000 drive they can run down to zero HZ with out any problems i do it often never had a problem.

Gene Bond · Jun 27, 2019

It's all about the cost.... Either will work.

If you need a brake on the motor, for when power is off, to hold the gate, I'd bet a servo would be cheaper than a motor/brake/encoder/vfd. Especially if you need something like IP-65. I'd add them up and see!

dginbuffalo · Jun 28, 2019

Thanks for the advice. The slide valve is actually internal to a compressor. The only thing sticking out is a stub shaft (from what I'm told). There will be a flowmeter close by that monitors flow and we will turn the screw CW/CCW to change the internal slide valve position.

Dhitzfelder · Jun 28, 2019

Interesting. Air compressor or refrigeration? What I've seen for that application is a cylinder activating the positioner, and solenoids porting oil or air to it based on pressure (or temperature in the case of a chiller...)

dginbuffalo · Jun 28, 2019

Dhitzfelder said:
Interesting. Air compressor or refrigeration? What I've seen for that application is a cylinder activating the positioner, and solenoids porting oil or air to it based on pressure (or temperature in the case of a chiller...)

It's actually a methane gas compressor. Basically, from what I understand is that it is moving a internal linear screw which is moving a pilot valve. The pilot valve ports high pressure oil to one side of the slide valve to move it. In the opposite direction, the pressure is relieved and outlet gas pressure pushes the slide valve back.

Dhitzfelder · Jun 28, 2019

Cool. Do you have information on how many turns travel it has? Does it have any form of feedback or at least indication?

dginbuffalo · Jun 28, 2019

Dhitzfelder said:
Cool. Do you have information on how many turns travel it has? Does it have any form of feedback or at least indication?

30 turns I believe and no form of feedback. We will need to develop our own.

shooter · Jun 30, 2019

I see no need to do it very fancy, just 2 relais to turn the motor for a moment, the rest is done internally. In normal compressors there can be two slidevalves: one for the ratio and one for the volume.
The volume should be zero on start, and can be controlled or internally with oil and some solenoid valves.
If you have a type and a nice diagram we will see what to be done.

VFD or Servo Control

dginbuffalo

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Dhitzfelder

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James Mcquade

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GaryS

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Gene Bond

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dginbuffalo

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Dhitzfelder

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dginbuffalo

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Dhitzfelder

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dginbuffalo

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shooter

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