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Old March 14th, 2006, 07:59 AM   #1
JamesWSY
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PID for Hydraulic Load

Hi all...
I used 1746-NO4V, Moog Servo Valve and signal converter VC2124 and load cell that mounted in front of hydraulic cylinder. The target is Load wheel that driven by 150 HP DC Motor. In stop condition I want to adjust the cylinder to touch the load wheel, example 1000 KG, servo valve will open and cylinder will move forward and if load reached 1000 KG then cyclinder will stop in this position to maintain 1000 KG. Could I use the PID? Or any Idea how to control the load?
Thank you for idea or suggestion.
James
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Old March 14th, 2006, 09:22 AM   #2
CharlesM
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You need a Delta

This web site should get you started.

http://www.deltacompsys.com/
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Old March 14th, 2006, 11:56 AM   #3
TConnolly
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Controlling force with a directional servo valve and a PID loop works very well but you will need to use a bi-polar PID. Here's the catch however - the SLC PID output is limited to an integer value 0 to 16383. Therefore to get bipolar operation you will need to offset the PID output so that 0 represents -100% command to the valve, 8191 represents 0% command to the valve, and 16383 represents +100% command to the valve. Whether the PID is forward acting or reverse acting will depend entirely upon how the ports of the servo valve are plumbed to the cylinder.

See this technote on creating a bipolar PID in the SLC: http://domino.automation.rockwell.co...8?OpenDocument

A hydraulic servo control PID loop must execute very fast - you might need to execute the loop a hundred or more times per second - so I suggest you place the PID in a STI subroutine.
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Old March 14th, 2006, 12:34 PM   #4
ndzied1
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James,

What you want to do is called position control with pressure/force override. I would highly recommend using some sort of hydraulic controller for this. They have the functions built in to do this sort of thing.

CharlesM's recommendation is a good one.

Other players (including Moog) have controlers as well.

I work for a Bosch Rexroth distributor who makes controllers also. To pick a specific controller, you would have to evaluate all of the operating modes of the axes (do you need manual operations?, communications required etc...)
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Old March 14th, 2006, 02:01 PM   #5
Peter Nachtwey
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The speed required depends on the ratio of the valve size to the cylinder volume. Small valves controlling pressure in big volumes of oil is relatively easy because the oil pressure doesn't change fast.

If the pressure is going to change dynamically then one must be prepared to use the derivative gain. It really is the key to tuning pressure loops. However, the time constants in PLC PIDs is in minutes. This is very slow relative to how fast the pressure can change. I know the time constants on hydraulic systems are measured in milliseconds.

Try the PLC if you already have it. If you need a real fast controller then you can get a RMC75 with just one analog in and analog out. If you add some digital IO you may not even need the PLC.
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Old March 14th, 2006, 05:30 PM   #6
kamenges
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A couple of months back I did something similar with a PLC. Whether you even have a chance is based on how fast you need to get to position as well as the flow capacity opf your servo valve versus the total hydraulic volume. In my application I didn't need to get to position very quickly.

I was using a hydraulic servo valve that I simply provided with an open loop command until the load cells indicated I reached some threshold force. At that point I switched to a PI controller in the PLC. With this system I stayed within about 5 pounds of setpoint in a 40,000 pound system. But keep in mind that this wasn't a very dynamic solution. I couldn't aggressively correct for error and it took a few seconds to reach setpoint.

As Peter said, if you have a little time try it in the PLC. If it doesn't work in the plc add a controller. If you don't have the time, don't even mess with the PLC implementation. The dedicated controllers do this sort of thing all the time and you will have a higher probability of initial success with one of them.

Keith
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