RSLogix 5000 PID Hydraulic Servo Valves for Position

timryder

Member
Join Date
Feb 2007
Location
Macomb MI
Posts
176
Hey Guys,

Got an application where the Engineer purchased some Hydraulic Servo Valves and would like me to write a PID to control the position of the axis the Hydraulics are moving. There is an Absolute rotary encoder hooked up to this rotational axis and the general idea is he'd like to input a position in engineering units and have this things aggressively move towards that position until it gets very close in which it should slow down and stop.

So what's the best way to accomplish this? PID? I've never used it before so some serious help would be needed. The Valve is setup as analog output 8 bit and I have the Encoder as a 0-10v input. They're all working and talking over EtherNet/IP to my CompactLogix PLC.


As usual all help is appreciated.
 
I don't think the PID would work, IF I understand the application. The PID will never close the valve (unless you have a large overshoot) which will cause the hydraulics to continue to move. Example: If I'm controlling a steam valve to control temp, when my temp is at SP the valve wouldn't close. It would stay open to continue maintaining the temp.



I would write something that compared the SP to the feedback position and then adjust the output based on how far apart they are.
 
If you had a Control Logix and SSI feedback I would say get a 1756-M02AS card.
This could be a complicated application depending on several factors.
I am confused by your statement about the encoder. You say the axis is rotational? What happens when the encoder makes a full revolution?
8 bit DtoA will only work for crude positioning.
PLC scan times are not fast. Neither are the PLC AtoD cards.

What model valve are you using?
What is the mass?
What is the time of motion and how far?
What is the accuracy?

Chances are a Compact Logix will not do the job.

I would replace the analog encoder with an SSI encoder with as many bits of resolution you can afford. 16 bits is usually enough and not too expensive. If the rotational axis must make multiple turns then a multi turn SSI encoder is required. I would then get a RMC75E-MA1 from Delta Computer Systems.

If the system is designed right the tuning and motion control can be done in minutes.
 

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