Hydraulic PID Controller

timryder

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

Looking for Recommendation of a Controller for Servo Hydraulic valves. We have an application with 4 Hydraulic cylinders that will have positional feedback on them and we need to accurately position and hold position. The system will have a few hundred recipes and the user will need to jog and position the hydraulic valves and be able to recall those positions. No cylinder is greater than a 4" dia so they aren't monsters with high flow. They're relatively slow positioning actions.

So we're going to be using servo control valves for the hydraulic cylinders and I am looking for an Allen-Bradley (compactlogix) compatible controller for these axis.

Does anyone have any recommendations?
Let me know if I didn't give enough information.

Thanks in advance!
 
For feedback it is highly recommended to use synchronous SSI type feedback.

You can likely use high response proportional valves and not real servo valves.

As a basic starting point you could use this controller.
RMC150E-S2

It plays really nice in Ethernet/IP with AB controllers.

RMC150.PNG
 
Hi, Tim. Without trying to get crossways with Peter, I used Bosch Rexroth HACD controllers which are a proportional valve controller. They are very precise, have adjustable gains, use either pressure or velocity as a control, connect real easy with ethernet and have readback for pressure, position and you can scale for a velocity readback.
Hope this helps.

I do not work for Bosch Rexroth. Since we are a distributor for them we must use their products when possible.
 
@saultgeorge

I need 3 axis of control with position feedback. I would like to establish a position scale for each of the axis so that I can tell the system what position to go to and hope the controller will execute that for me and hold position when there. I haven't picked out my feedback type yet, there will be 1 which can be easily an LVDT but two others which are a rotational axis type.

Do you think the HACD controllers would work well for that application?
 
Tim
We built a machine that has 2 cylinders that work opposite of each other, dispensing a slurry product. So there's 2 HACD's. Position feedback comes from an encoder on the power end of the cylinder and I scale that from using constants from the HACD into a scale block and use it in logic. Same with pressure and I did my own velocity and percent extended/retracted logic that shows up on the HMI. So sounds like you'd need 3 HACD's and it ought to work.
I'm just getting ready to leave, but you can search for HACD controllers and maybe get a little better info to see what they'll do. Our customer was so happy with the new machine and the control they bought another one from us.
 
A PID control is a poor choice for a positioning control the are designed for continuous process control Flow, Pressure, Line speed

a P Loop will work better for positioning used with a simple servo hyd valve
 
A PID control is a poor choice for a positioning control the are designed for continuous process control Flow, Pressure, Line speed

a P Loop will work better for positioning used with a simple servo hyd valve

Both the Delta RMC and Rexroth HACD are controllers purposely built for the control of hydraulics and should work very nicely in this application. I have used them both and personally, I think the RMC has a shallower learning curve.

From a functionality standpoint, they are a bit different. The HACD is what I would call "configurable" while the RMC is fully "programmable"
 
A PID control is a poor choice for a positioning control the are designed for continuous process control Flow, Pressure, Line speed

a P Loop will work better for positioning used with a simple servo hyd valve
Why do you say this?

A simple proportional gain will always have lag or following error. The extra time it takes to get into position, without feedforwards, will slow productions. The bandwidth for a proportional only control is about 1/4 of a PI or PD control.

Feedforwards are must.

If timryder wants a hydraulic servo controller to do the whole job without a PLC then the RMC150E is the only way to go. A huge advantage is the support in the US or anywhere there is Ethernet. The RMC supports many Ethernet protocols so it it compatible with just about any HMI.
For cheap will like C-more HMIs. For top of the line we like Red Lion HMIs but we support many.

This video was made using a RMC200 but it could be done with a RMC150E-H2.
https://deltamotion.com/peter/Videos/6DOF - Motion Profiles_Small.mp4
This video was made in India.
https://deltamotion.com/peter/Videos/Automate India 6DOF.mp4
The Indian engineers used our program to control electric servo motors.
I would hate to have to listen to Devo for a whole trade show.
The RMC controller is doing all the control in both examples. This is what Norm meant by being fully programmable as opposed to configurable where an external controller is required.

Timryder's application seems simple but I still think our tech support and ease of use is superior to all where every you are around the globe as long as you have internet access.
 
A PID control is a poor choice for a positioning control the are designed for continuous process control Flow, Pressure, Line speed

a P Loop will work better for positioning used with a simple servo hyd valve

Hello Gary,
please read this article. Its more than ten years old, but all what is written is very applicable today.
Cheers!
 
Pandiani pointed to some of my articles.
The first article is old. I was written when we only had our second generation hydraulic controllers. Actually, the issue of how to control a hydraulic cylinder depends on the damping factor. I can bore people with a lot of details but the short version is that a PID will work only if the damping factor is high.
When starting out, try proportional only. This will work if the required bandwidth is low.
Add the integrator. This will increase the bandwidth by about a factor of 4 and decrease the positioning error to 0 but there will be over shoot as the integrator winds up and unwinds. This is covered in the article.
Add velocity and acceleration feed forwards to the PI control. This is the simple 'sweet spot' assuming the hydraulic controller does not have advanced algorithms. The problem is that now there must be a target generator that updates the target velocity and acceleration every update. Most PLCs cannot do this. I am still looking for one that can. If the velocity and acceleration feedforward gains are tuned up right, the integrator will not wind up and cause overshoot.
PID should not be used unless the damping factor is so high that the open loop system does not act like a mass between two springs when the actuator is moved quickly.
This may occasionally happen when the mass is low.
Then there are more advanced algorithms like what the RMC uses. It is a PID with a second derivative gain plus velocity, acceleration and jerk feedfowards. This allows most of the damping to be done electronically. Electronic damping requires less energy that mechanical friction. This also allows for smaller diameter cylinders to get the same response. Smaller diameter cylinders mean lower cost initially and when running.
 
First, Thank you for the information
But I will still fall back on the code I developed well over 25 years ago, long before we had PID controls available in plc’s. It easy and simple to deploy with only a few lines of code. Easy to setup and can be used I many different applications with very little modification. Proven to be reliable with many years of use. Over the years it has been copied by other programmers for different applications.
 

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