Hydraulic Cylinder Control

[niteman9]

Hello, I have a system which consists of a AB 1769-L35E which has 4 hydraulic cylinders controlled by Rexroth servo valves (Each valve have the electronics built in and are controlled with +10V DC to -10V DC command from the PLC). Each cylinder has an encoder which provides feedback to the PLC. There are also pressure transducers on the system.

I have control over each cylinder manually and now need to program them to go to a given position or pressure.

What I need to do is move the cylinders to a given position or pressure. The speed will be determined by each Job.

What would be the best command to use for this purpose. Any help would be appreciated as I have never done any programming like this.

 

[ndzied1]

Your very best bet for robustness of control, ease of startup and future maintenance is to use a closed loop hydraulic controller.

These will have the proper types of closed loops you need to work with hydraulic motion control. Most PLC's have long scan times (compared to motion controllers) and only simple PID algorithms that work well for slow things like temperature and perhaps filling applications but normally don't give great results in motion control.

One such controller is the Delta Computer systems RMC150 which could talk to your PLC with the Ehternet I/P.

Have a look at http://deltamotion.com/

<disclaimer - I work for a distributor that sells the Delta Motion Controller>

 

[niteman9]

Thanks for the quick response.

Unfortunately I have what I have, which is to say an AB Compact Logix processor with analog output cards. Fortunately I don't think speed will be and issue on this process. (The Machine had a Moog controller and that was the driving force in replacing the control system). The customer wanted the Moog gone. He also did not want another "Black Box". He wanted all the control in a PLC.

The machine is a small hydro-forming machine. Two of the valves are used to advance the water connections to the part being formed. They advance to a set point position at the start of the cycle. This seals the water system on the part being formed. As the part is hydro-formed it will shrink and these cylinders will need to advance to keep the system sealed. This will be done by one of two methods. One is by maintaining a given hydraulic pressure on the cylinder or by the position following a pre-programmed profile as the part shrinks.

The other two valves are on the booster cylinders of the water system. These hydraulic cylinders are used like a piston pump for the water system. Only one booster will be used for a given job based on the overall pressure needed. The water system if filled with water and then closed off after the part. The booster moved compressing the water. If the booster reaches the end of travel before the desired pressure is reached it returns to home (Zero on the encoder). Then cycles again, doing this over until the required pressure is achieved.

I was planning on using the PID instruction but wasn't sure if there were other options. So the PID is what I should be using.

Again Thanks for the help.

 

[ndzied1]

What valve do you have?

 

[ndzied1]

Other things you have to thing about are what types of repeatability you need for position and pressure/force.

What speeds are the cylinders going to be traveling at?

Are the cylinders single rod or dual rod?

Are the valves mounted to the cylinders or remotely?

All of these things go into trying to give you a good direction to go.

So the PID is what I should be using.

I really think you should be using a motion controller since you talk about following a profile but minus that, your best bet is probably PID for the water connections. Do you need to profile the force for the hydroforming or just run the cylinder/pump back and forth at a steady rate?

 

[Karsten]

For positioning go for a PI controller with velocity feed forward and a virtual axis.

 

[dmargineau]

It could be done using on-board electronics internal position feedback proportional valves ("servo"-just like your Bosch-Rexroth) using "PID Motion" logic via FBD programming, however, this is quite some task even for a very experienced programmer... the Kp will not suffice...Ki will need to be extensively used and even the Kd might be required depending of the application...
As previously suggested, I'd look into an integrated Analog Motion module...Unfortunatelly only the A-B ControlLogix platform offers Analog Motion modules...and of course the third party 'black boxes"...

 

[Peter Nachtwey]

I thought I had replied to this thread but I can see that I must not have. Anyway.
niteman9, you are biting off a lot. I can provide a lot of reasons what you are trying to do will be difficult and at best achieve only mediocre results as best.

I am interested in why the Moog controller didn't work. It should be more capable than a compact logix. I always ask why people want to replace the controller because many/most times it isn't the controller's fault. It is bad hydraulic/mechanical design or bad implementation, not the controller.

What your customer needs is THE black box that Norm suggested. Those black boxes were intentionally painted black in defiance or in spite of the term 'black box'. You have to try one and get past the color. It is anything but a 'black box' in operation since it communicates easily with HMIs and PLCs and has better diagnostics.

Meanwhile read this about black boxes and hydro forming:
http://machinedesign.com/news/hydraulic-controls-transform-press-technology
The same as above applied to making aircraft parts.
http://www.metalformingmagazine.com/enterprise-zones/article.asp?aid=7194
an older application using a previous generation black box.
http://triformpress.com/triform_success_stories.php

http://www.ecs.net.nz/m14.asp?Page=/Motion/apps/hhtube.htm
Another old application. You can see by the date on the article that we have been at this for a while.
http://www.designnews.com/document.asp?doc_id=227050&dfpPParams=ind_182,aid_227050&dfpLayout=article
There are other examples but not everyone like to say what parts they used.

Now, where are the hydro forming success stories using compact logix?

We are patient.

It could be done using on-board electronics internal position feedback proportional valves ("servo"-just like your Bosch-Rexroth) using "PID Motion" logic via FBD programming,

How does one synchronize the motion and pressure profiles if the controls are on four different valves? Which on board electronics can control both pressure and position of the actuator? Most on board electronics only controls the spool position.

Peter Nachtwey
President
Delta Computer Systems
Maker of the black boxes

 

[dmargineau]

Peter,

There are quite a few systems out there implementing this kind of application on ControlLogix platforms; some using Analog Motion modules (1756-M02AE and/or 1756-HYD02) and some only "generic" AI and AO modules with Function Block "Feed Forward" PID logic (SCRV) in a sequential flow.
Again, it is not "mainstream" programming and quite "labor" intensive at development time, especially when tuning the system; obviously, the programming logic is proprietary but the concept is pretty straight forward.
The most critical components of the sytems I have worked with are the "Hybrid Type" actuators ("Oil behind Water") since the pressure within the water manifold would be the PV and the position of the actuator will represent the CV; also, accurate and fast processing feedback is a must.

 

[Peter Nachtwey]

dmarineau, have you implemented a hydro forming press?
I couldn't understand why you would make a comment about just using the on board electronics.

I agree that this would be a challenge even with the Control Logix. The Control Logix does a pretty good job of synchronizing axes by position. The real problem is the pressure control because the pressure control must be done in the Control Logix. There is too long a delay between reading the analog input, sending this data to the Control Logix, doing the PID, and then sending the control output to the M02AS especially when the requested packet intervals and the PLC scan times are asynchronous. This is the part that I don't think niteman9 or his customer understand, yet.

A common problem is using pressure sensors that don't respond quickly. Another problem with using PLC is that usually the analog to digital conversion isn't fast enough and there are filters on the analog inputs that slow down the response even more.

Peter,
Again, it is not "mainstream" programming and quite "labor" intensive at development time, especially when tuning the system; obviously, the programming logic is proprietary but the concept is pretty straight forward.

Hydro forming is not easy. Getting the PIDs for the basic position and pressure control is not that hard. It is the trial and error making the recipes for each type of piece to be formed that takes time.

The most critical components of the sytems I have worked with are the "Hybrid Type" actuators ("Oil behind Water") since the pressure within the water manifold would be the PV and the position of the actuator will represent the CV; also, accurate and fast processing feedback is a must.

I doubt most of my customers use the dual loop approach you suggest. I bet most consider the control output to the valve the CV and simply use the pressure control because the controller is fast.

 

[dmargineau]

Yes, I have...
The "feedback" and "command" Analog I/Os are 1756 Local Chassis (Backplane) "residents" and RPIed every 1ms; the sequential in nature Function Block Diagram programming logic is of a Priority 1, Scheduled Type with a Period of 3ms; obviously there are exactly 3 I/O updates for every Scheduled Task occurence; this would be the "original"setup first running without overlaps on Rev.15 L61s some five years ago.
Newer firmware L63s are capable of Task Periods half of the above listed ones and I am pretty sure that L7x are more capable yet.
Again, it is not easy by any means (it took months of development for each new system) and that's why I have suggested the deployment of a dedicated Motion Controller.