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Old June 17th, 2018, 09:56 PM   #1
Sanderson4401
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Hydraulic motion with PID

I need a bit of help with hydraulic positioning?. Basically I need the hydraulics to move to a position as quickly as possible and then decelerate to set point with minimal overshoot. I've currently programmed the motion using PIDs in Logix5000 and it works alright but I read this http://www.hydraulicspneumatics.com/...-Entertainment and thought it might be worth a try? To use feed forward I would have to change from PID in ladder to PIDE in function block correct? Any other worthwhile options? At this point my client probably won't be receptive to buying additional hardware.


Thanks!
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Old June 18th, 2018, 12:13 AM   #2
Peter Nachtwey
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I wrote that article. You should have used a hydraulic motion controller, either a 1756-M02AS or Delta Computer Systems RMC75 or RMC150. Your choice, we make them both.

Feed forwards are essential for rapid motion control. The problem with PIDs is that they require error to generate an output so there will always be a following error while moving. While coming into position the integrator will need to unwind by overshooting.

The cure is feed forwards. Feed forwards are used to predict the control output without error. To use feed forwards gains you need a target generator that generates a target position, velocity and acceleration that is constantly updated. The control output is estimated by multiplying the target velocity by the velocity feed forward gain and multiplying the target acceleration by the acceleration feed forward gain and summing the two values. These two values should then be used as the bias to the PID or PIDE.
Use the PIDE in velocity mode.

The problem is generating the target position, velocity and acceleration. That requires a lot of code to do right. It is easier to buy the hydraulic motion controller. In the long run it will pay off with better performance.
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Old June 18th, 2018, 10:09 AM   #3
Sanderson4401
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Quote:
Originally Posted by Peter Nachtwey View Post
I wrote that article. You should have used a hydraulic motion controller, either a 1756-M02AS or Delta Computer Systems RMC75 or RMC150. Your choice, we make them both.

Feed forwards are essential for rapid motion control. The problem with PIDs is that they require error to generate an output so there will always be a following error while moving. While coming into position the integrator will need to unwind by overshooting.

The cure is feed forwards. Feed forwards are used to predict the control output without error. To use feed forwards gains you need a target generator that generates a target position, velocity and acceleration that is constantly updated. The control output is estimated by multiplying the target velocity by the velocity feed forward gain and multiplying the target acceleration by the acceleration feed forward gain and summing the two values. These two values should then be used as the bias to the PID or PIDE.
Use the PIDE in velocity mode.

The problem is generating the target position, velocity and acceleration. That requires a lot of code to do right. It is easier to buy the hydraulic motion controller. In the long run it will pay off with better performance.
Without adding hardware, are there any best practices for using normal PID on hydraulics? I mentioned using addition hardware this morning and they want to try and make it work without for now. The application does't require extremely tight control and they have a similar machine operating in this manner.

Thanks for taking the time to respond!
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Old June 18th, 2018, 04:41 PM   #4
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Best Practices

Quote:
Originally Posted by Sanderson4401 View Post
Without adding hardware, are there any best practices for using normal PID on hydraulics? I mentioned using addition hardware this morning and they want to try and make it work without for now. The application does't require extremely tight control and they have a similar machine operating in this manner.

Thanks for taking the time to respond!
Sanderson,

You have been given the Best Practice solution. When doing hydraulic positioning....you can't get there with plc programming.

There are off-the-shelf hardware solutions that will meet your customers requirements without fail.

The Delta RMC-75E is perfect for a single axis system.

Bosch-Rexroth also offers a single axis hydraulic position controller, look for HACD.

Both the Delta RMC and BR HACD are world class electronic devices for hydraulic positioning.

Your next task is to ensure that your axis position transducer has good throughput capability.

Your final task is to make sure that your hydraulic valve has sufficient response time. There are many hydraulic vendors that offer servo tolerance, and servo-proportional tolerance valves that can net about 20mSec full volume step response or less.

Regards

Plastic
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Old June 18th, 2018, 05:15 PM   #5
VAN
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Quote:
Originally Posted by Peter Nachtwey View Post
Delta Computer Systems RMC75 or RMC150.
Any reason you aren't recommending the RMC200?
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Old June 18th, 2018, 06:06 PM   #6
Peter Nachtwey
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The RMC200 is gross overkill for single axis applications.
The RMC200 can control up to 32 axes. The processor is a dual core processor. The cores would spend a lot of time in an idle loop.

The RMC75E is the best ( most cost effective ) option of single axis applications.

There are many problems with using PLC only control. The inputs and outputs are slow and updated asynchronously to the scan. PID control with feed forwards doesn't perform well. Writing the target generator so feed forwards can be used is tricky and will take a lot of time and debugging.

The RMC75E has auto tuning. Better diagnostics.

Then there is the testing. The last time I wrote a target generator I tested it with > 200,000,000,000 combinations of commands and initial positions , velocities and accelerations. Yes, it took days. It was automated.

If anybody wants to play with a servo hydraulic simulator in python I have one here:
http://deltamotion.com/peter/py/HydSim.zip

Here is a video I made for a Chinese servo hydraulic forum.
I was attempting to say a few Chinese words for target position, velocity etc.
I can auto tune in less than two minutes if I am not talking and explaining.
http://deltamotion.com/peter/Videos/AutoTuning3.mp4
I am a moderator on a Chinese servo hydraulic control forum. The US hydraulic forums are dead.
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Old June 21st, 2018, 11:39 AM   #7
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You may have a hard time using the CLX PID instruction to control a hydraulic system. There are some dynamics in hydraulic systems that can be hard to handle with general purpose PID controllers.

If you aren't already, set up a virtual axis and issue motion commands to that axis to perform your target generator functions. You will have to make sure you can keep the virtual axis synchronized with your actual axis (which you can do by homing the virtual axis) but the motion commands on the virtual axis will generate commanded position, velocity and acceleration values that you can use for your feed forwards.

Keith
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Old June 21st, 2018, 12:22 PM   #8
Helliana
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I used a RMC75E for the first time last month. Overall, it seemed to work very well and was pretty simple to program. I only had one issue related to the PWM transducer signal i had, but Delta's free technical support solved the problem in under 5 minutes. That was by far the best technical support phone call I have ever experienced.



If you are doing hydraulic motion, i highly recommend using it. The controller was also a lot less expensive that I thought it would be. You can buy the entire controller for what it costs to get a SSI or PWM tranducer input card or ethernet gateway for a PLC.
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