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Hydraulic cilinder position control

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basparky:
Hello,

Today i found Oscat.de and downloaded the basic library and documentation. Great library and docu!!
I want to make a position control loop for 2 hydraulic cylinders. The cilinders are controlled by proportional valves with there pwm controllers. I need to greate a sollwert from minus 100% to 100%/-10V to 10V. Both cilinders have a position feedback 0-10V. length cilinder 200mm. I thought of using the CTRL_PID source to create the closed loop position control.
Could somebody please confirm if using the CTRL_PID is the right way to go?
Is there a way to determe the input parameters in front (behing my desk)?

The position off cilinder 2 is dependent from the actual position from cilinder 1. I can Use the actual position from cilinder 1 as Sollwert for cilinder 2. This in combination with a position check to make sure cilinder 2 is following correctly.
(I have no experience in controlling Proportional valves for position control  :()
Thanks in advance.

alexdrik:
Hello,

as far as i know, position control of hydraulic cylinders with proportional valves by plc is a difficult thing to do.
hydraulic by itself is not as constant as electric (oil volume is heat and preassure dependend, hoses are flexible, ...).
furthermore proportional valves may be non-linear, cylinders usually have two different preassure areas, ...

if you have slow movements, little disturbances and can accept some deviation, it might work. But tendencies to oscillate exist.
What kind of hydraulic machine do you want to create?

I hope you can prove me wrong.

Best regards
     Alex

basparky:
Hello Alex,

Thanks for your reply.

I know there are special servo valves for position control with hydraulics... In my case i need to use the supplied components..
The goal is to control a arm over 0 - 180 degrees. This arm needs to move from 0 to 180 degrees in 2 seconds. This with a smooth ramp ump and down curve. The arm is driven by a cilinder with a 200 mm stroke.
The accuracy is not very importand for the arm. The most important is a smooth movement and getting within 2 seconds from pos 0 to 180 degrees. The arm is mounted on a sled. This sled has also a 200mm cilinder.

The position of the sled depents on the actual angle of the arm. i thought of using the actual position of the arm in a select case function to select the wanted position setpoint for the sled.

I thought of using the PID function for both cilinders.

On other option is creating a ramp up and down function for the arm to create a smooth travel from 0 to 180 degrees. And using the actual poistion of the arm as sollwert for the pid loop of the sled.

What would you suggest?

Thanks in advance.

alexdrik:

Hello,

i have been talking to some of our hydraulic guys. They told me that it might work.

for 200mm stroke, a resolution of 12 bits typically is ok. analog meassurement can be used.
a recommended plc cycletime would be 2 ms.
an important thing is the valve. you will need one with zero overlap (e.g. 4WRSE...V1...)
PID-controller is a good approach.

Best regards
     Alex

basparky:
Thanks for asking your specialists!!
The 2 ms cyclus time is going to be a problem.. My plc runs at 10ms.

I will be on the site later this week. As soon as i have further test results i will posts.

Thanks!

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