Hydraulic Linear Motion Precision Controll

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17 Aug 2022 22:00 #249975 by andypugh

As far as I understand there is a velocity control mode on LinuxCNC which is used to control old servos with  Analog +-10V. And proportional valves also control the velocity of the Hydraulic cylinder.
So what do you think, is there a way of converting a +-10V signal into Step/Dir Pulses to let the stepper deviate from the zero position?
How is it possible to implement in software, or is it simpler to do in hardware, for example, to have an external microcontroller to convert the signals?
 

I think that it is easier to do it in software, but it isn't really a special velocity control mode. It's all just numbers and the controller has no idea what they really mean, that's up to you. 

But you can certainly have a PID controller take a position as command and feedback and output a value that you use as the velocity control input of a stepper motor, if that is what you want to do. 
 
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18 Aug 2022 12:31 #250014 by Todd Zuercher
I think for this you would actually be commanding the stepper motor with position commands, but those position commands would correlate with the velocity command of the hydraulic servo. So you'd command the stepper to move to X number of steps to move the control arm to Y angle resulting in opening the valve to move the hydraulics at N velocity. So in essence the position of the steeper would be the velocity command.

But in then end like Andy said, it's all just numbers, and the control doesn't know what the numbers really mean, It is up to you to scale and plug them in where they need to go to work. And I see no reason this couldn't work the same as any ordinary analog servo system. Where in a servo you have your +/-10v analog signal will be connected to your step generators position command (and scaled appropriately) instead of to a drive's analog velocity command input.
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18 Aug 2022 12:54 #250016 by Todd Zuercher
Start with an ordinary analog servo config, Then where the pid's output would be connected to the analog output command. You would connect that instead to your stepgen's position command. If you are using a Mesa hardware stepgen I wouldn't bother with using the usual PID to stepgen velocity command setup. I think i'd just connect directly to the stepgen's position command input.

You might find that the existing screws and other intermediate hardware between the stepper and the control valve are nessisary to have the required resolution to adequately control the valve with the stepper motor. (This is just a guess not having seen how it is really set up.) If that isn't the case simpler is almost always better.
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