Hydraulic Linear Motion Precision Controll

It certainly does, but hydraulic copy mills aren't crazy high accuracy, + - 5-10 thousandths for many of them if not greater.

They were never used for very high accuracy work afaik and mostly for surface contiur work where final critical dimensions were often milled after contour work afaik.

I know a few old iron guys, that's what they've indicated when i asked about a hydraulic copy mill for CNC cylinder head work

smc.collins wrote:

It certainly does, but hydraulic copy mills aren't crazy high accuracy, + - 5-10 thousandths for many of them if not greater.

As I understand it, copy lathes do pretty well. 

The thing is, CNC (specifically PID) ought to be able to compensate for the hysteresis in the control valve. 

Older hydraulic machine valves used dither to keep the spool from sticking near zero.

qualityhydraulics.com/blog/valves/what-dither-versus-pwm

This will help explain the problem.

Thank you all for the answers!
I have finally bought linear scales and started the installation.
I will tune it in the nearest future and share the results.

Another idea is to remove copy screws and other intermediate links between the stepper motor and proportional valve. I would like to connect the stepper motor directly to the proportional valve via the lever on the shaft of the stepper.
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?

Thank you very much for your suggestions and ideas.
Vasyl.

vanhalen90 wrote:

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. 
 

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.