Hydraulic Linear Motion Precision Controll

 
Hello, Everybody!
Half a year ago I have retrofitted the CNC mill which is fully hydraulically driven.
It is the Soviet Union Machine from the year 1991.
I have even incorporated ATC and automatic two-stage gearbox (Thanks to Eric Salo for his youtube videos).

3 axis motion has tracking hydraulic drive. It includes a stepper motor (Number 4 on the picture attached) with a screw(3) on a moving part of each axis and a proportional valve(2) that is connected to the stationary part of the actuator(1). Every time the screw shifts lever connected to the valve, the valve opens and lets reach the balance point of the system and to put the valve into its middle position. So proportional valve always follows the screw.
Righ as it is I have a positional accuracy of 0.02 - 0.04mm for machining square shape and drilling holes.
But once I want to machine a contour or holes/cylinders the diameter is 0.5-1mm bigger, depending on the speed of machining.
This happens because when the axis is moving it is always lagging behind in order to valve remain open and allow a constant velocity.
I am looking for ways to compensate for it. For me possible solutions are:
 - Install linear scale and tune the servo to compensate it.
    Will it be sufficient?
 - Install linear scale and connect stepper motor directly to the valve. 
   In this case, I will have to set up the controller to controll the velocity of the axis but not the position. How to do that?
   Also what is the best way mechanically to control the valve? 

I would be very thankful for any ideas and help in implementation.
If any additional explanation or information is needed just let me know.
I will keep you updated
Thank you
Vasyl.



 

In this case, I will have to set up the controller to controll the velocity of the axis but not the position. How to do that?

In hal stepgen you have a pin for "velocity or position" control.
For the rest i have no advice.

hydraulics, probably you have 2 options, convert to balls screws and servos, or slow the machine way way down so that the pid can tune it properly. Personally, I'd put the money into Servos and ball screws if possible. I'd want to go with position feedback and linear encoders

yes, but the hydraulic circuits in those machines are designed to operate like that, IE spool valves, high FQ solenoids etc. That machine is a tightly coupled system. I have my doubts that a soviet era hydraulic CNC machine would offer such accuracy. It very well may, but it seems unlikely.

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

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.