Hydrualic axis control

I'm sure I'm not the only one that has given thought to using hydraulics to move the axis on a cnc machine. there are numerous advantages to using a hydraulic cylinder to move the table on a mill as opposed to ball screws. zero backlash, virtually no wear, as fast or slow and as strong as needed. the problem is in getting perfect control.
And price ,, geez hydraulic cylinders are cheap compared to good ballscrews.
on the disadvantage side , a hydraulic pump would need to be running at all times regardless of motion.
I've given some thought to this (about as far as i've got so far) using a stepper or servo to control a spool valve and using glass dro scales to pinpoint actual location of the axis.
Seems doable and by using the glass scale as a reference the axis could be in exact position instead of an assumed position simply because the screw turned x rotations.
wear will always dictate the amount of error on a ball screw machine using an encoder on the drive motor to tell the motor where it should be ,, not so with hydraulics and glass scales . the machine will go to where the glass scales tell it to.
anybody out there with any new thinking novel ideas about how we can control such a system??

In the old days there was the servo valve, but open loop systems lack precision need for CNC. There must be some precision hydraulics of some type, but I have never came across them.

I remember way back , we used to have an old ditch witch (rt40 I think) that used hydraulics for steering ,(front and rear axle , switched a lever to decide which axle was steering),it had no mechanical linkage at all to the axles for steering, just hydraulic cylinders and the steering wheel was coupled to some type of valve no other mechanical connection ,, turning the wheel so far moved the cylinder so far ,, never dug deep enough into it to see how the steering valve worked ,,which is to say , it never failed or I'd have been working on it..

No reason why not, in the 80's Robert Bosch outlined the idea of a CNC control with proportional valves with linear encoder feedback.
Also in 1993 Universal Studios used servo valves in a simulated 8.3 earthquake attraction in the Florida theme park.
The technology is out there.
There was also a company that came out with a stepper controlled spool for precise control, and many CNC pipe bender manuf use the Hyd proportional valve technology today in their machines.
Nothing really new.
Al.

I'm the lead of prefabrication at a large steel fab shop, and the 3 spindle drill line we use to poke holes in beams, channel, plate, and whatever else we can stick in there uses hydraulics to control the movement on the heads. Hydraulic cylinders are controlled by an electronic valve (integrated steppers?) and monitored by encoders on a simple rack and pinion rail to provide feedback in a closed-loop system. They are accurate, fast and strong. If anyone is still interested, I can snap some pics of the electronics and control.

I would like to see some pictures ,,very interested in the valves/controls

There was a company a few years ago that came out with a stepper motor on the end of a Hyd cyclinder, the stepper motor controlled the integral valve spool in a similar way a hyd servo valve did.
I have not seen anything of them since.
Al.

The Valves look to be stepper controlled. Rexroth is the make. Well send pics this afternoon.

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