[anthony]

Greets all.
Curious more than anything else, but how are hydraulics (motors, pistons, etc) interfaced to CNC controls?

Is it all encoder feedback?

How does a CNC hydraulic press, or tubing bender, for example, know when and where to stop? and how do you set the pressure? it seems like an either-or situation.

I've done some work with steppers.. but that seems straightforward enough: x steps to the motor = x degrees of motion.

But with a hydraulic motor, for example, how do you get accurate motion when you're dealing with a fluid flow? does the motor just "turn-on" until an encoder tells it to stop? that doesn't come off as exceedingly accurate. And i'd imagine there is a lot of "hunting" when the hydraulics overshoot the mark.. just thinking of the response time to change in fluid flow direction.

Can someone shed some light on this?
Thanks,
Tony

[Cherezov Oleg]

Hi!
This to me too is very interesting . I hope, that at a forum there are experts on this theme. I look at a hydraulic drive as on an opportunity to simplify and facilitate a drive of an axis Z. But it is necessary to consider the price.

[Runner4404spd]

if running a screw to drive the axis, you would need a proportional flow control valve to control fluid flow to the motor. the encoder would be at the end of the screw and all the basic principles apply. the prop valve will have your acceleration and deceleration ramps programmed into it based on a frequency input usually.

now if you want to use a hydraulic cylinder i recommend building on with an LVDT in it to monitor position. you will still need a proportional flow control valve to set your acceleration and deceleration and it works the same way as an electric motor.

at least thats one way to do it.,

[anthony]

Runner, thanks for the quick reply.. but I'm still confused on the bit
that lies between the PC and the PF control valve.

Perhaps its my limited experience.. I've built a pretty decent plasma table using Campbell's I/O board, some geckos, and steppers.. so thats all I know.

Would an I/O board need to output a variable frequency signal? is there another module that goes in there?

Again, controlling hydraulics is completely foreign to me (other than on/off that is).. so I may be asking meaningless questions.

Thanks,
tony

[DareBee]

Even with hydraulic systems you still end up with servos, in the best systems anyway.

As mentioned an encoder can still be used where rotary action is present, Linear can be done with a glass scale system or Linear Resistance Transducer.

The most accurate control of a hydraulic actuator is a servo proportional valve.

[Al_The_Man]

You can use Hydraulic servo valves and linear scales, Bosch actually built a CNC machine using Hydraulic cyclinders this way.
There is also a company that I believe is still in existance that developed an interesting cheaper alternative, Instead of the costly servo valves which are operated by the Servo card and move the spool back and forth proportionaly, with a electro-magnetic lever, these valves are very prone to oil contamination and require very fine filtering, also costs in the $k's.
This development was to use stepper motors to move the spool under any stepper controller you want to use, Usually linear encoder is still used for precise positioning.
BTW Avey used to make a mill/drill that had a Hydraulic cyclinder for the Z and the feed was a binary combination of 8 flo valves, quite the set-up.
Al.

[RICHARD ZASTROW]

Anybody old enough to remember the original CINTIMATIC's? They were built starting in the early 1960's using hydraulic axis drives on round rail/bushing ways. Micro-switches riding on adjustable drum cams for Z axis. In summer the hydraulic oil reservoir would overheat thinning the oil. This resulted in overshooting commanded position, devestating in tapping. I hated those machines! Many were later retrofitted with electric servo's and were respectable as "pad milling" and drilling machines.

[Al_The_Man]

In '97 I retro-fitted a couple of around that vintage, they were basically point-to-point machines (no interpolated moves) positioned by hydraulic motor, they had 'Cincinnati Electronic Control' and encoders for feedback, IIRC the original control would anticipate the end of cut or position and slow the motor accordingly, I retained the encoders and Hydraulic motors and used a PC based Galil card, they are still running today making Railroad switch point parts.
Al.

[samco]

We have a K&T 3 axis mill that has hydraulic servos. (It finally died last year)
http://www.electronicsam.com/images/...on/hyservo.JPG
That is the servo - it is run by a proportioning vavle. (the valve actually has a little buzzer in it so it won't stick)

IT has a inner velocity loop using a tach - the outer position loop uses acupins. (see below)
http://www.electronicsam.com/images/...n/accupins.JPG

I have a tread going on here - converting it to EMC2
http://www.cnczone.com/forums/showthread.php?t=25929

IIRC its repeatablility was .0005

sam

[Cherezov Oleg]

And if to use the simple decision. Two hydrocylinders. By analogy to brake system of the car. Position plasma cutter depends only from a pressure of an electric current are sharp metal . System TНС has small moving on height.

[anthony]

Thanks for the info. Though I'm still not sure how (or how well) a hydraulic CNC would work.. at least now I've got some terminology to get me started on the reading.

Aside: advantages of using hydraulic motors to turn lead screws (or drive chains) instead of just really big servos? thoughts?

-Tony

[Runner4404spd]

on a small type setup the electric motor will win out hands down. however, on a large scale setup you will have a very hard time stalling hydraulic motors. they have a lot of torque and the torque is proportional to the pressure. with modern controls i have no doubt that you can do sweeps with the hydraulic servos.

[Ron22]

I have seen the old CINTIMATIC's too. I am in the process of installing Delta Tau controlers on one right now. If everything works out they have about 10-15 more there old beast to retrofit.
He has newer machines that were custom built that also have hydraulics on them.
The owner of the company I am doing the work for loves Hydraulic machine. He likes to make big cuts and really loads the axis.

[Geof]

Thanks for the info. Though I'm still not sure how (or how well) a hydraulic CNC would work.. at least now I've got some terminology to get me started on the reading.

Aside: advantages of using hydraulic motors to turn lead screws (or drive chains) instead of just really big servos? thoughts?

-Tony

A disadvantage of hydraulics is that normally the pump is noisy and a lot of the electrical energy that goes into driving the motor for the hydraulic pump ends up just making the oil hot but does not do useful work. The only advantages I can think are you can getting hulking torque in a small package and when you stall a hydraulic motor it doesn't start smoking.

[anthony]

Are hydraulic motors used on gantry-style machines?
If so, is it possible to drag hydraulic tubing back and forth like that?

From what I remember, that stuff tends to be pretty inflexible.

[Al_The_Man]

Are hydraulic motors used on gantry-style machines?
If so, is it possible to drag hydraulic tubing back and forth like that?

From what I remember, that stuff tends to be pretty inflexible.

Hydraulics are used all the time on gantry mill tables for applications like work clamps etc.
Al.

[Ron22]

Are hydraulic motors used on gantry-style machines?
If so, is it possible to drag hydraulic tubing back and forth like that?

From what I remember, that stuff tends to be pretty inflexible.

On the custom made hydraulic gantry I have seen they just mounted all the hydraulics on the bridge. The gantry is rack & pinion so this makes it a little easyer. Not sure about caring all the extra weight.

[DareBee]

Electric motors will be better in most cases.

Consider this, people really like to use air tools (sure they are cool).
Air tools are a HUGE waste of energy in a lot of cases. Eg I need to run a 3/8 drill for 4 hours in a 1 man shop. Do I use the air drill or my trusty 8amp Bosch electric.
A lot of people grab the air drill.
For you to run the air for 4 hours your 5 hp compressor will run about 30%? of the time.
Untill you get a lot of air tools running on that same compressor the electric will be more economical.

Same goes for hydraulics.

[Warpspeed]

There are many different ways to do this, but the basic idea is that you still use a stepper motor to determine position, speed, direction, and so on in the usual CNC manner, using all the familiar hardware and software.

You then connect a rotary spool valve between the stepper and whatever you are driving, and use hydraulic pressure to do the real work. Exactly the same idea as the power steering system in your car.

You and the steering wheel are replaced by a stepper motor. The power steering pump and hydraulic steering rack do the hard work. But the stepper determines the actual final position and how it actually gets there. The hydraulic system is just a giant force multiplier of the stepper position.

[Al_The_Man]

This is the stepper valve/cyclinder referred to in post #6
http://www.victorycontrols.com/DSSC.pdf
Al.