Re: [CAD_CAM_EDM_DRO] controlling hydraulics with Mach 2

Todd W. Syssa wrote:

>Anyone out there ever control hydraulics with Mach 2
>or other CNC PC based controller?
>
>I have a proportional hydraulic valve that uses 12 VDC
>and varies fluid with current (.6-1.2 amps) as well as
>an on off valve that uses 12 VDC to power the solenoid
>for each direction.
>
>I also have a linear encoder attached to the carraige
>of each slide and have gotten both encoders to work
>with the Mach 2 demo. My question is about converting
>the output used for a servo/stepper motor to making it
>useful for turning on and off the hydrualic
>cylinders...
>
>...is modifying a servo amplifier a possibilty? Any
>ideas would be helpful!
>
>

You can't do this with Mach2 because Mach 2 is not capable of
controlling true servos. Traditional analog servo systems use a
device for measuring velocity (a tachometer), and a method
for providing an approximate velocity from a voltage command.

Mach 2 just outputs step pulses, period. It has no provisions for
doing anything else, or closing the servo loop in software.

Your valve, however, is not suitable for proportional motion
control in four quadrants, which is what you need for a hydraulic
motion control system. The proportional servo valves used for
this purpose are capable of smoothly transitioning from motion
in one direction to motion in the opposite, and they do this several
hundred times a second even when attempting to stand still.

Commercial controls added "dither" to the servo command output
so as to keep the servo spool valve hydraulically floating on a cushion
of oil. The Allen-Bradley 7320 control's hydraulic drive board added
a 400 Hz sine-wave dither to the output, for instance. There's no way
your colenoid reversing valves could follow this rate.

Note that although Mach 2 has provisions for encoder input, that
is NOT used in any way to control motion in a closed loop fashion.
It is only used as a one-way DRO readout when in estop mode.

If you had a hydraulic valve capable of 4-quadrant operation without
discontinuities, I could show you how to make that work under EMC,
which DOES support true servo operation. But, you really don't
want to do this! It is a total nightmare. Check today, and see if you
can find any machine tool maker using closed-loop hydraulic servo control.
There is a reason why they all dropped this system as soon as totally
electric servo drive became practical.

Jon