Re: [CAD_CAM_EDM_DRO] replacing steppers with servos

cdoughtynz wrote:

> what is involved in replacing steppers with servos.?

You need encoders of sufficient resolution to position the machine to your
desired resolution. Belt drive (if used) and leadscrew pitch come into play
here. You need servo amplifiers or step-to-servo drives. If using servo
amps, you need a DAC to generate the velocity signals.

> I have got my steppers set up to run at a max speed of 750 RPM, would
> I need to change the gearing to suit the servos higher speed? If so
> what is the ideal speed for servos.

Not necessarily. it is possible to use very small, high speed servo motors
to move large machines, but there are a bunch of technical reasons why
that may not be desirable. In most cases, using a larger, lower speed
motor with a low belt reduction ratio, or even direct drive, may have
advantages. There is NO "ideal speed". Some motors are designed
to run at 900 RPM, some at 6000. Choosing a motor designed for 900 to
2000 RPM would be the best choice. With a 5 TPI screw, 120 IPM requires
the screw to turn at 10 rev/sec, or 600 RPM.

> Is there any problems tuning the drivers with the servo motors?

Yes, servo amps need to be tuned for the inertia and resonant characteristics
of the load. Large changes in load can unbalance the servo loop. Generally,
however, machine tools have so much frictional damping that this is not a
likely problem. An improperly tuned, high-power servo can develop
oscillations that can tip over a Bridgeport mill! But, carefully working through
the tuning, starting with a low current limit and working up in stages, while
keeping the E-stop button handy, should keep you safe.

> do
> they go out of adjustment with use?

Rarely. It would require some physical constant in the system to change, like
mass, power source, spring rate, etc.

> my steppers are 450 oz/in, what size servo would I need?

Note that stepper motors lost torque VERY rapidly as speed increases.
Servos have both a continuous torque and a peak torque. Your steppers
may only provide 100 Oz-In at top speed. A 100 Oz-In servo motor
(cont. rating) could produce 400 Oz-In peak all the way up to max speed.
(4:1 is a common ratio of peak:continuous torque rating.)

"Size" of a servo motor is related to power output and speed, so knowing the
size of the machine and the top rapid feed rate you want is required before I
can get more specific. Knowing the leadscrew type (Acme or ballscrew)
and pitch is also necessary.

Jon