Re: [CAD_CAM_EDM_DRO] Re: various types of AC/DC drives and their inputs?

Asim Khan wrote:

>Thanks Jon for your kind help!
>Please tell me if any drive has pwm and direction signal as inputs
>what kind of trouble we may have if we run it using STG using
>some electronic circuit that can convert the +/-10 V signal
>from the STG to PWM and Direction signal?
>or is this aproach (converting the Voltage command signal to pwm
>and dir) pefectly valid?
>
>

A servo with PWM input is most likely NOT a velocity servo, and may well not
be a torque servo, either. The "old standard" servo system had the CNC
control send
a velocity command signal to the servo amp. This was a signal where
voltage was
exactly proportional to the desired velocity. The actual velocity was
measured by a
DC tachometer, and an op amp compared the two and produced an error signal.
The DC gain of this circuit was usually in the 100,000 range. This
error signal
was then fed to a torque servo. With a traditional DC brush motor, the
armature
current was measured, and forced to be proportional to the error from the
velocity amp. This circuit allows enormous gain on the velocity amp without
instability.

Yes, you could build circuits to convert the +/- 10 V analog signal from
the STG
card to a PWM waveform. In fact, all modern analog-input servo amps do
exactly that.

But, if you feed the velocity command from the STG card to a plain PWM
amplifier,
you will find that it is much less stable that a true velocity servo
amp, and you will
have to keep the gain very low to prevent instability, due to the
motor's resistance,
inertia and inductance. I tried this as a temporary measure while
building my own
servo amps, and I can tell you, the performance was AWFUL! Huge
following errors,
both static and in motion.

Jon