Re: [CAD_CAM_EDM_DRO] Re: isolation, current and velocity, encoders

jmw@... wrote:

> Jon,
>
> I see that little Escap on the H&R web page. How do you tune its
> installation so that it delivers the desired (in my case) 7VDC per
> 1000 rpm? By drive pulley sizing, a potentiometer on the output, and
> what if the voltage output is non-linear in RPM?

You could, but most servo amps just need some reasonable
voltage, which is set to match the velocity command voltage.

In other words, if you have a max velocity of 120 IPM, and know that
the 2:1 drive and 5 TPI screw requires the motor to turn at 1200 RPM
for that velocity, you can set up like this:
1. the CNC control puts out +10 V for a 120 IPM move
2. The tach (wherever it is attached) puts out 5 volts at that
speed. So, turn down the command pot on the servo
amp so that it only gets 5 V to the resistor at the summing
node. Turn the tach pot all the way up.
3. Adjust these settings until the acceleration ramps are as
accurate as possible.

What I'm saying here is that there is NOTHING sacred about
7V / 1000 RPM. What you need is to arrange it such that a command
for some particular velocity is equaled at the error amp in the servo
amp by an equal signal from the tach. The servo amps usually have
pots to balance these signals over a pretty wide range.

Think about it this way: depending on the motor belt ratio, 120 IPM
could be anything from 100 RPM (direct drive with a coarse screw)
to 6000 RPM (very high belt ratio and/or fine screw). You just need
to balance the two signals at the servo amp.

The voltage output CAN NOT be non-linear. It is determined by
laws of physics, specifically conductors moving through a magnetic
field. A properly designed motor, with no load, will be VERY linear.
These motors are built using construction identical to their tachs,
and should satisfy all requirements. I have not actually used these
motors, but I have tested them on a scope to check for voltage
ripple (due to either magnetic field variations or commutator
effects) and have found them to be BETTER than my DC tachs.
My tachs had a lot of brush noise, these Escap motors were
CLEAN! I've never seen a better DC tach. (Now, this may be
due to the fact that they are new, and after some use there may
be a little brush noise, but they should work fine.)

Jon