Re: [CAD_CAM_EDM_DRO] Pittman Servomotor

Mike Haughton wrote:

> I have some Pittman servos taken from a HP7475 pen plotter. Can anybody
> supply details of these servomotors? I can't find them on the Pittman
> website.
> Details from the motor
> 9413D319
> 3140-0665 rev H
> 19.1 VDC
> 1-7-87

It is pretty easy to determine the basic motor constants. Kv is how many volts it
takes to get 1000 RPM. You can use a disc with a slot in it and a photo-sensor,
and count with an oscilloscope, if you have one. You can also use a neon bulb
with a 100 K Ohm resistor and rectifier, all in series. This bulb will blink 60 times
a second (at least in the US) and can be held next to a disc with a wide stripe
marked on it. Connect the motor to a variable DC power supply, and adjust for
16.67 pulses per second on the oscilloscope for 1000 RPM, and read voltage.
Or, set such that the disc only shows one stripe standing still (with the neon stroboscope)
and you should be at either 1800 or 3600 RPM. If you increase voltage to get two
stripes standing still, that would be 7200 RPM, but that might be tough on the motor.
Anyway, at 3600 RPM, divide the voltage by 3.6 and you have your Kv.

For Kt (the torque constant), mount a spool of some sort on the motor. A thread
spool might be perfect. Wind some heavy thread or light string on the drum a few
turns, and put some modest weight on the string. You might want to put a power resistor
of something like 1 - 10 Ohms in series with the power supply, and hook a DVM
in series to measure current. Adjust the voltage of the power supply until it just
starts to lift the weight. At this point, read the current. Kt is torque per amp.
So, knowing the radius of the drum and the weight, you can figure out the torque
to lift it in whatever units of measure you like. If the radius of the drum was one
inch, and the weight was 10 Oz, then lifting the weight required torque of 10 Oz-In.
If it took 1.5 Amps to do that, the Kt of the motor is 10/1.5 = 6.667 Oz-In/Amp.

>
> The encoder is made by HP and I would appreciate connection details (4 lead
> AMP flat cable). The encoder wheel is mylar with two tracks.

If you can find a part # on the encoder or read head (may look like HEDS-9000
or some similar) we can be more help.

Jon