motor parameters

Unfortunately servo motor datasheets often lack a good description of the parameters. There is a difference between line-to-line and phase values. Here are listed the most important motor parameters and how to determine them.

resistance conf0.r (ohm)

1. measure line-to-line resistance R
2. convert to phase resistance conf0.r = R/2

inductance conf0.l (henry)

1. measure line-to-line inductance L
2. convert to phase inductance conf0.l = L/2

moment of inertia conf0.j (kg*m^2)

1. read moment of inertia from datasheet

polepair count conf0.polecount ()

1. attach a power supply to the motor (limit the current)
2. turn the motor one rotation and count the number N of positions it snaps in conf0.polecount = N

torque constant conf0.psi (V*s/rad)

There is no common ground for the toque constant. Some manufacturers state the current in RMS, others in peak-to-peak. We also stumbled over values that were neither.

How to determine psi:

Scope

1. Drive motor at constant speed (lathe, power drill, ...)
2. Connect an oscilloscope between two phases and measure frequency F (Hz) and peak, line-to-line voltage U0 (V)
3. Convert to line-to-neutral voltage U1 = U0/sqrt(3)
4. Convert to amplitude U2 = U1/2
5. Convert to torque constant conf0.psi = U2/F/2.0/PI

STMBL

It's also possible to measure psi directly with STMBL. Maybe even automatically in a future version.

1. Disconnect the HV power, leave the servo connected.
2. Connect with Servoterm, drag&drop this file into Servoterm: https://github.com/rene-dev/stmbl/blob/newstuff/conf/psi.txt
3. Type stop and start
4. Turn the shaft of the motor. You can use your hand for a rough estimation, but it's probably better to drive with a cordless drill. The closer to the nominal RPM rating, the more accurate the result will be. There is no need to turn the shaft continuously, a quick short turn of one revolution is sufficient.
5. Type psi0.max_psi and you will get the peak psi value that was measured. You can set conf0.psi to the measured peak psi value now. If you did not drive the motor reasonably fast, the proper psi value is probably a couple percent higher.

KV

psi = 60.0 / #POLE_PAIRS / sqrt(3) / 2.0 / PI / KV

Nm/A

psi = (Nm/A) / 3.0 * 2.0 / #POLE_PAIRS

phase offset conf0.mot_fb_offset (rad)

TODO com_test