[kevincnc]

Actually I have 2 questions-

1) I have some small DC brush servos made by QMC. The label says 35 oz-in continuous torque, and 4.69 amps continuous current. What other specs can be derived from these two?

2) Does anyone know of an easy way to generate continuous and/or peak torque curves for DC servos?

Any input would be appreciated.

[Mariss Freimanis]

1) Kt = 35 in-oz / 4.69A = 7.46 in-oz / Amp
2) Kv = Kt / 1.351 = 7.46 / 1.351 = 5.52 V / 1,000 RPM

Because you don't give the rated voltage, I'll guess 24VDC

3) No load RPM = Vsupply / Kv = 24 / 5.52 = 4,345 RPM

Because you don't give the peak (stall) torque I'll guess Tstall = 5 * Tcont

4) RPM at rated load = (1 - Tcont / Tstall) * no-load RPM = 0.8 * 4345 = 3,476 RPM
5) Rated power output = (RPM * in-oz) / 1351 = 3476 * 35 / 1351 = 90 Watts

Mariss

[Al_The_Man]

KevinCNC, I have found alot of this information in technical papers by Superior Electric (steppers), Bodine Motors, Electro-craft to name a few.
This is some notes I made when I was looking for similar information and found them in various sources:
Personally I think that a simplified model of a DC motor can be derived assuming the armature inductance to be zero and ignoring the resonanceeffect. With these stipulations the equations are:
1. V=Ia R + Ke omega (Ia=armature current, R=armature resistance, Ke=electr. constant, omega=speed)
2. Tg=Kt Ia (Tg=costant, Kt=torque constant)
3. Tg=J d(omega)/dt (J=inertia, d(omega)/dt=accel.)
The DC motor transfer function is:
Gm(s)=(1/Ke)/(1+s(Rj/KtKe)), which can be written Gm(s)=(1/Ke)/(1+sTm)
where Tm=mechanical time constant.
To measure the parameters you are asking for, I suggests the following:
A. Measure with an ohm-meter the armature resistance, then apply voltage to the motor without load and measure the current and speed. From equation 1. you can easily derive Ke.
B. Apply nominal current to the motor (with the shaft locked) by means
of a variable voltage source. Measure the torque on the shaft. From this you can derive the torque constant Kt=Torque/Amp.
C. You will find that Kt is approx. equal to Ke
D. For the inertia you can obtain it by calculation from the size and
material of the rotor.

Note1: inductance can be ignored- the electrical time constant is
very short compared to the mech time constant so that it can usually be
ignored.
You can measure the mech time constant by running the motor up to
speed at no load, disconnecting the supply and letting it coast down- plot speed vs time and fit to exponential N=No(e^-t/Tm) time to drop to 36.8% of original speed is the time constant.

Note2: If it is a permanent magnet motor, you can determine the internal emf by spinning it at rated speed and measuring the open circuit voltage. The voltage at any other speed will be directly proportional to speed. To measure the winding resistance, lock the rotor so it doesn't turn and measure the current with a small voltage applied (so as not to exceed rated current) Don't bother using a multimeter's ohm range- not worth the effort.
For inductance, you should use a scope- apply a voltage, rotor locked and look at the current trace vs time.
This will be of the form i=K[1-e^Rt/L] where i is the current at time t.
In most cases the inductance can be ignored as its effects are generally swamped by the mechanical inertia in transient cases and is of little importance for steady state.
If there is anything in particular as far as spec's you need on these motors, list it here and I may be able to dig up a way of finding the answer.
BTW it may be worth downloading the 'CheatSheets' (in PDF) from Texonics inc. they have a raft of forumla's , definitions and conversions.
Al