Re: Victors Non-linear!!!
 

Measuring voltage is not going to tell much. PWM varies the power to the motor not voltage. A motor under no load takes very little power to run at full speed. It would be interesting to see a plot of rpm vs current draw. I would expect to see much different curves with a motor under load. A better measure of the victors would be to put the motor under a constant load and measure the power ouput of the victors motor system as the robot controller steps through the pwm variable.

Re: Victors Non-linear!!!
 

This would be true if a motor were strictly a resistor but it is not.
Power output of a motor at a given time is given by:
power = ((V-speed*kv)^2)/armature resistance
where speed is in rads/sec and Kv is the motor's CEMF speed constant.
as you can see power output is not porportional to the square of voltage unless you assume your robot is always moving at a single speed.

First off PWM DOES vary equivilant voltage NOT POWER. Any EE or anyone with any knowledge on these forums will back me up on this. in order for the speed controllers to vary power they would either have to have some sort of closed feedback loop based on current and voltage or have some knowledge of the system they are controlling including either feedback from a speed or torque sensor and the necessary system constants.

Secondy a plot of RPM vs. Current draw would be meaningless to say the least. Current draw cannot be related to speed without integral calculus, the moment of intertia of the driven system, the current torque constant of the motor, the armature resistance of the motor, the CEMF speed constant of the motor, the input voltage to the motor, etc.


The output of the victor is a duty cycle modulated square wave but its's frequency is high enough that for out purposes as well as modeling purposes it can be treated as if it were variable voltage DC. Since it is a square wave and it is of sufficiently hihg frequency its "RMS voltage" can be measured with accuracy without a "true RMS" meter.

If you want to model a PM DC brush motor, the relevant equation is as follows.

torque output=(Kt(Vin-speed*Kv))/r
where:
Kt is the torque voltage constant in newton-meters per amp
Kv is the speed CEMF constant in volts per radian per sec
r is the armature resistance in ohms
Vin is the input voltage in volts
if you plug this into our friend newton's second law and solve for speed vs time for a particular voltage input, you will find that when there are no other forces in the system(ie motor is spinning freely), speed will be porportional to voltage input(after time for acceleration/decelleration is allowed).

Re: Victors Non-linear!!!
 

I'm going to say you can't acurately measure the voltage with a voltmeter by the definition of RMS.

VRMS = 0.7 × Vpeak
VRMS = 0.7 * 12V = 8.4V

Since we're pulsing +12V DC (changing the duty cycle), theoreticly, the voltage should always be 8.4 Volts.

EDIT: This is only true for a sine wave (AC voltage). Give me a minute for a square wave.

EDIT2: Because our duty cycle changes it throws off the RMS calculation big time. The above equation is true for a 50% duty cycle square wave.

Re: Victors Non-linear!!!
 

Whoa Boys,
I stop looking at the boards for a few minutes and things go a little crazy on me. The PWM output (or input for that matter) is not able to be accurately read on most multimeters as Kevin specified above. The input circuitry for most meters, Fluke general line included, is designed to measure AC signals below 400Hz in most cases. The conversion electronics are designed to give reasonable RMS datum on nearly pure sine waves. With the Victor output being a PWM square wave at a much higher frequency than 400 Hz, very few meters will give any indication that would be linear. Two methods that would give you accurate data would be to measure the duty cycle with a scope or with a PWM to average convertor. The second may be easy to build. Check out the ARRL Radio Amatuers Handbook for full wave bridge power supplies. Build up the bridge and capacitor filter, (two components from Radio Shack, bridge rectifier and 100 microfarad capacitor) and add a little load resistance (say 470 ohms at 1/2 or 1 watt.) in parallel with the cap. Measure the voltage across the resistor for different PWM inputs and you should see what you are expecting to see in linearity. (Give the output a chance to settle a few seconds before making the measurement.) The bridge will also allow you to measure reverse currents as well since the bridge will automatically make the switch for you. Remember that this test is only looking for representative numbers, not absolute, dead nuts, accuracy. It is just easy to build and get valid, representative data.
Mike, the RMS value of a square wave of varying pulse duration can be calculated for each pulse by averaging the voltage over the pulse repetition, i.e. the duty cycle. If the duty cycle is 40% then a 12 volt pulse would average 4.8 volts, a 10% pulse would be 1.2 volts, etc.(average is the secret word here!) You can, of course, write an integral that describes the action or even use Fourier transforms for other than pure sine waves. Since RMS is theoretically intended to give the same "heating power" of an AC signal as the equivalent DC voltage would. (pure sine wave or not) True reading RMS voltmeters as Kevin suggested above actually make this measurement. The 0.707 * PEAK Voltage only holds true for pure sine waves and is the solution to the integral for a pure sine wave. Add a little distortion to the signal and you can throw your calculations out the window.

Re: Victors Non-linear!!!
 

But isn't speed proportional to voltage for a given load? (ie speed=V*someConstant). In that case, we have

power = ((V-V*k*kv)^2)/armature resistance
and hence
power = ((V*(1-k*kv))^2)/armature resistance



Anyway, I think there is some very obvious and simple evidence that your data is wrong. I would guess that most of the community gathered data for RPM/torque/current curves were gathered using Victors to control voltage, and all these curves report the expected linear curve, and therefore, the Victor must be outputting a linear voltage.

Re: Victors Non-linear!!!
 

Let's get a better look at this. The equation that Rick quoted is input electrical power derived from Ohm's Law for power, P=V^2/R. In the case of the motor, the available voltage for determining armature current (for DC permanent magnet motors) is the power source voltage-the counter EMF produced by the motor. CEMF varies with speed, hence the introduction of the speed constant and CEMF spec as a reduction in the available voltage. V-(speed*kv)=voltage available to motor winding. Then the reults are simply plugged into Ohm's Law for power as above. Unfortunately, armature resistance is not the only loss that affects total series resistance. Brushes, wiring, armature connections, etc. are all losses that will affect current through the armature and heat will change all of these. All of the losses summed together (electrical and mechanical) determine the output power vs. input power, or efficiency. That is why efficiency is never 100%!

Max, the data sheets are provided by manufacturers and cannot use controllers. All of the variables must be tightly controlled in order for porduction specs to be guaranteed. Imagine what would happen if only half of the motors in the four million CD players you just made, could not meet RPM as specified.

Re: Victors Non-linear!!!
 

I've measured the Victor 884's PWM frequency as 120 Hz.

Re: Victors Non-linear!!!
 

Greg,
I have not checked myself in a while (OK, a couple of years) but I was under the impression they were about 2kHz. Still the rise time of the pulses get into trouble when the input is frequency limited. Even a 50% duty cycle would not have enough measureable energy beyond maybe the first or second odd harmonic to be accurate, I would think.

Re: Victors Non-linear!!!
 

Well the trick there would be that we don't have an actual DC signal going into the motor. I'm relatively certain that the inductance of the motor will have an effect on the square wave going into the motor. And I'm too lazy to try to figure out the transfer function of a square wave with variable duty cycle, so I was just going to make a Simulink model of the whole electro-mechanical system and be done with it. Assuming I can find data on the frequency of the PWM output of the Vics, that is.

EDIT: This link claims the frequency is 120Hz. http://www.enigmaindustries.com/links.htm
Also, Victor 885? have these always been around or are we getting new Vics this year? I'll grant you they're overpowered for our applications, though.

Re: Victors Non-linear!!!
 

The 884 I checked is about six months old and it was defiantly 120 Hz. It's quite possible the frequency was changed at some point. I think the 883s were 2 kHz, but I've never actually checked one of those.

Re: Victors Non-linear!!!
 

I was not referring to the manufacturer provided sheets which we can obviously trust the voltages on. Though I can't seem to find it right now, there were some measurements taken by FIRST members with a tachometer, and I assume, a Victor for voltage control.

Re: Victors Non-linear!!!
 

I think what you're seeing is caused by low a PWM switching frequency. If a speed controller's PWM period is too long, longer than the current rise/fall time caused by inductance, you don't get the smoothing you would with a higher PWM frequency.

Best way to check if the duty cycle is linear would be to use an oscilloscope.

Re: Victors Non-linear!!!
 

<christmas carol>
All I want for Christmas is an Agilent O-Scope
</christmas carol>

I can't wait to get back to the lab after the Holidays! I'll come back with some readings and maybe some screen shots off our oscilloscope.

Re: Victors Non-linear!!!
 

I have a frequency counter with a duty cycle function as well as a scope. When i get home I will definately repeat my tests measuring duty cycle. I will also note the output frequency. Even if you assume that my pwm vs voltage graph is entirely bogus, that still doesn't explain the pwm vs speed graph which looks just like it. One of the less obvious reasons for the pwm vs. speed graph in the first place is that a motor under no load converges on a speed porportional to its input voltage. A PM DC brush motor that is under no mechanical load will act as a linear voltage to speed transducer and likewise a motor under no electrical load will act as a speed to voltage transducer. The second graph was partially to confirm the first.

Re: Victors Non-linear!!!
 

Greg et al,
The reference in Kevin's post: http://www.enigmaindustries.com/links.htm lists the 884 as 120Hz and the 883 as 2kHz, hence my confusion. (being around for a long time does get in the way.) Greg, are you in a position to check rise time where you are right now?

Jim,
(I am confused and keep calling you Rick) You tell us that the motor is unloaded but it is connected to the FP gearbox. That is a load. The numbers in your table suggest the large amount of friction present at low speeds in that FP gearbox. At low speeds, a motor still must overcome internal losses due to friction, brush pressure and magnetic influences. On the FP motor in particular, there are only three commutator segments which means three windings. Depending on the duty cycle, energy may only be supplied to the motor for a small portion of the brush/commutator cycle at low speeds. If analyzed, you may find that current is only flowing to one winding at a time instead of two. (two gives better torque/output/higer current) In contrast, the drill motor has several windings/commutators and the spec sheets illustrate the difference in these two motors. Ironically, the graph you displayed in your earlier post is a typical charge current graph of an inductor. This may indicate the effect of the duty cycle vs. inductance or not.