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Originally Posted by Max Lobovsky
I think this is actually good news for most FIRST applications. Power is, of course, proportional to voltage squared and this curve looks to be approximately 1/x^2 so that way, PWM values are close to proportional to power.
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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.
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Originally Posted by Gdeaver
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.
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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.
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Originally Posted by Kevin Sevcik
Erm, so I'm confused. I was under the impression that the Vics have a PWM output instead of a variable voltage output. As such, you can't really measure the output with a standard multimeter. This would be why Mike Betts was talking about checking it with a scope.
This has not bearing on if the RPM output is linear, however. I think I'll do a Simulink model tonight and see how RPM varies with duty cycle, if I can get enough data on motors and the PWM output.
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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).