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Originally Posted by Mike Betts
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I do not know why the Victors are non-linear. IFI claims that that they are linear ( see this) but this assertion is not backed up by experiment.
I have attached my data, taken with this year's kit frame, gearbox, CIM motors and wheels using the kit Hall effect sensors counting sprocket teeth with the frame on blocks (free speed except for bearing and gear friction).
However, this is getting off topic...
Regards,
Mike |
Mike,
Continuing your off topic comments...
I think I have an explanation for the data you have and it is consistent with IFI's claim of linear PWM output.
In my day job, we were breaking gear teeth on an actuator when the actuator was lightly loaded. No problem, we thought, we will just PWM the motor input and slow the actuator down. To our surprise, unless we got to very low PWM values, the free speed of our actuator stayed pretty much constant with respect to PWM value (we generated a chart very like the one you have shared with us).
Here is what we think we understand now: It boils down to the fact that we THINK that Vmotor = Vbatt X (PWM %) but that is only an approximation and it does not always hold true, especially if the PWM is the type that switches between Vbatt and OPEN CIRCUIT*, as my actuator driver and Victors do.
This is what happens as best as I can explain it. During the ON pulse, the motor accelerates (unless it is at its Vbatt Free Speed). During the OFF pulse the motor just coasts (recall that the motor is lightly loaded and that Newton's Laws apply: objects in motion remain in motion unless something slows it down). So... ...over successive ON pulses, the motor keeps accelerating until it reaches the Vbatt free speed.
The approximation of Vmotor = Vbatt * (PWM %) is much better for stall torque by the way.
Comments?
Joe J.
* There are other PWM types. For example the H-bridge could switch between Vbatt and Dynamic Brake (by holding both motor leads to Vbatt or both to Ground). Another example is that the H-bridge could switch between Vbatt and -Vbatt (in this case "off" is a 50% duty cycle which hits the motors with Vbatt for half the time and -Vbatt for half the time)
These other driver schemes are not as crazy as they may seem at first.
In fact, based on my non-FIRST experience with these types of drive schemes, I STRONGLY URGE IFI to offer these drive methods to FIRST teams in the future. They are useful in 2 ways. #1 If used for drive motors, the robot acts much more like a driver is commanding "speed" rather than commanding "gas" so the robot is easier to drive. #2 if used for large robot arms, the arms are orders of magnitude easier to control.
On the down side, they do pound more heat into the FETs but I'm an M.E. not a E.E. ;-) Bottom line, they make for well behaved robots. We should use them, even if they do stress the Victor a bit more. In my opinion, the Victors have the design margin in them. JJ