Arduino PWM output

[slijin]

Originally Posted by sand500

I have a VEX Motor Module(3-wires) that our teacher had from a Vex Kit. I have the Arduino to slowly rise the PWM from 0-255 and then lower it down. What I have found is that the motor starts moving when the PWM is at around 40. I know that around 120, the motor starts slowing down and at 170, it stops completely. Once it is at 200, the motor starts again but in the opposite direction, slow at first but gets faster around 250. At 255, the motor stops completely and at 254, it starts again. Again the motor doesnt move when the PWM is between 200 and 170 and after that, it turns again in the original direction and stops at 40.

I dont know why this is happening but the duty cycle frequency was wrong, then why does it stop completely at 255? When I hook up the pwm and ground on the arduino to a regular dc motor that you would find in an electronics kit, the motor works just like how pwm should.

I'm not too familiar with the VEX system, so take what I say with a grain of salt, but here's what I see happening:

The VEX motor module has built-in deadbands at 1) the neutral, which allows a user to feed a noisy joystick signal straight to the motor and 2) the ends, which either acts as a safety, to facilitate interfacing with the VEX system (due to hardware/programming constraints etc), or it was a design decision.

Hooking up this signal to a regular DC motor produces exactly that kind of behavior, not because it's how your signal "should" behave, but because it's how motors are expected to behave in response to such signals.

[sand500]

Originally Posted by slijin

I'm not too familiar with the VEX system, so take what I say with a grain of salt, but here's what I see happening:

The VEX motor module has built-in deadbands at 1) the neutral, which allows a user to feed a noisy joystick signal straight to the motor and 2) the ends, which either acts as a safety, to facilitate interfacing with the VEX system (due to hardware/programming constraints etc), or it was a design decision.

Hooking up this signal to a regular DC motor produces exactly that kind of behavior, not because it's how your signal "should" behave, but because it's how motors are expected to behave in response to such signals.

Why does the higher end of the pwm, have a smaller range, 200-255, while the lower end of the pwm has a bigger range, 40-170. When I was running the motor, I could see the motor hit a higher speed in the 40-170 range than in the 200-255. So why would a motor rotate faster in one direction than in another direction?

[Gdeaver]

Use the servo library.

[slijin]

Originally Posted by sand500

Why does the higher end of the pwm, have a smaller range, 200-255, while the lower end of the pwm has a bigger range, 40-170. When I was running the motor, I could see the motor hit a higher speed in the 40-170 range than in the 200-255. So why would a motor rotate faster in one direction than in another direction?

Again, purely hypothetical here:

There are multiple reasons that this could happen, both electrical and mechanical.

On the electrical side, it could be an intentional design point (unlikely). It could also be that it's designed to take a different type of pulse (also unlikely). The most probable reason is an amalgam of 1) manufacturing inconsistencies and 2) a design "flaw" stemming from the circuit design (see the discussions about Victor and Jaguar signal driving; I can't do an explanation of these mechanisms justice, nor will I attempt to).

On the mechanical side, it could be a damaged bearing/bushing or shaft that gunks up in one direction and offers no resistance in the other (almost like a ratchet).