Motor Controller 29 with 12-bit PWM?

VEX V5 General Discussion General Forum
1

This might be a stupid question (and likely an @jpearman question), but would the Motor Controller 29 work with a 10 or 12 bit PWM signal (as opposed to the 8 bit PWM signal provided by the Cortex on all the 3 wire ports)?

So two questions…

  1. Would it work at all?
  2. Would I actually get the extra precision?

Thanks everyone (or most likely James)!

Also, bonus question:

  1. I’m struggling with this board. I think it can be used to power VEX motors, however Adafruit didn’t list a maximum input voltage or amperage. The datasheet seems to classify this as an LED driver not a servo control board and lists values that are way too small for the intended application. Given that the stall current of a 393 is almost 5A, and this board has 16 connectors, that means in theory the draw could be 80A.

So would using this board be like reasonable or completely crazy?

2

Don’t know about the first two, but for the last one, any Adafruit product for motors or servos is going to be based around the tiny ones they sell. I doubt that board would handle even a single 393 doing anything more than free running. I’d say completely crazy.

3

Yes, the pwm signal is really an analog signal, how you generate it and to what precision is not relevant.

I doubt it but it depends on how the MC29 measures the pulse width which I don’t know. Practically I don’t think you would get any benefit.

4

I mean, IDK, the MC29 handles the actual high current stuff. You’re basically just passing that current over the V+ and GND rails on this board, basically the board is just a wire. As long as the traces can handle the current flow, why wouldn’t that work?

5

You actually posted this… (see attachment).

Anyway this is the PIC in the MC29, it seems to have a 10 bit ADC soo… I’m guessing you’re just going to get 10 bits of precision out.

So @jpearman, is that Adafruit board going to cook or do you think I can use it to drive say 6 393’s?

MC29_anatomy.jpg
MC29_anatomy.jpg599×565 60.6 KB

6

You could use that board. V+ goes directly the the center pin on the motor/3wire connector, that would be unregulated battery. Vcc needs to be 5V or less and you need to pull that from somewhere else, don’t connect to the 7.2V battery. But why? What are you trying to do?

7

It’s nothing to do with the ADC. The pic will use a timer to measure the pulse width, whether that’s hardware or software I don’t know. The precision of the timer will theoretically determine the precision. Then the PIC needs to generate the actual motor PWM waveform, again timers are used, we don’t know how. It could be measured but I never have time for these experiments anymore.

8

Take over the world, as usual. :stuck_out_tongue:

I want to replace the Cortex with this and a RPi3, the RPi3 doesn’t have enough PWM’s to play with though.

Makes sense, no worries this is/was an odd question, I don’t expect you to spend any time on it. Thanks a bunch!

I already planned on having a 5V regulated source from the battery to power the RPi3, makes sense. Thanks for the warning.

9

Hey @jpearman,

While I have you here, do you have any idea why VEX used two different MOSFET’s on the MC29’s H bridge? I’m not an expert on H-bridges but that seems strange.

10

You use 4 FETs to create an H-Bridge, two N-channel and two P-channel. Somewhere I posted diagrams on the forum but I can’t find them right now. Scroll down through this page and you will see the explanation as to why.
http://modularcircuits.tantosonline.com/blog/articles/h-bridge-secrets/mosfets-and-catch-diodes/