Help: BBB Antweight Drive Kit

Hi, I hope this is the right place to ask:
I recently decided to get into bot building, and got the antweight drive kit from BBB.
I soldered on the motors, mounted it on a rather simple bottom plate and added an rx - and it mostly works,

  • I can drive forward in a straight line
  • I can turn left
  • I can turn right

However, if I pull back the throttle to drive backward… it drives backward, but also the drive doesn’t do the same speed, so it becomes a (quite tight) curve.

I’ve tried multiple different radios, with freshly created models, just to make sure I didn’t carry over any setting that might cause this, so I can rule out the radio.

I replaced the RX - just in case - and because I don’t know how I could test it any better (as I’ve never used RX with PWM outputs before). Issue persists.

now I have no experience with brushed motors, but my guess would be that its like with brushless ones: if an issue exists turning in one direction, it should exist when running the other direction too?

which leaves the BBB dual ESC - which sadly came without a manual.

Has anyone run into a similar issue? If so did you find a solution?

Does anyone have any documentation on the dual esc itself, information what firmware it is running (or better, if/where the source for the firmware is available), or really any advice on how to debug this any further without?

This sounds like it can be solved by using the trims on your transmitter. It’s pretty common and a usual part of setting up a robot.

A slight timing in the motors themselves, weight distribution and traction can all affect the handling. Very rarely will you get a robot that drives laser straight forward and backwards. Especially if it’s 2wd.

I always try and balance it out so you can get fairly straight all round. The way I set it up is by setting the robot up off its wheels and move the stick very slightly until a motor starts moving - normally one will start before the other. Then I’ll adjust the trim until they are both moving at the same pace, then trying it on the floor and seeing how it behaves. Rinse and repeat until satisfied.

thank you for taking the time to answer!

however, I feel you missed the part where I mentioned that driving forward is pretty straight, and the issue only occurs when trying to drive backward - if i’d try to trim for the issue, then I’d start not driving straight forward anymore.

This is what I was getting at with saying it is rare that it will drive perfectly straight forward and backwards. You can balance it out a little bit, try and compensate so it’s not so extreme in one direction. Yes you will lose a bit off the forward but if it stops you going in circles reversing it is normally worth it.

Other options could involve disabling the mixing, using dual rates to slow one side and running an additional mixer inline but that is pretty convoluted for the benefit.

Can you remove the mechanics from the question by removing the motors from the chassis and either removing the wheels or keeping them off the ground? They should turn roughly the same speed if you go straight forward, back, left or right (opposite directions of you’re going left/right). The problem with testing by driving is that, depending on the mechanics, you could have zero drive in reverse on one side but the other wheel will still drag it backwards and quite happily pivot.

There are two possible reasons I can see:

  1. The reverse side of the H bridge on one side has failed. In this case the motor in question will be fine forward but just won’t spin at all when you go back or turn towards that side
  2. The motor, gearbox, wheel or chassis on one side is binding up on something in reverse only, in which case once you take the problem out of the driving context it will work fine.

alright, further testing:

  • both wheels are definately turning into both directions
  • one of the two is running slower, while going backwards, on low-throttle, the higher the throttle climbs the more they run similar - meaning at full-throttle, i can almost drive backward in a straight line too

Hm, that sounds pretty normal unfortunately. There will always be some variation between motors, and they are generally offset to run better in one direction. My experience has generally been similar to what you’re describing, i.e., they run more consistently at high throttle.

You may have been unlucky there in terms of getting a bit more variation between your two motors but I’m not sure what you’re seeing is unusual. It’s something that may just have to be corrected by getting the feel for the driving…

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