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Skid steer robot not turning

Sat Sep 15, 2018 5:05 pm

I'm trying to use a piconzero to drive a four-wheeled robot (two micro-metal motors attached to each connector) and whilst it can happily go forwards and backwards, I can't get it to turn. The code is looking ot just turn one side of the motors forwards and the others backwards. The wheels are just stalling.

Is there some magic formula for how far apart the wheels should be on each side compared to the width of the robot? Is the piconzero board just not up to the task? Is it the motors? The tyres? The flooring (wood)? Is there something just really simple I'm missing here that there's good instructions out there for (nothing obvious seemed to show up on a forum search).


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Re: Skid steer robot not turning

Sat Sep 15, 2018 5:13 pm

So you have code that can drive each motor forwards, and code that can drive each motor backwards?

But you can't make it turn by driving two motors on one side forward, and the other two motors backwards?

That sounds like a logical or implementation error in your code. But no-one will know for sure unless you share it....

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Re: Skid steer robot not turning

Sat Sep 15, 2018 5:22 pm

Sorry perhaps I didn't explain it well enough, or where I'd got to well enough.

If I take the robot off the floor then I can see the wheels turning as expected - one side going forwards and the other backwards. And it's the example piconzero example code I'm using, so I'm pretty sure the software side is ok. Which must mean it's down to my motor positioning (I believe).

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Re: Skid steer robot not turning

Sat Sep 15, 2018 5:33 pm

So that sounds entirely physical/mechanical.

You've got a solid, probably rectangular chassis? And you want one side to go forwards and the other side to go backwards.

Maybe the wheels/motors can't simultaneously drive longitudinally at the same time as skidding sideways?

I'm not a mechanical engineer, but I think slip-differential drives for road vehicles had to be invented to overcome this problem

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Re: Skid steer robot not turning

Sat Sep 15, 2018 8:43 pm

Sounds like its purely a mechanical or power problem.

It might be that your wheels a rubber and are gripping the surface so well they can turn and slip sideways.

your wheel spacing may be to great and so will not allow the chassis to turn.

lastly your power supply for the motor or the motors them selves my not have the power to turn both sets of wheels in opposite directions at the same time due to the high load under these conditions.

more details about the hard ware set-up and the power supply for the motors might help us locate the problem.

pictures are always good, up load them to a n image sharing site, imgur.com is the one I use and post links here.
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Re: Skid steer robot not turning

Sun Sep 16, 2018 1:05 am

Sounds like lack of torque output from motors.

It requires more torque to skid steer then just drive forward or backward without needing to break traction to skid steer.

These little micro-metal motors r preety low powered but I have used them for small skid steer robots successfully.

I did only have a quick look over the piconzero and didn't see the specs for forward voltage across the H-Bridge. All H-bridge to use up some power (i.e there will be a voltage drop across the H-Bridge). If you have a multi meter then measure the output voltage at the motor block terminal then look up the working voltage of the motors.

I make some guesses here. R u running the motors from the 5v of the RPi or do you have separate battery connected to the input terminal. If your using the 5v from RPi then allow for a little over 1v drop across the H-Bridge then you will be getting less than 4v at the motors and if the motors require 6v to drive well then this will explain your lack or torque to skid steer.

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Re: Skid steer robot not turning

Mon Sep 17, 2018 9:30 am

Can the robot drive the motors on a slippery (smooth wood, tile, etc) surface? If it can, but not on carpet or grass then you are slightly underpowered. If it cannot skid even on a smooth surface then you are way underpowered. Skid steering is the easiest to build mechanically, but is the least precise/controllable and does require high torque.
There is no magic length:width ratio that I know of, but in general the wider the robot in relation to its length the better. I have had 4 wheel robots which are slightly longer than square that cannot spin on the spot on carpet, but can on a smooth floor. As the wheel-surface grip improves they need to be steered more gradually with both motors going the same way but one faster than the other.
As has already been mentioned, look for a voltage drop across the motor controller. This will vary with the controller: L298 controllers are cheap and easy to get, but generally have at least a 1V drop. Better controllers are more expensive but more power gets to the motor.

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Re: Skid steer robot not turning

Mon Sep 17, 2018 10:53 pm

I'm agreeing with the other posters.

Either the gear ratio is much to low (too little torque, too much speed), or the overall mechanical setup is dodgy.

As far as I can tell, the chip which the piconzero uses (DRV8833) is pretty sound and will drive the motors as hard as they can go (with the power supply).

You should be able to drive one side forwards and the other back. If you can do that with the wheels off the ground, but not with the wheels on the ground, then they're not giving enough torque.

What gear ratio motors do you use? What supply voltage? what wheel size? How heavy is the robot?

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