Motor Bias (or whatever it's called)

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bitternboy
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Motor Bias (or whatever it's called)

Post by bitternboy »

Hey, throughout my time making robots I've been plagued by a problem where drive is biased to one side when driving forward and to the other when reversing. Only recently did I find out this was caused by mismatched motors when Rory kindly explained it to me. A quick google only provided fixes involving rotary encoders which is beyond the tech that's ever been used in ants and redistributing the weight (as Rory suggested) isn't always an option. My main confusions are a) why has this been particularly bad for robots I've built and b) is there a way to tune the motors in some way to prevent this?
I was hoping someone could shed some light on this for me :-?
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haz
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Re: Motor Bias (or whatever it's called)

Post by haz »

Trims can normally solve this.
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bitternboy
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Re: Motor Bias (or whatever it's called)

Post by bitternboy »

They can solve it in one direction but just make it worse in the other direction
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Remote-Controlled Dave
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Re: Motor Bias (or whatever it's called)

Post by Remote-Controlled Dave »

If you have a posh transmitter you can probably use some of the finer points of mixing to solve it. I tend to just adjust my driving style though. Very few of my 2WD bots go entirely straight so I tend to correct for motor bias on the joystick.
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Shakey
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Re: Motor Bias (or whatever it's called)

Post by Shakey »

Whenever I have motor bias I just trim it so forwards is straight. One of flippers is great going forwards but can't reverse in a straight line to save it's life but it never really matters.
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Rapidrory
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Re: Motor Bias (or whatever it's called)

Post by Rapidrory »

*could be caused by mismatched motors; I'm yet to have actually done much research into the problem, but it sounds possible(mismatched as in slightly different build tolerances in the motors, not different types of motor). Trim will fix it so it drives forward straight by correcting the bias of the motors until they match, however when you reverse the motors, the mismatch between them would be accentuated by the trim, causing it to turn. The same effect is caused if you have more weight on one side with a 2wd robot, as the offset weight will cause more drag on one side meaning that motor has to be driven slightly harder. If you use an ESC with mixing built in, then things such as tweaking the travel adjust has no effect as it adjusts both motors equally.

Though having said that, I made a front wheel drive robot once which would only drive straight in reverse and not forward, no matter what I did with the trim. This seemed to be due to having the center of mass far behind the driving wheels, so conversely if you had the center of mass far in front of the driving wheels (as in a nose heavy rear wheel drive robot), you'd be able to drive straight forwards but not backwards.

Basically, try using the trim to see if you can adjust it to drive straight in reverse (ignoring what forwards is doing for now). If you can get it to drive straight, then the what's in first paragraph's the problem, if not then the second paragraph is.. If you can work out which then we can come up with a solution based on that... possibly :L

Have you got a photo of one of your problematic bots? Might give a better idea of possible problems..
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bitternboy
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Re: Motor Bias (or whatever it's called)

Post by bitternboy »

I'll put a photo up when I get back. On a side note, I do get a sense of deja vu as the worst case 've ever had of this was on a 4 wheel pushbot called Robot of the Opera back in 2010. I started a thread about it and proceeded to rant and despair about it in my angsty 15 year old way. --> http://robotwars101.org/forum/viewtopic.php?f=10&t=1310
Nobody knew why it was doing it, so I replaced every component except the motors and ESC and swapped the motors around a bit but it didn't work. Sent the ESC back to technobots and they said they couldn't find anything wrong with it.
Convinced it's the same problem :-?
Jonathan Atkinson
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Re: Motor Bias (or whatever it's called)

Post by Remote-Controlled Dave »

Side note - Robot of the Opera was ace.
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peterwaller
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Re: Motor Bias (or whatever it's called)

Post by peterwaller »

This has come up a few times without being fully sorted.
One thing I find helps is not just setting the trim so both motors are stationary as most controllers have a dead zone.
The trick is to adjust the steering trim so that both motors start at exactly the same time.
Thinking about it because the motors are facing in opposite directions one motor is running foward and the other in reverse for forward drive.
While motor timings are set to try to ensure motors run at equal speed in either direction that may not always be the case.
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Re: Motor Bias (or whatever it's called)

Post by Rapidrory »

On a similar note: if you steer slightly, one wheel will usually start turning before the other. If you set the stick so it's just starting to steer, you can then adjust the forwards and reverse trim to get both motors starting at exactly the same time, allowing you to turn on the spot even at low speeds. Another good trick I find is to put your throttle stick full forward, then wind back the travel adjust until the motors just start to change pitch (aka, slow down). If you repeat this for reverse and steering then it gives you better control without affecting top speed.

None of this is particularly relevant to solving the problem of reversing straight, but hopefully it'll help a bit :L
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NanoTwo Motor Controllers: https://nutsandbots.co.uk/product/nanotwodualesc
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