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[DPRG] Odometry and backlash

Subject: [DPRG] Odometry and backlash
From: steve at txtulip.com steve at txtulip.com
Date: Mon Jan 5 09:22:23 CST 2015

Marcus,

         You determined that there is backlash in your gearbox and you are correct saying that CNC compensates for this, but that is not a complex problem. You determine how much error there is and just subtract that amount of motion anytime you change directions. Depending on how accurate you are trying to be, might make this an unmanageable problem. If you are trying to put a straw in a soda bottle, 1 degree is disastrous over a football field but if you're trying to hit a wide reciever in the endzone... the guy ought to be traded if he can't catch that thing. You could mostly solve this problem by using odometry as a rough estimate and, I think Doug suggested this, using vision to determine how much error you have and reset your location accordingly. MY personal suggestion is to drop a wheel attached to an encoder and read that instead of the motor. reading a motor doesn't allow for slippage of the wheels but "reading the ground" could give a more accurate distance provided it stays in contact with the ground. All gearboxs will develop backlash due to wear. I used to work on pen plotters and the really big ones had a spur gear system. Eliminating backlash from wear was part of the repair procedure. There was a mechanism build in for that but I doubt you can do that with a regular gearbox.
   I AM surprised you can't just mathematically compensate for that backlash but again, it depends on how much accuracy you are trying to get and how far you are traveling before resetting your counter.

Steve

 

 

 

-----Original Message-----
From: David Anderson <davida at smu.edu>
To: dprglist <dprglist at dprg.org>
Sent: Mon, Jan 5, 2015 12:13 am
Subject: Re: [DPRG] Odometry and backlash


Hi Markus,

It would be good to know exactly where the problem lies.   How have you 
determined the rather large variance in calibration runs?  Was this done 
using the UMBMark?   Large left-hand and right-hand squares?

Is the large variance observed in  the stopping point of a series of 
similar, say, left-hand runs?   Or is it between the left-hand and 
right-hand runs?  Those are different problems.

~1 degree slop in the gear train is probably manageable, depending on 
how you are doing the navigation.   If it is a single calculation at the 
start of a run or line segment, then ~1 degree theta error at the start 
will throw the robot way off in X,Y at the end.   If instead the 
odometry navigation is a continuously running calculation as the robot 
travels, then ~1 degree of error becomes less and less significant as 
the robot approaches its target, and basically insignificant when the 
robot is at the target.


Don't give up on your drive-train yet. :)

dpa





On 01/04/2015 02:21 PM, Markus Lampert wrote:
> Hi,
>
> I am adding odometry to one of my robots and found a rather large variance in my calibration runs. As it turns out my gear motor has some backlash resulting in ~1 degree of heading "wiggle room". The quad encoder sits on the motor shaft, not on the wheel itself which means they don't pick up on the backlash at all.
>
> I could not find any methods for backlash compensation other than for CNC machines, which doesn't seem to translate to mobile robots.
>
> Is there a method to compensate for it or am I in the business of finding a 'better' drive train for my robot?
>
> Thanks in advance,
> Markus
>
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