Message thread on Webots Programming

e-puck simulation: wheels become disconnectedby Helen Rankin (2) 2017-03-15 18:12:51
We have been working with the e-puck robot in Webots. However, one thing myself and others have noticed is that over time, the wheels of the e-puck become disconnected as a result of impacting several objects. This is becoming quite a problem, as this can cause the e-puck to move in unexpected ways (or not at all).

This can happen with no errors in the console, but the problem is exacerbated if there are a few "ODE Message 3: LCP internal error, s <= 0 (s=0.0000e+00)" errors. The main observable impact is the robot movement and the wheels spinning strangely. I've observed the wheels even rotating along the wrong axis, like a coin being flipped.

To replicate:
Open the genetic algorithm sample world (in samples-> curriculum -> advanced_genetic_algorithm). Allow it to run through. You will see the fitness increase and rapidly drop off - this is not caused by the GA itself, but rather when the wheels start to become an issue. In extreme cases you may observe the wheels wobbling or appearing farther apart. In later generations the e-puck will not move, although the wheels will be spinning.

Have others encountered this? And does anyone know a viable solution?
Re: e-puck simulation: wheels become disconnectedby Fabien Rohrer (Webots PRO - 1261) 2017-03-16 09:38:02
Hi Helen,

We can indeed reproduce this annoying issue in our last version. I tried to workaround it modifying simply the supervisor, but I didn't found a simple solution.

=> We will do our best to fix this important bug for the next release and keep you informed here.

Meanwhile, you could think about implementing the GA a layer above, and use Webots as a slave from a higher level script. I mean, to create one Webots instance per fitness evaluation. This would allow you to get rid of the supervisor functions to reset the robot position.

Fabien Rohrer
Re: e-puck simulation: wheels become disconnectedby Tom Norton (Webots PRO - 59) 2017-03-30 14:56:33
Dear Helen,

I can confirm this issue is resolved and the fix will be included in our next release.


Tom Norton
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