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4wheel skid-steer - weird behaviour

asked 2015-11-19 08:25:09 -0600

charlesfox gravatar image

I've set up a 4-wheeled skid-steer (differential drive) sim and am testing turn-on-the-spot. With a plugin I set both right wheels to go forward, velocity 1, and the left ones backwards, velocity 1.

Result is that for 90 degrees, the robot turns on the spot as expected. But the it goes in a straight line for a couple of meters, then turns another 90 degrees, and so on to make a kind of rounded-corners square path.

I've tried playing with surface friction params but not seemign to make much difference. Is this is known issue with 4wheel skid steer sims or could something else be wrong?

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answered 2015-11-19 13:01:15 -0600

hsu gravatar image

Which model are you using? If you share the model maybe someone on the forum can offer some insight.

A working example of in-place-rotation using Gazebo's built-in joint velocity controller can be found here (PID vel-in torque-out):

https://youtu.be/yRxu6QoS15Q

Using model from Robotnik with some parameter modification from here:

https://bitbucket.org/osrf/gazebo_mod...

Inserted into an empty world with Gazebo model insert tab.

This model uses friction pyramid with fdir1 specified. But now we have cone friction that might work as well (see https://bitbucket.org/osrf/gazebo/src... for example on how to enable cone model right now).

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I am looking at this again and can reproduce similar (but different) weird behaviours by starting with the standard Pioneer 3AT model from the gazebo library. The basic version of it works fine. But I get the behaviours if I place the front and back axles further apart, eg. 0.5m apart. I wonder, is this some fundamental property of skid steers , or perhaps it needs the friction param to be updated for each new configuratin? Can anyone suggest friction params for 1m spacing?

charlesfox gravatar imagecharlesfox ( 2016-01-06 09:35:29 -0600 )edit
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Asked: 2015-11-19 08:25:09 -0600

Seen: 1,630 times

Last updated: Nov 19 '15