Robo-Rats Locomotion: Skid-steer Drive


Skid-steer locomotion is commonly used on tracked vehicles such as tanks and bulldozers, but is also used on some four- and six-wheeled vehicles.  On these vehicles, the wheels (or tracks) on each side can be driven at various speeds in forward and reverse (all wheels on a side are driven at the same rate).  There is no explicit steering mechanism--as the name implies steering is accomplished by actuating each side at a different rate or in a different direction, causing the wheels or tracks to slip, or skid, on the ground.  

In the above left figure, the wheels on the left side are driven forward and the wheels on the right side are driven in reverse at the same rate.  The result is a clockwise zero radius turn about the center of the vehicle shown in the right figure.

Note that throughout the turn the wheels are required to skid on the ground, with the front and rear pair of wheels skidding more than the center pair.  Skidding has some disadvantages including tire/track wear but for tracked vehicles there is no alternative.  (Vehicles that use skid-steer usually are off-road types such as construction equipment and tanks--the reduced friction of a non-paved surface helps to reduce tire/track wear.)  In the "real world" these disadvantages are offset by the simplicity of the drive system.  However, in the "robot world" skidding is a severe disadvantage because of the negative effect it has on odometry: wheels that are skidding are not tracking the exact movement of the robot.  Since odometry is a very important sensor for position determination, skid-steer is not commonly used on robots with sparse sensing (no video cameras or sonar) that require accurate position determination (such as finding mapped "food items" in the Robo-Rats competition).

Skid-steer is closely related to the differential drive system, replacing the caster wheel with extra drive wheels. It has the same disadvantage: moving in a straight line requires the wheels on each side to be turning at the same speed, which can be difficult to achieve.  The advantage of skid-steer is increased traction and no "caster wheel effect".

Below is a photo of a commercially available 4-wheel skid-steer robot from ActivMedia:

Motors:

2 - One for each side of the robot.

Pros:

Simplicity - No explicit steering mechanism.

Traction - Multiple drive wheels on each side gives greatly increased traction, especially on rough terrain (even greater for tracked vehicles).

Cons:

Control - Straight-line travel can be difficult to achieve (see comments on differential drive).

Odometry - Skidding causes wheels to lose contact with the ground which means odometry sensors cannot accurately track the position of the vehicle.


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Last modified: 04/04/01 22:30