Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

196 Chapter 6 Steering History
Figure 6-7
Synchronous drive
mechanism must be able to rotate the drive wheel in either direction as
much as is needed. This requires an electrical slip ring in the steering
joint. Slip rings, also called rotary joints, are manufactured in both standard
sizes or custom layouts.
One type of mechanical solution to the problem of powering the
wheel in a drive/steer module has been done with great success on several
sophisticated research robots and is commonly called a syncrodrive.
A syncro-drive (Figure 6-7) normally uses three or four wheels.
All are driven and steered in unison, synchronously. This allows fully
holonomic steering (the ability to head in any direction without first
requiring moving forward). As can be seen in the sketch, the drive
motor is directly above the wheel. An axle goes down through the center
of the steering shaft and is coupled to the wheel through a right angle
gearbox.
This layout is probably the best to use if relying heavily on dead reckoning
because it produces little rotational error. Although the dominant
dead-reckoning error is usually produced by things in the environment,
this system theoretically has the least internal error. The four-wheeled
layout is not well suited for anything but flat terrain unless at least one
wheel module is made vertically compliant. This is possible, but would
produce the complicated mechanism shown in Figure 6-8.