Page 245 - Robot Builder's Bonanza

214 MOVING YOUR ROBOT

Holonomic drives are distinctive in that they allow motion in any direction, at any time.
They can go straight ahead, then suddenly move 90° sideways— all without changing the
orientation of the vehicle. A ball demonstrates holonomic movement: it can instantaneously
travel straight, then move in any direction of the compass.
The common trait of holonomic steering systems is that the robot is omnidirectional, able
to move in both the x and y directions with complete freedom. The most common form of
holonomic robot base uses three motors and wheels, arranged in a triangle.

How Omnidirectional Steering Is Achieved
How exactly does this work? In the majority of robots that use holonomic steering, the secret
is in the wheels. There are three wheels, each driven by a separate motor. Each wheel has
rollers around its circumference— wheels within the wheels, as shown in Figure 20- 6. The
rollers are at some angle to the main wheel. The rollers provide traction to the wheel when
the wheel is turning, but the rollers also let the wheel “slip” sideways to make turns.
The robot moves “forward” by activating any two motors; it turns by adjusting the speed
or direction of any and all three of the motors (see Figure 20- 7). These types of wheels were
originally designed for materials handling, as a substitute for conveyor belts, but recently
they’ve found new use in robot propulsion.

Other Forms of Omnidirectional Steering
Other forms of holonomic bases involve drive wheels that can each be independently rotated.
These go by various names, such as synchronized omnidirectional. The rotation of each

Figure 20- 6 An example wheel
used in a holonomic robot. Instead of Figure 20- 7 A holonomic vehicle steers by controlling the speed
a solid rubber tire, the rim of the and direction of all of its wheels. For example, it goes forward by
wheel is composed of several rollers equally powering two wheels and letting the third “drag.” It spins by
that are set at opposite angles to the equally powering all three wheels. Turns are achieved by altering the
rotation of the wheel. speed and/or direction of each wheel.

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