Page 138 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

Live Reckoning

The worst configurations for odometry are skid-steered vehicles that have a high degree
of coupling between driving and steering, and also depend on slippage between points of
contact and the ground

A tank, for example, has no steering as such, but depends on driving its two treads at
different speeds to accomplish steering. If one tread has better purchase than the
other, the intended amount of turning may not occur. The same is true for wheeled
2
vehicles that use skid steering and differential drive .
A vehicle using Ackerman steering (e.g., a common passenger car) has a steering me-
chanism that is independent of the drive mechanism, but depends on the vehicle
moving forward (or backward) while the steering is deflected before the steering has
an affect on its heading. If the car’s drive wheels slip, and it does not move forward
the amount indicated by its odometry, then a heading error will occur. For this
reason, Ackerman systems are somewhere between Synchro-drive vehicles and skid
steered vehicles in the quality of their inherent odometry.
The choice of a drive system may have many other considerations besides its inher-
ent odometry quality. If the system does not have good inherent odometry, then it is
essential to supplement the odometry with the appropriate sensors. One of the most
common sensors for this purpose is the gyroscope. Optical gyroscopes are now available
that can provide the vehicle with an excellent sense of direction. Unfortunately, the
cost of an optical gyroscope is usually only reasonable in higher-end applications.

Picking coordinate systems

A robot will normally use two coordinate systems, one for global reference and one
for local reference. The global reference system is concerned with where the robot is
in its environment and it has no inherent center. For this reason, global systems are
almost always Cartesian.

The local reference system, on the other hand, is used to integrate data from sensors
into the robot’s sense of things. This process involves vector math, and is inherently
centered on the robot. Therefore, the natural coordinate system for this purpose is a
polar system.

2 Skid steering uses two or more powered but non-steered wheels on each side of the vehicle.
Differential drive uses two powered wheels on either side of the platform with the other wheels
being casters.

121