Sensors and Methods for Mobile Robot Positioning

28 Part I Sensors for Mobile Robot Positioning
Truck A
Castor
Castor
Drive
wheel
Drive
wheel
Drive
wheel
Truck B \book\clap30.ds4,
Drive
wheel
clap30.wmf, 07/19/95
Figure 1.15: The compliant linkage is
instrumented with two absolute rotary
encoders and a linear encoder to
measure the relative orientations and
separation distance between the two
trucks.
Figure 1.16: The University of Michigan's MDOF vehicle is a dualdifferential-drive
multi-degree-of-freedom platform comprising two
TRC LabMates. These two "trucks” are coupled together with a
compliant linkage, designed to accommodate momentary controller
errors that would cause excessive wheel slippage in other MDOF
vehicles. (Courtesy of The University of Michigan.)
1.3.8 Tracked Vehicles
d min
d max
Track
footprint
Figure 1.17: The effective point of contact for a skid-steer vehicle is
roughly constrained on either side by a rectangular zone of ambiguity
corresponding to the track footprint. As is implied by the concentric
circles, considerable slippage must occur in order for the vehicle to
turn [Everett, 1995].
Yet another drive configuration for
mobile robots uses tracks instead of
wheels. This very special implementation
of a differential drive is
known as skid steering and is routinely
implemented in track form
on bulldozers and armored vehicles.
Such skid-steer configurations
intentionally rely on track or wheel
slippage for normal operation (Figure
1.17), and as a consequence
provide rather poor dead-reckoning
information. For this reason, skid
steering is generally employed only
in tele-operated as opposed to autonomous
robotic applications, where the ability to surmount significant floor discontinuities is more
desirable than accurate odometry information. An example is seen in the track drives popular with
remote-controlled robots intended for explosive ordnance disposal. Figure 1.18 shows the Remotec
Andros V platform being converted to fully autonomous operation (see Sec. 5.3.1.2).