| Robots used in hazardous environments
need a high degree of mobility with good precision and robustness to actuator
failures. In this paper, a novel gear train is proposed to satisfy the requirement
for such a mechanism. The gear train is developed based on the following
principles: (i) the mechanism is omnidirectional for a high degree of mobility,
(ii) the mechanism uses only conventional tirewheels for high precision,
and (iii) the mechanism uses only three motors for no actuation redundancy;
at the same time, it has robustness to actuator failure so that when any
one of the motors is not functioning properly, regardless of which one is
not and regardless of the configuration at the moment of failure, the posture
is controllable with the other two working actuators. By virtue of the proposed
gear train, in an actuator failure, the entire structure becomes similar
to a differentially driven two-wheeled mobile robot subject to nonholonomic
constraints. The nonholonomic constraints, inherent in the mechanism and
stemming from an actuator failure, are crucial to maintain the controllability
of the robot posture when omnidirectional mobility is lost due to actuator
failure. Controllability is proved and control laws are presented for both
the omnidirectional mode, when all three motors are functioning, and the
non-omnidirectional mode, when one of the motors is locked due to a failure.
The omnidirectional mobility and robustness to an actuator failure is verified
by experimental results. |
