Page 1086 - The Mechatronics Handbook

FIGURE 39.3 Top level fault tree for a three-branch parallel manipulator.

the failure of the parallel manipulator, unless the device is redundantly actuated/sensed or has a redundant
unsensed joint(s) for the given task.
The common failure modes of active, passive sensed, and passive unsensed joints are the joint break
and joint jam. The only failure modes of passive unsensed joints are the common joint failures. The
failure modes of passive sensed joints include sensor failure, in addition to the common failure modes.
In this case, the motion of the joint cannot be measured and the joint will be reduced to a passive
unsensed joint. The major failure modes of active joints could be classified as actuator failure, transmis-
sion failure, and sensor failure. As a result of an actuator failure, the active joint degrades to a passive
sensed joint, provided that the joint is back drivable; otherwise, the joint must be locked and the
corresponding branch and the parallel manipulator will lose one DOF and an actuation. Because of a
transmission failure, the actuator fails to drive the joint, and the active joint could only be used as a
passive sensed joint. When the sensor of an active joint fails, although the actuator may operate properly,
the motion of the joint cannot be controlled as there will be no reliable information available on the
joint motion; hence, the active joint is degraded to a passive unsensed joint.
Subsystem Failures
The branches of a parallel device could be categorized as active or passive branches. An active branch
possesses at least one active joint to provide a required force and to facilitate a suitable loci for the branch
end location. A passive sensed branch has at least one sensed joint and its main function is to constrain the
loci of the branch end position. Neither joint of a passive unsensed branch is sensed, and the branch is
mainly used to constrain the motion of the mobile platform.
A branch of a parallel manipulator could fail because of component (link, joint) failures. As well, a
branch will not follow its assigned path if it is in the workspace boundary, or at an internal singularity
(where it loses one or more DOF). Therefore, the mechanical failure modes of a branch include branch
break, loss of DOF, and loss of sensing/actuation.

Mechanical System Failures
A parallel manipulator could fail because of component and/or subsystem failures. Therefore, the
mechanical failures of a parallel manipulator include loss of the DOF, loss of the actuation, loss of the
motion constraint, and uncertainty configurations. A summary of the mechanical failure levels, modes,
effects, and causes of parallel manipulators has been tabulated in Table 39.1.

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