Page 1085 - The Mechatronics Handbook

The sensor noise could be taken care of by statistical analysis on sensor readings. The uncertainties in
the system model could be taken care of by estimating the effect of parameter uncertainties and compen-
sating for it in the FDI system, or by minimizing the sensitivity of the FDI system to these uncertainties.
The detection scheme should also be capable of monitoring the degradation of the system, as well as
evolution and progress of failure over time (and predicting the failure), and responding to each accordingly.

39.6 Example Mechatronic System: Parallel
Manipulators/Machine Tools

Parallel structured machine tools consist of multiple serial branches/legs acting in parallel on a common
mobile platform with the spindle being connected to the mobile platform. Parallel manipulator-based devices
have the advantages of not requiring actuation of base distal joints and of having their active joints acting
in parallel on the mobile platform. These advantages can lead to parallel machine tools having desirable
stiffness, accuracy, and dynamic characteristics, which, in turn, will provide high material removal rate
(high product volume) with tight tolerances and in-process inspection capability (on-machine measure-
ments of workpieces, fixtures, and tools during and after manufacturing process without breaking setups).
The failure analysis of parallel machine tools should include failures of parallel architecture, as well
as failure of cutting tool, in addition to software failures.

Parallel Architecture Manipulators (Based on a Paper by Huang
and Notash, 1999)

The following discussion will focus on the design orientated failure analysis of the mechanical system of
parallel manipulators/machines.
Parallel manipulators consist of a base platform (stationary link), a mobile platform (end effector),
and multiple branches/legs connecting the base and mobile platforms. Figure 39.2 depicts an example
of a six-branch parallel manipulator.
The mechanical failure modes of manipulators could be classified as joint failure (component), link
failure/breakage (component), branch failure (subsystem), end effector failure (subsystem), and device
failure (system). Figure 39.3 represents the top level FTA of a three-branch parallel manipulator/machine.
Component Failures
Parallel (closed-loop) manipulators possess both active joints (joints that are sensed and actuated) and
passive joints (unactuated joints which could be sensed or unsensed). Therefore, their failures could be
due to the failures of active, passive sensed, or passive unsensed joints. The failure of any joint will cause

FIGURE 39.2 Example of a six-branch parallel manipulator/machine.

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