Page 238 - Sensors and Control Systems in Manufacturing

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for measuring load vibrations via current fluctuations. Motor current
signature analysis uses many of the same techniques as vibration
analysis for interpreting the signals. But motor current signature
analysis is nonintrusive because motor current can be measured any-
where along the motor power cables, whereas a vibration sensor or
transducer must be mounted close to the machine element of interest.
The limited bandwidth of the signals associated with motor drive sig-
nature analysis, however, may restrict its applicability.

4.7.4 Acoustics
A qualified operator can tell from the machine-generated noise that a
fault is developing. Consequently, It is natural to extend this concept
to automatic diagnosis. The operator has access to subtle innate pat-
tern recognition techniques, and thus is able to discern sounds from
myriad background noises. Any diagnostic system based on sound
would have to be able to identify damage-related sounds and separate
the information from the ambient noise. Acoustic sensing (looking for
sounds that indicate faults) is a nonlocal noncontact inspection
method. Any acoustic technique is subject to outside disturbances,
but is potentially a very powerful tool, provided that operating con-
ditions are acoustically repeatable and that the diagnostic system can
effectively recognize acoustic patterns.

4.7.5 Temperature
Using temperature as a measurement parameter is common, particu-
larly for equipment running at high speed, where faults cause enough
waste heat to raise temperature significantly. This method is gener-
ally best for indicating that a fault has occurred, rather than the
precise nature of the fault.

4.7.6 Sensors for Diagnostic Systems
Assuming an automated diagnostic system is required, the necessary
sensors are normally mounted permanently at their monitoring
sites. This works well if data are required continuously, or if there are
only a few monitoring locations. However, for those cases where
many sites must be monitored and the data need not be continuously
received during operation of the flexible manufacturing cell, it may
be possible to use the same sensor or transducer, sequentially, in the
many locations.
The robot is well-suited to gathering data at multiple points with
a limited number of sensors and transducers. This would extend the
mandate of the robot from simply moving workpieces and tools
within the flexible manufacturing cell (for production) to include
moving sensors (for diagnostic inspection).
Within the flexible manufacturing cell, a robot can make measure-
ments at sites inside its work space by taking a sensor or transducer