User Manual

Page 48
Figure 58. Threadbreak sensor
Machine Monitoring
The fidelity of a shielded piezo film sensor in musical instruments led to the development of vibration
sensors for machines. In its simplest mode, piezo film vibration sensors behave essentially like dynamic
strain gages. The film does not require an external power source, yet typically generates signals greater
than strain gages after amplification. A typical piezo film sensor produces four orders of magnitude
higher voltage signal than a foil-type strain gage, and two orders higher than semiconductor types. The
frequency response of the piezo film strain gage is also superior.
The extreme sensitivity is due to the form of the piezo film material. The low thickness of the film
results in a very small cross sectional area. Thus very small longitudinal forces create very large stresses
within the material.
Piezo film sensors can be affixed to a vibrating surface and monitor the amplitude and frequency of the
vibrating structure. The sensors can cover larger areas than normal strain gages so any direct
comparisons should be performed in uniform strain fields for meaningful results. Obviously, point-
type transducers may be used where required, although the low capacitance of the small sensor area will
require additional consideration. Operation down to fractions of Hz can be achieved by either
conventional charge amplifiers or, since signal levels are relatively high, simple high impedance FET
buffer circuits.
Bearing Wear Sensors
A shielded piezo film sensor has been used to monitor bearings for wear and evidence of spall. The
sensors are permanently affixed to the outer surface of the bearing race with epoxy. The low mass and
thin profile allow its use as a built-in nondestructive testing sensor, rather than the time consuming use
of accelerometers for periodic fault-condition checks.
Fan Flow Sensor
A laminated beam type sensor is used in ducted airflow as a centrifugal fan failure sensor. The presence
of air flow is detected by the vibrations in the sensor caused by the turbulence of air flow at about 100
Hz. The absence of this signal is used for trigger electronics. The sensor and switching electronics are
based around a TL084 quad bi- FET op amp, with typical input signals of 80 mV. Sensor reliability is
the key feature. Since the sensor is subjected to virtually no operating stresses, it has an indefinite
working life.
Thread Break Sensor
Textile plants require the continuous monitoring of often
thousands of lines of thread for breakage. An undetected
break event can require that a large volume of material be
discarded, as the labor costs to recover the material exceed
the manufacturing cost. Drop switches, where switch
contact closure occurs when the thread breaks, are very
unreliable. Lint fouls the contact points, resulting in no
output signal. A piezoelectric film vibration sensor,
mounted to a thin steel beam, monitors the acoustic signal
caused by the abrasion of the thread running across the
beam, analogous to a violin string (Figure 58 ). The absence
of the vibration instantly triggers the machinery to stop.
Thread tension and defects in man-made fibers are also
monitored with piezo film textile sensors.