Measuring and testing – Dynamometers – Responsive to torque
Patent
1992-10-15
1996-12-17
Chilcot, Richard
Measuring and testing
Dynamometers
Responsive to torque
73862321, 73DIG4, G01L 302
Patent
active
055855717
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
This invention relates to a method and apparatus for measuring strain arising from stress. Many applications call for strain measurement including static and dynamic loading of structures and components and for the subsequent derivation of information from such measurement. The present invention is particularly, but not exclusively, concerned with the measurement of dynamic torque arising when power is transmitted by way of a rotating shaft.
TECHNICAL FIELD
For the measurement of static and dynamic strain conventional resistive strain gauges are widely used. They are characterised by resistive elements incorporated into bridge circuit so that changes in resistive elements due to stress in components to which at least some of the elements are secured are quantified by way of the bridge circuit.
Torque Measurement
A resistive strain gauge can be used for measurement of torque transmitted by a shaft to which the gauge are securely attached. Slip rings on the shaft are used to feed signal inputs to and to recover signal outputs from the gauge. Changes in the geometry of the gauge arising from twisting of the shaft during torque transmission are monitored by way of the slip rings. However the use of slip rings has a number of disadvantages. Brush drag on the slip rings produces errors which are significant for measurement of signals representing lower torque values and drag can vary with the friction conditions and as wear occurs. Inertia effects limit the acceleration to which the shaft bearing the slip rings can be subjected. The rings and brushes generate electrical noise.
The measurement of torques can extend from on the one hand the very small torques arising from viscosity associated with the use of small scale laboratory mixers to, on the other hand, the very large torques occurring in transmission shafts of aero and marine propulsion units. It has been found that in general slip ring systems are not readily applied to the measurement of torque of less than about 2 Newton meter. Typically the diameter of the gauge section of a shaft becomes so small that insufficient area is available to provide for the mounting of strain gauge of suitable size and the size of the electrical noise generated results in a signal to noise ratio which prejudices effective use. At the other extreme on very large shafts the use of slip rings and the associated equipment can lead to access and housing problems.
As an alternative to resistive strain gauges optical torque transducers have become available for the purposes of shaft torque measurement. In an optical transducer radially extending segmented gratings are mounted on the shaft. Output from an array of light sources passes through the gratings to fall on a bank of photocells. The signal output from the photocells varies directly with the torque applied to the shaft. Accuracy of measurement is unaffected by the shaft speed or the torque range. The intensity of the light sources can be monitored to ensure that photocell output does not vary due to light source variation.
Currently these optical/electronic gauges are widely used for torques extending in ranges from 0 to 10 milli Newton meters to 0 to 5000 Newton meters and for rotational speed between 0.5 to 30000 rpm. While the technical advantages of the optical electronic systems are substantial they are costly. In particular the segmented gratings are expensive to manufacture. In addition a periodic need to replace lights in the array imposes a design constraint inasmuch as the gauge needs to be accessible for maintenance and calibration.
BACKGROUND ART
In what follows reference is made to a `surface acoustic wave resonator`. Such a resonator is made up of a microstructure deposited on a piezoelectric substrate. The microstructure is formed by at least one pair of interleaved comb-like (`interdigitated`) electrodes deposited as a thin metal conducting layer on the substrate. FIG. 15 shows a basic model of a surface acoustic wave device 10 having input electrode 11 interleaved with output electrode 1
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patent: 3878477 (1975-04-01), Dias et al.
patent: 4096740 (1978-06-01), Sallee
patent: 4100811 (1978-07-01), Cullen et al.
patent: 4107626 (1978-08-01), Kiewit
patent: 4573357 (1986-03-01), Meunier
patent: 4623813 (1986-11-01), Naito et al.
Radio, Fernsehen Elektronik, vol. 34, No. 8, Aug. 1985, (Ost-Berlin, DE), R. Thoma et al.: "Sensoren auf der Grundlage akustischer Oberflachenwellen", pp. 480-483.
Lonsdale Anthony
Lonsdale Bryan
Chilcot Richard
Dougherty Elizabeth L.
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