Sensor Technology Ltd

Dynamic rotary torque measurement using surface acoustic waves

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Courtesy of Sensor Technology Ltd

The subject of torque measurement has previously been addressed extensively. Most of the existing transducers either rely on low voltage analogue systems (e.g. conventional resistive strain gauge) or complicated mechanical assemblies by physically assessing displacement, i.e. optical systems. This paper identifies a new, potentially low cost, non-contact, frequency domain strain torque sensor1 utilising SAW (Surface Acoustic Wave) Technology for surface strain measurement. The sensor has a short axial length making it flexible in terms of integration into a variety of applications, encompassing both static and dynamic measurement. The paper presents a technical description of the resulting transducer, regarding its operation and construction.

Background
The siress-slrain relationship is fundamental lo the study of the mechanics of materials and is used to measure many physical quantities such as axial force, bending moment, torque, pressure, acceleration and temperature. In order to practically assess the stress state in a specimen it is imperative to measure the strain. Simple computation' of the strain, along with material properties, provides a method for determining the stress state. Effective practical measurement of strain has long been a problem. The following is a summary of the existing methodologies for strain assessment.

Mechanical devices evolved as the tlrst type of strain gauge e.g. extensometer, which relies on the displacement of levers to indicate the strain. Another example is that of the photoelectric strain sensor, which utilises the displacement of light passing through gratings separated at a set distance; sensing of variation in light intensity by photocells provides a signal indicative of the strain. On the whole, such methods are bulky, difficult to use and mostly limited to static strain analysis.

Optical techniques, such as photo-elasticity, holography or Moire* methods for strain analysis prove to be both accurate and sensitive. 1 lowever, the apparatus and intricacy of the optical processes generally restricts the practise of such methods to specially prepared laboratories.

Existing electrical devices for measuring strain can utilise any of the following methods; capacitive, inductive, piezoelectric, resistive and piezo-resistive techniques can be identified as the main contributors to the solution of practical strain measurement. However, all rely on the interrogation of low power density analogue signals, highly susceptible to amplitude-modulated noise.

Of all the techniques described, resistive strain gauges have emerged as the dominating technique for strain measurement. The gauge is generally bonded onto the specimen and provides a low cost solution with the following characteristics: short gauge length, small physical size, small mass, moderate signal error resulting from temperature fluctuations and capability of measuring both static and dynamic strain.

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