• White, Nm
• Rakowski, R.
• Tudor, Mj
• Jones, B.
• Yan, T.
• Beeby, Steve

Abstract

Metallic resistive strain gauges are widely used in measuring devices for physical quantities such as load, pressure and torque. The gauges are bonded to the surface of the sensing structure at strategic points to obtain an appropriate level of strain. Typically in a load cell the strains at the gauges do not exceed 1500 microstrain at the rated load. With a four-gauge fully active Wheatstone bridge circuit, a nominal output signal is about 3 mV/V of bridge excitation for the maximum level of 1500 microstrain at the full load, based upon a gauge factor of 2. If the bridge excitation voltage is 10 V, which is determined by the gauge resistance, the gauge grid area and the heat-sink characteristics of the load cell material, the maximum output voltage of the bridge at the full load will be about 30 mV. Despite many favourable factors of the metallic resistive strain gauges, the limiting factors are that the output signals are quite low and very often the measurement accuracy is limited by the signal-to-noise ratio. Also the installation of strain gauges is normally labour intensive. Furthermore, to obtain a measurable output signal, the surface strain is usually designed to approach the proportional elastic limit of the sensing structure. For this reason strain-gauges-based load cells can seldom withstand overloads of more than double the rated full range load. Strain gauges have for many years been the primary sensors in the fields of measurement for load, pressure and torque. However, some instrument manufacturers of load, pressure and torque measurement devices have moved away from using resistive strain gauges. Since early 1980’s, Shinko Denshi Co. Ltd. has developed metallic resonant tuning fork balance and since early 1990’s, Avery Berkel and Weigh-Tronix (now Avery Weigh-Tronix) have developed quartz resonant tuning fork weighing scales, and Druck Ltd has developed silicon resonant pressure sensors. Further commercial developments are taking place to enhance device manufacturability, to enable wireless/batteryless operation of the resonant sensors, and to make measurement on stiff structures at much lower strain levels possible.<br/>A resonant sensor is a device with an element vibrating at resonance of which the resonance frequency is a function of the measurand. The output of a resonant sensor is a quasi-digital frequency signal, which does not require accurate measurement of the amplitude of the analogue voltage signal. The frequency signal is compatible with digital circuitry eliminating the need for analogue-to-digital conversion. The resolution achievable using a resonant sensor is much higher than alternative strain gauge sensors as the frequency can be measured with greater accuracy, for example the resonance frequency of the quartz tuning fork in watches is used as an accurate time base. Resonant sensors also have good long-term stability since the resonance frequency is not dependent on the amplitude of the electrical signals, but rather the mechanical properties of the sensor element. Resonator sensors often have a high mechanical quality factor (Q-factor), which leads to a high sensitivity and low power consumption. Resonant sensors have been made in a wide range of types, sizes and materials. This paper reports upon the development of metallic resonant sensors based on a triple-beam tuning fork structure with thick-film printed piezoelectric elements.

Topics

• impedance spectroscopy
• surface
• Silicon
• weighing