Abstract
Circular vibrator sensors for the measurement of atmospheric pressure, temperature and humidity by means of their natural frequency change have been developed. Uniform pressure over a circular plate supported at the edge causes the axisymmetric deformations to be a quasi shallow spherical cap, which results in the natural frequency change. The practical examples are shown in Fig. 1 where example(a)is for the atmospheric pressure measurement. For temperature or humidity, a similar deformation effect is achieved when bi-material arrangement is employed for the plate. The bi-material is composed of two thin layers with different thermal or humid expansion coefficient bonded together. That is, the higher expansion in one layer due to temperature or humidity change results in bend to form a quasi shallow spherical cap. In this case, the boundary condition at the edge must be other than "clamped". Examples(b)and(c)in the figure is for the temperature or humidity measurement. Example(b)is the case "simply supported" and example(c)"free". Two electrostrictive transducers are provided on both surfaces of the plate. One is to drive the flexural vibration while the other to receive. This is a sort of narrow bandpass electromechanical filters(See Fig. 3, 7 and 10). Proper connection of a reasonable amplifier between the transducers causes oscillation at the natural frequency of the sensor which changes with pressure, temperature or humidity. It is seen in Figs. 4 to 9 that change of the natural frequency is linear against pressure, temperature or humidity over relatively wide range, and the sensitivity is practically large enough. Sensors are suitable for felemetering and digital measurement applications. Theoretical estimation together with the experimental results is also discussed.
