Fluid jet angular velocity sensors utilizing sensing elements for sensing the speed of rotation are well known in the art. U.S. Pat. No. 3,500,690 to Schuemann, U.S. Pat. No. 4,020,700 to Lopiccolo et al., and U.S. Pat. No. 3,581,578 to Schuemann, all disclose fluid jet angular velocity sensors having a pair of sensing elements for sensing the speed of rotation about an axis perpendicular to a "plane of sensitivity".
The sensing elements are usually positioned symmetrically about a reference jet axis with each element on opposite sides and at equal distances therefrom. A fluid jet is directed along the reference jet axis from a nozzle which cools the sensing elements in substantially equal proportions in the absence of sensor rotation. Due to the well-known Coriolis effect, the fluid jet impinges nonsymmetrically, i.e., the fluid jet "bends" in the presence of sensor rotation. Because of the well-known characteristic of fluid jets in which the higher velocity fluid particles are concentrated at the center of the jet and the lower velocity particles around its periphery, the sensing elements are cooled in different proportions whenever the fluid jet impinges nonsymmetrically upon the sensing elements.
One source of unrepeatability in prior art angular rate sensors is caused by the basic properties of the piezoelectric material (PZT) used to construct the pump diaphragm. The PZT material is subject to temperature hysteresis. This shows up as a change, for example, in the pump impedance (and hence in the flowrate) at room temperature when the pump is either heated or cooled to the test limits of positive 155 degrees Fahrenheit or negative 35 degrees Fahrenheit and then returned to room temperature. This error (in terms of the original values) gradually disappears if the pump is kept at room temperature, but it can take as long as a week for this to occur. This phenomenon is well-known for materials with high dielectric constants and also affects capacitors.
The use of PZT pumps involves additional special problems. The optimum frequency is difficult to achieve using PZT diaphragms because the PZT pump cannot be driven harder than about six volts due to power supply limitations. In addition, with a PZT diaphragm, the deflection is a direct function of voltage and thickness. Changing the thickness is a very time consuming manufacturing operation and it has been found that there is a definite limit on minimum thickness because of manufacturing difficulties. Thus, both minimum frequency and maximum deflection are limited by properties of the PZT material itself.
The PZT pump suffers from differential expansion problems and PZT pumps require specialized manufacturing techniques. The PZT material has a very low coefficient of expansion which requires the anvil 28 to be made of INVAR to match it, but that results in an anvil material which does not match the coefficient of the nozzle block.
Another source of temperature sensitivity is the use of extremely thin sensing wires. A further source of temperature sensitivity is temperature hysteresis effects in the sensor itself.
Thus, in practice it has been found that angular velocity sensors of this type are highly sensitive to temperature variations. A need exists to find ways to minimize temperature sensitivity in angular rate sensors of this kind.