The present invention relates to elastic surface wave accelerometers and more particularly to those having at least two oscillators connected to transducer means, located on two faces of a bending or deflecting structure having a fixed end. The transducer means are designed so as to excite and collect the elastic surface waves, whereof it is known that the propagation speed varies as a function of mechanical bending stresses. According to a first construction, the transducer means form a transmission line, whose time lag fixes the phase displacement of an oscillator loop. According to a second construction, the transducer means are placed in a resonant cavity having grids and are connected to electrical means for maintaining the oscillation of the cavity. The frequency representing the oscillation to be measured results from the subtraction of two oscillating frequencies, but the latter are subject to a thermal drift, which can falsify the measurement of weak accelerations. When the two oscillators are coupled, the oscillation of one of them can lead to the other oscillating at the same frequency. This phenomenon occurs when the natural frequencies of the two oscillators are very close. To combat this phenomenon, it is possible to move the oscillators apart, or adequately shift their oscillating frequencies, but then the compensation of the thermal drifts is not ideal. This type of problem is encountered in accelerometers, wherein the bending structure is a simple lamellar member fixed at one end. Thus, the transducer means overhang the two main faces of the lamellar member and for acceleration sensitivity reasons, said member only has a limited thickness. The mechanical assembly conditions of the lamellar member can lead to undesirable stresses at the fixing point. These stresses vary as a function of the ageing of the materials and their relaxation, which take place subsequent to the initial assembly.
Apart from the electrical disconnection and fixing problems referred to hereinbefore, there is also a problem of ensuring a measurement of the acceleration component to be measured. The use of a single bending lamellar member involves a perfectly symmetrical fitting of the seismic mass to ensure that the acceleration cannot give rise to torsional deformations of the said member. Thus, the torsional rigidity depends on the inertia moment of the cross-section of the lamellar member and the latter is not sufficient to oppose a torsional movement. The solution of the problem of the mechanical disengagement relative to the deformable structure can be envisaged, provided that the sensitivity of the accelerometer is not sacrificed. To this end, it is also necessary to carefully position the elastic surface wave measuring means, because it would not be advantageous to use the entire length of a bending beam for the circulation of the elastic surface waves. Thus, no matter whether a delay line or an elastic surface wave resonator is used, it must be borne in mind that each oscillator may only be excited on a single oscillation mode. If a significant length is given to a delay line or resonator, there is a risk of confining the modes to a point such that the selectivity of the transducers or the reflector grids not making it possible to ensure the desired monomode operation. In practice, this leads to elastic surface wave measuring means covering a distance of about 100 wavelengths at a frequency of approximately 100 MHz. The reduction in the resolution corresponding to a limitation of the range of the measuring means can be largely compensated by a better utilization of the bending stresses, on which depends the variation of the propagation speed of the elastic surface wave.
USPTO Backgrounds
USPTO Backgrounds