BACKGROUND OF THE INVENTION

This invention relates to a vibratory transducer for measuring temperature, where the dimensions of the transducer are selected to avoid spurious modes of operation and thus provide accurate temperature readings over a wide range.

Double bar transducer elements formed in the shape of tuning forks have been proposed for a variety of uses including use as an accelerometer element (U.S. Pat. No. 3,238,789), use as a force transducer (U.S. Pat. No. 4,372,173), and use as a temperature sensor (U.S. Pat. No. 4,437,773). In the last mentioned patent, a so-called quartz thermometer utilizes the torsional mode of vibration (each tine rotates about its own longitudinal axis 180.degree. and out of phase with the other tine) to measure temperature changes. It is noted in the patent that by proper orientation of the axes of the quartz crystal used in the tuning fork--length of the tuning fork is arranged to be generally parallel to the X axis, among other things--the torsional mode frequency will change with temperature. Thus, by measuring change in the frequency of vibration, a measure of temperature change can be made.

It has been discovered that with the dimensions given in U.S. Pat. No. 4,437,773, for the tuning fork (resonator) there disclosed, a number of spurious modes of operation develop over the operational range of the device and these spurious modes result in a lowering of the mechanical Q of the device, a shift in frequency, and a possible cessation of oscillation at the desired natural resonant frequency. The cause of this anomalous behavior is that at certain frequencies, the torsional mode frequency coincides with the frequency of another vibrational mode. The coincidence of modes allows some of the electrical energy driving the desired mode to be diverted into the spurious mode resulting in an abrupt shift in frequency or cessation of oscillation, either of which renders the resonator useless as a temperature sensor.

Some of the spurious modes include (1) flexure or oscillation of the bars (tines) in phase in a direction normal to the plane of the resonator or transducer (f.sub.p), (2) flexure or oscillation of the bars 180.degree. out of phase in a direction normal to the plane of the transducer (f.sub.w) (3) oscillation in phase in the plane of the transducer (f.sub.b), and (4) oscillation out of phase in the plane (f.sub.a). In addition to the fundamental frequencies of these spurious modes there are also overtone frequencies which can likewise cause problems. These spurious modes may be excited by the pumping motion of the structure resulting from the bars flexing, by the piezoelectric effect (assuming piezoelectric material is used) if the structure's geometry is poorly chosen, and by the nonlinear elastic behavior of the transducer material. The existence of these spurious modes of operation has not been recognized in the past in torsional mode tuning forks.

Another factor to be considered for torsional mode temperature transducers is the need for configurations which will allow use of photolithography and chemical etching for fabrication. Such fabrication techniques provide cost advantages, miniaturization and tight dimensional control.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a resonator temperature transducer which is accurate over a wide range of temperatures.

It is a further object of the invention to provide such a transducer which is relatively easy to fabricate.

It is also an object of the invention to provide such a transducer which is constructed to facilitate ease of fixing in place for use.

It is still another object of the invention to provide such a transducer constructed to minimize the possibility of certain spurious modes of oscillation.

The above and other objects of the invention are realized in a specific illustrative embodiment of a resonator temperature transducer which includes a pair of elongate, generally parallel and spaced apart bars made of a piezoelectric material and coupled together at one end in the form of a tuning fork. The thickness of each of the bars is t, the width is w, and the length between the free and fixed ends is L. The distance from the free end to the point at which the bars are connected is m. The dimensions t, w, L, and m are selected so that certain spurious modes of operation may be avoided. In particular, avoidance of these modes requires careful selection of the values of ratios t/w, m/w, and L/m.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the invention will become apparent from a consideration of the following detailed description presented in connection with the accompanying drawings in which:

FIG. 1 shows a perspective view of a torsional mode vibratory force transducer made in accordance with the principles of the present invention; and

FIGS. 2 through 6 is a chart showing the range of values useable for t/w and m/w for fixed L/m ratios for operation of exemplary transducers free from spurious modes.

DETAILED DESCRIPTION

FIG. 1 shows a double-bar vibratory transducer 4 composed of a pair of generally parallel bars 8 and 12 coupled together at one end and joined to a significantly enlarged or widened mounting end portion 16. A slot separates the two bars 8 and 12. The widened end portion 16 is used for mounting the transducer on suitable support structure 28. Advantageously, the transducer is made of a piezoelectric material such as quartz, with the crystallographic axes of the material being oriented so that the torsional mode frequency of vibration of the transducer will change with temperature. (Such orientation is discussed in U.S. Pat. No. 4,437,773.)

The various dimensions of the transducer are labeled in the drawing, with t representing the thickness of the bars, w representing the width of the bars, L representing the length of the bars from the fixed end to the free ends (i.e., in FIG. 1 the distance between the significantly enlarged mounting portion 16 and the free end), and m representing the length of the slot. The width of the portion between the fixed end and the point where the two bars are joined is generally equal to 2 w plus the slot width, but can be wider by as much as w without significantly affecting the spurious mode frequencies described herein. Exemplary values and ranges of values for these dimensions will be given later.

An oscillator circuit 32 is coupled to thin electrode films or coatings 36 and 40 disposed on various surfaces of the transducer 4 as shown in the drawing. (The placement of the electrode films to obtain torsional mode vibration is discussed further in U.S. Pat. No. 4,437,773.) Application of A.C. signals by the oscillator 32 to the electrode films 36 and 40 produces stress forces in the bars 8 and 12 to cause the bars to vibrate in a torsional motion in 180.degree. phase opposition. That is, the bars 8 and 12 are caused to alternately twist in a well known manner and at a characteristic or desired natural resonant frequency.

When the transducer 4 is subjected to a change in temperature, the frequency of vibration of the bars is caused to change, and the magnitude of the change serves as a measure of the temperature change. The oscillator 32 follows in frequency the frequency of the transducer and so the change in frequency of the transducer can be measured by simply measuring the output frequency of the oscillator. A conventional counter and display 44 may be connected to the oscillator 32 to provide a read out of the temperature.

The significantly enlarged mounting portion 16 of the transducer 4 is provided to give certainty to the location of the fixed ends, i.e., to the dimension L, regardless of the locations at which the end of the transducer is bonded or otherwise fixed to support structure. Without the enlarged end portion 16, the dimension L could vary from one device to another since L would then be measured from the edge of the attachment joint to the free ends. Since it is difficult to mount (attach) consistently to the same location each time a device is used, consistency and preciseness in the dimension L would be hard to achieve. For reasons to be discussed later, it is important that the dimensions of the transducer 4, including the dimension L, be carefully selected and maintained to avoid certain undesirable modes of operation.

Before describing specific illustrative dimensions of applicant's invention some of the principles governing the operation of torsional mode vibratory transducers will be discussed.

It has been discovered, as earlier discussed, that certain spurious modes of oscillation may develop in a double-bar torsional mode vibratory transducer. These modes are (1) flexure or oscillation of the bars in a direction normal to the plane of the transducer and in phase (f.sub.pn), and (2) flexure of the bars in a direction normal to the plane, but out of phase (f.sub.wn), (3) oscillation in the plane of the transducer with the bars in phase (f.sub.bn), and oscillation in the plane with the bars out of phase (f.sub.an), where n indicates the nth order mode. When the desired natural resonant frequency equals the frequency or twice the frequency of oscillation of these spurious modes, the mechanical Q of the transducer is dramatically descreased because acoustic energy is transferred to the undesired mode which pulls down the desired resonant frequency, and thus any reading of temperature is either non-existent or incorrect. It is thus desirable to construct the transducer so that these spurious modes are avoided, i.e., so that the desired natural or characteristic frequency (or multiples thereof) of the transducer do not equal the spurious mode resonant frequencies.

The desired resonant or characteristic frequency f for a double-bar torsional mode transducer constructed in accordance with the structure shown in the drawings is given to close approximation by: ##EQU1## where S.sub.55 and S.sub.66 are the shear compliances in the device planes tm (plane coinciding with the length m and thickness t), and wm (plane coinciding with the length m and width w), and where Z is derived from the density P and shear compliance S.sub.66 of the material. It should be noted that the magnitude of Z varies with temperature because the density and shear compliance S.sub.66 vary with temperature which contributes the main part of the temperature dependence of f.sub.T, the phenomenon used here to measure temperature. Z is equal to G.sqroot.1/PS.sub.66, with G being a constant of proportionality independent of temperature and determinable by experiment as explained hereinafter. The material parameters S.sub.55 and S.sub.66 change with crystallographic orientation if crystalline material is used.

The resonant frequencies of the spurious modes of oscillation of the transducer have been determined by experimental and theoretical examination to be as follows: ##EQU2## where various values of n designate the order of the mode (fundamental and harmonic resonances), where ##EQU3## and where E is Young's modulus and P is the density of the material, and where the values L, m, t, and w are dimensions described above. The various values for A given below were experimentally determined:

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    A.sub.a1 = 3.05     A.sub.b1 = 8.57
    A.sub.a2 = 18.99    A.sub.b2 = 41.82
    A.sub.a3 = 51.92    A.sub.b3 = 96.97
    A.sub.p1 = 3.91     A.sub.w1 = 4.96
    A.sub.p2 = 22.12    A.sub.w2 = 30.91
    A.sub.p3 = 59.09    A.sub.w3 = 71.82
    A.sub.p4 = 114.1    A.sub.w4 = 89.80
    A.sub.p5 = 189.9    A.sub.w5 = 143.4
    A.sub.p6 = 281.8    A.sub.w6 = 180.8
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