Resonator Transducer System With Temperature Compensation

This invention relates to a resonator transducer for simultaneously measuring force or gas density, and temperature.

Resonator systems for measuring various force parameters such as pressure, acceleration, weight, resonator surface film stress, etc., and other parameters such as density of the medium in which the systems are placed are well known. Typically such systems include a vibratory element whose frequency of resonation varies with variation in the force parameter or other parameter to which the vibratory element is subjected, an oscillator for causing the vibratory element to resonate, and processing circuitry for determining the variation in frequency of resonation of the vibratory element and thus the variation in the parameter to which the vibratory element is subjected.

It is also known that the frequency of resonation of most vibratory elements is affected by the temperature to which the vibratory element is exposed. Thus, errors may be introduced in the measurement of force or other parameters by resonator transducer systems as a result of temperature variations. Such errors can be significant and can seriously affect the accuracy of any measurements made.

There are a number of approaches for compensating for temperature variation when making force parameter measurements with vibratory element resonator transducers. One way is measuring temperature with a conventional analog temperature sensor and then using analog to digital conversion to provide digital temperature information. See for example G. R. Cucci, U.S. Pat. No. 4,311,053. One disadvantage of this approach is the need for both frequency and analog measurements and analog to digital conversion.

The most common approach for quartz crystal vibratory elements appears to be the utilization of two quartz crystal resonators, both of which are exposed to the operating environment (temperature), but only one of which is subjected to the force parameter to be measured. The output of the resonator exposed only to temperature is used to correct or compensate for temperature induced errors in the force parameter measurements made by the other resonator. See, for example, E. P. EerNisse, "Vacuum Applications of Quartz Resonators", Journal of Vacuum Science and Technology, Vol. 12, No. 1, Jan./Feb., 1975, pages 564-568; and H. E. Karrer and J. Leach, U.S. Pat No. 3,561,832, issued Feb. 9, 1971. The disadvantage of this approach is that the two resonators cannot be in precisely the same physical location and thus will not be exposed to precisely the same temperature. Also, unless the resonators are exactly the same in dimension, characteristics, etc., they will not respond in exactly the same way to temperature variation, and yet such exactness in response is necessary to accurately compensate for temperature variation.

Another proposed approach for compensating for, or at least determining, temperature variations in quartz resonator transducers is that a single quartz crystal be used in two different modes of oscillation. See J. A. Kusters and J. Leach, "Dual Mode Operation of Temperature and Stress Compensated Crystals", Proc. 32nd Annual Symp. on Frequency Control, 1978, pages 389-397. Here, a single crystal is driven by an oscillator to resonate in the so-called fast-shear mode or "B" mode, and also in the slow-shear mode or "C" mode. In the C mode, the frequency of oscillation of the crystal is fairly temperature independent whereas in the B mode, the frequency of oscillation varies with variation in temperature. The so-called dual mode oscillator arrangement was proposed not for force parameter measurement with temperature compensation, but rather for producing an output signal frequency which is temperature compensated. Among the disadvantages of this arrangement are the difficulty in driving a crystal in both the B and C modes simultaneously and the consequent requirement of a more complicated oscillator driver, the problems created by the closeness of the frequencies of the B and C modes and the attendant requirement of very precise filters to eliminate cross talk, and the errors which can be introduced using the B mode of oscillation which has what are termed "activity dips" (spurious frequency responses at certain temperatures). The closeness of the frequencies of the B and C modes also causes phase noise in the C mode frequency output when the B mode is resonating.

It is an object of the invention to provide a new and improved apparatus and method for compensating for temperature variation in resonator transducers.

It is another object of the invention to provide a simple and yet accurate method and apparatus for temperature correction in resonator transducers.

It is a further object of the invention to provide temperature compensation in a single resonator transducer.

It is also an object of the invention to provide a single resonator transducer capable of measuring both force or other parameters and temperature to thereby compensate for either temperature introduced errors or force or other parameter introduced errors.

The above and other objects of the invention are realized in a specific illustrative embodiment of a resonator transducer system which includes a vibratory element such as a crystal which resonates at least two frequencies f.sub.1 and f.sub.2 selected from the fundamental frequency and its overtone frequencies in response to an oscillatory electrical signal. The crystal is selected so that the two frequencies f.sub.1 and f.sub.2 both vary with variation in force (or other parameter such as density of the medium surrounding the crystal) applied to the crystal and with variation in temperature to which the crystal is exposed, with the magnitude or scale factor of variation for one of the selected frequencies being different from that for the other selected frequency. Also included is an oscillator for causing the crystal to resonate at the two frequencies, detection apparatus for detecting the two frequencies of resonance of the crystal and for producing signals V.sub.1 and V.sub.2 representing the two selected frequencies, and a processor responsive to the signals V.sub.1 and V.sub.2 for producing a signal F representing the force (or other parameter) applied to the crystal and a signal T representing the temperature to which the crystal is exposed. The force measurement is compensated for temperature variations in the crystal and the temperature measurement is compensated for variations in the force applied to the crystal.

Advantageously, the crystal is adapted to resonate in a mode which is free of significant activity dips in both the selected frequencies. It has been found that this is the case for the C mode of vibration for SC-cut quartz crystals and to a lesser extent for the C mode of AT-cut quartz crystals and the B mode of BT-cut quartz crystals. Other crystal configurations which have been found suitable for producing the two desired frequencies f.sub.1 and f.sub.2 are a double-bar, double-ended arrangement resonating in a flexure mode (see U.S. Pat. No. 4,372,173) and a double-bar, single-ended arrangement also resonating in the flexure mode for measuring fluid density of the surrounding medium (see copending U.S. application, Ser. No. 506,314.)

In accordance with one aspect of the invention, a pair of filters are connected in parallel with each other between the crystal and the oscillator, with one filter being designed to pass signals having frequencies in a band generally centered about the frequency f.sub.1 and the other filter being designed to pass signals having frequencies in a band centered about the frequency f.sub.2. A similar pair of filters advantageously are connected in parallel with each other between the oscillator and the frequency detection apparatus.

Because a single crystal is used, the two modes of oscillation used--frequency f.sub.1 and frequency f.sub.2 --are exposed to the same environment, thus removing any error which may be introduced by using two crystals. Also, the oscillator driver for causing the crystal to resonate in the two frequency modes is simple in construction and the two modes of resonance are free of significant activity dips which could cause spurious readings.

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 is a schematic view of a resonator transducer system made in accordance with the principles of the present invention and utilizing a single oscillator driver;

FIG. 2 is a schematic view of another embodiment of the invention which utilizes two oscillator drivers;

FIG. 3 is a perspective, fragmented view of a double-bar, double-ended resonator adapted to oscillate in the flexure mode; and

FIG. 4 is a perspective, fragmented view of a double-bar, single-ended resonator adapted to oscillate in the flexure mode.

Referring to FIG. 1, there is shown a resonator transducer system capable of measuring a force parameter (which will hereafter be referred to simply as "force") and temperature in accordance with the present invention. The system includes a vibrating element 4 adapted to resonate free of significant activity dips. It has been found that for the thickness shear mode of vibration, the SC-cut, AT-cut and BT-cut quartz crystals meet this criterion, and that SC-cut quartz crystals are preferred because they are more stable in transient temperature environments and have better short term frequency stability. In addition, the flexure mode of vibration, among others, in both the double-bar, double-ended resonator configuration (see U.S. Pat. No. 4,372,173) and the double-bar, single ended configuration (see copending U.S. patent application, Ser. No. 506,314) have been found to meet this criterion.

An oscillator 8 is coupled to the crystal 4 by way of a pair of filters 12 and 16 which are connected in parallel with each other. The oscillator 8 produces an oscillatory signal which is supplied via the filters 12 and 16 to the crystal 4 to cause the crystal to resonate at two (or more) frequencies f.sub.1 and f.sub.2 selected from the fundamental frequency and its overtone frequencies. Filter 12 passes oscillatory signals whose frequencies are within a band of frequencies centered about the first selected frequency f.sub.1, whereas filter 16 passes oscillatory signals having frequencies in a band centered about the second selected frequency f.sub.2. The frequencies at which the crystal 4 resonates are dependent both upon the force (such as pressure) to which the crystal is subjected, and the temperature to which the crystal is exposed. The crystal is selected and formed so that the variation in the first selected frequency with a change in temperature is by a different scale factor from that at which the second selected frequency varies with variation in temperature. See, for example, A. R. Chi, Temperature Behavior of AT-cut Quartz Resonators", Proc. 10th Annual Symp. on Frequency Control, 1956, pages 47-59. The same is also true for variations in force applied to the crystal 4. It is this phenomenon which is utilized in a unique and yet simple fashion for producing both force (or other parameter) and temperature measurements using a single crystal resonator transducer, or alternatively for producing a force measurement which is temperature corrected or a temperature measurement which is force corrected.

The oscillator 8 follows the frequency of oscillation of the crystal 4 and produces a signal having the same two selected frequencies as the crystal and this signal is supplied to a second pair of filters 20 and 24. Filter 20 passes signals having frequencies in a band generally centered about the first selected frequency f.sub.1 and filter 24 passes signals having frequencies in a band generally centered about the second selected frequency f.sub.2. Filter 20 supplies its frequency signal to a counter 28 and filter 24 supplies its signal to a counter 32. Counter 20 produces a digital output signal representing the first selected frequency of the received signal and counter 32 produces a digital output signal representing the second selected frequency of the received signal. These digital outputs are supplied to a microprocessor 36 which processes the signals to determine the magnitude of the force applied to the crystal 4, and the temperature to which the crystal is exposed. The force and temperature measurements are supplied to a display unit 40 where the measurements are visually displayed. The oscillator 8, filters 12, 16, 20 and 24, counters 28 and 32, microprocessor 36 and display 40 are all conventional units.

The change in the two selected frequencies (preferably the fundamental and third overtone frequencies or fundamental and fifth overtone frequencies) with force and temperature can be characterized by algebraic equations as follows: ##EQU1## where f.sub.01 and f.sub.02 are the first and second selected frequencies at a predetermined reference temperature T.sub.0 and reference force F.sub.0. Also, T is the change in temperature from the reference temperature and F is the change in force (or other parameter) from the reference force to which the crystal is subjected. The inverses of these equations are: where a.sub.F, b.sub.F c.sub.F, d.sub.F, e.sub.F, f.sub.F are predetermined constant force coefficients, and where a.sub.T, b.sub.T, c.sub.T, d.sub.T, e.sub.T, f.sub.T and g.sub.T are predetermined constant temperature coefficients.

The coefficients in the equations (3) and (4) are readily determined by applying known forces and temperatures to the crystal and determining the constant coefficients in the equations by curve fitting the equations to the known data. With the coefficients being determined, and having two equations and two unknowns, the two unknowns can be readily determined by conventional processing techniques. In effect, equations (3) and (4) are solved by the microprocessor 36 to produce a measure of force F and temperature T which are both self-compensated. Other parameters such as fluid density, along with temperature, could be determined in a similar fashion.

FIG. 2 shows another embodiment of the invention which includes a single crystal 54 driven by two oscillators 58 and 62 to resonate at two selected frequencies f.sub.1 and f.sub.2. Filters 66 and 70 are coupled in series respectively to oscillators 58 and 62 to pass signal frequencies f.sub.1 and f.sub.2 to the crystal 54 to thereby cause the crystal to resonate at these frequencies. The oscillators 58 and 62 follow respectively the frequencies of oscillation f.sub.1 and f.sub.2 of the crystal 54 to produce and supply to counters 74 and 78 oscillatory signals having frequencies f.sub.1 and f.sub.2. The counters 74 and 78, in turn, produce digital output signals representing frequencies f.sub.1 and f.sub.2 respectively, and these signals are supplied to a microprocessor 82. The microprocessor 82 processes the signals, as in the FIG. 1 embodiment, to determine the force (or other parameter being measured) and temperature for visual display on display unit 86. The processing is carried out in the same fashion as described for the FIG. 1 embodiment.