Temperature Sensor and Method Using a Single Rotated Quartz Crystal

The invention relates to a temperature sensor including an oscillator circuit comprising a crystal resonator in the form of a substantially plane-parallel plate cut from a quartz crystal, the crystal resonator having, in response to an electric field, a mode of oscillation with a substantially linear frequency-temperature characteristic over a selected temperature range, and electronic oscillator means to generate and apply the electric field to the crystal resonator.

Such a temperature sensor is disclosed in U.S. Pat. No. 4,079,280.

The said United States patent discloses a temperature measuring method in which use is made of the substantially linear frequency-temperature characteristic in the temperature range between -20.degree. C. and 50.degree. C. of a quartz crystal resonator having a (yxwl) 21.93.degree./33.93.degree. orientation which is made to vibrate in a given mode (B mode or so-called fast shear mode).

The advantages of temperature measurement using a piezoelectric crystal when compared with such measurement using other sensors which provide an electrical signal (resistance sensor, thermocouple), are a high accuracy and the possibility of direct digital processing of the measured values.

The said other kinds of temperature sensors provide an analogue output. Said output may be a low voltage in the case of a thermocouple or of a measurement output relating to resistance from, for example, a Pt resistance thermometer. This output is first converted, by means of a circuit which is sometimes termed a transmitter, into a normalised information stream and is then multiplexed and converted via an A/D converter into digital information if a microprocessor is to be used for further processing.

This results in the said sensors being expensive to use, while a further disadvantage is that each converter will involve a loss of accuracy. Thus a sensor is to be preferred which provides digital information directly. A disadvantage of the above-mentioned known sensor which uses a crystal resonator, however, is that the resonator in question is difficult to make in a reproducible manner in mass production since it requires a doubly rotated cut. Moreover, a strong unwanted resonance proves to be present close to the main resonance, which necessitates a more complex design of the oscillator. It is an object of the invention to provide a temperature sensor with a crystal resonator having a singly-rotated cut with a substantially linear frequency-temperature characteristic over a selected temperature range.

This object is achieved in that the temperature sensor of the kind described in the opening paragraph comprises a crystal resonator in the form of a crystal resonator plate formed using a crystal cut rotated singly through an angle .theta. about the X axis (piezoelectric axis) of the crystal, .theta. being in one of the ranges -29.degree..ltoreq..theta..ltoreq.-16.degree. and +39.degree..ltoreq..theta..ltoreq.+45.degree. and that the major faces of the plate are provided with respective electrodes to make the plate vibrate in a thickness shear mode.

The sign of the angle .theta. conforms with the IRE standards on piezoelectric crystals (1949) published in the Proceedings of the IRE, Volume 37, pp. 1378-95. December, 1949.

It has been found that the resonators having a singly-rotated crystal cut thus selected (which ensures a manufacture in large numbers with good reproducibility and at comparatively low costs), dependent on the angle over a given part of the temperature range of approximately -25.degree. C. to approximately 275.degree. C., combines a very linear, temperature-frequency variation wiht a sufficiently large temperature coefficient. Over a temperature range of 20.degree. to 80.degree. C. the most linear variation is found at an angle .theta. of -26.degree.. Over the temperature range from 0.degree. C. to 150.degree. C. at an angle of -23.degree.. Over the temperature range from -25.degree. C. to 95.degree. C. at an angle of -28.degree. . Over a temperature range from 50.degree. to 275.degree. C. at angles of -17.degree. and +43.degree..

The invention also relates to a crystal resonator for a temperature sensor as described above.

An embodiment of the invention will be described by way of example with reference to the drawing, in which

FIG. 1 shows a first orientation of a quartz crystal resonator,

FIG. 2 shows a second orientation of a crystal resonator,

FIGS. 3a and 3b are a plan view and a cross-sectional view, respectively, of a crystal resonator having electrodes for use in an oscillator circuit, and

FIG. 4 shows a plot of the frequency deviation .DELTA.f versus the temperature T for a crystal resonator having the orientation shown in FIG. 2.

The two main properties which have made quartz so successful are the piezoelectric effect in the material as a result of which mechanical displacements can be associated interactively with electrical variations, and the very low damping factor associated with mechanical oscillations. The piezoelectricity is directly associated with the crystal structure. In FIG. 1 the usual axes of a quartz crystal are shown. The Z axis, the optical axis, is an axis of three-fold symmetry, the X-axis, the piezoelectric axis, is one of the three equivalent axes of two-fold symmetry. It is known that the sensitivity of the resonant frequency of a quartz crystal with respect to temperature depends on the orientation with which a quartz plate has been cut from the crystal. A once-rotated cut is obtained by rotating the cut from a plane perpendicular to the Y axis (the "Y" cut) through an angle .theta. about the X axis. The cut according to the invention has been rotated once counter-clockwise (-29.degree..ltoreq..theta..ltoreq.16.degree.: FIG. 1) or rotated once clockwise (39.degree..ltoreq..theta..ltoreq.45.degree.: FIG. 2). These cuts prove to give rise to a predominantly linear frequency-temperature characteristic in selected temperature ranges. FIG. 3a is a plan view of a quartz crystal resonator with electrodes and FIG. 3b is a cross-sectional view thereof. Quartz crystal 1 has the form of a plane-parallel plate of thickness d. The resonant frequency is substantially determined by the thickness of the plate but is reduced slightly by the mass of electrodes 2 and 3. As a result the central part has a slightly different resonant frequency from the surrounding part. This weakens the coupling of the central part with respect to the holder parts 4 and 5 between which the crystal plate 1 is clamped: the energy is trapped at the centre ("energy trappine"). Thus the Q factor becomes higher as a result of the presence of the electrode material. FIGS. 3a and 3b are not drawn to scale for reasons of clarity. In fact, the thickness d of the crystal plate 1 is very small. It is, for example, a few tenths of a millimeter, while the main dimensions l and b are each, for example, 10 mm.

In order to ensure that the crystal resonator 1, provided with two electrodes and clamped in the manner shown in FIGS. 3a and 3b, has a substantially linear frequency-temperature characteristic in a selected temperature range, it is cut to an orientation as shown in FIG. 1 or in FIG. 2.

It is achieved by the manner of clamping shown that, when an H.F. electric field of suitable frequency is applied to the electrodes 2 and 3, the resonator 1 will vibrate in a so-called thickness shear mode.

In flat crystal plates there are three major vibration modes. These are "thickness vibrations": acoustic standing waves over the thickness of plate. They are indicated by a, b and c in the sequence of decreasing wave speed. In single cuts the displacements in mode a are longitudinal (this is a displacement perpendicular to the plate) and in modes b and c they are transversal to the plate (shear).

A (hydrothermally grown) quartz crystal from which the plates are to be sawn is clamped with one degree of freedom in the desired orientation in a sawing machine after determining the crystal axes. The orientation is checked by means of X-ray diffraction to an accuracy of approximately 0.01.degree. in .theta.. In the present case .theta. was adjusted to be 43.degree. 00. After sawing, the plates are lapped to approximately the correct thickness between lapping disks. During the lapping process the plates are accommodated in small holes in a sheet of synthetic resin. A radio receiver connected to the lapping disks gives a signal when the required frequency is reached.

In the present case the plates are ground to a thickness of 326 .mu.m so as to make resonators for a nominal frequency of 8.85 MHz which will operate at the fundamental of the b mode. The present type of resonators operate generally at frequencies between 1 and 250 MHz. The plates are then stacked, ground to the desired shape, separated from each other and etched. Etching is necessary to remove the damage resulting from lapping and to prepare the surface for the vapour deposition of the electrodes. The electrodes are, for example, of silver or gold. When they have been applied, each of the plates is mounted in its holder. The assembly is provided with a glass envelope which is evacuated, or is provided with a metal envelope which is filled with gas.

The frequency-temperature characteristic of nine resonators manufactured in the above-described manner was measured in a tube furnace. For that purpose the resonators were incorporated in a conventional oscillator circuit which will not be described in detail. The temperature was recorded with a chromel-alumel thermocouple. From the measured temperature and the measured frequency a best fit straight line was computed by means of the smallest quadrant method indicating the relationship between the frequency and the temperature. All straight lines obtained in this manner show the same variation as that shown in FIG. 4. The temperature coefficient is on an average 409 Hz/.degree. C. In the temperature range between 50.degree. C. and 275.degree. C. the departure of the real frequency-temperature curve from the said straight line is less than 1.degree. C.

For the other crystal cuts other temperature ranges will be found in which the frequency-temperature characteristic is predominantly linear.

For a cut angle .theta. of -26.degree. this is the temperature range from 20.degree. C. to 80.degree. C.

For a cut angle .theta. of -23.degree. this is the temperature range from -20.degree. C. to 150.degree. C.

For a cut angle of -28.degree. this is the temperature range from -25.degree. C. to 95.degree. C.

For satisfactory reproducibility it is of importance that, when a given cut angle has been chosen, the plates manufactured using this orientation should not depart by more than b 1.5' from said given angle.