FR2489510A1 - QUARTZ THERMOMETER - Google Patents

Abstract

The thermometer essentially consists of a quartz resonator (2) excited in torsional mode which delivers an electrical signal whose frequency is representative of the temperature to which it is subjected. The angle phi of cut of the quartz and the ratio b DIVIDED a between the thickness (b) and the width (a) are chosen so that the second order temperature coefficient is zero. A linear relationship between the temperature and the electrical signal delivered is thus obtained.

Description

 The present invention relates to a quartz thermometer.

 More specifically, the present invention relates to a thermometer in which the temperature-sensitive element is a quartz resonator vibrated by an oscillator circuit so as to deliver an electrical signal whose frequency is representative of the temperature at which is submitted the resonator.

 Temperature measuring devices using a quartz resonator as a sensor are already well known. It is indeed well known that the frequency of such a resonator varies with temperature. The relationship between the oscillator frequency of quartz and the temperature can be represented by the following polynomial expression f T fTo 1 + a (T - To) + ss (T - To) 2 + in which fTo, a, ss and To are constants, T the temperature to be measured, and f T the frequency of the quartz oscillator subjected to this temperature.

 It is not enough, in general, that the frequency depends on the temperature T, but it is very advantageous that this dependence is as linear as possible.

The essential advantage of this type of thermometer, compared to all other known devices, lies in the fact that it performs direct temperature / frequency conversion, that is to say that information on the temperature is presented. in quasi-digital form. Another advantage of this type of thermometer is that frequency is a physical quantity which is measured with a resolution as large as desired if the measurement time is increased accordingly.

I1 already exists on the market of thermometers provided with a probe containing a vibrating quartz crystal and whose frequency depends on the temperature in a very linear way. A first device, manufactured by the
Hewlett-Packard USA company uses cut quartz
LC excited at a frequency of 28 MHz. This instrument has a sensitivity of the order of 1000 Hz per OC. In the device manufactured by Tokyo Dempa, the oscillating quartz has a YS cut and is excited with a frequency of the order of 10 MHz. This device has a sensitivity of the same order of magnitude as that which was mentioned above.

 These high-precision instruments are however very expensive and the quartz resonators used as sensitive elements have the drawback of being very bulky, since they are manufactured using a technology which does not allow easy miniaturization. On the other hand, their manufacturing method is not suitable for mass production. Finally, as their vibration mode corresponds to a frequency equal to or greater than 10 MHz, the electronic circuits which process the signal delivered by the quartz resonator consume a relatively high amount of electrical energy.

 To avoid these drawbacks, a first object of the invention is to provide a quartz thermometer having good linearity as a function of temperature and which is relatively compact.

 A second object of the invention is to provide such a thermometer in which the resonator is easy to manufacture by modern machining techniques, such as chemical etching.

 A third object of the invention is to provide such a thermometer in which the electronic circuit is relatively simple.

 A fourth object of the invention is to provide such a thermometer which operates at lower frequencies than those of the devices of the prior art to reduce the consumption of electrical energy.

 To achieve these aims, the invention consists in using as a thermometer a quartz resonator excited in torsion and whose long length is substantially parallel to the X axis of the quartz. This resonator can be constituted by a substantially parallelepiped bar or by a tuning fork.

 According to a preferred characteristic of the invention, the ratio between the values of the small and large sides, of the cross section of the resonator, that is to say respectively its thickness and its width, is between 0 and 0, 8.

 According to another preferred characteristic of the invention, the resonator is oriented so that its short side makes an angle between + 40 and + 900 or between - 500 and - 900 with the Y axis of the quartz.

These objects and advantages of the invention as well as others will appear more clearly on reading the following description of several preferred embodiments of the invention. The description refers to the accompanying drawings, in which
- Figure 1 is a perspective view of a tuning fork-shaped resonator usable in the invention, the figure also showing the different parameters for defining the tuning fork;
- Figure 2 is a perspective view of a quartz bar used in the invention, the figure further showing the different parameters of the bar definition;
FIG. 3 shows curves giving the temperature coefficient ss of the second order of a quartz excited in torsion for different values of the cutting angle + with the axis Y of the quartz as a function of the ratio b / a of the dimensions of the thickness and width of the quartz element;
- Figure 4 shows an exemplary embodiment of the electrical circuit for processing the signals delivered by the resonator;
- Figures 5a and 5b show time diagrams explaining the operation of the processing circuit; and
- Figure 6 shows thermometer adjustment curves.

 As already explained, the thermometer of the invention comprises on the one hand a quartz resonator the electrodes of which are located and supplied to energize the quartz element in torsion and on the other hand an electronic circuit for displaying the temperature measured by the resonator.

 Referring to Figures 1 and 3, there will first be described a first embodiment of the invention according to which the resonator is constituted by a tuning fork 2 made of quartz. The tuning fork 2 comprises a base 4 by which it is fixed to the pedestal 6. I1 of course also includes two branches 8 and 10 whose cross section is substantially rectangular. This straight section has a short side of length b or thickness of the tuning fork and a large side a or width of the branch of the tuning fork.

Each branch of the tuning fork 8 and 10 thus has the shape of a rectangular parallelepiped. In FIG. 1, the directions corresponding to the length L, the width a and the thickness b of a branch of the tuning fork are marked by the axes
Y 'and Z'. FIG. 1 also shows the orientation of the axes X ', Y' and Z 'of the resonator with respect to the axes X,
Y and Z of quartz.

According to the invention, the tuning fork is cut in such a way that its length is arranged substantially along the axis X of the quartz, that is to say that the directions X and
X 'are substantially combined. On the other hand, the Y 'axis of the tuning fork makes an angle with the Y axis of the quartz. The same goes of course for the Z 'axis with respect to the Z axis.

 As already explained, the tuning fork must be twisted. For this, metallizations are deposited on the faces corresponding to the width a, and on the sides corresponding to the thickness b of each branch of the resonator 2. The electrodes facing each other are brought to the same potential, and the electrodes placed on the faces of one of the branches are brought to the same potential as the electrodes carried by the sides of the other branch. This arrangement also makes it possible to vibrate asymmetrically the branches 8 and 10 of the tuning fork in order to compensate for their respective kinetic moments.

 In the example shown in FIG. 1, the electrodes 12 (only one of which is visible), arranged on the faces of the branch 8 and the electrode 14 disposed on the sides and on the end of the branch 10 are connected to a common terminal which is referenced 16. Similarly, the electrodes 18 (only one of which is visible) disposed on the faces of the branch 10 and the electrode 20 disposed on the sides of the branch 8 and its end are connected to the terminal municipality which is referenced 22.

Furthermore, to obtain a good linearity between the temperature and the frequency delivered by the thermometer, it is necessary to choose on the one hand the angle
b <p and on the other hand the value of the ratio å in certain
with forks. To optimize the linearity of the response, it is also necessary to respect a relationship between the value
b chosen for <p and the value of the ratio.

at
More precisely, the angle <p must be between - 900 and - 500 or between 400 and 900. I1 must,
b moreover, that the ratio a is between 0 and 0.8.

Figure 3 gives on the ordinate the value of the coef
-9 o -2 ficient ss expressed as 10 9 C 2 as a function of the va
b their of the ratio b plotted on the abscissa for different va
at their angle <P expressed in degrees.

These curves therefore give the pairs of optimal values of å and <p to effectively cancel the coef
a ficient ss. The curves giving the value of the coefficient a of the first degree term as a function of the temperature
b show that, for the pairs of values of <p and of å chosen, this coefficient has a suitable value, which therefore gives good sensitivity to the thermometer.

As an example, a thermometer was produced with a torsionally excited diapason quartz having the following characteristics
<p = + 900
a = 0.22 mm b = 0.55 b = 0.12 mm a
L = 2.4 mm (length of a branch of the tuning fork)
In this case, the excitation frequency is 608 Hz and the temperature coefficients of this thermometer are
a = 33.10 C ss = O;
It is important to observe that the choice of the value + 900 for the parameter <p is particularly interesting. Indeed, this choice allows to obtain by chemical machining a tuning fork having clear lateral faces and perpendicular to the upper and lower surfaces. In addition, it should be noted that the working frequency of 608 kHz is very reduced compared to the working frequencies quartz thermometers according to the prior art, the working frequencies of which are around ten MHz.

 Figure 2 shows another resonator usable in the thermometer according to the invention. The resonator 2 'has the shape of a parallelepiped bar of width a' and thickness b '. The electrodes 13 and 13 'are arranged on the upper and lower faces of the bar 2'. They are connected to the same terminal 17. The electrodes 15 and 15 'are arranged on the sides of the bar 2'. They are connected to the same terminal 19. Thus, the bar 2 'vibrates in torsion. The thermal characteristics of the bar 2 ′ are substantially identical to those which have been described in connection with FIG. 1 and the curves in FIG. 3 remain valid for the bar.

 Referring now to Figure 4, there will be described an embodiment of the electronic processing circuit which, with the resonator, constitutes the thermometer.

In general, the thermometer comprises: an oscillating measurement circuit A including the temperature sensor constituted by the resonator; a reference oscillating circuit B; means C for comparing the signals delivered by the oscillators A and 3; and means
D display of the temperature and reset of the measurement system.

 The oscillating measurement circuit A comprises the measurement resonator 2 or 2 ′ described above and a maintenance circuit of known type. This circuit comprises the amplifier 30 with, in feedback, the resonator 2. Capacities 32 and 34 are mounted respectively between the input and the output of the amplifier on the one hand and the ground on the other hand. This circuit is completed by an isolation amplifier 36.

 The reference oscillating circuit B has a substantially identical structure and comprises the amplifier 38 with, in feedback, the reference resonator 40.

This circuit further comprises the capacitors 42 and 44. It should be noted that the capacitor 44 is adjustable in order to be able to adapt the reference frequency as will be explained later.

 The comparison means C essentially comprise two counters by M (M: whole number) referenced 46 and 48 respectively, a third counter 50 and a generator 52 of electric pulses with adjustable frequency. More precisely, the clock inputs 46a and 48a of the counters 46 and 48 receive the pulses delivered by the oscillating circuits A and B. The outputs 46b and 48b of the counters 46 and 48 are connected to the two inputs of an EXCLUSIVE OR gate 54 The variable frequency source 52 preferably consists of a generator 52a which delivers a pulse signal at a fixed high frequency and the output of which is connected to a programmable divider 52b. At the output of the divider 52b, the signal therefore has the frequency 4 adjusted by the division rate of the divider 52b. Of course, the generator 52 could also consist of a generator with controllable frequency.

The output of the generator 52 and the output of the gate 54 are respectively connected to the two inputs of an AND gate 56. The clock input 50a of the third counter 50 is connected to the output of the AND gate 56. The outputs of the counters 46 and 48 are connected respectively to input D and to clock input 58a of a bistable
D 58 whose function is to give the sign of the temperature, that is to say the order in which the rising edge of the signals delivered by the meters 46 and 48 occurs respectively.

The display and reset circuit D includes a latch 60, the inputs of which are connected to the binary outputs 501 to 50 of the counter 50. The latch 60
n is itself connected to a decoding and display assembly 62. The display assembly 62 includes a particular input 62a connected to the output of the bistable 58 to display the sign of the temperature relative to OOC.

 The outputs 46b and 48b of the counters 46 and 48 are also respectively connected to the two inputs of an AND gate 64. The output of the gate 64 is connected to the input of the monostable 66 which is of the "non-re-excitable" type. The output 66a of the monostable 66 delivers a control signal LD which is applied on the one hand to the input of the second monostable 68 and on the other hand to the control input 60a of the latch 60. The monostable 68-delivers a pulse which is applied to the reset inputs 46c, 48c and 50c of the counters 46, 48 and 50 respectively.

The operation of the thermometer which has just been described is as follows
The counter 46 counts the pulses delivered by the measurement oscillator A. When the counter has counted M pulses, its output goes to logic state 1, as shown in the diagram (1) of FIGS. 5a and 5b.

Simultaneously, the counter 48 counts the pulses delivered by the reference oscillator B. When the counter has counted M pulses, its output goes to logic level 1, as shown in the diagram (2) of FIGS. 5a and 5b. Let Tt be the time that the counter 46 takes to count M pulses and T the time that the counter takes
r 48 counting M pulses. The signal L which appears at the output of the EXCLUSIVE OR gate 54 has a logic level 1 between the instants tl and t2
1 2 the outputs of counters 46 and 48 go to logic state 1. Signal L therefore has logic level 1 for a duration T t TT. In the case
r of FIG. 5a, the time T t is less than the time Tr, that is to say that the temperature to be measured is greater than the reference temperature. On the contrary, in the case of FIG. 5b, the time Tt is greater than the time Tr, therefore the temperature to be measured is less than the reference temperature.

The counter 50 receives on its input 50a the pulses of the signal P delivered by the AND gate 56. I1 therefore receives the pulses delivered by the generator 52 between the instants tl and t2. Consequently, from the instant t2 (fig. 5a), the content of the counter 50 is equal to the number of pulses N emitted by the generator 52 during the time Tt - T. This number N is representative
r of the temperature T to be measured as will be explained later. In addition, at time t2 (fig. 5a), the door
ET 64 sees its output passing to logic level 1, since its two inputs are at logic level 1. The output of the monostable 66 therefore passes to logic level 1 during the time T1 fixed by its time constant, which gives the signal LD. The LD signal delivered by the monostable 66 controls the unlocking of the latch 60 and the number N of pulses is applied to the decoding and display system 62. The falling edge of the LD signal causes the state of the mono output to change. stable 68 which passes to logic level 1 during the time T2 fixed by its time constant. This pulse resets counters 46, 48 and 50 to zero. When the falling edge of this pulse appears, the device is ready for a new temperature measurement.

In summary, the counter 50 counts the pulses of the signal delivered by the pulse generator 52 in the window limited by the instants t1 and t2. The number
N of pulses represents the temperature T to be measured, the relationship between N and T being very linear thanks to the choice of the resonator 2.

 The thermometer is adjusted by acting on the frequency fC of the signal delivered by the generator 52 and on the value of the capacitance 44 of the reference oscillator.

 FIG. 6 shows several lines for different settings D1, D2 and D3 which give the linear relationship between the temperature T and the number N of pulses counted by the counter 50. The modification of the frequency fC makes it possible to modify the proportionality ratio between temperature T and the number of pulses counted.

It therefore acts on the value of the angle Y (Y1XY2) On the contrary, an action on the capacitor 44 modifies the reference value, that is to say the number No of pulses corresponding to the temperature OOC. In Figure 6, the line D1 corresponds to an optimal setting since No = 0 for T = 0 and that the angle y = y2 corresponds to a good sensitivity of the thermometer.

To adjust the value of the capacity 44, the sensitive part of the thermometer (oscillator A) is placed in an atmosphere at 0 ° C. The time Tt therefore corresponds to
o the temperature O C. The capacity 44 is modified until the counter 50 does not count any pulse from the generator 52. In this situation, T = Tt and the oscillator
rt
B therefore delivers a signal at a reference frequency corresponding to T = OOC. This setting is of course independent of the setting of the frequency fC.

 The value of the angle y is adjusted by acting on the frequency f C without modifying the capacitance 44.

 If the resonator has a coefficient a of the first degree of 30.10 600C and if M (counting rate of counters 46 and 48) is worth 2-105, an increase in temperature of 1 0C involves a reduction in the counting time of the Ous. If therefore the frequency f C is chosen equal to 10 MHz (period TC = O. lps) N will be worth 100. Consequently, if the frequency of the resonator '2 at T = OOC is equal to 600 kHz, such a thermometer can measure temperature differences of 0.010C.

 In the example considered two successive measurements will be separated by a time interval less than or equal to 400 ms if the time constants t1 and T2 of the monostables 66 and 68 are chosen judiciously.

 The circuit which has just been described constitutes only one embodiment particularly well suited to the problem to be solved. It is clear, however, that other circuits could be used. It suffices that these circuits include a device which makes it possible to measure the frequency difference between the signal delivered by the measurement oscillator and the signal delivered by the reference oscillator, and a device for displaying this measurement preferably in digital form.

 It follows from the preceding description that, thanks to the choice of the resonator and of its excitation mode, the thermometer which is the subject of the invention has a simple structure and therefore inexpensive.

 It is also clear that the machining of quartz is simplified, whether it is a tuning fork or a bar. In particular, when the angle <p is 900, the resonator can easily be machined by chemical attack.

 The treatment circuit has a simple structure and allows easy adjustment of the thermometer. In addition, because the resonator works at a frequency of the order of 600 kHz, the power consumption is reduced.

 In addition, despite these multiple simplifications, the thermometer makes it possible to obtain a temperature measurement with good resolution (0.010C) and a sufficiently low measurement periodicity (of the order of 400 ms) to allow rapid variations in temperature.

Claims (8)

 1. Quartz thermometer, characterized in that it comprises  - a resonator (2,2 ') comprising at least one elongated element (8,10) of quartz whose length (L) is substantially parallel to the axis X of the quartz;  - means comprising electrodes (12, 14, 18,20) disposed on said elongated element (8,10) to create in the quartz an electric field causing said elongated element (8,10) to vibrate in torsion, said resonator ( 2.2 ′) delivering a signal whose frequency variations are a substantially linear function of the temperature; and  - an electronic circuit (A, B, C, D) for processing the signal delivered by said resonator (2,2 ') and supplying at its output a quantity representing the temperature to which said resonator is subjected.  2. Thermometer according to claim 1, characterized in that said resonator is a quartz bar (2 ') parallelepipedal with a substantially rectangular cross section.  3. Thermometer according to claim T, characterized in that said resonator is a tuning fork (2) each branch (8,10) in the form of a parallelepiped with a rectangular cross section.  4. A thermometer according to any of claims 2 and 3, characterized in that the ratio between the lengths of the small (b, b ') and the large (a, a') side of the cross section is between 0 and 0, 8.  5. Thermometer according to any one of claims 2 to 4, characterized in that the short side (b, bt) of the cross section makes with the axis Y of the quartz an angle <p between + 400 and + 900 or - 500 and - 900.  6. Thermometer according to claim 5, characterized in that the angle <p is substantially equal to + 900.  7. Thermometer according to any one of claims 1 to 6, characterized in that said electronic circuit comprises  - a reference oscillator B with adjustable frequency;  - Means (C) for comparing the frequency of the signal delivered by said resonator (2) and said reference oscillator B and producing an indication of temperature from the difference between the frequencies of these signals; and  - Means (62) for displaying said temperature indication.  8. Thermometer according to claim 7, characterized in that said comparison means (C) comprise  - Means (46) for counting M pulses delivered by said resonator (2) and for delivering a first signal when the M pulses are counted;  - means (48) for simultaneously counting M pulses delivered by said reference oscillator (B) and for delivering a second signal when the M pulses are counted;  - means (52) for delivering a measurement signal with adjustable frequency; and  - Means (50) for counting the pulses of said measurement signal between said first and second signals.