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The present invention relates to a temperature detector using a surface
acoustic wave device of which the frequency characteristic varies
responding to the temperature of a component base plate thereof and, more
particularly, a temperature detector capable of detecting in a wireless
manner the temperature of the base plate composing the surface acoustic
wave device at a plate remote from this surface acoustic wave device.
There have been publicly known various temperature detecting means, for
example, a means for detecting the change in the resistance value of a
thermistor responsible to the change of temperature, a means for detecting
the change in the electromotive force of a thermocouple responsible to the
change of temperature, a means for detecting the change in the quantity of
thermal expansion of a matter itself as in the thermometer, and so on.
A rising demand exists these days to detect by a wireless means the
temperature of a matter or in a space located apart from the place where
temperature detection is made. None of the above-mentioned means can meet
this demand. A first means using the wireless means detects the infrared
rays emitted from a matter, for example. However, this first means must be
provided with a semiconductor infrared-ray-sensor or a piroelectric device
of relatively high cost. In addition, it is necessary in the first means
to correct the emissivity of the surface of a matter of which temperature
is to be detected. The first means can be therefore employed in particular
fields.
A second means using the wireless means is of the type which modulates a
carrier wave in the form of AM (amplitude modulation), FM (frequency
modulation), or PCM (pulse code modulation) using measured temperature
data, and which transmits the modulated wave by means of the wireless
means to a place where the temperature detection is to be made. However,
because of high cost, this second means can be applied only to a
temperature detection system of relatively large scale.
As a third means using the wireless means is the well known means which
uses a crystal vibrator. The increasing use of micro-computers these days
has caused this third means to be developed for the purpose of obtaining
measured temperature data in a digital manner. It is usually required that
a crystal vibrator minimize the change in the oscillating frequency
thereof due to changes in temperature. However, a vibrator in which the
change in the resonant frequency due to changes in temperature is large
can be obtained by selectively applying Y-cut or LC-cut to the crystal
plate thereof. When the vibrator thus obtained is used to form an
oscillator and the oscillating frequency thereof is measured by a counter,
the temperature of this vibrator can be detected in a digital manner. This
makes it possible to detect temperature with high accuracy and good
linearity. However, the third means has the following drawbacks: this
vibrator is not suitable for mass production because the vibrators must be
formed and trimmed independently from one another; and since the
mechanical vibration phenomenon is due to the whole of the vibrator, it is
difficult to use the vibrator itself stuck to the inner face of a metal
case which has a large heat transfer rate, and therefore it is necessary
to support the vibrator by means of a supporting rod arranged inside the
metal case, so that the response time during which the vibrating frequency
of the vibrator varies due to the change in the atmospheric temperature
outside the metal case becomes large because the temperature of air, for
example, outside the case must be transfered through the gas or air inside
the case to the vibrator.
Accordingly, the object of the present invention is to provide a
temperature detector using a surface acoustic wave device suitable for
mass production which enables the response time due to the change in
atmospheric temperature to be short, the digitization of measured
temperatures to be easy, and measured temperatures to be transmitted in
wireless manner.
The temperature detector of the present invention includes a surface
acoustic wave device and comprises a transmitting section including an
oscillator having a surface acoustic wave device of which the frequency
characteristic varies corresponding to the temperature of a base plate
thereof, the oscillator generating an oscillation output of a frequency
corresponding to the frequency characteristic of the surface acoustic wave
device and a first antenna coupled to the oscillator for transmitting the
oscillation output; and a receiving section including a second antenna for
receiving the output of the first antenna, a means for detecting the
oscillation frequency of the oscillator from the output of the second
antenna and a signal processing circuit for processing the output of the
detecting means to generate at least one of a temperature display and
control signals of the base plate of the surface acoustic wave device.
This invention can be more fully understood from the following detailed
description when taken in conjunction with the accompanying drawings, in
which:
FIG. 1 is a perspective view showing an example of the surface acoustic
wave resonator employed in the temperature detector of the present
invention;
FIG. 2 is a cross-sectional view taken along the line II--II in FIG. 1;
FIG. 3 is a circuit diagram showing an example of the transmitting section
of the temperature detector according to the present invention and
including the surface acoustic wave resonator shown in FIG. 1;
FIG. 4A is an equivalent circuit of the circuit shown in FIG. 3;
FIG. 4B is a view partly showing a modification of the circuit shown in
FIG. 3;
FIG. 5 is a graph showing the relation between the temperatures of the base
plate of the surface acoustic wave resonator included in FIG. 3 and the
changing rate in the oscillation frequencies of the oscillator shown in
FIG. 3;
FIG. 6 is a block diagram showing an example of the receiving section of
the temperature detector according to the present invention; and
FIG. 7 is a block diagram showing another example of the transmitting
section of the temperature detector according to the present invention.
In FIG. 1 a surface accoustic wave resonator 1 includes a Y-cut and
Z-propagation base plate (piezoelectric base plate) 2 of lithium niobate,
an interdigital transducer 3 arranged in the center and on the surface of
this base plate 2, about 200 pieces of reflecting strips 4 arranged at the
both sides of the transducer 3, and terminals 5 respectively connected to
a pair of electrodes forming the transducer 3. The electrodes of the
transducer 3 and the reflecting strips 4 are formed by vapor-depositing a
thin film of aluminium on the surface of the base plate 2 and then
applying photo-etching thereto. The film thickness of aluminium is 2000 A,
and the line width as well as the space of the electrodes of the
transducer 3 and the reflecting strips 4 are about 12 .mu.m, respectivley.
The element 1 can be formed about 50 units on a base plate of which
diameter is about 5 cm by means of the usual IC (integrated circuit)
forming process. These 50 units of element are separated one from another
by cutting. The area of the base plate having one unit element is 1.5
mm.times.10 mm and the thickness thereof is 0.5 mm.
The above-mentioned surface acoustic wave resonator shows the same
resonance characteristics as that of a crystal resonator, and it is taught
in the U.S. Pat. No. 3,886,504 (C. S. Hartmann et al) that this surface
acoustic wave resonator can be used as a filter or an oscillator
component. However, it should be noted that the following large
differences exist between the crystal resonator and the surface acoustic
wave resonator: in the case of the crystal resonator the whole plate of
the resonator vibrates mechanically while in the case of the surface
acoustic wave resonator the mechanical vibration is generated and
propagated centering on a portion extremely adjacent to the surface of the
base plate; the oscillation frequency characteristics has no relation to
the shape of the base plate; and it is the shape and arrangement of the
electrodes of the transducer 3 and the reflecting strips 4 that determine
the frequency characteristics, particularly the resonance frequencies are
determined mainly by the width as well as the spacing of the electrodes of
the transducer and the reflecting strips 4. For these reasons, the surface
acoustic wave device having resonace frequencies in a band higher than 30
MHz, that is, in the VHF or UHF band can be easily produced in mass
production scale.
The shape of the base plate can be quite freely selected as described
above, so that the thickness of the base plate can be made thin. In
addition, since the shape of the back face of the base plate can also be
freely selected, the base plate is suitable for forming a temperature
sensor of which the response time due to the change in atmospheric
temperature is short by sticking the base plate directly to a metal plate
having a large heat transfer rate.
The Y-cut and Z-propagation piezoelectric base plate of lithium niobate has
the following advantages: The changing rate of frequency characteristic
thereof corresponding to a temperature of the base plate is large: The
Y-cut angle deviation ranging .+-.5.degree. and the Z-propagation
direction angle deviation ranging .+-.0.5.degree. cause the deviation of
surface acoustic wave speed to be only the range of .+-.0.3 m/s. This
deviation of .+-.0.3 m/s corresponds to a temperature change of
.+-.1.degree. C. of the base plate. This means that the above-mentioned
deviations do not prevent the mass production of the surface acoustic wave
devices, that characteristic deviations among the surface acoustic wave
devices are substantially small and that the acoustic wave devices thus
produced are suitable for temperature sensors.
Referring to FIG. 1, the element 1 is stuck onto an iron plate 6 which is
0.4 mm thick and 12 mm.times.15 mm wide, for example, through a flexible
adhesive layer 7 such as epoxy resin, as shown in FIG. 2. A cover (not
shown) is attached onto this iron plate. The terminals 5 are attached to
the iron plate 6 through insulating members. This construction prevents
the resonance frequency of the element 1 from being changed due to the
distortion of the base plate 2 of the element 1 which is caused by the
distortion of the iron plate 6 due to thermal expansion or other reasons,
as the distortion of the plate 6 is absorbed by the flexible adhesive
layer 7.
In a transmitting section shown in FIG. 3, numeral 8 represents TOSHIBA's
linear IC TA-7131p, which has external connection terminals 1a-7a and
includes a bias stabilizer 9, resistors R1-R4, and transistors Tr.sub.1
and Tr.sub.2. The negative pole of a power source 10 (battery, 1.5 V) is
earthed and the positive pole thereof is connected through the terminal 7a
and the resistor R4 to the collector of the transistor Tr.sub.2, directly
to the collector of the transistor Tr.sub.1, through the resistor R2 to
the input terminal of the stabilizer, and through the resistors R2 and R1
to the base of the transistor Tr.sub.1, respectively. The emitters of
transistors Tr.sub.1 and Tr.sub.2 are connected to each other through the
resistor R3 and an output of the stabilizer 9 is supplied to the base of
the transistor Tr.sub.2. The terminals 2a-6a are connected to the base of
the transistor Tr.sub.1, the emitter of the transistor Tr.sub.1, the
emitter of the transistor Tr.sub.2, the base of the transistor Tr.sub.2,
and the collector of the transistor Tr.sub.2, respectively. One terminal 5
of the surface acoustic wave device 1 is earthed and the other terminal 5
thereof is connected to the terminal 2a, and to the terminal 3a through a
capacitor C.sub.1. One end of a capacitor C.sub.2 is earthed and the other
end thereof is connected to the terminal 3a, and to the terminal 5a
through a capacitor C.sub.3. The terminal 4a is earthed. The terminal 6a
is connected to a receiving antenna 12 (which may be an ordinary
conductor) through a capacitor C.sub.4. In this embodiment, the element 1
is designed to have a resonance frequency of 74.80 MHz at 25.degree. C.
FIG. 4A shows an equivalent circuit of the oscillating portion included in
the circuit shown in FIG. 3. As shown in FIG. 4A, the electric equivalent
circuit of the surface accoustic wave device 1 is represented by a series
resonance circuit of r, Co and Lo, and an electrostatic capacitor C.sub.T
between the electrodes of the inter-digital transducer 3. The series
resonance circuit and the capacitor C.sub.T are connected is parallel. The
impedance viewed from a point 13 in the direction of the IC circuit 8 is
shown by a series circuit including a capacitor Cx and a negative
resistance -R. The value of Cx is determined mainly by the values of
capacitors C.sub.1 and C.sub.2 and the characteristics of the transistor
Tr.sub.1 shown in FIG. 3. When a condition that the negative resistance
.vertline.-R.vertline. is larger than the resistance r is fulfilled,
oscillation is maintained. In this embodiment, the oscillation frequency
of the oscillator is determined to be 75.00 MHz at 25.degree. C. This
oscillation output is amplified by the transistor Tr.sub.2 and the
amplified power is applied to the antenna 12 to be transmitted. The
current values supplied from the power source 10 at the time of
oscillation are in the range of about 1 mA to 1.5 mA. Electric power of 75
MHz and about 1 mW is emitted from the antenna 12. In the actual
application of this invention, it is desirable to operate the oscillator
by way of a battery and to reduce the power consumption thereof. To attain
this object, it is desirable to operate the oscillator intermittently for
controlling a temperature intermittently as well. For this, as shown in
FIG. 4B, the oscillator can be periodically operated by periodically
closing a switch 14a by the output of a timer 14b, said switch 14a being
arranged between a power source 10 and a terminal 7a.
A receiving section will be described with reference to FIG. 6. The radio
wave emitted from the transmitting antenna 12 is received by a receiving
antenna 15. The output (having a frequency of 75 MHz) of the antenna 12 is
picked up through a band pass filter 16, which has a center frequency of
75 MHz and a band width of .+-.2 MHz, and then amplified by a high
frequency amplifier (IC TA 7124p for TOSHIBA's television). The outputs
from a crystal oscillator 19 of 75.167 MHz and an amplifier 17 are applied
to a mixer 18 as inputs and the output of the difference frequency between
both inputs is picked up through a low frequency amplifier 20 and then
amplified by an amplifier 21. The output frequency of the amplifier 21 is
counted by a counter 22. TOSHIBA's digital IC TC 4029p was used as the
counter. The output of the counter 22 is subjected to a predetermined data
process in a data processor and a temperature display or control signal is
picked up from this data processor. FIG. 6 shows a case where the
temperature detector of the present invention is employed in an air
conditioner which is controlled by a micro-computer 23 and located at the
corner of a room. In other words, FIG. 6 shows the case where a signal of
4 bits is applied from the counter 22 to the micro-computer 23 and a
temperature display signal 24 (which displays the temperature of the base
plate in the surface acoustic wave device located in the center of the
room) and a temperature control signal 25 (which controls indirectly the
temperature of the base plate 2 in such a way that the temperature of the
center of the room becomes equal to a predetermined value) are emitted as
outputs. TOSHIBA's 4 bits IC TMP-4315 can be used as the micro-computer.
FIG. 5 shows a characteristic curve 27 which is drawn by plotting the
temperatures of the temperature sensor (the temperatures of the base plate
of the surface accoustic wave device 1 shown in FIG. 1) on the axis of
abscissa and the frequency change rates (PPM) on the axis of ordinate,
said frequency change rates being obtained by normalizing the difference
frequencies detected by the receiving section (FIG. 6) by 75 MHz.
Measurement was made by housing the oscillator in a thermostatic vessel
and locating the receiving section 2 m apart from the vessel. The
temperature of the temperature sensor was displayed to be 26.degree. C. by
the receiving section when about 2 minutes had passed after the
temperature sensor was moved from the theremostatic vessel of 35.degree.
C. to a place of which room temperature was 25.degree. C. In other words,
this shows that measurement accuracy of about 90% was attained after 2
minutes. It has been found that the response time due to the change in
temperature meets practical needs.
Though the case where the temperature detector of the present invention is
applied to the air conditioner has been described above, the temperature
at a predetermined portion of a moving or rotating body can be measured by
the wireless means at a place spaced apart from the body. The transmitting
section of the present invention can be formed by a publicly well known
oscillator, in which a loop is formed by a surface acoustic wave delay
line and an amplifier, as well as the surface acoustic wave resonator
(shown by numeral 1 in FIG. 1). For example, as shown by the principle
block diagram in FIG. 7, it may be arranged so that the output of a first
inter-digital transducer 3a of the surface acoustic wave delay line is
supplied to an amplifier 28 and the output of the amplifier 28 is then
supplied to the antenna 12 while being fed back through a second and the
first inter-digital transducers 3b and 3a to the amplifier 28, said
surface acoustic wave delay line being formed by arranging the first and
second inter-digital transducers 3a and 3b at both ends of a base plate 1a
.
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