JPH0641887B2 - SAW force sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a force sensor used for measuring or detecting a mechanical physical quantity such as stress, pressure, strain, displacement, etc. S using surface acoustic wave (SAW: Surface Acousic Wave) propagating on the surface of solid material
The present invention relates to a SAW force sensor that applies the pressure-sensitive function of an AW delay line.

[Conventional technology]

Strain gauges whose resistance value changes according to strain applied are widely used in sensors that measure mechanical physical quantities such as stress, displacement, strain, and pressure. This strain gauge is generally used by being attached to a spring material. Conventionally,
In order to detect a mechanical physical quantity, a method of forming a Wheatstone bridge with a strain gauge and a resistor, converting a resistance value change of the strain gauge into a voltage change and outputting the voltage change is adopted. In addition to the above, there is also one that utilizes a change in resistance value due to stress of diffusion resistance formed in a semiconductor. This type of sensor employs a method of forming a Wheatstone bridge like the strain gauge and outputting a change in resistance value as a change in voltage. A major feature of this type of sensor is that a silicon substrate is used and a pressure sensitive portion and an electric circuit for signal detection are integrated on the same substrate. Further, temperature compensation for temperature changes is also performed on the same substrate by utilizing the temperature sensing function of the semiconductor. As described above, the sensor of the type that uses the diffusion resistance of the semiconductor is a more practical sensor, and is a sensor that has been downsized and mass-produced.

Since the relationship between the stress and the strain of an object is uniquely determined, the stress and the strain may be used synonymously without any particular distinction.
For example, an example in which the stress sensor and the strain sensor are the same can be given. Therefore, in this invention, the term force sensor is used.

In addition, SA is another technology that belongs to force sensors.
Some use the pressure-sensitive function of the W delay line. This SAW
When stress is applied to the delay line where the surface acoustic wave propagates, the SAW propagation velocity changes in proportion to the magnitude of the force.
As a result, a piezoelectric substrate having the property that the delay time of the SAW delay line changes is used. If the oscillator is configured using the SAW delay line having this characteristic, it is possible to realize the SAW force sensor that outputs the frequency change amount proportional to the applied force. SAW realized in this way
The force sensor has good resolution, and can be a sensor with higher accuracy than various types of force sensor using the conventional resistance change. In addition, S that constitutes the sensor
The AW oscillator has a great feature that it can be easily configured by a SAW delay line also serving as a sensor section and an amplifier.

[Problems to be solved by the invention]

The SAW sensor is not limited to the force sensor, but the majority of the SAW sensor constitutes a SAW oscillation circuit, and the measured amount is displayed by a change in the natural oscillation frequency of the oscillation circuit. In this way, the method of displaying the amount of change in frequency is
This is because the SAW delay line itself is a passive device, and is not a device of the type that spontaneously emits energy or that converts the emitted energy of the DUT into another form and outputs it. Therefore, when the SAW delay line is used for the sensor, first, the SAW delay line operates in a stable state, that is, the SAW delay line is used as an oscillating element, that is, the SAW delay line works in cooperation with the SAW delay line. It is necessary to create a state of steady oscillation at the natural frequency of.

However, since the piezoelectric substrate used for the SAW delay line has a temperature coefficient of SAW velocity, it has a property of reacting sensitively to ambient temperature, and when an oscillator is constructed, the natural oscillation frequency fluctuates. There is. Therefore, in order to obtain a sensor with higher sensitivity, a method of compensating for the frequency fluctuation due to the ambient temperature is required. It is also necessary to simultaneously compensate for fluctuations in the oscillation frequency due to changes over time in the SAW delay line.

Therefore, in the conventional SAW force sensor, two oscillators are configured by using two SAW delay lines each having substantially the same or symmetrical (congruent) shape because of the necessity of compensating for the variation due to temperature and aging. The one is used for compensating for changes in temperature and aging, and the other is used for detecting force, and a beat signal of each oscillation circuit is extracted. However, this method cannot function as a sensor with higher accuracy, and thus another method is needed.

[Means for solving problems]

Therefore, in the present invention, a piezoelectric substrate having a characteristic that the SAW speed changes with an applied force is used.
An electrode for transmitting and receiving an AW (also called an interdigitated electrode or IDT. TDT is an abbreviation for Interdigital Transducer) is provided, and, for example, between these two electrodes, that is, a transmitting electrode and a receiving electrode, A heating resistor that generates heat according to the input power is provided, and two SAW delay lines that are structurally substantially the same or symmetrical (congruent) constitute two oscillators. And for compensating for changes with time, the other oscillator was used for detecting force. It should be noted that the heat generating resistor may be provided anywhere on the piezoelectric substrate, as long as heat can be transmitted to the surface of the piezoelectric substrate where the SAW propagates between the transmitting electrode and the receiving electrode, but as described above, It is preferable to provide it between the transmitting electrode and the receiving electrode in terms of response speed. Further, the two heating resistors have the same resistance value.

Further, a phase-locked loop circuit (PL) is constructed by an oscillator, a reference frequency oscillator, a phase comparator, and a low-pass filter for compensating for temperature and aging variation configured as described above.
L: Phase-Locked Loop circuit).

In this PLL circuit, the oscillation frequency of the compensation SAW oscillator is made to follow the oscillation frequency of the reference frequency oscillator, and when it follows, a current flows through the heating resistor on the compensation SAW delay line. This current is the temperature,
This is to compensate for fluctuations in the oscillation frequency of the SAW oscillator due to changes over time. This utilizes the temperature change of the SAW delay line due to the heating resistor based on the temperature sensing function of the SAW oscillator. This SAW oscillator is usually a voltage controlled oscillator (VCO) that constitutes a PLL circuit.
scillator).

The control current obtained by the above-described configuration is passed through the heating resistor, which is one of the constituent elements of the SAW oscillator for detecting the other force, so that the temperature and the variation with time are compensated. It can be provided as a sensitive SAW force sensor.

[Work]

That is, a PLL circuit including a SAW oscillator, a reference frequency oscillator, a phase detector, and a low-pass filter for compensating for temperature and aging, and an SA for detecting force.
High-precision SAW by configuring the sensor with a W oscillator
A force sensor can be realized.

Now, when the surface temperature of the two SAW delay lines changes, each SAW oscillator shows the same amount of frequency change. This is because the two SAW delay lines are provided close to each other. At this time, S for compensating for temperature and aging changes
The AW oscillator operates so as to be pulled back to the same oscillation frequency as the oscillation signal of the reference frequency oscillator by the PLL circuit having the above configuration. In order to perform such an operation, in the present invention, a current necessary for heat generation is made to flow through a heating resistor provided on a compensation SAW delay line, and the value of this current is controlled by an output signal of the PLL circuit. Is taking. Based on such control, this current becomes an amount according to the temperature fluctuation, and the heating resistor on the SAW delay line that constitutes the SAW oscillator for force detection, which has the same configuration as the SAW oscillator for compensation, simultaneously This allows the temperature to be compensated as a detecting oscillator. Further, it is considered that the frequency changes of the two SAW oscillators due to changes over time occur equally because they have the same configuration. Here, by flowing the current detected by the compensation oscillator through the heating resistor on the detection SAW delay line, compensation can be performed by a mechanism similar to that of temperature.

〔Example〕

FIG. 1 shows an embodiment of the SAW delay line used in the present invention. On the first and second piezoelectric substrates 1a and 1b (which may be separate or the same substrate), first and second transmitting electrodes 2a and 2b for emitting SAW, and the first and second transmitting electrodes 2a and 2b, respectively. And the SAWs emitted from the second transmitting electrodes 2a and 2b, respectively.
First and second SAW delay lines 4a that are structurally and substantially the same or symmetrical (congruent) are provided by providing first and second receiving electrodes 3a and 3b for receiving , 4b are formed. In addition, S is provided on the first and second delay lines 4a and 4b.
First and second heat generating resistors 5a and 5b which absorb electric power from the outside and generate heat are provided in a portion where AW is propagated. The first and second piezoelectric substrates 1a and 1b are fixed to the substrate fixing member 6.
The first transmitting electrode 2a and the first receiving electrode 3a are the first amplifier (oscillator) 7a, and the second transmitting electrode 2b and the second receiving electrode 3b are the second amplifier (oscillator). ) 7b, respectively, to form first and second SAW oscillators 8a, 8b. Then, the first transmitting electrode 2a and the first receiving electrode 3
a, the first SA composed of the first amplifier (oscillator) 7a
W oscillator 8a is used for compensation of temperature and aging,
A second SAW oscillator 8b including a second transmitting electrode 2b, a second receiving electrode 3b, and a second amplifier (oscillator) 7b is used for force detection. One of all electrodes is connected to ground potential. The first and second heating resistors 5a and 5b are connected in series, and the other of the first heating resistors 5a is connected to the frequency control terminal 9 and the second heating resistor 5b.
The other of is connected to the ground potential. The force is applied by using the means 10 for exerting the force to be measured on the second SAW delay line having the above-mentioned configuration.

FIG. 2 shows an embodiment of the SAW force sensor of the present invention. The first transmission electrode 2a is formed on the first piezoelectric substrate 1a.
And a first receiving electrode 3a and a first heating resistor 5a are formed, and a first SAW delay line 4a for compensating for temperature and aging is formed.
And One of the first transmitting and receiving electrodes 2a and 3a is connected to the ground potential, and the other is connected to the first amplifier (oscillator) 7a, respectively, and the first SAW oscillator 8a for compensation is connected to the first SAW oscillator 8a. Constitute. The output of the first SAW oscillator 8a is taken out to the outside via the first output buffer 11a. The signal output through the first output buffer 11a is compared in phase and frequency by the output signal of the reference frequency oscillator 12 and the phase comparator 13, and the comparison result is output as a voltage. Then, the error voltage output from the phase comparator 13 is input to the low-pass filter in the feedback means 14, and only the low frequency component is extracted and output. When this output signal is fed back to the first heating resistor 5a of the compensation first SAW oscillator 8a, the oscillation signal of the compensation first SAW oscillator 8a matches the oscillation frequency of the reference frequency oscillator 12. Frequency and phase. In this way, the phase-locked loop (PLL)
It constitutes the circuit (15). Here, a constant current for heat generation is previously applied to the first heat generating resistor 5a,
Temperature compensation is realized by increasing or decreasing this with a signal output from the circuit. Next, the piezoelectric substrate 1b, the second transmitting electrode 2b, the second
A second SAW delay line 4b for force detection is constructed by the receiving electrode 3b and the second heating resistor 5b. One of these electrodes is connected to the ground potential, and the other is connected to the second amplifier (oscillator) 7b, and the second SAW for detection is used.
This constitutes the oscillator 8b. When a force is applied to the second SAW delay line 4b constituting the second SAW oscillator 8b using the means 10 for applying the force to be measured, the second SW delay line 4b
The AW oscillator 8b exhibits a frequency change according to the force. This second
The output signal of the SAW oscillator 8b of the second output buffer 11
It is input to a means for measuring the oscillation frequency via b, for example, a commercially available frequency counter 16. A high-sensitivity SAW that compensates for temperature and aging by flowing a control current for compensation obtained by the PLL circuit through the second heating resistor 5b provided on the second SAW delay line 4b for detection. A force sensor can be configured.

〔The invention's effect〕

As described above, in the SAW force sensor according to the present invention, the PLL circuit is configured using the SAW oscillator for compensation, and the characteristics of the sensor with temperature and changes with time are detected by the current value, and the current is detected. It is a high-performance sensor because it is fed back to the oscillator for use. The compensation of these sensors can be performed without impairing the sensitivity of the sensor to the force because the accuracy of the piezoelectric substrate is utilized by utilizing good temperature characteristics.

Further, since the current value output from the low-pass filter in the feedback means is an output value mainly based on the temperature change, it can be used as a signal for detecting the temperature of the sensor.

Furthermore, since the SAW force sensor of the present invention outputs a frequency value suitable for digital signal processing as a measurement value, it can be directly used for measurement and control using a microcomputer. Furthermore, since the SAW force sensor according to the present invention utilizes the good pressure-sensitive characteristics of the piezoelectric substrate,
The sensor can have high sensitivity.

Therefore, the SAW force sensor according to the present invention has high industrial utility value.

[Brief description of drawings]

FIG. 1 shows S constituting the SAW force sensor of the present invention.
An example of an AW delay line is shown. FIG. 2 shows an embodiment of the SAW force sensor of the present invention. In the figure, 1a and 1b are the first and second piezoelectric substrates, and 2a and
2b is the first and second transmitting electrodes, and 3a and 3b are the first and second transmitting electrodes.
Receiving electrodes, 4a and 4b are the first and second SAW delay lines,
5a and 5b are first and second heating resistors, 6 is a substrate fixing member, 7a and 7b are first and second amplifiers (oscillators), and 8a and 8b.
Is a first and second SAW oscillator, 9 is a frequency control terminal, 10 is a means for exerting a force to be measured, 11a and 11b
Is a first and second output buffer, 12 is a reference frequency oscillator, 13 is a phase comparator, 14 is a means for feeding back, 15
Is a phase-locked loop circuit, and 16 is a means (frequency counter) for measuring the oscillation frequency.

Claims (2)

[Claims] 1. A first piezoelectric substrate (1a) having a first heating resistor (5a) which absorbs electric power from the outside to generate heat, and a first piezoelectric substrate (1a) provided on the first piezoelectric substrate. A first transmitting electrode (2a) for emitting a first surface acoustic wave and a first receiving electrode (3a) provided on the first piezoelectric substrate for receiving the first surface acoustic wave. First surface acoustic wave delay line consisting of and (4a)
A first oscillator (8a) that cooperates with the first surface acoustic wave delay line and oscillates at the natural frequency of the first surface acoustic wave delay line; and a first oscillator (8a) in series with the first heating resistor. A second piezoelectric substrate (1b) having a second heating resistor (5b) connected to it, and a second piezoelectric substrate (1b) provided on the second piezoelectric substrate and emitting a second surface acoustic wave. The first surface acoustic wave includes a transmitting electrode (2b) and a second receiving electrode (3b) which is provided on the second piezoelectric substrate and receives the second surface acoustic wave. A second surface acoustic wave delay line (4b) placed adjacent to the delay line and having a substantially congruent shape; cooperating with the second surface acoustic wave delay line, A second oscillator (8b) oscillating at the natural frequency of the surface acoustic wave delay line; a reference frequency oscillator (12); a phase of an output signal of the reference frequency oscillator and a phase of an oscillation signal of the first oscillator. And A phase comparator (13) for comparing and outputting an error voltage proportional to the difference, and means (14) for feeding back the output of the phase comparator to the first and second heating resistors, A phase-locked loop circuit (15) adapted to match the oscillation frequency of the first oscillator with the frequency of the reference frequency oscillator; and means for exerting a force to be measured on the second piezoelectric substrate.
A SAW force sensor comprising: (10); and a means (16) for measuring an oscillation frequency of the second oscillator, for detecting an external force applied to the second surface acoustic wave delay line. 2. The SAW force sensor according to claim 1, wherein the first piezoelectric substrate (1a) and the second piezoelectric substrate (1b) are one substrate. .