JPH0548961B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a force sensor used to measure or detect mechanical physical quantities such as stress, pressure, strain, and displacement, and particularly relates to a force sensor used for measuring or detecting mechanical physical quantities such as stress, pressure, strain, and displacement. Surface acoustic waves (SAW) propagating on the surface of
This paper relates to a SAW force sensor that applies the pressure-sensitive function of a SAW delay line using acoustic waves.

[Conventional technology]

In sensors that measure mechanical physical quantities such as stress, displacement, strain, and pressure, strain gauges whose resistance value changes depending on externally applied force are widely used. This strain gauge is usually used by being attached to a spring material. In order to detect mechanical physical quantities, a method is used in which a strain gauge and a resistor constitute a Wheatstone bridge, and a change in the resistance value of the strain gauge is converted into a voltage change and output.

In addition, there is also a method that utilizes the fact that a diffused resistor formed on a semiconductor substrate changes its resistance value due to stress applied from the outside. This type of sensor also has a Wheatstone bridge structure similar to the strain gate described above, and employs a method of outputting changes in resistance as a voltage. The major feature of this type of sensor is that it uses a silicon substrate, so the pressure sensing part and the electric circuit for signal detection can be integrated on the same substrate, and temperature compensation is also done on the same substrate by using the temperature sensing function of the semiconductor. This is what is happening above. Sensors of this type that utilize semiconductor diffused resistance are more practical sensors, and are also the sensors that are most likely to be miniaturized and mass-produced.

Furthermore, there is another technology that belongs to the force sensor category that utilizes the pressure-sensitive function of the SAW delay line. In this case, when stress is applied to the piezoelectric substrate used for the SAW delay line, the propagation speed of the SAW changes approximately in proportion to the magnitude of the stress, and as a result, the SAW delay line It has the property that the delay time changes.
Therefore, by configuring an oscillator using a SAW delay line having this characteristic, it is possible to realize a SAW force sensor that outputs a frequency change proportional to an externally applied force. This SAW force sensor has good resolution and can be made into a sensor with higher accuracy than various conventional force sensors that utilize resistance value changes. Additionally, the SAW oscillator that makes up the sensor also serves as the sensor section.
Another great feature is that it can be easily configured using a SAW delay line and an amplifier.

[Problem that the invention seeks to solve]

The SAW force sensor according to the present invention is different from the conventional one.
Similar to the SAW sensor, a SAW oscillator is configured using an amplifier and a SAW delay line, and the
By applying external force to the SAW propagation path
The basic principle is to change the delay time of the SAW delay line and change the oscillation frequency of the SAW oscillator. The SAW force sensor based on this principle is
Since the piezoelectric substrate that makes up the SAW delay line uses the pressure-sensitive characteristics with good linearity, stability and
Both accuracy and accuracy are satisfactory. In particular, the reproducibility of sensor operation is good, with almost no hysteresis observed. This is because crystals such as LiTaO ₃ and LiNbO ₃ are used as the piezoelectric material because they do not easily store the energy of externally applied forces.

As described above, the SAW force sensor according to the present invention has good characteristics, and on the one hand, the piezoelectric substrate is made of LiTaO ₃ ,
Because LiNbO ₃ is used, the operation is stable and the temperature coefficient of the SAW propagation velocity has good linearity. When a SAW oscillator is constructed using a SAW delay line having this characteristic, the oscillation center frequency can be changed with good linearity in response to temperature changes in the piezoelectric substrate of the SAW delay line. This kind of good quality inherent in piezoelectric substrates
The temperature characteristics of SAW propagation velocity have not been fully utilized in SAW sensors including conventional SAW force sensors.

[Means to solve the problem]

In this invention, a piezoelectric substrate exhibiting pressure-sensitive characteristics and temperature-sensitive characteristics with good linearity is used.
Transmitting and receiving electrodes (also called interdigital electrodes and IDT) for transmitting and receiving SAW.
Interdigital Transducer)
For example, a heating resistor that generates heat by power supplied from an external power source is provided between the transmitting and receiving electrodes to configure a SAW delay line. The heating resistor may be provided anywhere on the piezoelectric substrate, but in order to obtain good response characteristics, the best place is the part where the SAW propagates between the transmitting electrode and the receiving electrode. In this case, the SAW delay line is formed as a cantilever beam with one end fixed to the substrate fixing member in order to function as a force sensor.

The transmitting electrode and receiving electrode of the SAW delay line configured in this manner are connected via an amplifier to configure a SAW oscillator using a positive feedback circuit. In this SAW oscillator, when an external force is applied to the SAW delay line to cause distortion in the SAW propagation path of the piezoelectric substrate, the oscillation frequency changes approximately in proportion to the applied external force. The amount of change in this oscillation frequency, that is,
By detecting the difference in the oscillation frequency of the oscillator before and after an external force acts on the SAW delay line, it can be used as a SAW force sensor.
Furthermore, in this SAW force sensor,
For example, the piezoelectric substrate constituting the SAW delay line,
A heating resistor is provided on the surface. When power is supplied to this heating resistor from the outside, the heating resistor is heated according to the power, and the substrate temperature of the piezoelectric substrate changes based on that, and the SAW
Since the propagation speed can be set as the propagation speed, the center frequency of the SAW oscillator when no force is applied can be controlled to an arbitrary value. This function allows the oscillation frequency of the sensor to be set to the optimum measurement frequency that matches the characteristics of a frequency detector, such as a frequency counter, that detects the output signal of the force sensor.

[Effect]

As mentioned above, on the surface of the piezoelectric substrate,
A transmitting electrode for emitting a SAW, a receiving electrode for receiving the SAW, and a heating resistor for applying heat to the piezoelectric substrate between the transmitting electrode and the receiving electrode are provided. Configure a delay line, and further combine the SAW delay line and an amplifier to create a SAW
By configuring an oscillator and detecting the amount of change in the oscillation frequency using a means for counting the oscillation frequency of the SAW oscillator, such as a frequency counter,
It is possible to realize a SAW force sensor that can measure the magnitude of an external force applied to a SAW delay line as a force to be measured. Also, due to this configuration
In the SAW force sensor, power is also supplied externally to the heating resistor provided on the SAW delay line to cause the heating resistor to generate heat, thereby controlling the temperature of the SAW propagation path of the piezoelectric substrate that constitutes the SAW delay line. By doing so, it is also possible to control the oscillation center frequency of the SAW force sensor. This control of the oscillation center frequency utilizes the good temperature characteristics of the piezoelectric substrate, so
This can be done with high precision by supplying power from outside.

〔Example〕

FIG. 1 shows one embodiment of a SAW delay line used in the present invention. A transmitting electrode 2 for emitting SAW and the transmitting electrode 2 are disposed on the surface of the piezoelectric substrate 1.
A receiving electrode 3 for receiving the SAW emitted from the SAW is provided to form a SAW delay line 4. and,
A heating resistor 5 that generates heat in response to externally supplied power is provided in a portion of the piezoelectric substrate constituting the SAW delay line 4 where the SAW propagates. This heating resistor 5
may be provided anywhere on the piezoelectric substrate 1, but
When controlling the SAW speed, the best method is to provide the SAW propagation path on the surface of the piezoelectric substrate. However, if a heating resistor is provided in the part where the SAW propagates, the SAW will be reflected by the heating resistor, causing ripples in the frequency passband when used as a SAW delay line, and as a result, the linearity of the SAW force sensor will be impaired. Sometimes. Therefore, when emphasis is placed on linearity, the heating resistor 5
is the back surface or the front surface of the piezoelectric substrate 1.
It is better to provide it in an area where SAW does not propagate. Also,
In this case, by providing the heat generating resistor 5 over a wide area, the temperature gradient due to heat generation can be reduced. The SAW delay line 4 configured in this manner is fixed to the substrate fixing member 6. In this case, one of the transmitting electrodes 2 and one of the receiving electrodes 3 is connected to an amplifier 7 to form a SAW oscillator 8.
The other electrode of the transmitting electrode 2 and the receiving electrode 3 is connected to the ground potential. Further, one end of the heating resistor 5 is connected to the power supply terminal 9, and the other end is connected to the ground potential. has a configuration like this
The piezoelectric substrate 1 of the SAW delay line 4 is pressurized by means 10 for applying the force to be measured.

FIG. 2 shows an embodiment of the SAW force sensor of the present invention. In this embodiment, a transmitting electrode 2, a receiving electrode 3, and a heating resistor 5 are formed on a piezoelectric substrate 1 to constitute a SAW delay line. In this case, the heating resistor 5 may be provided on the back surface of the piezoelectric substrate. Then, one electrode of the transmitting electrode 2 and the receiving electrode 3 is connected to the ground potential, and the other electrode is connected to the amplifier 7 to connect the SAW
The oscillator 8 is configured. Configure this oscillator 8
Means for applying the force to be measured to the SAW delay line 1
Pressurize at 0. One end of the heating resistor 5 is connected to the ground potential, and the other end is connected to the frequency control power source 11 via the power supply terminal 9 to supply power to the heating resistor 5 and control the temperature of the piezoelectric substrate 1. . The oscillation signal of the SAW oscillator 8 is output to the outside via an output buffer 12, and the output signal is input to a means 13 for measuring the oscillation frequency, such as a commercially available frequency counter. . With the above configuration, it is possible to realize a SAW force sensor that detects an external force applied to the SAW delay line 4. In this SAW force sensor, a heating resistor 5 is provided in the SAW delay line 4 constituting the SAW oscillator 8, and the SAW Since the speed can be controlled, the oscillation center frequency of the SAW oscillator 8 can be controlled as a result.

〔Effect of the invention〕

As described above, in the SAW force sensor according to the present invention, the oscillation center frequency of the oscillator can be controlled externally with high accuracy by controlling the power supplied to the heating resistor that constitutes the surface acoustic wave delay line. This is a practical sensor with the ability to stably detect quantitative measurements with good linearity. With this performance, the oscillation center frequency of the frequency signal output by the sensor can be set to an optimal value according to the performance of the frequency measurement means such as a frequency counter that detects the frequency, and the sensor's inherent pressure-sensitive characteristics can be used effectively.

The SAW delay line, which is the sensor part, is manufactured using photolithography technology, so it can be manufactured at low cost and with uniform performance. Another great feature of this sensor is that it can be easily configured with a SAW delay line and one amplifier. Furthermore, since the output signal is a frequency signal suitable for digital signal processing, the sensor is ideal for control and measurement using a microcomputer.

As described above, the SAW force sensor according to the present invention is a sensor with high industrial value.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of a SAW delay line constituting the SAW force sensor of the present invention. Figure 2 shows
1 shows an embodiment of the SAW force sensor of the present invention. In the figure, 1 is a piezoelectric substrate, 2 is a transmitting electrode, 3 is a receiving electrode, and 4 is a surface acoustic wave (SAW).
Delay line, 5 is a heating resistor, 6 is a board fixing member, 7
is an amplifier, 8 is a surface acoustic wave (SAW) oscillator, 9
10 is a power supply terminal, 10 is a means for applying a force to be measured, 11 is a power supply means (frequency control power source), 12 is an output buffer, and 13 is a means for measuring an oscillation frequency (frequency counter).

Claims (1)

[Claims] 1. A piezoelectric substrate 1, a transmitting electrode 2 provided on the surface of the piezoelectric substrate for emitting surface acoustic waves, and a transmitting electrode 2 provided on the surface of the piezoelectric substrate for receiving the surface acoustic waves. a surface acoustic wave delay line 4 comprising a receiving electrode 3 and a heating resistor 5 provided on the piezoelectric substrate and generating heat by absorbing power; power supply means for controlling power supplied to the heating resistor; 11; an oscillator 8 that cooperates with the surface acoustic wave delay line and oscillates at the natural frequency of the surface acoustic wave delay line; means 13 for measuring the oscillation frequency of the oscillator; means 10 for applying a force to be measured, and before applying the force to be measured, the power supply means controls the power supplied to the heating resistor to set the oscillation frequency of the oscillator to a predetermined value. , the SAW force sensor applies the force to be measured to the surface acoustic wave delay line, counts changes in the oscillation frequency of the oscillator, and detects an external force applied to the surface acoustic wave delay line. 2. The SAW force sensor according to claim 1, wherein the heating resistor 5 is provided on the back surface of the piezoelectric substrate 1. 3. The SAW force sensor according to claim 1, wherein the heating resistor 5 is provided on the surface of the piezoelectric substrate 1.