JPH0641886B2 - SAW force sensor - Google Patents

Description

The present invention relates to a force sensor used for measuring and detecting mechanical physical quantities such as stress, pressure, strain, and displacement, and more particularly to solid state sensors. S using surface acoustic wave (SAW: Surface Acoustic Wave) propagating on the surface of a substance
The present invention relates to a SAW force sensor that applies the pressure-sensitive function of an AW delay line.

[Conventional technology]

A strain gauge whose resistance value changes according to the strain applied is widely used in a sensor that measures a mechanical physical quantity such as stress, displacement, strain, or pressure. This strain gauge is generally used by being attached to a spring material. In order to detect a mechanical physical quantity, a method of forming a Wheatstone bridge with a strain gauge and a resistor and converting a resistance value change of the strain gauge into a voltage change and outputting it is adopted.

In addition to the above, there is also one utilizing 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 a 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 pressure sensitive portion and an electric circuit for signal detection can be integrated on the same substrate because a silicon substrate is used. Further, temperature compensation is also performed on the same substrate by utilizing the temperature sensing function of the semiconductor. As described above, the sensor of the type using the diffusion resistance of the semiconductor is a more practical sensor, and is a sensor that has been downsized and mass-produced.

In addition, as other technologies belonging to the force sensor,
Some use the pressure sensitive function of the SAW delay line. SAW
The piezoelectric substrate used for the delay line has a property that when stress is applied to the portion where the surface wave propagates, the SAW propagation velocity changes in proportion to the magnitude of the force, and as a result, the delay time of the SAW delay line changes. have. If an oscillator is configured using the SAW delay line having this characteristic, a SAW force sensor that outputs a frequency change amount proportional to the applied force can be realized. This SAW force sensor has a good resolution, and can be a sensor with higher accuracy than conventional force sensors that utilize changes in resistance value. In addition, the SAW oscillator that constitutes the sensor has an S
It has a great feature that it can be easily constructed by an AW delay line and a vibration enhancer.

[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. 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 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, in cooperation with the SAW delay line, the SAW delay line is used as an oscillating element, and its natural frequency is changed. It is necessary to create a state of steady oscillation.

However, the piezoelectric substrate used for the SAW delay line is SA
Since it has a temperature coefficient of W velocity, it reacts sensitively to the ambient temperature, and when an oscillator is constructed, a problem arises that the natural oscillation frequency fluctuates. To solve this problem,
The SAW oscillator for detecting the ambient temperature and the SAW oscillator for detecting the change in the natural oscillation frequency are composed of SAW delay lines of the same shape. Is being compensated. As described above, temperature compensation can be performed by extracting the beat of two signals, but the conventional SAW force sensor does not have a structure in which the advantages of the method of extracting the beat are fully utilized.

[Means for solving problems]

The present invention uses a piezoelectric substrate that exhibits pressure-sensitive characteristics and temperature-sensitive characteristics, and electrodes for transmitting and receiving SAW on this substrate (also referred to as interdigitated electrodes or IDT. IDT is an Interdig.
(Ital Transducer) is provided to form two SAW delay lines that are structurally substantially the same or symmetrical (congruent). A heating resistor that generates heat by an externally applied electric power is provided in the SAW propagation portion of either one of the two SAW delay lines. Two SAW oscillators are formed by using these two SAW delay lines, one of which serves as a temperature compensation oscillator and the other of which serves as a signal detection oscillator. The signals output from the two oscillators are input to the mixer and output as beat signals, so that a temperature-compensated signal can be obtained. When power is externally supplied to the heating resistor provided on one of the SAW delay lines, the SAW
The oscillation frequency of the SAW oscillator including the delay line changes, and the center frequency of the beat signal output from the mixer changes.
By controlling the power applied to the heating resistor in this way, the center frequency of the beat signal can be changed to an arbitrary frequency.

[Work]

The configuration described above, that is, one of the SAs
By compensating for the oscillation frequency of the SAW oscillator for signal detection and the SAW oscillator for temperature compensation that are configured by providing a heating resistor on the W delay line, temperature compensation becomes possible, and power is externally applied. Thus, the center frequency of the beat can be controlled arbitrarily. With this frequency control function, for example, when detecting a small force, the oscillation frequencies of the two oscillators are brought close to each other, while when measuring a large force, the oscillation frequencies of the two oscillators are separated to obtain an appropriate resolution. Is possible. It is also possible to determine the optimum oscillation frequency that matches the performance of the frequency counter, and it is possible to provide a more practical SAW force sensor.

[Embodiment] FIG. 1 shows an embodiment of a SAW delay line used in the present invention. On the piezoelectric substrates 1a and 1b, transmitting IDT electrodes 2a and 2b for emitting SAW and receiving IDT electrodes 3a and 3b for receiving SAW emitted from the transmitting IDT electrodes are provided. IDT electrode 2a for transmission and IDT for reception
The heating resistor 4 that generates heat in accordance with the input power is provided in the portion where the SAW propagates between the electrodes 3a. The two SAW delay lines 5a and 5b are formed as described above. Here, the two SAW delay lines are structurally substantially the same or symmetrical (congruent).
And the shape. The SAW delay line formed by the transmitting IDT electrode 2a and the receiving IDT electrode 3a includes a temperature compensation function and can freely set the temperature (hereinafter referred to as temperature compensation), while the transmitting IDT electrode 2b , And I for reception
The SAW delay line formed by the DT electrode 3b is a detection SAW delay line for detecting force. IDT electrodes for transmission 2a, 2b,
One side of each of the receiving IDT electrodes 3a and 3b and the heating resistor 4 is connected to the ground potential. Also, the IDT for transmission
The other side of each of the electrode 2a and the receiving IDT electrode 3a is connected to an amplifier 6a to form an oscillator 7a in which the temperature can be freely variably set including temperature compensation. This circuit is hereinafter called a temperature compensation oscillator. On the other hand, the transmitting IDT electrode 2b
The other side of the receiving IDT electrode 3b is connected to the amplifier 6b to form a detecting oscillator 7b. The other side of the heating resistor 4 is connected to the frequency control terminal 8. The piezoelectric substrates 1a and 1b are fixed to the substrate fixing member 9.

FIG. 2 shows an embodiment of the SAW force sensor according to the present invention. An IDT electrode 2a for transmission, an IDT electrode 3a for reception, and a heating resistor 4 are provided on the piezoelectric substrate 1a, and SA for temperature compensation is provided.
The W delay line 5a is formed. One side of each IDT electrode 2a, 3a and heating resistor 4 is connected to the ground potential, and the other side of each IDT electrode 2a, 3a is connected to an amplifier 6a to form an oscillator 7a for temperature compensation. To do. The signal of the oscillator 7a is output via the output buffer 10a. Next, the transmission IDT electrode 2b and the reception IDT electrode are formed on the piezoelectric substrate 1b.
3b is arranged to form a force detection SAW delay line 5b. One side of each IDT electrode 2b, 3b is connected to ground potential and the other side is connected to an amplifier 6b, respectively, to form an oscillator 7b for detection. The signal of the oscillator 7b is output via the output buffer 10b. The output signals of the two oscillators 7a and 7b thus obtained are input to the mixer 11 to obtain a beat signal. The beat signal passes through the low pass filter 12 and is output as a sine wave. This signal is taken out at the output terminal 13. The signal obtained with such a configuration is a signal that compensates for sensor fluctuations due to temperature fluctuations. The other side of the heating resistor 4 provided on the SAW delay line for temperature compensation is connected to the frequency control terminal 8, and the center frequency of the beat can be freely controlled by supplying electric power from the outside. be able to.

〔The invention's effect〕

As described above, the SAW force sensor according to the present invention can arbitrarily determine the center frequency according to the measurement target. For example, when measuring a small force, a sensor with higher accuracy can be obtained by bringing the oscillation frequency of the temperature compensation signal and the oscillation frequency of the detection oscillation signal close to each other. Since this oscillation frequency control utilizes the temperature characteristic of the piezoelectric substrate having good accuracy and linearity, it can be performed without deteriorating the sensitivity of the sensor to the force. Further, this SAW force sensor requires a frequency counter in order to output the detected force as a frequency signal. In accordance with the performance of this frequency counter, the center frequency can be changed in the SAW force sensor according to the present invention to output a signal having an optimum frequency.

On the other hand, since this sensor is an output signal having a frequency signal suitable for digital signal processing, it is a type of sensor suitable for signal processing using a microcomputer.

As described above, 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.
It is a figure which shows one Example of an AW delay line. FIG. 2 is a diagram showing an embodiment of the SAW force sensor of the present invention. In the figure, 1a and 1b are piezoelectric substrates, and 2a and 2b are transmission IDs.
T electrodes, 3a and 3b are IDT electrodes for reception, 4 is a heating resistor,
5a and 5b are SAW delay lines, 6a and 6b are amplifiers, and 7a and 7b are SA
W oscillator, 8 is a frequency control terminal, 9 is a substrate fixing member, 10a and 10b are output buffers, 11 is a mixer, 12 is a low-pass filter, and 13 is an output terminal.

Claims (2)

[Claims] 1. A first heating device having a heating resistor (4) on its surface that absorbs electric power from the outside to generate heat, and is heated by the heating resistor.
Piezoelectric substrate (1a), a first transmission electrode (2a) for emitting a first surface acoustic wave provided on the first piezoelectric substrate (1a), and the emitted first electrode (2a). First surface acoustic wave delay line including a first receiving electrode (3a) for receiving the first surface acoustic wave
(5a); and a first oscillator (7a) that cooperates with the first surface acoustic wave delay line (5a) and oscillates at the natural frequency of the first surface acoustic wave delay line (5a); A second piezoelectric substrate (1b); a second transmitting electrode (2b) for emitting a second surface acoustic wave provided on the surface of the second piezoelectric substrate (1b), and Second receiving electrode (3b) for receiving the second surface acoustic wave
And a second surface acoustic wave delay line (5b), which is placed adjacent to the first surface acoustic wave delay line and has a substantially congruent shape; A second oscillator (7b) which cooperates with the delay line (5b) and oscillates at the natural frequency of the second surface acoustic wave delay line (5b); an output signal of the first oscillator; The output signal of the second oscillator is mixed with the first surface acoustic wave delay line or the second surface acoustic wave delay line.
SAW consisting of a surface acoustic wave delay line and a mixer (11) that outputs a beat of a frequency corresponding to the force applied from the outside.
Force sensor. 2. The SAW force sensor according to claim 1, wherein the first piezoelectric substrate and the second piezoelectric substrate are the same piezoelectric substrate.