JPH0641888B2 - SAW force sensor - Google Patents

Description

The present invention relates to a force sensor used for measuring and detecting a mechanical physical quantity such as stress, pressure, strain, displacement, etc. 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 and temperature-sensitive characteristics 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 the strain gauge and a resistor and converting a resistance value change of the strain gauge into a voltage change and outputting the voltage change is adopted.

In addition to this, in consideration of the fact that the resistance value of the diffusion resistance formed in the semiconductor changes due to stress, there is also one that utilizes such a change in resistance value. This type of sensor comprises a Wheatstone bridge similar to the strain gauge,
Although the method of outputting the change in resistance value as voltage is adopted, the major feature is that the silicon substrate is used, so that the pressure sensitive part and the electric circuit for the signal detector can be integrated on the same substrate. Temperature compensation for changes is also performed on the same substrate using the temperature sensing function of the semiconductor. Thus, 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.

Further, as another technique belonging to the force sensor, there is a technique utilizing the pressure sensitive function of the SAW delay line. In this case, when a force is applied to the SAW delay line where the surface acoustic 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. A piezoelectric substrate having characteristics is used. When an oscillator is constructed using a SAW delay line having this characteristic, a SAW that outputs a frequency change amount proportional to the applied force.
A force sensor can be realized. The SAW force sensor realized in this manner has a good resolution and can be a sensor with higher accuracy than the 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 an amplifier.

The majority of the SAW force sensor is a SAW oscillation circuit, and the measured amount is displayed by a change in the natural oscillation frequency of the oscillation circuit. The method of displaying the amount of change in frequency is such that the SAW delay line itself that constitutes the SAW oscillation circuit is a passive device and spontaneously emits energy or is measured. This is because it is not a type of device that converts the emitted energy of a substance into another form and outputs it.

Therefore, when the SAW delay line is used not only for the SAW force sensor but also for the sensor, the SAW delay line operates in a stable state, that is, the SAW delay line works in cooperation with the SAW delay line to determine the natural frequency of the SAW delay line. It is necessary to configure a SAW oscillator that oscillates steadily.

However, since the piezoelectric substrate used for the SAW delay line has a temperature coefficient of the SAW velocity, it has a property of reacting sensitively to the ambient temperature, and when the oscillator is constructed, the inherent oscillation frequency fluctuates. There's a problem. Therefore, in order to obtain a sensor with higher sensitivity, a method of compensating for frequency fluctuations 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, the same applicant as the present application first applied for the “SAW force sensor” and configured the SAW oscillator with a SAW delay line and an amplifier to compensate for fluctuations due to temperature and aging so that good oscillation stability can be obtained. The SAW force sensor shown in FIG.

That is, the SAW force sensor that can obtain this good oscillation stability is the SAW of the SAW delay line that constitutes the oscillation circuit.
It is a sensor that positively changes the W propagation speed by heat, force, electric field, etc., and detects and displays the oscillation frequency of the SAW oscillator due to the change.

[Problems to be solved by the invention]

However, this SAW force sensor must use the amount of change in the oscillation frequency as an output signal due to its structural limitation. Recently, however, like many conventional force sensors, the measured amount is output as a voltage or current value. An analog signal output type SAW force sensor is also desired.

[Means and Actions for Solving Problems]

In the present invention, a piezoelectric substrate whose SAW velocity changes linearly with applied heat and force is used, and electrodes for transmitting and receiving SAW on this substrate (also referred to as interdigitated electrodes or IDTs). IDT is Interdigital Trans
and a heating resistor that absorbs electric power supplied from the outside to generate heat is provided between the two electrodes through which the SAW propagates, that is, the transmitting electrode and the receiving electrode. Configure the SAW delay line. The location where the heating resistor is provided is not limited to the SAW propagation portion of the piezoelectric substrate, and may be provided anywhere on the piezoelectric substrate.
However, considering the response speed of the sensor, it is best to provide it in the portion where the SAW propagates. Further, the SAW delay line is fixed to the substrate fixing member to form a cantilever beam in order to improve the pressure sensitive function as a force sensor.

Then, the transmitting electrode and the receiving electrode of the SAW delay line are connected via an amplifier to form a SAW oscillator by a positive feedback circuit. The SAW oscillator configured as described above is a SAW oscillator.
When an external force is applied to the piezoelectric substrate forming the delay line, the SAW propagation velocity changes, and as a result, the oscillation frequency changes. Further, when power is externally supplied to the heating resistor provided on the piezoelectric substrate forming the SAW delay line, the substrate temperature of the piezoelectric substrate is changed by the heat generated by the heating resistor, and the SAW delay line is changed. The SAW propagation speed of is changed. As a result, the oscillation frequency of the SAW oscillator changes as in the case where a force is externally applied to the piezoelectric substrate.

Therefore, the SAW oscillator, a reference frequency oscillator that always oscillates at a constant oscillation frequency, an output oscillation frequency of the SAW oscillator and an output oscillation frequency of the reference frequency oscillator are compared, and an error frequency signal that is a comparison output, That is, by configuring a PLL (phase-locked loop) circuit with a phase comparator that outputs a beat signal and a means for feeding back the output of this phase comparator to the heating resistor provided on the surface of the SAW delay line. The oscillation frequency of the SAW oscillator can be made constant.

That is, in the PLL circuit, when a force to be measured is applied to the SAW delay line forming the SAW oscillator, S
The oscillation frequency of the AW oscillator changes. Then, the changed oscillation frequency is compared with the oscillation frequency of the reference frequency oscillator by the phase comparator, and the difference between the two frequencies is output as the comparison output, that is, the error frequency signal. When this error frequency signal is sent as a feedback signal to the heating resistor of the SAW delay line, the heating resistor generates heat in proportion to the magnitude of the error frequency signal, and the heat changes the substrate temperature of the piezoelectric substrate, Along with this, the oscillation frequency of the SAW oscillator is returned to the oscillation frequency before the force to be measured is applied to the piezoelectric substrate. Therefore, at this time, the force applied from the outside can be detected by detecting the value of the current flowing through the heating resistor.

As described above, the piezoelectric substrate having the pressure sensitive characteristic and the temperature sensitive characteristic is used, and the SAW delay line in which the heating resistor that generates heat by the power supplied from the outside is provided is formed on the piezoelectric substrate. The SAW delay line is fixed to form a cantilever structure.
By constructing an oscillator using the SAW delay line having such a structure, power is supplied to a heating resistor provided on, for example, the surface of the piezoelectric substrate that constitutes the SAW delay line, or the piezoelectricity is externally supplied. By applying an external force to the substrate, the oscillation frequency of the SAW oscillator can be changed independently. Therefore, the SAW oscillator having the pressure sensitive characteristic and the temperature sensitive characteristic, the reference frequency oscillator, and the S
A phase comparator that compares an output signal (oscillation frequency) of an AW oscillator with an output signal (oscillation frequency) of the reference frequency oscillator and outputs a beat signal (error frequency signal) that is the comparison result, and the phase comparator Means for returning the output signal of the SAW to the heating resistor provided on the surface of the SAW delay line.
PLL having a function of keeping the oscillation frequency of the W oscillator constant
If the circuit is configured, the device having this configuration serves as a SAW force sensor that outputs an analog signal.

In the device of this configuration, when an external force is applied to the SAW delay line that constitutes the SAW oscillator, the oscillation frequency of the SAW oscillator changes. This amount of change is detected by the phase comparator and output as an error frequency signal having a magnitude corresponding to the amount of change, that is, a beat signal. The beat signal is applied to a heating resistor provided on the surface of the SAW delay line, and acts as a feedback signal for returning the oscillation frequency of the SAW oscillator to a value before the external force to be measured is applied. Since the magnitude of this signal has a value according to the external force to be measured, this signal keeps the oscillation frequency of the SAW oscillator constant. Further, this feedback signal is an analog signal, and by outputting this signal, a SAW force sensor that outputs an analog signal can be realized.

〔Example〕

FIG. 1 is a diagram showing an embodiment of a SAW delay line used in the present invention. On the surface of the piezoelectric substrate 1, a transmitting electrode 2 for emitting a SAW and a receiving electrode 3 for receiving the SAW emitted from the transmitting electrode 2 are provided.
The W delay line 4 is formed. Then, the heating resistor 5 is provided on the surface of the SAW delay line 4 where SAW is propagated. The heating resistor 5 may be provided on the surface of the piezoelectric substrate 1 where SAW does not propagate or on the back surface of the piezoelectric substrate 1. The piezoelectric substrate 1 on which the SAW delay line 4 is formed is fixed to the substrate fixing member 6 to form a cantilever beam. Then, one of the transmitting electrodes 2 of the SAW delay line 4 and one of the receiving electrodes 3 are connected via an amplifier 7 to form a SAW oscillator 8 by positive feedback. Here, one end of the heating resistor 5 provided on the SAW delay line 4 is connected to the power supply terminal 9
In addition, the other of the transmitting electrode 2 and the receiving electrode 3 and the other end of the heating resistor 5 are connected to the ground potential. An external force is applied to the SAW delay line 4 configured as described above by using a means 10 for exerting a force to be measured, so that the SAW oscillator 8
The oscillation frequency of changes. A basic SAW force sensor can be configured by making the amount of change in the oscillation frequency correspond to the force applied from the outside. The SAW oscillator 8 has a frequency control function, and the frequency is controlled by externally supplying power to the power supply terminal 9 to control the amount of heat generated by the heating resistor 5 provided on the surface of the SAW delay line 4. You can do it.

FIG. 2 shows an embodiment of the SAW force sensor according to the present invention. The transmission electrode 2 and the reception electrode 3 are provided on the surface of the piezoelectric substrate 1 to form the SAW delay line 4. A heating resistor 5 that generates heat by electric power supplied from the outside is provided in the SAW propagation portion of the surface of the SAW delay line 4. One electrode of each of the transmitting electrode 2 and the receiving electrode 3 is connected through an amplifier 7, and the other respective electrode is connected to a ground potential to form a SAW oscillator 8. The oscillation signal of the SAW oscillator 8 having this configuration is output via the output buffer 11. The output signal of the reference frequency oscillator 12 and the output signal of the SAW oscillator 8 output via the output buffer 11 are input to the phase comparator 13 and compared in phase and frequency. The error frequency signal output from the phase comparator 13 is
It is input to the heating resistor 5 provided on the SAW delay line 4. It should be noted that this error frequency signal may be passed through a low pass filter in the feedback means 14 and only the low frequency component may be taken out and added to the heating resistor. The PLL circuit 15 having the feedback function as described above is configured, and the phase comparator 13
A SAW force sensor that outputs an analog signal can be realized by outputting the output signal of the above through the output terminal 16.

〔The invention's effect〕

As described above, the SAW force sensor according to the present invention is one in which the PLL circuit is configured using the oscillator with the SAW delay line, and the conventional SAW force sensor is
This is a new type of SAW force sensor that outputs an analog signal that could be realized only by using a D / A converter. Since the output analog signal uses the pressure-sensitive characteristic and the temperature-sensitive characteristic, which are originally provided in the SAW delay line and has good linearity, as a principle of the sensor, the SAW force sensor according to the present invention is not used as a conventional force sensor. It can be a sensor with no high precision.

In addition, since it is a type of force sensor that outputs an analog signal, it is also a great feature that many analog signal processing technologies that have been conventionally accumulated can be used as they are. In particular, since it constitutes a PLL circuit, it is optimal as a sensor for a control system using feedback.

Various circuits used for the PLL circuit are available on the market in the form of ICs, and by using these elements, a highly accurate and inexpensive analog output type sensor can be realized.

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.
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, 1 is a piezoelectric substrate, 2 is a transmitting electrode, 3 is a receiving electrode, 4 is a surface acoustic wave delay line (SAW delay line), 5
Is a heating resistor, 6 is a substrate fixing member, 7 is an amplifier, 8 is a surface acoustic wave oscillator (SAW oscillator), 9 is a power supply terminal,
Reference numeral 10 is a means for exerting a force to be measured, 11 is an output buffer, 12 is a reference frequency oscillator, 13 is a phase comparator, 14 is a feedback means, 15 is a PLL circuit, and 16 is an output terminal.

Claims (1)

[Claims] 1. A piezoelectric substrate (1) having a heating resistor (5) to which a force can be applied from the outside, and a transmitter for emitting a surface acoustic wave provided on the surface of the piezoelectric substrate. Electrode (2)
And a surface acoustic wave delay line (4) comprising a receiving electrode (3) provided on the surface of the piezoelectric substrate for receiving the surface acoustic wave.
A surface acoustic wave oscillator (8) which cooperates with the surface acoustic wave delay line and oscillates at a natural frequency of the surface acoustic wave delay line; a reference frequency oscillator (12); and an output signal of the reference frequency oscillator. And a phase comparator (13) that compares the output signal of the surface acoustic wave oscillator and outputs a phase difference signal proportional to the difference, and means for feeding back the output of the phase comparator to the heating resistor ( 14) and a phase-locked loop circuit (1) in which the oscillation frequency of the surface acoustic wave oscillator matches the frequency of the reference frequency oscillator.
5) and; an output terminal (16) for outputting the phase difference signal; and a means (10) for applying a force to be measured to the piezoelectric substrate, by detecting the output of the phase comparator A SAW force sensor that detects a force applied to the piezoelectric substrate.