JPS63238438A - Force sensor - Google Patents

Abstract

PURPOSE:To detect dynamic force and static force with high sensitivity and good response, by mounting switching FET and impedance converting FET. CONSTITUTION:Gas is introduced from an introducing port 3 and the pressure thereof is transmitted to an electrode 17. Whereupon, not only the electrode 17 but also a piezoelectric body 18 are distorted and the internal spontaneous polarization corresponding to the strains of both of them is generated and charge is induced in the electrodes 17, 19 of a piezoelectric element 5 with said polarization. The detection of voltage due to this charge is performed by such a method that a voltage pulse of a predetermined cycle is applied to the gate terminal of switching FET (Tr1) through a terminal 14 to intermittently bring switching FET (Tr1) to a continuity state and detected as the pulse like voltage intermittent between both terminals, that is, the terminals 13, 15 of an output resistor Rs. Since the detected voltage corresponds to the pressure of the gas, the pressure of the applied gas can be known from the magnitude of the detected voltage. By this method, dynamic and static pressures can be detected with high sensitivity and good response.

Description

DETAILED DESCRIPTION OF THE INVENTION 1 Chestnut Yo L Sara Riko■ This invention relates to a force sensor using a piezoelectric element that can detect not only dynamic force but also static force.

BACKGROUND ART Conventionally, Bourdon tubes, bellows, diaphragms, and the like have been used as sensors for detecting static forces, but all of these have the problem of having many moving parts and being large in size. Furthermore, all of these force sensors have slow response speed and low sensitivity.

In addition, in recent years, force sensors using diffused semiconductors, capacitance type, inductance type, and the like have been proposed.

Although these are more compact than the ones mentioned above, they still have many moving parts and 1. There are problems such as slow response speed and low sensitivity, and the performance is rather inferior to those mentioned above.

On the other hand, piezoelectric bodies are known as force sensor elements. Therefore, when using a piezoelectric material, the force sensor can be made compact by selecting the material, and it can also provide high-speed response and
It has the advantage of being able to achieve high sensitivity, use in a wide temperature range, and extend its life cycle. However, at present, it is not used for static force detection. This is due to the following reasons.

In other words, piezoelectric sensors use piezoelectric materials such as crystal, lead zirconate titanate porcelain (PZT), and polyvinylidene fluoride (PVDF). When added to generate strain, the magnitude of spontaneous polarization changes. Therefore, the electric charge induced in the surface electrode in response to this change in spontaneous polarization is passed through an external load resistor, which converts it into voltage, and the magnitude of the force can be known as the magnitude of the voltage. .

However, since charge is consumed by the external load resistance, the voltage gradually decreases in response to a static force.

This is from the magazine "Sensor Technology J, Vol.

3, No. 10, pp. 33-40 (1983), when static pressure is applied to a piezoelectric material, a corresponding voltage is initially obtained, but the voltage gradually decreases; This is because it has type characteristics. Therefore,
Efforts have been made to connect the piezoelectric body to a charge amplifier to minimize the amount of charge attenuation, but this is still insufficient and only dynamic forces such as pulsating pressure can be detected.

This is the biggest problem with piezoelectric force sensors.

FIG. 1 shows an example of a conventional force sensor using a piezoelectric material, where 103 is a piezoelectric material, and 101 and 102 are piezoelectric materials 1.
03 is a surface electrode provided, 104 is a field effect transistor (FET) for impedance conversion, 105 (RQo
) is the external load resistance.

However, in this force sensor, the external load resistor 105 is grounded, and therefore the impedance conversion FET10
Although there is no effect on the voltage due to temperature fluctuations in the leakage current of 4, the value of the external load resistor 105 is increased considerably, for example, 5.
Even if it is 0 GΩ, there is no change in the so-called differential type characteristics. Then, the detected voltage corresponds to the amount of change in the fluctuating force.

The above-mentioned problem is caused by the presence of the external load resistor 105 that consumes charge as current. Therefore, as shown in Fig. 2, the external load resistance 105 in the force sensor shown in Fig. 1 is eliminated, and the FE for impedance conversion is
It is also possible to use the high input resistance of T104. By doing so, it is possible to detect static forces with a fast response speed and high sensitivity, which was previously impossible. However, when the temperature changes, the impedance conversion FET10
Another problem is that the output fluctuates due to the leakage current of No. 4, and electrostatic damage to the gate is likely to occur.

In order to prevent this, it is possible to use FETs that have a diode inserted between the gate and source to prevent electrostatic damage, but at present, FETs that are small and can be used at high frequencies are being used. Because it is difficult to obtain FETs with high withstand voltage, the output of this conventional force sensor becomes unstable when used in areas with large temperature changes, and
F due to damage to the gate if used for a long period of time.
There is also a decline in ET performance, resulting in low reliability. Another problem is that the types of piezoelectric materials are limited, such as the difficulty of using piezoelectric materials with high internal resistance.

Problems that the invention aims to solve The purpose of Jin's invention is to solve the problems of conventional force sensors, to be able to detect not only dynamic force but also static force, and to be able to detect high-speed force. The purpose of the present invention is to provide a force sensor with excellent sensitivity and responsiveness.

Means for Solving the Same Point In order to achieve the above object, the present invention includes a piezoelectric element formed by forming electrodes on both sides of a piezoelectric body, and an electric chopper circuit connected to the piezoelectric element. And, the electric chopper circuit includes (a) a switching field effect transistor and an impedance conversion field effect transistor, and (b) a drain terminal of the switching field effect transistor is connected to one electrode of the piezoelectric element. (c) the source terminal of the switching field effect transistor is connected to the gate terminal of the impedance conversion field effect transistor; (d) the gate terminal of the switching field effect transistor is connected to a pulse generator; (e) A leak resistance is connected between the source terminal of the switching field effect transistor and the other electrode of the piezoelectric element; A force sensor is provided, characterized in that an output resistor is connected between the electrodes.

In this invention, the piezoelectric body used in the piezoelectric element is crystal, lead zirconate titanate porcelain (PZT), lanthanum zirconate lead porcelain (PLZT), ZnO1L i Ta
O3, PbTi 03, LiNbO2, triglycine sulfate tJ, (TGS>, polyvinylidene fluoride (PVDF)
), polyvinylidene fluoride/trifluoroethylene copolymer (PVDF-TrFE), or, for example, P
This is a so-called composite piezoelectric material made by mixing inorganic piezoelectric material powder into VDF. These are usually 5mm thick
In the following, those with an area of 0.01 m2 or less will be used. Although the shape varies depending on the use, it is preferably rectangular, disc-shaped, or ring-shaped. Note that the piezoelectric body is usually supported by a suitable holder.

The electrodes formed on both sides of the piezoelectric body described above, for example, for detecting gas pressure, are preferably formed by vapor deposition, sputtering, plating, etc. of gold, aluminum, nichrome, nickel, or stainless steel.

For devices that detect liquid pressure every once in a while, it is preferable to use gold, nickel, or stainless steel plate by vapor deposition, sputtering, or plating, or a stainless steel plate.

In general, phosphor bronze plates, carbon steel plates, stainless steel plates, etc. are preferable for detecting tensile force or compressive force. In any case, in a corrosive atmosphere, it is preferable to cover with a protective film such as a corrosion-resistant polymer thin film or stainless steel thin film. In addition, when an electric chopper circuit is formed directly on a silicon substrate, it is also possible to use it in place of an electrode.

An electrical chopper circuit for intermittently detecting voltage due to charges induced in the electrodes of a piezoelectric element includes two FETs for switching and impedance conversion, and two resistors, a leak resistance and an output resistance. . Thus, the train terminal of the switching FET is connected to one electrode of the piezoelectric element, and the source terminal is connected to the gate terminal of the impedance conversion FET. Further, the gate terminal of the switching FET is connected to a pulse generator. Roughly speaking, the leak resistance is the switching FET.
The output resistor is connected between the source terminal of the impedance conversion FET and the other electrode of the piezoelectric element. In addition,
When forming an FET on a silicon substrate, each terminal of the FET is usually made of aluminum, which makes it difficult to connect to other parts, so the silicon substrate is glued onto a circuit board made of plastic or other material. A copper print is provided on the circuit board for relaying, and the copper print and each terminal of the FET are bonded.

In this case, it is preferable to connect this circuit board t leak resistance and the output resistance.

A piezoelectric element and an electric chopper circuit are usually housed in a case made of metal that is a good electrical conductor such as aluminum, iron, or brass, conductive plastic, or plastic coated with conductive paint. Ground it or connect it to the common line of the power supply to eliminate external electrical noise.

When a force is applied to the acting piezoelectric body from either side, the piezoelectric body deforms and bends. However, when a piezoelectric material is deformed, the magnitude of its internal spontaneous polarization changes depending on the amount of deformation. At this time, the amount of charge changes in the electrode of the piezoelectric element so as to maintain a balance with the spontaneous polarization. If the acting force is constant, the internal spontaneous polarization will also be constant, and the potential difference due to the generation of charges, that is, the voltage, will always be constant. This voltage is intermittently made conductive through a switching FET and detected as a voltage across the output resistor. At this time, the leak resistance protects the gate terminal of the impedance conversion FET from electrostatic damage. FE for switching
When T becomes conductive, a potential difference, that is, a voltage, appears across the output resistor due to the generation of charges, but at this time, since there is a leak resistance, the charges leak and the voltage drops somewhat. When the switching FET becomes non-conductive, charge leakage disappears and the voltage at the electrode of the piezoelectric element returns to a value corresponding to the spontaneous polarization inside the piezoelectric body. Therefore, the voltage detected here is in the form of a pulse, but the voltage can be synchronized with a signal that intermittently turns on the switching FET using a sample-and-hold circuit, and the voltage can be extracted digitally, or the voltage can be extracted digitally, or - the next lag transmission The force is detected by extracting it analogously using an operational amplifier with a function.

In principle, the response speed of the output is estimated to be less than a picosecond, which corresponds to the rate of change of the internal polarization of the piezoelectric body, but in reality it depends on the response speed of the switching FET and the response speed of the impedance conversion FET. limited. but,
It is quite fast, less than 10 microseconds. - Since the voltage due to the charge on the surface of the piezoelectric element is detected, there is almost no reduction in charge due to the conventional external load resistance, and a highly sensitive output of 1q is achieved.

Large blind screen An embodiment of the present invention will be described with reference to a case where gas pressure is measured.

In FIGS. 3 to 5, the main body of the force sensor is housed in a base 1 and a case 2 made of aluminum with good heat dissipation. The case 2 is provided with a gas inlet 3, and the base 1 is provided with a communication port 9 to the atmosphere and terminals 10, 12 to 16 for electrical connection with the outside. The terminal 10 is connected to the base 1 and the case 2, and is used to prevent electrical noise from the outside.
This is a ground terminal.

The main body part is kept airtight with the gas inlet 3.
an upper holder 4 that also serves as a terminal for the electrode 17 of the piezoelectric element 5;
It has a piezoelectric element 5, a support base 7 for fixing the piezoelectric element 5, and a lower holder 6 which also serves as a terminal for an electrode 19 of the piezoelectric element 5, and an electric chopper circuit is arranged therein. The electric chopper circuit is formed on a silicon substrate 8 and a glass fiber reinforced plastic circuit board 11 to which the silicon substrate 8 is bonded. Switching FET (
Trl) and impedance conversion FET (Tr2
> is formed on the silicon substrate 8. Further, on the circuit board 11, a leak resistance RCI of 300 MΩ and a leak resistance RCI of 2
0 is connected to the output resistance Rs of Ω, and two FE
T, that is, the relay of the connection between the switching FET (Tri), the impedance conversion FET (Tr2>'s drain terminal, gate terminal, source terminal, upper holder 4 and lower holder 6, and each terminal 10.12 to 16). This copper print, the terminals of each FET, and terminals 10, 12 to 16 are connected by bonding according to Fig. 3.Then, the upper holder 4 and the lower holder 6 and the copper print on the circuit board 11 are connected by soldering lead wires.

The inlet 3 and the support base 7 are made of polyacetal and have electrical insulation and mechanical strength.

The upper holder 4 and the lower holder 6 are made of ring-shaped brass, and a lead wire is attached to one end thereof.

The piezoelectric element 5 includes electrodes 17 and 19 provided on both sides of a piezoelectric body 18, and these electrodes 17 and 19 are made of a vapor-deposited film of aluminum. Thus, the piezoelectric cord 5 is bonded to the upper holder 4 and the lower holder 6. An adhesive is used from the inlet 3 to the support base 7 to prevent gas from leaking.

Next, the electric chopper circuit will be explained.

3 to 5, the lower holder 6 in contact with the electrode 19 of the piezoelectric element 5 is connected to the train terminal of the switching FET (Trl), and the source terminal is connected to the gate terminal of the impedance conversion FET (Tr2). and one side of the leakage resistor RO.Switching FET
The gate terminal of (Trl) is connected to the terminal 14 provided on the base 1. FET for impedance conversion
(The train terminal of Tr2> is connected to the terminal 12, and the source heir is connected to one of the output resistors Rs and the terminal 13. Also, the upper holder 4 which is in contact with the electrode 17 of the piezoelectric element 5 The other resistor RCI is connected to the other output resistor R5, and is further connected to a terminal 15. These connections are made by soldering or bonding on a copper print on the circuit board 11 to achieve the above-described sensor action. To explain, gas is introduced through the inlet 3 and its pressure is transmitted to the electrode 17.

Then, the piezoelectric body 1B is distorted together with the electrode 17, internal spontaneous polarization occurs in accordance with the distortion, and charges are induced in the electrodes 17 and 19 of the piezoelectric element 5 accordingly. Detection of voltage using this charge is performed using a switching FET (Tri >
A voltage pulse of a predetermined period is applied to the gate terminal of the switching FET (Trl) through the terminal 14 to make the switching FET (Trl> conductive intermittently. Detected as voltage.

Since the detected voltage corresponds to the gas pressure, the applied gas pressure can be determined from the magnitude of the detected voltage. Note that a positive voltage is applied to the terminal 12, and a positive voltage is applied to the terminal 15.
A negative voltage is applied to each.

This invention relates to a switching F for intermittently detecting voltage due to charges induced on the surface of an electrode of a piezoelectric element.
Since an electric chopper circuit including an ET and an impedance conversion FET is provided, static pressure, which has been the biggest drawback of piezoelectric force sensors, can be detected with high sensitivity and at high speed. Furthermore, by selecting the material for the piezoelectric body, it becomes possible to make it small and lightweight, use it in a wide range of temperatures, and have a long life cycle.

Therefore, water pressure, where other force sensors were traditionally used,
It can now be used to measure atmospheric pressure and hydraulic pressure, from static pressure to dynamic pressure. There are many uses for water pressure, such as water pressure measurement in water and sewage systems and automatic operation of water pumps based on water pressure measurements. In particular, for water pressure measurement in cold regions, it can replace Bourdon tube type or semiconductor type force sensors with poor temperature characteristics.

The sensitivity of atmospheric pressure is higher than that of other force sensors, so it can be used for pressure control management of pneumatic feeding equipment, leak testing of containers, liquid level measurement using gas back pressure, and liquid level measurement using gas back pressure. In addition to being used for density measurements, it can also be used to measure absolute atmospheric pressure. Among these, the force sensor of the present invention is most suitable for differential pressure measurement because the pressure difference is generally very small.

In terms of hydraulic pressure, it can be used to measure pressure in hydraulic drive circuits under hydraulic tlJ control, which is widely used in recent mechatronics.

In addition, we can detect the force applied to a car's handbrake as strain on the steel plate from the brake wire, detect and control the direction of travel based on the difference in air pressure, measure the amount of fuel gas injected, and measure the amount of fuel gas injected. It can be used to measure and control the amount of air mixed with

[Brief explanation of the drawing]

1 and 2 are circuit diagrams showing a conventional force sensor,
3 to 5 are schematic diagrams showing one embodiment of the force sensor of the present invention, with FIG. 3 being a circuit diagram, FIG. 4 being a longitudinal sectional view, and FIG. 5 being a bottom view. 1: Base 2: Case 3: Inlet 4: Upper holder 5: Piezoelectric element 6: Lower holder 7: Support base 8: Silicon substrate 9: Atmospheric communication port 10: Terminal 11: Circuit board 12: Terminal 13: Terminal 14: Terminal 15: Terminal 16: Terminal 17: Electrode 18: Piezoelectric body 19: Electrode Trl Niswitching field effect transistor Tr2
: Field effect transistor for impedance conversion Rg: Leak resistance R5: Output resistance

Claims (1)

[Scope of Claims] A piezoelectric element having electrodes formed on both sides of a piezoelectric body, and an electric chopper circuit connected to the piezoelectric element, and the electric chopper circuit is (a) for switching. a field effect transistor and an impedance conversion field effect transistor; (b) a drain terminal of the switching field effect transistor is connected to one electrode of the piezoelectric element; (c) a source terminal of the switching field effect transistor is connected to one electrode of the piezoelectric element; (d) the gate terminal of the switching field effect transistor is connected to a pulse generator; (e) the source terminal of the switching field effect transistor and the piezoelectric element; A leak resistance is connected between the other electrode of the piezoelectric element, and (f) an output resistance is connected between the source terminal of the impedance conversion field effect transistor and the other electrode of the piezoelectric element. A force sensor featuring: