JPS6182104A - Electrostatic capacity type range finder - Google Patents

Abstract

PURPOSE:To measure a range with high precision without any influence from a body nearby an electrostatic capacity type sensor by providing the electrostatic capacity type sensor with a sensor shield device which applies nearly the same voltage as a voltage applied to the electrostatic capacity type and shields the sensor. CONSTITUTION:The sensor 10 is arranged opposite a body S to be measured and when an AC signal having specific frequency is outputted from an oscillator 11, the output is amplified by a signal amplifier 12 to a specific value and then applied to the sensor 10 through a capacitor C1. At the same time, the output of the amplifier 12 is applied to a high-input impedance type amplifier 14 through the capacitor C1 and then applied to an auxiliary sensor 13 through a coupling capacitor C2. Consequently, the voltage applied to the sensor 13 is nearly equal to the voltage applied to the sensor 10 and no current flow between both sensors. The AC voltage, however, is applied to the sensor 10 from the amplifier 12, so the detection sensitivity of the sensor 10 in the direction where the auxiliary sensor 13 is provided decreases, but the detection sensitivity at the surface side facing the body S to be measured does not vary. Consequently, the detection direction has directivity.

Description

[Detailed description of the invention] [Technical field of invention] The present invention relates to improvements in capacitive distance meters.

[Technical background of the invention]

Some distance meters use a capacitive sensor and measure the distance to a measured object based on changes in capacitance detected by the sensor.

Hitherto, this type of capacitance detection means has been known, part 1.
One type uses a charge amplifier, and the other uses a dummy capacitor and a capacitance type capacitor to form a bridge circuit, which minimizes the capacitance of the capacitance type sensor. There are devices that detect minute changes in capacitance of a sensor capacitor by converting the change into a voltage signal using a bridge circuit and extracting it.

FIG. 5 is a configuration diagram of a conventional feedback amplification type capacitance meter, and this capacitance meter has a capacitor C8 of a capacitance type sensor.
A change in static capacitance is detected from a voltage difference between a voltage obtained in response to this change in capacitance and an alternating current voltage of constant amplitude output from the oscillator 1. Specifically, the capacitor C6 of the capacitive board sensor is connected to the negative input terminal of the operational amplifier 2, the oscillator 1 is connected to the positive input terminal of the operational amplifier 2, and the output terminal of the operational amplifier 2 is connected to the negative input terminal of the operational amplifier 2. It is connected to the negative input terminal of the amplifier 2 via the feedback capacitor CM to constitute a negative feedback type amplifier.

In the above-described capacitance meter, an alternating current voltage e1 of a fixed frequency and constant amplitude is applied from the oscillator 1 to the operational amplifier 2, and in this state, the capacitance of the capacitor CB changes to the measured object (not shown). ), the operational amplifier 2 outputs a voltage eK corresponding to the value of the CN/C8 ratio. This output voltage eK is expressed by the following equation.

In other words. As is clear from this equation (1), if the output voltage ei of the oscillator 1 and the value of the negative feedback capacitor CN are fixed, the output voltage eK when feedback is added is
is a value corresponding to a change in capacitance of capacitor C3.

Therefore, by measuring the output voltage θ of the amplifier 2, the value of the capacitance of the capacitor Cs can be indirectly detected. Note that FIG. 6 shows the output characteristics with respect to the ratio of CN/C8 when the output voltage ei of the oscillator 1 is set to IV.

By the way, the capacitance value of the capacitor C8 of the capacitive sensor with respect to the relative distance between the capacitive sensor and the object to be measured is expressed by the formula: S −ζ C=□゛...(2) D It will be done. Here, S is the cross-sectional area of the capacitive sensor, ε is the dielectric constant between the capacitive sensor and the object to be measured, and D is the relative distance between the capacitive sensor and the object to be measured. Therefore, the capacitance change of capacitor C3 is
It is non-linear with respect to the relative distance, and becomes minute as the relative distance becomes longer.

[Problems with background technology]

However, the conventional feedback amplification type capacitance meter has the following problems. In other words, ■ If there is another object in the vicinity of the capacitive sensor, especially a charged or conductive object, the capacitive sensor will be affected by this object.
The change in capacitance of the sensor no longer corresponds to the relative distance to the object to be measured. Further, there is no means for preventing the influence of other objects.

■ Output voltage e of operational amplifier 2. As shown in FIG. 6, since the output voltage e0 is non-linear, the output voltage e0 must be made linear by passing it through a linear amplifier, which complicates the circuit.

■ Generally speaking, the ratio of the cross-sectional area to the measurement span of a capacitive sensor is limited to about 1. For example, with a capacitive sensor with a cross-sectional area of about 32 mm, the measurement span for two measurements is about 30 mm. Nasi,
Measurement Suno J? There is a request to expand the scope of the project.

■ As can be seen from Equation (1), if the amplification degree is increased (the value of CM is set large) in order to magnify and detect minute changes in the capacitance of the sensor, the absolute value of the output voltage eK is too large and becomes saturated, making measurement impossible.

[Purpose of the invention]

The present invention was developed based on the above-mentioned circumstances, and the purpose of the present invention is to provide a static electric current that can prevent the influence of objects existing near the sensor, provide linear output characteristics, and enable highly accurate distance measurement. Our objective is to provide a capacitive distance meter.

[Summary of the invention]

The present invention configures a bridge circuit with a reference side impedance and a measurement side impedance having a capacitance type sensor whose capacitance changes depending on the distance from the object to be measured. Approximately the same voltage as that applied to this capacitance type sensor is applied to the capacitance type sensor, and a sensor shielding means is provided to shield the capacitance type sensor to prevent effects from objects that exist near the capacitance type sensor. A voltage proportional to the difference between the output voltages on the side and measurement sides is positively fed back to the signal amplifier circuit that supplies an AC signal to the bridge circuit to obtain a signal indicating the distance between the capacitive sensor and the object to be measured. This is a capacitive distance meter.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described with reference to FIG. In the figure, A is a bridge circuit, which has a plurality of impedances Zl.

A capacitance type sensor 10 (hereinafter referred to as a sensor) that detects a capacitance that changes according to a reference side circuit such as z2, a capacitor C, and a distance t from the object to be measured S.
It consists of a different measurement side circuit. An alternating current signal of a predetermined amplitude and a predetermined frequency outputted from an oscillator 11 is applied to this bridge circuit A via a signal amplifier 12 having an r-in compensation negative feedback circuit resistor R1rRN. . Furthermore, each connection point a between each reference side circuit and the measured side circuit.

b are individually connected to two input terminals of the feedback differential amplifier 15. This differential amplifier 15 has connection points a and b.
This is to find the voltage difference between the signal voltages eIll r e B@ that appear in . Further, the output end of the feedback differential amplifier 15 is connected to the positive input end of the signal amplifier 12 via the adder circuit 16, forming a so-called positive feedback circuit. 17
is a feedback amount adjustment circuit, which has the function of adding the output voltage of the signal amplifier 12 to the addition circuit 16VC and adjusting the amount of positive feedback to the signal amplifier 12.

Further, the sensor 10 is provided with a sensor shielding means that applies substantially the same voltage as the voltage applied to the sensor 10 to shield the sensor 10. That is, this sensor shielding means protects the sensor 10 from the object to be measured S.
A first shield body 13 (hereinafter referred to as an auxiliary sensor) formed to cover the opposite surface and the outer surface side is provided, and receives a high signal voltage appearing at the connection point of the circuit on the side to be measured. It is configured to be applied to the auxiliary sensor 13 via an impedance type amplifier 14 and a coupling capacitor C1.

Note that the ratio of the capacitance Cf of the auxiliary sensor 13 to the capacitance of the coupling capacitor C2 is C2/(4 is 104 to 105
The amplification factor of the amplifier 14 is set to 1.

Next, the operation of the distance meter configured as described above will be explained. When the 7/10 is placed opposite to the object to be measured S and an AC signal of a predetermined frequency is output from the oscillator 11, this AC signal is amplified to a predetermined value by the signal amplifier 12 and then passed through the capacitor C1. added to sensor 10.

At the same time, the output voltage of the signal amplifier 12 is applied to the high input impedance type amplifier 14 via the capacitor C1, and is further applied to the auxiliary sensor 1 via the coupling capacitor C2.
Added to 3.

As a result, the voltage θ8 applied to the auxiliary sensor 13 is approximately the same value as the voltage applied to the sensor 10 (the output voltage obtained according to the change in capacitance of the sensor 10) aSl, and the current between both sensors 10 and 13 is will not flow. However, since an AC voltage is applied to the sensor 10 from the signal amplifier 12 with low output impedance, the sensor 10
Although the detection sensitivity on the upper side of 0, that is, the direction in which the auxiliary sensor 13 is provided, decreases, the detection sensitivity on the side facing the object to be measured S does not change. However, directivity occurs in the detection direction. Therefore, even if an object other than the object S to be measured exists above the sensor 10, the sensor 10 will not be affected by that object.

FIG. 2 is an output characteristic diagram of the sensor 10, and is 4an
This is an example using a sensor IQ having a 4 cm surface area. As is clear from this output characteristic diagram, the output characteristic (A) of the sensor 10 is approximately linear. In addition, the output characteristics (b) are
The object to be measured S is provided on the side where the auxiliary sensor 13 is provided, and the relative distance between the sensor 10 and the object to be measured S in this state is changed.

Therefore, the output voltage egl obtained by the capacitance change according to the distance between the sensor 10 and the measured object S is sent to one input terminal of the feedback differential amplifier 15. The other input terminal has an impedance Zl,
The output voltage eg of the signal amplifier 12 divided by Z2,
is input. Then, from this feedback differential amplifier 15, a difference voltage e between the input voltages egl + ea2, =
(e phantom-eax) Gls is output and supplied to the adder circuit 16. G11i represents the amplification degree of the amplifier 15.

Furthermore, this differential voltage e8 is added to the output signal of the feedback amount adjustment circuit 17 in the adder circuit 16 and applied to the other input terminal of the signal amplifier 12. Therefore, the output voltage e0 of the signal amplifier 12 at this time has a value corresponding to the distance between the sensor 10 and the object S to be measured. Here, the output voltage e0 is expressed by the following equation. That is, Ml; eII3/e0 Hz = Sit/ ea = Cx/ (C1+ 0
1). Note that N is the amplification degree of the signal amplifier 12 (amplification degree without positive feedback, N=RN/Rs), K is the positive feedback rate of the feedback amount adjustment circuit 17, and Cx is the capacitance value of the sensor 10. As can be seen from this equation (3), the output voltage e1 of the oscillator 11 and the amplification degree N r 01 of each amplifier 12゜15
If the positive feedback rate is 8% and each value of Ml is fixed, the output voltage e0 will change according to the change in the capacitance C of the sensor f10. By measuring, the distance t between the sensor 10 and the object S to be measured is determined.

In this way, in the distance meter of the present invention, 10 is provided with an auxiliary sensor 13, and this auxiliary sensor 13 is provided with a sensor 1.
An AC voltage is applied from a signal amplifier 12 that applies an AC voltage to 0 through an amplifier 14 with a high input impedance ring to make it the same potential as the sensor flO and shield it, and further output voltage eB1 of the sensor 10 and output voltage eg2 of the signal amplifier 12. Since the difference voltage between the auxiliary sensor 13 and the auxiliary sensor 13 is positively fed back to the signal amplifier 12 and the distance t is calculated from the output voltage e0 of the signal amplifier 12 at this time, the detection sensitivity of the sensor 10 is First, only the side facing the object to be measured S is used.

Specifically, the difference in detection sensitivity between the sensor 10 on the side to be measured S and the side on the auxiliary sensor 13 is about 20 dB, which improves the S/N ratio against external influences (disturbances). Therefore, even if an object other than the object S to be measured exists in the vicinity of the sensor 10, for example on the upper side or side, it will not be affected by the object, and the output voltage e0 of the sensor 10 will depend on the distance between the objects S to be measured. The value will be determined accordingly.

Furthermore, the output characteristics are linear as shown in FIG. 2, and there is no need to use linear live as in the conventional case. Furthermore, the measurement span can be increased by 3 to 5 times compared to the conventional method.

Next, a second embodiment of the present invention will be described with reference to FIG. In this figure, the same parts as in FIG. 1 are given the same reference numerals, and detailed explanation thereof will be omitted. This distance meter has a configuration in which a second shield body 20 having the same shape as the auxiliary sensor 13 is provided so as to cover the auxiliary sensor 13, and the second shield body 20 and the object to be measured S are commonly connected and grounded. It becomes. Therefore, the second shield body 20 is kept at ground potential and completely cuts off the electric field generated from the auxiliary sensor 13. Due to this, the detection sensitivity of the auxiliary sensor 13 side of the sensor 1o will be significantly reduced. FIG. 4 is an output characteristic diagram of the sensor 1o shown in FIG.
The output characteristic (b) of o to the auxiliary sensor 13 side is constant, which indicates that the detection sensitivity has decreased.

Further, a capacitance C1 is generated between the auxiliary sensor 13 and the second shield body 20, but even if this capacitance CK is generated, the signal source (high input impedance Impedance Z of type amplifier 14)
o is very small (10 or less), and the ratio between the capacitance CK and the capacitance of the coupling capacitor C2 is C3≧10~10CK
By setting CK, the influence of the static capacitance CK can be almost ignored.

Therefore, it goes without saying that the second embodiment has the same effects as the first embodiment, and is also less susceptible to the influence of other objects than the first embodiment.

〔Effect of the invention〕

According to the present invention, a sensor shield means is provided for shielding the capacitance sensor by applying substantially the same voltage as the voltage applied to the capacitance sensor to the capacitance sensor. The difference between the output voltages on each side is positively fed back to the signal amplifier to obtain an output voltage according to the distance from the signal amplifier, which prevents the influence of objects near the object to be measured, and also allows linear It is possible to provide a capacitance distance meter that can obtain output characteristics and measure distances with high accuracy.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a first embodiment of the capacitive distance meter according to the present invention, FIG. 2 is an output characteristic diagram of the distance meter shown in FIG. 1, and FIG. A configuration diagram showing a second embodiment of a capacitive distance meter, FIG. 4 is an output characteristic diagram of the distance meter shown in FIG. 3, FIG. 5 is a configuration diagram of a conventional capacitive distance meter, and FIG. 6 is an output characteristic diagram of the distance meter shown in FIG. 5. 10... Capacitance sensor, 11... Oscillator, 12
...Signal amplifier, 13...Auxiliary sensor, 14...
High input impedance amplifier, 15... Differential amplifier for feedback, 16... Adder circuit, 17... Feedback amount adjustment circuit, Zl, Zl... Impedance, CI. C2...Coupling condition/su, S...Measurement object. Figure 1 Figure 2 Z distance Am 1 (mm) Figure 3 Figure 1 Figure 4 Measurement distance 1 (mm)

Claims (3)

[Claims] (1) A bridge circuit consisting of a plurality of reference side impedances and a measurement side impedance having a capacitance type sensor whose capacitance changes depending on the distance from the object to be measured, and an oscillator connected to this bridge circuit. A signal amplification circuit supplies an AC signal of a predetermined amplitude and a predetermined frequency output from the bridge circuit, and receives each output voltage on the reference side and measurement side of the bridge circuit, and applies a voltage proportional to the difference between these output voltages to the signal. The present invention is characterized by comprising a feedback differential amplifier circuit that provides positive feedback to the amplifier circuit, and sensor shielding means that applies substantially the same voltage as the voltage applied to the capacitance type sensor and shields the capacitance type sensor. Capacitive distance meter. (2) The sensor shield means includes a high input impedance amplifier circuit that amplifies the output voltage of the signal amplifier circuit, and is connected to this amplifier circuit via a coupling capacitor, and is arranged opposite to the object to be measured of the capacitive sensor. and a first shield body provided on the opposite side.
The capacitance distance meter described in section 1). (3) The sensor shield means includes a high input impedance amplification circuit that amplifies the output voltage of the signal amplification circuit, and is connected to this amplification circuit via a coupling capacitor and faces the object to be measured of the capacitance type sensor. a first shield body provided on the opposite side to the side to be measured; and a second shield body provided outside the first shield body and connected to the object to be measured and grounded.
A capacitive distance meter according to claim (1), comprising a shield body.