JP2008046080A - Capacitance sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To detect accurate distance between a detecting object and a detecting electrode without being affected by disturbance. <P>SOLUTION: This capacitance sensor has the detecting electrode 1, and outputs, as a detected value, ground capacitance Cx of the detection electrode 1 varied by approach of the detecting object 2. The capacitance sensor has an initial capacitance C<SB>0</SB>in a signal line or the like connected to the detection electrode 1, and a composite capacitance C output as the detected value becomes the sum of the capacitance Cx and initial capacitance C<SB>0</SB>between the detection electrode 1 and detecting object 2. When the composite capacitance C as the sum of the capacitance Cx and initial capacitance C<SB>0</SB>is measured at a distance of at least two existing or calculable points, the distance L between the detection electrode 1 and detecting object 2 can be measured based on the difference between the composite capacitances C at the two points. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a capacitance sensor that detects the position of a detected object based on the capacitance between the detected object and a detection electrode.

Conventionally, the capacitance between the detected object and the detection electrode is converted into a detected value such as voltage or oscillation frequency, and the position of the detected object is detected based on the detected value. A capacitance sensor is known (Patent Document 1).
JP 7-29467 A

  However, in such a capacitance sensor, when the object to be detected contains moisture or when a metal is close to the detection electrode from the outside, the capacitance detected by these disturbances changes, and the accurate The position of the detected object cannot be detected. As a result, there arises a problem that the electrostatic capacitance sensor malfunctions.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a capacitance sensor that realizes improved measurement accuracy with respect to disturbance.

  The capacitance sensor according to the present invention includes a detection electrode whose capacitance changes due to the proximity of a detected object, a capacitance detection circuit that detects the capacitance of the detection electrode, and a capacitance detection circuit. In a capacitance sensor including a control circuit that detects a distance between the detected object and the detection electrode based on the detected capacitance, the control circuit is configured to position the detected object at a measurement start point. The first capacitance detected by the capacitance detection circuit during the operation, and the capacitance detection circuit when the measurement object is positioned at a point moved a predetermined distance from the measurement start point. The position of the object to be detected is detected from the difference value from the second capacitance detected in step (1).

  In the capacitance sensor, the control circuit performs a predetermined process when the capacitance detected by the capacitance detection circuit exceeds a preset threshold value. The variation of the capacitance including the initial capacitance is obtained from the difference value between the first capacitance and the second capacitance, and the threshold value is corrected by the variation. Can be configured.

  Furthermore, in the capacitance sensor, the measurement start point is known, and the control circuit calculates a dielectric constant of the object to be detected from a difference value between the first capacitance and the second capacitance. Can be configured. Further, the measurement start point is known, and the control circuit is configured to obtain an area of the detected object from a difference value between the first capacitance and the second capacitance. it can.

  Further, in the capacitance sensor, the control circuit calculates a position of the detected object and the detected capacitance from a difference value between the first capacitance and the second capacitance. It can be configured to obtain and store the relationship.

  According to the capacitance sensor of the present invention, when the detected object approaches, by obtaining the detection value from the difference value between the first distance and the second distance, the initial capacitance of the signal line, It is possible to offset the influence of the capacitance generated by the disturbance. As a result, the distance between the object to be detected and the detection electrode can be detected accurately.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

(First embodiment)
FIG. 1 is a schematic diagram showing a capacitance sensor according to the first embodiment of the present invention.

The capacitance sensor according to the first embodiment is configured to include the detection electrode 1 and outputs the ground capacitance Cx of the detection electrode 1 that changes depending on the proximity of the detected object 2 as a detection value. It is. Further, the electrostatic capacitance sensor has an initial electrostatic capacitance C ₀ in a signal line or the like to which the detection electrode 1 is connected, and the combined electrostatic capacitance C output as a detection value is the detection electrode 1 and the detected object 2. the sum of the electrostatic capacitance Cx and the initial capacitance C ₀ between.
C = Cx + C ₀ (1)
Here, the electrostatic capacitance Cx of the detection electrode 1 that changes depending on the proximity of the detection object 2 is ε: dielectric constant, S: area of the detection electrode 1 covered by the detection object 2, L: detection electrode 1 and the detection object 2 The distance between the detection electrode 2 and the detection electrode 2 can be used as follows.
Cx = εS / L (2)
By substituting equation (2) into equation (1), the following equation is obtained.
C (L) = (εS / L) + C ₀ (3)
Therefore, the combined capacitances C (La) and C (Lb) that are output when the distance between the detection electrode 1 and the detected object 2 are La and Lb are expressed by the following equations.
C (La) = (εS / La) + C ₀ (4)
C (Lb) = (εS / Lb) + C ₀ (5)
Here, if the distance between the distances La and Lb is the distance ΔL, the relational expression between the distance La and the distance Lb is as follows.
Lb = La−ΔL (6)
Substituting equation (6) by subtracting equation (5) from equation (4),
C (La−ΔL) −C (La) = {εS / (La−ΔL)} − (εS / La)
... (7)
Further, when this equation is expanded, the following equation is obtained.
La ² −ΔLa− [εSΔL / {C (La−ΔL) −C (La)}] = 0 (8)
In Expression (8), the electrostatic capacity C (La−ΔL) and the electrostatic capacity C (La) are the synthetic statics output when the distance between the detection electrode 1 and the detected object 2 is the distances La and Lb. The capacitance C is obtained by actual measurement. Here, if the distance ΔL, the dielectric constant ε, and the area S are known, the distance La can be calculated. Note that the speed v at which the detected object 2 and the detection electrode 1 approach each other is known in advance, and the distance ΔL is obtained by multiplying the proximity speed v by the time t until the distance Lb moves from the distance La. be able to.

Thereby, as shown in FIG. 2, even if the initial capacitance C ₀ is C ₀₁ or C ₀₂ , the detected capacitance is detected from the detected synthetic capacitance C without depending on the initial capacitance C _0. The distance La of the object can be obtained.

Next, a case where an influence due to disturbance is applied to the capacitance sensor will be described. When the object to be detected 2 contains moisture, or when a disturbance such as a metal approaching the detection electrode 1 from the outside occurs, the capacitance output from the capacitance sensor changes. When the capacitance that is increased by the influence of such disturbances and the capacitance C _1, the capacitance to be output and the sum of the capacitance C ₁ by the electrostatic capacitance Cx, the initial capacitance C ₀ and the disturbance Become.
C ′ (L) = (εS / L) + C ₀ + C ₁ (9)
Here, when the capacitance C ₁ by influence of disturbance becomes a steady state, the distance La, the combined capacitance C that is output in Lb 'can be expressed as follows.
C ′ (La) = (εS / La) + C ₀ + C ₁ (10)
C ′ (Lb) = (εS / Lb) + C ₀ + C ₁ (11)
Next, subtracting equation (11) from equation (10), substituting equation (6), and expanding the equation,
La ² −ΔLa− [εSΔL / {C ′ (La−ΔL) −C ′ (La)}] = 0
(12)
Is obtained. Here, the electrostatic capacitance C ′ (La−ΔL) and the electrostatic capacitance C ′ (La) are the electrostatic capacitances C ′ (Lb) and C ′ (La) output at the distances Lb and La, respectively. Can be obtained. Here, if the distance ΔL, the dielectric constant ε, and the area S are known, the distance La can be calculated from Expression (12). Thus, even when a disturbance occurs, the distance L can be obtained from the detected capacitance C ′ without depending on the initial capacitance C ₀ and the capacitance C ₁ due to the disturbance.
(Second Embodiment)
FIG. 4 is a schematic view of a capacitance sensor according to the second embodiment of the present invention. The second embodiment includes the capacitance sensor in the first embodiment as a sensor unit, and is configured to output a detection signal such as an alarm output in accordance with a capacitance change due to the proximity of the detected object 2. It has been done.

This capacitance sensor includes a detection electrode 1, a capacitance detection circuit 3, and a control circuit 4. The detection electrode 1 is connected to the capacitance detection circuit 3. The capacitance detection circuit 3 includes, for example, an oscillation circuit, and has a combined capacitance C including a capacitance Cx due to the proximity of the detected object, an initial capacitance C ₀ , and a capacitance C ₁ caused by disturbance. In response to this, a pulse signal Po whose duty ratio changes is generated and output to the control circuit 4. The control circuit 4 smoothes the pulse signal Po input from the capacitance detection circuit 3 and converts the detection value Vout which is a digital value.

Further, the control circuit 4 calculates the initial capacitance C _{0 of the system} and the capacitance C ₁ due to disturbance from the detected value Vout based on the combined capacitance C of at least two points whose distances are known or can be calculated. The electrostatic capacity C (L) predicted at an arbitrary position is calculated. Further, the control circuit 4 obtains a threshold value Cth at the distance Lth to be detected by substituting the distance Lth to be detected into the obtained expression C (L), and converts it to the voltage Vth by a method described later. The control circuit 4 is configured to compare the magnitude relationship between the threshold value Vth and the detection value Vout and to output an alarm to the outside according to the determination result.

  FIG. 5 is a circuit diagram illustrating a specific configuration example of the capacitance detection circuit 3.

  The capacitance detection circuit 3 includes two comparators 5 and 6 and an RS flip-flop circuit (hereinafter referred to as “RS-FF”) in which outputs of the comparators 5 and 6 are input to a reset terminal R and a set terminal S, respectively. ) 7, a buffer 8 that outputs the output DIS of the RS-FF 7, a transistor 9 that is ON / OFF controlled by the output DIS of the RS-FF 7, and a trigger signal generation circuit 10 that generates a constant pulse signal. Configured.

  The comparator 6 compares the trigger signal TG output from the trigger signal generation circuit 10 as shown in FIG. 6 with a predetermined threshold value Vth1 divided by the resistors R1, R2, and R3, and synchronizes with the trigger signal TG. Output the set pulse. This set pulse sets the Q output of RS-FF7.

  This Q output turns off the transistor 9 as a discharge signal DIS. When the transistor 9 is in an off state, the sensing electrode 1 and the ground are charged at a speed determined by a combined capacitance C and a time constant by a resistor R4 connected between the input terminal and the power supply line VDD.

  As a result, as shown in FIG. 6, the potential of the input signal Vin increases at a speed determined by the combined capacitance C. Here, when the input signal Vin exceeds a threshold value Vth2 determined by the resistors R1, R2, and R3, the output of the comparator 6 is inverted and the output of the RS-FF 7 is inverted.

  As a result, the transistor 9 is turned on, and the charged charge of the detection electrode 2 is discharged through the transistor 9. Therefore, the capacitance detection circuit 3 outputs a detection pulse signal Po that oscillates at a duty ratio based on the capacitance C between the detection electrode 1 and the ground. The detection pulse signal Po generated in this way is output to the control circuit 4 and converted into a detection value Vout such as a voltage.

  FIG. 7 is a timing chart showing the operation of the capacitance sensor configured as described above.

When the sensor power supply is turned on, the capacitance sensor performs self-diagnosis to determine whether the detection electrode 1 functions normally during time T1. Next, when the self-diagnosis is completed, the synthetic capacitance C (La) is measured at the distance La to obtain the detection value Va. When the time T2 elapses after obtaining the detection value Va, the composite capacitance C is measured again to obtain the detection value Vb. Acquired electrostatic capacitance C (La) and C (Lb), in the control circuit 4 is substituted into Equation (8) or formula (12), the static by initial capacitance C ₀ and the disturbance in the system was measured An offset amount (C ₀ or (C ₀ + C ₁ )) including the capacitance C ₁ is calculated.

Assuming that this electrostatic capacity sensor is set to output an alarm when the distance L between the detected object 1 and the detection electrode 2 is 15 mm, the control circuit 4 is obtained from the above. By substituting the offset amount (C ₀ or (C ₀ + C ₁ )) and the distance of 15 mm into the expression (3) or the expression (9), a capacitance C (15 mm) as a threshold value is obtained. As described above, the control circuit 4 obtains the electrostatic capacitance C (15 mm) serving as the threshold value, and converts the electrostatic capacitance C (15 mm) into the threshold voltage Vth15 mm by the same method as the detection value Vout. Furthermore, the control circuit 4 compares the magnitude relationship between the threshold voltage Vth15mm obtained in this way and the detection value Vout, and outputs a detection signal when the detection value Vout exceeds the threshold Vth15mm to output an external electric circuit. Alarm output (not shown).

  FIG. 8 shows a flowchart of this capacitance sensor.

  First, at t = 0, the first capacitance C (La) is measured to obtain the detection value Va, and counting of the elapsed time t is started (S1). Here, the magnitude relationship between the measured detection value Va and the maximum detectable value Vlim that can be measured is compared (S2). If the detected value Va is greater than the maximum detected value Vlim, an alarm is output and the operation is terminated (S13). ). On the other hand, when the detected value Va is smaller than the maximum detected value Vlim, the magnitude relationship between the elapsed time t and a preset time T2 is compared (S3).

  Here, when the elapsed time t is smaller than the time T2, the counting of the elapsed time t is continued until the elapsed time t exceeds the time T2 (S4), and when the elapsed time t exceeds the time T2, the counting of the elapsed time t is stopped. (S5) The capacitance at the elapsed time t = T2 is measured, and the detected value Vb is output (S6). Here, the distance ΔL that is the difference between the distance La and the distance Lb can be obtained by multiplying T2 by the proximity velocity v of the detected object 2 predicted in advance.

  Next, a difference voltage Vc obtained by subtracting the detection value Va from the detection value Vb is obtained, and it is determined whether or not the difference voltage Vc is larger than the minimum detectable value Vmin that can be detected (S7). When the difference voltage Vc exceeds the minimum detection value Vmin, the elapsed time t is counted (S8).

  Here, when the differential voltage Vc becomes larger than the minimum detection value Vmin, the magnitude relationship between the differential voltage Vc and the preset maximum threshold value Vth20mm is compared (S9), and the differential voltage Vc is larger than the maximum threshold value Vth20mm. If an alarm is output, the operation is terminated (S13). Note that the maximum threshold value Vth20 mm is calculated by the threshold value setting method described above, for example, at a maximum distance of 20 mm when the distance L to be detected is 15 mm ± 5 mm.

  On the other hand, if the differential voltage Vc is smaller than the maximum threshold value Vth20mm, as explained in the principle, the threshold value Vth15mm is calculated using the detection values C (La) and C (Lb) obtained by the two-point measurement. (S10) The detection value Vout is measured until the detection value Vout exceeds the threshold value Vth15mm (S11). If the detected value Vout exceeds the threshold value Vth15 mm (S12), an alarm is output and the operation is terminated (S13).

Thus, by measuring the capacitance C (C ′) at two points and obtaining the initial capacitance C ₀ and the capacitance C ₁ due to disturbance from the differential voltage Vc, a threshold corresponding to the system is obtained. The value Vth can be set. Therefore, by comparing the threshold value Vth in consideration of the initial capacitance C ₀ and the electrostatic capacitance C ₁ due to disturbance with the detection value Vout, accurate detection is possible regardless of the influence of the initial capacitance C ₀ and disturbance. The proximity of the detected object 2 can be detected.

  In the description of the embodiment, it has been described that the electrode area S of the detection electrode 1 is known. However, when the electrode area S is unknown, the threshold Vth is set by performing the following operation. It is possible to set.

First, the capacitance is measured at two points where the distance L is known. For example, the detected object 2 or the detection electrode 1 is moved from the position where the distance L is 0 mm to set the distance L to 10 mm and 20 mm, and the output of the capacitance detection circuit 3 at this time is acquired. Here, the capacitance C output by the distances of 10 mm and 20 mm is as follows.
C (10 mm) = (εS / 10 mm) + Cx (13)
C (20 mm) = (εS / 20 mm) + Cx (14)
Also, subtracting equation (14) from equation (13) and solving for area S,
S = {C (10 mm) −C (20 mm)} / {(ε / 10 mm) − (ε / 20 mm)}
... (15)
If the dielectric constant ε is known, the area S can be obtained. By further adding this operation, the threshold value Vth can be calculated even if the area S is unknown.

  On the other hand, when the area S is known, the dielectric constant ε can be measured from the equation (13). Further, since the change in the dielectric constant ε varies depending on, for example, the change in the moisture content, the capacitance sensor of the present invention can also be used for humidity measurement.

  In the present embodiment, the detection object 2 is configured to be close to the detection electrode 1, but the detection electrode 1 or the capacitance sensor may be configured to be close to the detection object 2. Further, instead of converting the capacitance as a detection value into a voltage, an oscillation circuit that outputs a pulse wave having a duty ratio according to the detection value or outputs an oscillation frequency according to the detection value may be configured. .

  In the present embodiment, the distance L and the dielectric constant ε are calculated by arithmetic processing. However, for example, a memory storing a data table is connected to the control circuit 4, and the capacitance C or detection at an arbitrary position L is detected. It can also be configured to monitor the value V. Thus, by determining the current distance and threshold value based on the data table, the position of the detected object can be instantaneously measured.

It is a schematic diagram of the sensor part of the capacitance sensor in the first system. It is a figure which shows the relationship between distance and an electrostatic capacitance. It is a schematic diagram of the sensor part of the capacitance sensor in the second system. It is a figure which shows the structure of the electrostatic capacitance sensor which concerns on this invention. It is a figure which shows the specific structure of the electrostatic capacitance type detection circuit of the electrostatic capacitance sensor. It is a figure which shows the timing chart of the electrostatic capacitance type detection circuit of the electrostatic capacitance sensor. It is a figure which shows the timing chart of an electrostatic capacitance sensor. It is a figure which shows the flowchart of an electrostatic capacitance sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Detection electrode, 2 ... Object to be detected, 3 ... Capacitance detection circuit, 4 ... Control circuit, 5, 6 ... Comparator, 7 ... RS flip-flop circuit, 8 ... Buffer, 9 ... Transistor, 10 ... Trigger signal generation circuit.

Claims (5)

A sensing electrode whose capacitance changes depending on the proximity of the detected object;
A capacitance detection circuit for detecting the capacitance of the detection electrode;
In a capacitance sensor comprising: a control circuit that detects a distance between the detected object and the detection electrode based on the capacitance detected by the capacitance detection circuit;
The control circuit includes:
A first capacitance detected by the capacitance detection circuit when the detected object is located at a measurement start point, and a point where the measured object is moved a predetermined distance from the measurement start point. A capacitance sensor, wherein the position of the detected object is detected from a difference value from the second capacitance detected by the capacitance detection circuit when positioned. The control circuit includes:
A predetermined process is executed when the capacitance detected by the capacitance detection circuit exceeds a preset threshold value,
From the difference value between the first capacitance and the second capacitance, a variation in the capacitance including the initial capacitance is obtained, and the threshold value is corrected by the variation. The capacitance sensor according to claim 1, wherein: The measurement starting point is known,
2. The capacitance according to claim 1, wherein the control circuit obtains a dielectric constant of the object to be detected from a difference value between the first capacitance and the second capacitance. Sensor. The measurement starting point is known,
The capacitance sensor according to claim 1, wherein the control circuit obtains an area of the detected object from a difference value between the first capacitance and the second capacitance. . The control circuit includes:
The relationship between the position of the detected object and the detected capacitance is obtained from the difference value between the first capacitance and the second capacitance, and stored. The capacitance sensor according to claim 1.

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