JP5033489B2 - Capacitance change detection circuit - Google Patents

Description

  The present invention relates to a capacitance change detection circuit suitable for use in a capacitance type touch switch that detects an operator's touch operation based on a change in impedance (capacitance) between electrodes.

  2. Description of the Related Art Conventionally, a touch switch that detects when a part of a human body contacts or approaches a conductive electrode member and activates a desired device is widely known. In such a touch switch, the electrode member functions as a capacitance sensor, and a change in the impedance (capacitance) of the electrode when the human body touches the electrode is read to detect on / off of the touch switch. It is like that.

In such a touch circuit, it is necessary to detect a change in capacitance (impedance change) as described above. As a conventional technique for detecting a change in capacitance, an electrode and an oscillation circuit are connected to each other, Techniques have been proposed in which the frequency or amplitude of the oscillation circuit changes when the capacitance changes (see, for example, Patent Documents 1 to 3 below). A technique for detecting a change in electrostatic capacitance between a pair of two or more electrodes has also been proposed (see, for example, Patent Documents 4 and 5 below).
JP 2003-46383 A JP-A-11-345552 Japanese Patent Application Laid-Open No. 11-190751 Japanese Patent Laid-Open No. 2004-340661 JP 2006-177895 A

  However, the conventional technique has a problem in that many components are required to configure the oscillation circuit, resulting in an increase in cost. In particular, it is disadvantageous to form a circuit configuration that simultaneously performs signal processing of many independent capacitance detection targets. In addition, even in a circuit configuration that does not use an oscillation circuit, a configuration in which an analog arithmetic element for calculating charge / discharge current is frequently used has caused a similar problem.

Furthermore, when used as a human contact or proximity sensor, a configuration that requires two or more electrodes per detection means not only requires more electrical circuit elements but also a dielectric attached between the two electrodes. There is a risk of false detection due to sexual substances.
More specifically, since the circuits disclosed in Patent Documents 1 and 2 use coils and bipolar elements, the power consumption is large and the cost is increased (see FIG. 2 of Patent Document 1 and FIG. 1 of Patent Document 2). In addition, since the circuit does not continuously change the capacitance according to the proximity of the human body, the sensitivity is not good and the sensitivity setting is difficult (see FIG. 6 of Patent Document 1).

  In addition, the circuits disclosed in Patent Documents 3 to 5 require switching elements (see reference numeral 2 in FIG. 1 of Patent Document 3 and reference signs S1 to S13 in FIG. 2 in Patent Document 4), and operational amplifiers (patent The use of a plurality of reference numerals 22 to 24 in FIG. 1 in Document 3, reference signs A4 to A7 in FIG. 2 in Patent Document 4, and reference numerals IC1 and IC2 in FIGS. become.

  The present invention has been devised in view of such problems, and an object of the present invention is to provide a highly accurate capacitance change detection circuit that achieves cost reduction with a simple configuration.

An electrostatic capacitance change detection circuit according to the present invention senses a change in a minute electrostatic capacitance, and in the capacitance change detection circuit that detects the presence or absence of an input based on the change amount, an oscillation source that outputs a square wave, and the oscillation A first resistor to which a square wave output from the source is input, an inverting means for inverting and outputting the square wave output from the oscillation source, and a square wave output via the inverting means are input. A second resistor; a detection electrode connected downstream of the second resistor; and a third resistor connected downstream of the detection electrode; and termination of the first resistor and the third resistor. it is characterized by side are connected.

When the resistance values of the first resistor, the second resistor, and the third resistor are r ₁ , r _2, and r ₃ , respectively, the resistance values r ₁ , r _2, and r ₃ are r ₁ = r ₂ + it is not preferable to satisfy the relationship of r _3.
The resistance values r ₁ , r _2, and r ₃ preferably satisfy the relationship r ₁ : r ₂ : r ₃ = 2: 1: 1.

  According to the capacitance change detection circuit of the present invention, there is an advantage that the capacitance change can be detected with high accuracy, and the cost can be reduced because a bipolar element, an operational amplifier and a switching element are not used. it can.

1 is a schematic circuit diagram showing the overall configuration of the circuit, and FIG. 2 is a diagram for explaining the operation of the circuit according to an embodiment of the present invention. .
As shown in FIG. 1, the capacitance change detection circuit according to the present embodiment is formed as a half-bridge circuit mainly including a resistance element and a C-MOS inverter element. In this circuit, the square wave signal from the detection electrode 2 and the inverted signal of the square wave signal are added, and the addition result is amplified to detect a change in capacitance and to detect the touch operation of the operator. It is configured to determine the presence or absence.

  In the following, this circuit is provided with a high-frequency oscillation source 1 that outputs a square wave, and an inverter (inversion means) 3 that inverts a square-wave signal output is connected to the high-frequency oscillation source 1. ing. The high-frequency oscillation source 1 is an AC power source that alternately outputs currents having two different voltage values of V1 (High) and V0 (Low) at a predetermined frequency. For example, in this embodiment, V1 = 5 [V]. , V0 = 0 [V] is output.

  The inverter 3 inverts and outputs the input AC current, and outputs an AC signal in which the output values V1 and V0 are inverted in reverse phase to the AC current output from the high-frequency oscillation source 1. It is supposed to be. That is, after the inverter 3, V0 is output while the high-frequency oscillation source 1 outputs V1, and conversely, V1 is output while the high-frequency oscillation source 1 outputs V0. Yes.

Further, as shown in the drawing, a signal path is branched between the inverter 3 and the high-frequency oscillation source 1, and a resistor (first resistor) R1 is interposed in the branched path.
On the other hand, a resistor (second resistor) R2 and a resistor (third resistor) R3 are connected in series after the inverter 3, and a touch electrode (detection electrode) 2 is interposed between these two resistors R2 and R3. Is intervening.

  Further, the termination side of the resistor R3 and the termination side of the resistor R1 are joined, and signals of these two paths are combined. Further, an amplifier (amplifying means) 5 is provided on the subsequent stage side of the resistors R1 and R3, and the amplitude of the above-described synthesized AC signal is input to the amplifier 5 and amplified. Further, a detecting means 6 is connected downstream of the amplifier 5 to convert the amplitude of the input AC signal into a DC signal to detect a change in capacitance and determine an operator's touch operation.

In the present embodiment, the amplifier 5 includes an inverter 41, a resistor 42, a plurality of capacitors 43, 44, and the like, and an input signal is amplified by the amplifier 5. The detection means 6 has a function of converting an input alternating current into a direct current, and includes diodes 61 and 62, a capacitor 63, a resistor 64, and the like.
Note that the above-described amplifier (amplifying means) 5 and detection means 6 are known techniques, and the configuration thereof is not particularly limited.

By the way, the resistances R1, R2, and R3 described above have resistance values r ₁ , r ₂ , and r ₃ , respectively, and are set to satisfy the relationship r ₁ = r ₂ + r ₃ . Thereby, the voltage value in the path of the resistor R1 and the voltage value in the path of the resistors R2 and R3 when the operator is not operated are kept the same. The resistance values r ₁ , r _2, and r ₃ preferably satisfy the relationship r ₁ : r ₂ : r ₃ = 2: 1: 1. Therefore, in the present embodiment, r ₁ = 200 [kΩ], r ₂ = 100 [kΩ], and r ₃ = 100 [kΩ] are set.

Therefore, for example, when the operator does not touch the touch electrode 2, the AC signal from the high-frequency oscillation source 1 is inverted and output through the inverter 3, and then output through the resistors R2 and R3. On the other hand, a signal from the high-frequency oscillation source 1 is directly input to the resistor R1.
In this case, as described above, the resistance values r ₁ , r ₂ , and r ₃ of the resistors R ₁ , R ₂ , and R ₃ satisfy the relationship r ₁ = r ₂ + r ₃ , and thus are output via the resistor R 1. The AC signal and the AC signal output via the resistors R2 and R3 are square waves having the same voltage value and the same amplitude and having opposite phases.

  For this reason, the value (voltage value) obtained by combining these two signals cancels out the output value, and the amplitude becomes 0 [V]. Therefore, even if the amplitude of the synthesized signal is amplified by the amplifier 5 after that, the output remains substantially 0 [V], and the output value becomes 0 [V] even if it is converted to direct current by the detection means 6 in the subsequent stage. For this reason, in the detection means 6, when the output value is 0 [V], it is determined that the electrostatic capacity has not changed, and it is determined that the touch operation by the operator has not been performed.

On the other hand, when the operator (object to be detected) touches the electrode 2, the impedance changes because a high-frequency current flows to the ground side (GND) through the operator, and therefore, the high-side voltage between the resistors R2 and R3. As the value decreases, the voltage on the Low side increases.
In this case, when the non-inverted signal and the inverted signal are superposed, the resistor R1 is set by an amount corresponding to the decrease in the high-side voltage value and the increase in the low-side voltage value of the AC signal output via the resistors R2 and R3. An amplitude is generated in a composite value with the AC signal output via the AC signal, and an AC signal is generated. For this reason, by amplifying this amplitude and converting it into a direct current, a voltage value of a predetermined level is output from the detection means 6.

Accordingly, the detection means 6 determines that the capacitance of the electrode 2 has changed when a voltage of a predetermined value or more is detected, that is, the touch operation of the touch electrode 2 by the operator has been performed.
Since the capacitance change detection circuit according to the embodiment of the present invention is configured as described above, its operation will be described below with reference to the time chart of FIG. Here, FIG. 2 is a time chart showing a change in state when the operator touches the detection electrode 2 at time t1 from a state where the operator does not touch the detection electrode 2, and (a) shows the output of the high-frequency oscillation source 1. (B) shows the output change after passing through the inverter, (c) shows the voltage change at the detection electrode, and (d) shows the output of the high-frequency oscillation source 1 downstream of the resistor R1 and downstream of the resistor R3. (E) shows a signal converted into a DC signal by the detecting means.

Now, as shown in FIGS. 2A and 2B, changes in the output from the high-frequency oscillation source 1 and the output after passing through the inverter are constant regardless of the presence or absence of the operator's touch operation, and they are squares of opposite phases. A wave is formed.
Further, in the detection electrode 2, the voltage decreases by an amount proportional to the resistance value r2, but the waveform itself is in phase with the output after passing through the inverter, as shown in FIGS. When the operator touches the electrode 2 at t1, the impedance of the electrode 2 changes, and as shown in FIG. 2C, the amplitude of the high-frequency signal changes greatly compared to before the touch operation.

For this reason, as shown in FIG. 2 (d), the combined signal obtained by superimposing the signal downstream of the resistor R1 and the signal downstream of the resistor R3 is out of phase with each other when no touch operation is performed. Although the AC signal cancels out due to the square wave and the voltage value becomes constant and the amplitude disappears, the AC signal is generated due to the decrease in the voltage during the touch operation.
Therefore, when the amplitude of this AC signal is amplified by the amplifier 5 and converted to DC by the detection means 6, characteristics as shown in FIG. 2 (e) can be obtained, and the presence or absence of touch can be determined.

  Therefore, according to the capacitance change detection circuit according to the present embodiment, the capacitance change (based on the amplitude of the combined signal of the AC signal output from the high-frequency oscillation source 1 and the AC signal obtained by inverting this signal ( With a simple configuration of detecting (impedance change), it is possible to reliably detect the capacitance change at low cost. In particular, in such a circuit, when the electrode 2 is not touched, the amplitude of the combined signal becomes 0. Therefore, it is possible to easily increase the sensitivity by amplifying the combined signal with the amplifier 5.

Further, in this circuit, a resistor element and a C-MOS inverter are used, and a bipolar element, a switching element or the like is not used, so that the cost can be further reduced.
Incidentally, FIG. 3 is a diagram showing an example of a configuration when the present invention is applied to a plurality of detection electrodes 2. As shown in the figure, in order to execute signal processing of a plurality of detection electrodes 2, only signal processing means (amplification means 5 and detection means 6) need be connected in parallel. On the other hand, it can be used as a common part. Therefore, the number of parts can be reduced, and the cost can be further reduced by applying such a configuration to an operation panel or an operation device having many touch switches.

  Next, a modification of the present embodiment will be described with reference to FIG. The circuit diagram of the modification shown in FIG. 4 is the same as the circuit shown in FIG. 1 except that only the arrangement of the inverter 3 is changed from the circuit shown in FIG. In this case, the inverted signal is input to the resistor R1, and the square wave signal from the oscillation source 1 is directly input to the resistor R2. The square wave input to the resistor R2 is output via the detection electrode 2 and the resistor R3, and is superimposed on the inverted signal output via the resistor R1.

Therefore, in this case, only the phases of the two AC signals are inverted as compared with the case of FIG. 1, and substantially the same operation effect as the circuit of the embodiment shown in FIG. 1 can be obtained.
Although the embodiments of the present invention have been described above, the present invention is not limited to such embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, the present invention may be applied to a matrix circuit. In this case, for example, it can be applied to a keyboard using a touch switch.

1 is a schematic circuit diagram illustrating an overall configuration of a capacitance change detection circuit according to an embodiment of the present invention. It is a figure for demonstrating the effect | action of the electrostatic capacitance change detection circuit which concerns on one Embodiment of this invention. It is a figure which shows an example of a structure at the time of applying the electrostatic capacitance change detection circuit which concerns on one Embodiment of this invention to a some detection electrode. It is a figure which shows the modification of the electrostatic capacitance change detection circuit which concerns on one Embodiment of this invention.

Explanation of symbols

1 High-frequency oscillation source (oscillation source)
2 Touch electrode (detection electrode)
3 Inverter (reversing means)
4 Inverter 5 Amplifier (Amplification means)
6 Detection means R1 1st resistance R2 2nd resistance R3 3rd resistance

Claims (3)

In a capacitance change detection circuit that senses a change in minute capacitance and detects the presence or absence of an input based on the amount of change,
An oscillation source that outputs a square wave;
A first resistor to which a square wave output from the oscillation source is input;
Inverting means for inverting and outputting a square wave output from the oscillation source;
A second resistor to which a square wave output through the inverting means is input;
A sensing electrode connected downstream of the second resistor;
A third resistor connected to the downstream side of the detection electrode;
A capacitance change detection circuit, wherein the terminal ends of the first resistor and the third resistor are connected. When the resistance values of the first resistor, the second resistor, and the third resistor are r ₁ , r _2, and r ₃ , respectively, the resistance values r ₁ , r _2, and r ₃ satisfy the following relationship: The capacitance change detection circuit according to claim 1.
r ₁ = r ₂ + r _{3 3. The} capacitance change detection circuit according to claim ₂ , wherein the resistance values r ₁ , r ₂ and r ₃ satisfy the following relationship.
r ₁ : r ₂ : r ₃ = 2: 1: 1

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