JP5284834B2 - Capacitive proximity sensor - Google Patents

Description

The present invention relates to a proximity detection sensor that detects a proximity object by detecting a change in capacitance.

  2. Description of the Related Art Conventionally, a sensor that detects a nearby object by detecting a change in electrostatic capacitance is known. As a document disclosing this type of sensor, for example, there is Patent Document 1 below. In the embodiment disclosed in Patent Document 1, a sensor that detects a change in electrostatic capacitance due to an object approaching a handrail as a detection electrode has a capacitance value in a static state depending on the length of the handrail, the thickness of an insulator, and the like. Corresponding to the change of the sensitivity, it corresponds to the sensitivity change by changing the value of the capacitance of the capacitor. A neon tube is connected to the handrail as an electrostatic spark removing means.

Japanese Patent No. 3600796

  An object of the present invention is to automatically adjust the sensitivity with respect to a change in the shape and installation state of the detection electrode in a capacitance type proximity detection sensor that detects a nearby object by detecting a change in capacitance, Another object of the present invention is to provide an easy-to-use and low-cost proximity sensor that can be used without adding a new element corresponding to electrostatic sparks.

  In order to solve the above-described problems, a first invention is a proximity detection sensor that detects that an object to be detected has approached a detection electrode from a change in capacitance. Capacitance measurement means, filter value calculation means for outputting a filter value from which a rapid change in the measured value measured over time is output, and proximity detection determination value calculation for outputting a proximity detection determination value determined according to the filter value Proximity detection sensor comprising means and determination means for detecting that the measured value has changed by more than the determination value from the filter value.

A second invention according to claim 1 is the proximity detection sensor according to claim 1, wherein the detection electrode is a conductor laid substantially parallel to the ground, and its length is changed in accordance with a detection range.

According to a third aspect of the present invention, in the first or second aspect, the capacitance measuring unit includes an oscillating unit, the oscillating element of the oscillating unit is a CMOS inverter, and a detection electrode is connected to the other end of the resistor connected to the output terminal. A proximity detection sensor characterized by connecting a coil and one end of a resonance circuit by a capacitor each having one end grounded.

According to the present invention, even if the shape and installation state of the detection electrode are changed, the proximity detection determination value can be appropriately adjusted by the proximity detection determination value calculation means, and the sensitivity can be automatically corrected. Moreover, it is possible to cope with electrostatic sparks even though the circuit uses a low-cost CMOS inverter.

It is a block diagram which shows the whole Example of this invention. It is a circuit diagram which shows the oscillation means Example of this invention. It is a flowchart of the software Example of this invention.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing the entire embodiment of the present invention. The detection electrode 11 in FIG. 1 has a capacitance between the ground and the like, and has a constant value depending on the shape and distance of the electrode. When the object to be detected approaches the detection electrode, the capacitance changes in the direction in which the capacitance increases as the object approaches.

Specifically, when the sensing electrode is a cylinder having a radius of 4 mm and installed on the ground 0.2 m, the capacitance C1 per unit length can be obtained by the following calculation formula. When the length is 1 m, C1 = 2πε / log (2h / a) = 12.1 pF.
(A: radius of cylinder, h: height from ground, ε: dielectric constant)
If the length of the detection electrode is doubled in order to widen the detection range, the capacitance C2 becomes 24.2 pF. Also, it can be seen that the capacitance per unit length does not change if the diameter a of the detection electrode is increased by the same ratio even when the size of the detection object is different and the height h of the electrode is changed. The installation at the height of 0.2 m assumes the detection of a cat. However, when detecting a larger object to be detected such as a dog, the height is adjusted to 0. What is necessary is just to make 4 m and a detection electrode radius into 8 mm.

The increase in capacitance ΔCx when the same object approaches the detection electrode when the height and radius are the same and the length is 1 m and 2 m is the same if the size of the object is sufficiently smaller than 1 m. The respective capacitances are C1x = 12.1 + ΔCx, C2x = 24.2 + ΔCx.

In this embodiment, the capacitance measuring means 12 in FIG. 1 is realized by an oscillation circuit whose oscillation frequency changes depending on the capacitance, and an oscillation frequency measuring means for measuring the oscillation frequency. An example of an oscillation circuit is shown in FIG. 21 and 22 operate as CMOS inverters, 21 operates as an oscillation element, and 22 operates as a buffer. This circuit does not use any special parts to prevent spark discharge from the outside, but connects the coil 23, capacitor 25, and resistor 26 to the detection electrode to disperse the static input, thereby protecting the CMOS inverter that is particularly sensitive to static electricity. Yes. A resonance circuit is constituted by the coil 23, the capacitor 24, the capacitor 25, the detection electrode, and the electrostatic capacitance Cx of the ground, and the oscillation frequency f0 is obtained by the following equation.
f0 = 1 / 2π√ (L23 · C24 · (C25 + Cx) / (C25 + c24 + Cx))
If C24 >> C25 and C24 >> Cx, it can be approximated as follows.
f0 = 1 / 2π√ (L23 · (C25 + Cx))

Specifically, if L23 = 33 mH, C25 = 5 pF, and the oscillation frequency f10 = 211.87 kH in the steady state, the length of the detection electrode can be calculated as 1 m with Cx = 12.1 pF. Assuming that the capacitance that the detected object increases and ΔCx1 = 1 pF, the oscillation frequency f10Δ1x = 205.93 kH when the detected object approaches is obtained. The difference is 5.94 kH. Similarly, when the detection electrode is 2 m, f20 = 162.13 kHz and f20Δ1x = 159.43 kHz, respectively. The difference is 2.70 kH. Thus, when the length of the detection electrode is changed, the steady oscillation frequency also changes, and even when the same object to be detected approaches, the frequency change width also differs.

The oscillation frequency measuring means can be realized by counting the number of pulses of the oscillation circuit that are input at regular intervals. Specifically, it is realized by software that takes in the number of pulses integrated every 100 mS into a microcomputer, and the entire flowchart is shown in FIG. Each time a pulse is input, the count is incremented by one in 31 pulse count-up routines, and is captured in 33 pulse number count capture routines every 100 mS. The frequency is calculated by multiplying the count number for every 100 mS by 10 by the frequency calculation 33. The latest calculated frequency is fxn.

The filter calculation means 13 in FIG. 1 filters the frequency value calculated every 100 mS to remove instantaneous changes or very slow changes. Specifically, this is realized by sequentially storing the previous calculated frequency as fxn-1 and the previous frequency as fxn-2, and obtaining the average value for 100 times. If the value of this filter calculation result is fy, fy = (fxn + fxn−1 +... + Fxn−99) / 100
It can be expressed. This is realized by the filter operation 35 in FIG.

The proximity detection determination value calculation means 14 in FIG. 1 determines the proximity determination value fH based on the value of fy. The length of the detection electrode is determined by a value of 1 m, 2 m, or fy, and an oscillation frequency value that changes due to a change in capacitance when the detection object approaches is set as fH.
When fy> (f10 + f20) / 2, fH = 5.94 kH
When fy <(f10 + f20) / 2, fH = 2.70 kH
Although the proximity determination value fH is switched in two steps in this embodiment, it can be obtained from a calculation formula even if the number of stages is increased. In some cases, the calculation formula may deviate due to the influence of stray capacitances of circuits, elements, etc., and the frequency fy and the determination value fH may be obtained by actual measurement and mapped.

  The determination means 15 in FIG. 1 compares fxn, fy, and fH, and determines whether or not the detected object is close by the detection determination routine 37 in FIG. When the object to be detected approaches, the capacitance increases. However, since the oscillation frequency decreases, it is determined that the proximity is detected when fxn decreases. fxn and fy−fH are compared, and if fxn <fy−fH, it is determined that the object is to be detected and a detection signal is output. In this embodiment, the proximity detection is determined based on the frequency value, but it is also possible to determine the proximity detection when ΔCx1> 1 pF by calculating the capacitance Cx (the length of the detection electrode) and ΔCx1 from the oscillation frequency. Similarly, the filter calculation means and the proximity detection determination means can also be realized by electrostatic capacity. It can also be realized by replacing the capacitance with a voltage value, a duty ratio, or the like.

Claims (3)

In a proximity detection sensor that detects that an object to be detected has approached a detection electrode from an increase in capacitance of the detection electrode, an oscillation circuit in which an oscillation frequency decreases due to the increase in capacitance, and the oscillation frequency over time Frequency measuring means for measuring, frequency filter value calculating means for outputting a frequency filter value fy from which an early change in the frequency measurement value fxn measured over time is removed, and a plurality of predetermined numbers corresponding to the frequency filter value fy Proximity detection determination frequency value calculating means for switching and outputting the proximity detection determination value frequency fH, and that the frequency measurement value fxn is lower than the frequency filter value fy by the proximity detection determination value frequency fH or more (fxn <fy−fH). Proximity detection sensor comprising proximity detection frequency determination means for detecting 2. The proximity detection sensor according to claim 1, wherein the detection electrode is a conductor laid substantially parallel to the ground, and its length is changed in accordance with a detection range. 3. The oscillation element of the oscillation circuit according to claim 1, wherein the oscillation element of the oscillation circuit is a CMOS inverter, and the other end of the resistor connected to the output terminal thereof is connected to one end of the resonance circuit by a capacitor and a capacitor each having one end grounded. And the other terminal of the said resonance circuit was connected to the input terminal of the said CMOS inverter, The proximity detection sensor characterized by the above-mentioned.

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