JP3579908B2 - Capacitive proximity sensor - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a separated capacitive proximity sensor in which a sensor unit serving as a sensor and a main circuit unit of a switch are separated.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a separation type proximity switch has been proposed in which a detection electrode and an oscillation circuit are used as a sensor unit and a sensor unit and a detection switch main circuit unit are separated. FIG. 3 is a circuit diagram showing a configuration of a sensor unit of such a conventional proximity switch. As shown in the figure, the detection electrode 1 is an electrode arranged on the object detection surface of the sensor unit, and is connected to the base of the transistor Q1. The transistors Q1 and Q2 constitute an amplifier, and a part of the output is extended to a position close to the detection electrode 1, and the capacitance C1 therebetween serves as a negative feedback capacitance. The capacitor C2 is a positive feedback capacitor. When the capacitance between the detection electrode 1 and the ground terminal increases due to the proximity of an object, the oscillation circuit starts oscillating due to a decrease in the amount of negative feedback due to the capacitance C1. Since such an oscillation circuit is provided in the sensor section as it is, a proximity switch (hereinafter referred to as a main circuit section) excluding the sensor section and the oscillation circuit has three lines in the power supply and for supplying an oscillation output to the main circuit section. Was connected.
[0003]
[Problems to be solved by the invention]
As described above, the conventional sensor-separated capacitance proximity switch has a disadvantage that the structure in the sensor unit is complicated because the oscillation circuit is provided in the sensor unit as it is.
[0004]
In the conventional oscillation circuit, a part of the amplitude output is extended to a position close to the detection electrode 1 to form a negative feedback capacitance. However, the capacitance varies depending on each device. There is a disadvantage that the characteristics are not stabilized.
[0005]
The present invention has been made in view of such a conventional problem, and has as its technical object to simplify the structure of a sensor unit and stabilize characteristics.
[0006]
[Means for Solving the Problems]
The present invention relates to a first conductor layer used as a sensing electrode, a second conductor layer provided between the first conductor layer and the first conductor layer via an insulator layer, and a side opposite to the first conductor layer from the second conductor layer . , A sensor unit having a third conductor layer grounded and having a grounded third conductor layer, a first conductor layer and a second conductor layer having one core wire and one covering wire, one ends of which are respectively provided in the sensor unit And the first conductor layer and the second conductor layer are connected via the core wire and the covering wire of the shield cable, respectively, the first conductor layer is connected to the input end, and the second conductor layer is connected to the output side. Buffer circuit, an oscillation circuit that oscillates at a frequency based on a change in capacitance between the first conductor layer and the object, and a detection circuit that detects proximity of the object based on a change in the oscillation frequency of the oscillation circuit. And a main circuit section.
[0007]
[Action]
According to the present invention having such features, the sensor unit is constituted by at least the first to third conductor layers, and determines whether or not the object is approaching between the first conductor layer and the ground by using the first conductor layer and the object. It is determined as a change in capacitance between the two . The first conductor layer and the second conductor layer are connected to respective input / output terminals of a buffer circuit constituting an oscillation circuit of the main circuit section by a shielded cable. Therefore, these electrodes are always at the same potential, and charging and discharging are not performed during this period, so that the capacitance between these electrodes is negligible. The first and second conductor layers are eliminated from the external influence by the third shield ground pattern. If an object approaches the first conductor layer in this state, the capacitance between the first conductor layer and the ground changes. Then, an oscillation circuit of the main circuit portion oscillating at a frequency corresponding to the capacitance is used, and the proximity of an object is detected by a change in the oscillation frequency.
[0008]
【Example】
FIG. 1 is a diagram showing a front part of a sensor section and an amplifier section of a capacitance type proximity switch according to an embodiment of the present invention. In the figure, the sensor section 11 is constituted by a printed circuit board 12 having three layers. The pattern of the first conductor layer formed on one surface of the printed circuit board 12 is a detection electrode 12a arranged to face the object detection area, and the pattern inside the printed circuit board 12 shields the detection electrode 12a. Conductor layer pattern, that is, the common-mode shield pattern 12b. The pattern of the third conductor layer formed on the other surface of the printed circuit board 12 is referred to as a ground pattern 12c. The shield ground pattern 12c is a pattern for reducing the influence of external noise on the detection electrode 12a and the in-phase shield pattern 12b. Then, the patterns 12a and 12b are connected to the core wire and the covering wire of the shielded cable 13, respectively, and connected to the main circuit portion 14 side. The core wire to which the detection electrode 12 a is connected in the main circuit section 14 is connected to the input terminal of the buffer circuit 15. The output terminal of the buffer circuit 15 is connected to the shielded wire of the shield line 13 and is connected to the input terminal of the Schmitt trigger inverter 16. The feedback resistor R is connected between the output terminal of the Schmitt trigger inverter 16 and the input terminal of the buffer circuit 15.
[0009]
Here, when an object grounded to the detection electrode 12a approaches, the capacitance Cd therebetween increases. The buffer circuit 15 and the Schmitt trigger inverter 16 constitute an oscillation circuit 17 that oscillates with the capacitance Cd and the feedback resistance R as time constants, and the output thereof is connected to a period counter 18. The cycle counter 18 measures the oscillation cycle of the oscillation circuit, and its output is provided to a linearizer 19. The linearizer 19 linearizes the change in the cycle as a change with respect to the distance to the object. The output of the linearizer 19 is input to the display circuit 20 and the comparison circuit 21. The comparison circuit 21 discriminates the input signal by a predetermined threshold value, and is output by the output circuit 22 as a signal for determining the presence or absence of an object.
[0010]
Next, the operation of this embodiment will be described. First, when the power is turned on, the power supply voltage of the Schmitt trigger inverter 16 immediately rises as shown in FIG. Since the Schmitt trigger inverter 16 has a certain hysteresis, its input level is initially 0 level, and the capacitor Cd is gradually charged via the feedback resistor R. Therefore, the potential of the electrode 12a gradually increases as shown in FIG. When the output exceeds the threshold of the Schmitt trigger inverter 16, the output is inverted. FIG. 2C shows the output of the Schmitt trigger inverter 16. After the inversion, the capacitance of the capacitor Cd is discharged via the resistor R.
[0011]
Thereafter, as shown in FIGS. 2 (b) and 2 (c), charging and discharging are repeated at a constant cycle, and oscillation is performed at this frequency. If the object is not approaching, Cd has a small value and oscillates at a high frequency. If the distance between the object and the object is d, the oscillation cycle is substantially proportional to the distance d between the detection electrode 12a and the object. Therefore, although the oscillation period becomes long, the period to the object can be displayed by counting the period by the period counter 18 and linearizing it through the linearizer 19. In this case, since the area of the object needs to be fixed, the distance to the same object is measured. The linearizer 19 is calibrated in advance based on the output when the object is set at a specified position. In this way, the distance to the object can be accurately displayed. It is also possible to output a switching signal based on the distance to the object.
[0012]
According to such a configuration, since the detection electrode 12a and the in-phase shield pattern 12b are connected to the input / output terminal of the buffer circuit 15 via the shield cable 13, they always have the same phase and the same voltage. Therefore, the detection electrode 12a is not affected by the capacitance between the detection electrode 12a and the common-mode shield pattern. Therefore, the sensor section having the detection electrode and the electronic circuit section can be separated by the shield cable 13.
[0013]
In the present embodiment, the third pattern is grounded as an earth pattern. However, when the case of the sensor unit 11 is not formed of a metal body and cannot be grounded to a metal body, the sensor unit 11 is connected via a shielded wire. It shall be connected to the ground voltage of the main circuit.
[0014]
【The invention's effect】
As described above in detail, according to the present invention, since the sensor section is formed of three conductor layers , the structure of the sensor section is extremely simplified, and the characteristics can be stabilized.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a capacitance type proximity switch according to an embodiment of the present invention.
FIG. 2 is a time chart illustrating the operation of the present embodiment.
FIG. 3 is a circuit diagram showing a configuration of a sensor unit of a conventional capacitance type proximity switch.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 Sensor part 12 Printed circuit board 12a Detecting electrode 12b In-phase shield pattern 12c Shield ground pattern 13 Shield cable 14 Main circuit part 15 Buffer circuit 16 Schmitt trigger inverter 17 Oscillation circuit 18 Period counter 19 Linearizer

Claims (1)

A first conductor layer used as a sensing electrode, a second conductor layer provided between the first conductor layer and the first conductor layer via an insulator layer, and a side opposite to the first conductor layer from the second conductor layer provided through the insulator layer, and a sensor section having a third conductor layer which is grounded,
A shielded cable having one core wire and one covered wire, one end of which is connected to the first conductor layer and the second conductor layer of the sensor unit, respectively;
Said first conductor layer and second conductor layer is connected through the core wire and the sheath wire of each said shielded cable, the first conductor layer is input, a buffer circuit in which the second conductor layer connected to the output side, the first A main circuit unit having an oscillation circuit that oscillates at a frequency based on a change in capacitance between the one conductor layer and the object, and a detection circuit that detects proximity of the object based on a change in the oscillation frequency of the oscillation circuit; And a capacitance type proximity sensor.

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