JPH0634307A - Capacitance type displacement sensor - Google Patents

Abstract

PURPOSE:To accurately measure a distance based on capacitance to an object. CONSTITUTION:This displacement sensor is provided with a detection electrode 1, capacitance Cd determined by a distance to an object, and a Schmitt trigger inverter 2, to an input end of which the detection electrode 1 is connected and which has feedback resistance R. Oscillation is conducted with a frequency determined by the feedback resistance R of the Schmitt trigger inverter 2 and a capacitance value. By measuring this oscillation frequency by means of a period counter 3, a distance to an object can be measured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a capacitance type displacement sensor for detecting a distance to an object using capacitance.

[0002]

2. Description of the Related Art An electrostatic capacitance type proximity switch for detecting the presence or absence of an object by an electrostatic capacitance is used. In such a capacitance type proximity switch, a specific terminal of the oscillation circuit is used as an electrode, and oscillation starts when the capacitance of the electrode increases due to the proximity of an object. Therefore, the output is envelope-detected to determine the presence or absence of an object.

[0003]

However, in such a conventional capacitance type proximity switch, the oscillation condition greatly depends on the variation of the circuit constants, and the individual difference of the proximity switch becomes large. Moreover, since the temperature drift is large, the object could not be detected stably. Further, since the distance to the detection object is detected as a capacitance value, it is necessary to bring the electrodes close to the detection object. However, if the distance between the electrode and the other electronic circuit section is increased, the fixed capacitance value of the electrode increases. Therefore, the electrode part and the electronic circuit part are separated,
There is a drawback that it is difficult to construct a so-called amplifier-separated sensor.

The present invention has been made in view of the above-mentioned conventional problems, and it is a technical object to make it possible to measure the approach of a detection object with high accuracy. In addition to the above problems, it is a technical object of the invention of claim 2 to separate the sensor unit from other electronic circuit units so that the sensor unit can be downsized.

[0005]

According to the invention of claim 1 of the present application, a sensing electrode, a sensing electrode at the input end, and a feedback resistor connected between the input and output ends have hysteresis, and the sensing electrode and the sensing object are provided. A Schmitt trigger inverter that oscillates in a cycle based on a capacitance and a feedback resistance between the two, and a cycle counter that detects an oscillation cycle of the Schmitt trigger inverter, and detect a distance to an object based on the cycle. It is characterized by.

According to a second aspect of the present invention, the shield wire having one end of the core wire connected to the detection electrode, the other end of the core wire of the shield wire and the input end of the feedback resistor are connected, and the output end thereof has a Schmitt trigger. A buffer circuit connected to the input terminal of the inverter and the ground line of the shield wire is provided.

[0007]

According to the present invention having such characteristics, the oscillation output continuously oscillated by the Schmitt trigger inverter is obtained by the oscillation frequency determined by the capacitance between the sensing electrode and the object and the resistance value of the feedback resistance of the Schmitt trigger inverter. To be The distance to the object is detected by detecting this oscillation cycle. Further, in the invention of claim 2 of the present application, the detection electrode and the input end of the Schmitt trigger inverter are connected by a shield line and a buffer circuit, and the output of the buffer circuit is connected to the ground line of the shield line. Therefore, the potential difference between the detection electrode and the input end of the Schmitt trigger inverter and the detection electrode disappears. Therefore, it is possible to increase the distance between the sensor circuit and the electronic circuit portion, and it is possible to realize a separate sensor of the sensor portion.

[0008]

1 is a block diagram showing the overall construction of a capacitance type proximity sensor according to an embodiment of the present invention. In the figure, the electrode 1 is an electrode having a certain area and is fixed to the front surface of the proximity switch. A Schmitt trigger inverter 2 having hysteresis is connected to this electrode 1. A feedback resistor R is connected between the input and output ends of the Schmitt trigger inverter 2 as shown in the figure. If an object exists between the electrode 1 and the grounded end, a capacitance is generated between them. This capacitance Cd is shown by the following equation. Cd = εS / d where S is the area of the electrode 1, d is the distance to the object, and ε is the permittivity in air.

The Schmitt-drigger inverter 2 oscillates with the electrostatic capacitance Cd and the feedback resistance R as a time constant, and its output is given to the cycle counter 3. The period counter 3 measures the oscillation period of the oscillator circuit by the Schmitt trigger inverter, and its output is given to the linearizer 4. The linearizer 4 linearizes the change in the cycle as a change with respect to the distance to the object. The output of the linearizer 4 is displayed by the display circuit 5, compared with a predetermined threshold value by the comparison circuit 6, and output from the output circuit 7 as a determination signal of the presence or absence of an object.

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 2 rises immediately as shown in FIG. 2 (a). Since the Schmitt trigger inverter 2 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 1 gradually rises as shown in FIG. When the threshold value of the Schmitt trigger inverter 2 is exceeded, the output is inverted. FIG. 2C shows the output of the Schmitt trigger inverter 2. After the inversion, the capacitance of the capacitor Cd is discharged via the resistor R.

Thereafter, as shown in FIGS. 2 (b) and 2 (c), charging / discharging is repeated at a constant cycle, and oscillation is performed at this frequency.
If the object is not approaching here, Cd has a small value and thus oscillates at a high frequency. When an object approaches, the distance to the object is d, and the oscillation period is almost proportional to the distance d between the sensing electrode 1 and the object. Therefore, the cycle is counted by the cycle counter 3, and the linearizer 4
The distance to the object can be displayed by linearizing via. In this case, since the area of the object needs to be fixed, the distance to the same object is measured. Further, the linearizer 4 is to be calibrated in advance by the output when this object is set at a prescribed position. In this way, the distance to the object can be displayed accurately. It is also possible to output a switching signal based on the distance to the object.

Next, a second embodiment of the present invention will be described. In this embodiment, the sensor portion having the detection electrode and the electronic circuit portion can be separated by connecting the detection electrode 1 and the input end of the Schmitt trigger inverter 2 with a shield wire. That is, the detection electrode 1 is connected to one end of the core wire of the shielded wire 11, and the input end of the buffer circuit 12 is connected to the other end. Further, the output terminal of the buffer circuit 12 is connected to the Schmitt trigger inverter 2 described above. Further, the output end is connected to the ground line of the shield wire 11. Feedback resistor R
Is connected to the output of the Schmitt trigger inverter 2 and the input end of the buffer circuit 12. Other configurations are similar to those of the first embodiment. Here, the capacitance of the electrode between the detection electrode 1 and the ground line of the shield wire is defined as Co.

According to this structure, the connection point d between the core wire of the shield wire 11 and the detection electrode 1 and the connection point e between the ground wire of the shield wire and the input end of the buffer circuit 12 are always in phase and at the same voltage. Becomes Therefore, the influence of the capacitance Co on the detection electrode 1 is eliminated. Therefore, the shield wire 11 can separate the sensor section having the detection electrode from the electronic circuit section.

[0014]

As described in detail above, according to the present invention, since the Schmitt trigger inverter is used to constantly oscillate and the oscillation cycle thereof is detected, the distance to the object can be accurately detected. You can Claim 2
According to the invention, the electronic circuit section and the sensor section can be separated, and the sensor section can be downsized.

[Brief description of drawings]

FIG. 1 is a block diagram showing an overall configuration of a capacitance type sensor according to a first embodiment of the present invention.

FIG. 2 is a time chart showing the operation of this embodiment.

FIG. 3 is a block diagram showing a configuration of a capacitance type displacement sensor according to a second embodiment of the present invention.

[Explanation of symbols]

 1 Detection Electrode 2 Schmitt Trigger Inverter 3 Period Counter 4 Linearizer 5 Display Circuit 6 Comparison Circuit 7 Output Circuit 11 Shielded Wire 12 Buffer Circuit

Claims (2)

[Claims] 1. A sensing electrode, a sensing resistor at the input end, and a feedback resistor connected between the input and output ends, having a hysteresis, and based on the capacitance and the feedback resistance between the sensing electrode and the sensing object. A capacitance type displacement sensor, comprising: a Schmitt trigger inverter that oscillates according to a cycle; and a cycle counter that detects an oscillation cycle of the Schmitt trigger inverter, wherein a distance to an object is detected based on the cycle. . 2. A shield wire having one end of a core wire connected to the detection electrode, the other end of the core wire of the shield wire and a feedback resistor connected to an input end thereof, and an output end thereof being an input end of the Schmitt trigger inverter. 2. The capacitance type displacement sensor according to claim 1, further comprising: a buffer circuit connected to the ground line of the shield line.