JP4858382B2 - Proximity sensor - Google Patents

Description

  The present invention relates to a high-frequency oscillation type proximity sensor.

  Conventionally, a high-frequency oscillation type proximity sensor has been proposed as a proximity sensor that detects a detection object made of a metal body (conductor) or a magnetic body in a non-contact manner.

The high-frequency oscillation type proximity sensor has an LC resonance circuit unit including a parallel circuit of a detection coil and a capacitor. This proximity sensor utilizes the phenomenon that, when the object to be detected approaches the detection coil that constitutes the LC resonance circuit unit, eddy current loss occurs due to electromagnetic induction and the effective resistance value (impedance) of the detection coil changes. The object to be detected is detected. In other words, when the impedance of the detection coil changes, the oscillation conditions of the LC resonance circuit section also change. Therefore, from the state in which the LC resonance circuit section is oscillated, the oscillation of the LC resonance circuit section stops or the oscillation amplitude attenuates by a predetermined value or more. At this time, it is determined that the detected object exists (see, for example, Patent Document 1).
JP 2005-295248 A

  In the conventional proximity sensor described above, if the distance between the detected object and the detection coil is longer than the distance (detection distance) at which the oscillation of the LC resonance circuit unit stops or the oscillation amplitude is less than a predetermined value, the detected object is detected. If it is determined that the detected object does not exist within the detection range of the coil and is shorter than the detection distance, it is determined that the detected object exists within the detection range of the detection coil. It is a digital value that has only two values of whether or not it exists.

  As described above, in the conventional proximity sensor, if the distance between the detected object and the detection coil becomes shorter than the detection distance, it is determined that the detected object exists. That is, the detected object is in the detection range of the detection coil. It was possible to obtain a digital detection signal indicating whether or not the detected object exists, but only how much the detected object is relative to the detection coil. Since it cannot be detected until it is approaching, an analog detection signal indicating the distance between the detected object and the detection coil cannot be obtained.

  The present invention has been made in view of the above points, and an object of the present invention is to provide a proximity sensor that can detect the distance to a detected object in addition to detecting the presence of the detected object.

  In order to solve the above-described problems, in the invention of claim 1, an LC resonance circuit unit including a detection coil and a capacitor used for detection of an object to be detected, an oscillation circuit unit that oscillates the LC resonance circuit unit, and an LC resonance A monitor unit for detecting the oscillation amplitude of the circuit unit, a control unit for setting the negative conductance of the oscillation circuit unit to a critical value at which the LC resonance circuit unit can oscillate based on the oscillation amplitude detected by the monitor unit, and an object to be detected A signal processing unit that creates a detection signal indicating a distance between the detection coil and the detection coil, the oscillation circuit unit including an amplification circuit that outputs a current corresponding to an oscillation voltage of the LC resonance circuit unit, and a current output from the amplification circuit A current feedback circuit that supplies a feedback current corresponding to the magnitude to the LC resonance circuit section, and a variable resistance section that is inserted between the output terminal of the amplifier circuit and the ground to adjust the magnitude of the current output from the amplifier circuit. Yes The control unit sets the resistance value of the variable resistance unit so that the negative conductance of the oscillation circuit unit becomes the critical value, and the signal processing unit creates the detection signal based on the resistance value of the variable resistance unit. It is characterized by that.

  According to the first aspect of the invention, the condition that the LC resonance circuit section oscillates is that the absolute value of the negative conductance of the oscillation circuit section is equal to or larger than the absolute value of the conductance of the detection coil. When the negative conductance is a critical value at which the LC resonant circuit unit can oscillate, the absolute value of the negative conductance can be considered to be equal to the absolute value of the sense coil conductance, where the conductance of the sense coil is The change in eddy current loss due to the distance between the detected body and the detection coil, that is, the change in the distance between the detection coil and the detected body, and the negative conductance of the oscillation circuit unit equal to the conductance of the detection coil is The feedback current supplied to the LC resonance circuit unit and the oscillation amplitude of the LC resonance circuit unit are determined. The feedback current supplied to the LC resonance circuit unit is determined by the amplifier circuit. Since it increases or decreases according to the current to be applied, the resistance value of the variable resistance unit that adjusts the magnitude of the current output from the amplifier circuit can be used as a value indicating the distance between the detection coil and the detected object. By using the resistance value of the variable resistance unit, an analog detection signal indicating the distance between the detected object and the detection coil is used instead of a digital detection signal indicating whether or not the detected object exists as in the conventional example. In addition, since it is possible to obtain a digital detection signal indicating whether or not the detected object exists by thresholding such a detected signal, in addition to detecting the presence of the detected object, The distance to the object to be detected can also be detected.

  According to a second aspect of the present invention, in the first aspect of the present invention, the variable resistor section includes a series or parallel circuit of one or more fixed resistors and one or more variable resistors.

  According to the second aspect of the present invention, the resistance value of the fixed resistor can be used as an offset of the resistance value of the variable resistor portion, and compared with the case where only the variable resistor is used, the detected object and the detection coil Distance resolution (positional accuracy) can be improved.

  According to a third aspect of the present invention, in the first or second aspect of the invention, the variable resistor section includes a variable resistor composed of a digital potentiometer capable of setting a resistance value with a digital code, and the control section includes the oscillation unit. A digital code for setting the resistance value of the variable resistance unit so that the negative conductance of the circuit unit becomes the critical value is output to the variable resistance unit and the signal processing unit. The detection signal is created based on the output digital code.

  According to the invention of claim 3, since the resistance value of the variable resistance unit can be obtained by only reading the digital code output from the control unit by the signal processing unit, for example, the control that the variable resistance unit outputs from the control unit The resistance value of the variable resistance portion can be easily obtained as compared with the case where the resistance value is set by turning the switch on and off by a signal.

  According to a fourth aspect of the present invention, in the third aspect of the present invention, the monitor section detects the oscillation amplitude of the LC resonance circuit section and outputs an analog signal indicating the oscillation amplitude, and the oscillation output by the detection section. An A / D converter that converts an analog signal indicating amplitude into a digital signal and outputs the digital signal to the control unit, and the control unit creates the digital code from the digital signal indicating the oscillation amplitude output from the A / D converter It is characterized by doing.

  According to the fourth aspect of the present invention, the processing speed (responsiveness and followability of change in the resistance value of the variable resistor portion) can be improved compared to the case where the resistance value of the variable resistor portion is changed by an analog circuit, and the detected object is detected. Even when the moving speed of the body is fast, the negative conductance of the oscillation circuit unit can be quickly set to a critical value that allows the LC resonance circuit unit to oscillate.

  According to a fifth aspect of the present invention, in the third aspect of the invention, the control unit includes a comparison unit that compares the oscillation amplitude detected by the monitor unit with a predetermined threshold value, and the digital unit is compared with a comparison result of the comparison unit. Whether to change the resistance value of the potentiometer is determined, and when changing the resistance value of the digital potentiometer, the digital code is changed one by one.

  According to the fifth aspect of the present invention, when the resistance value of the variable resistance portion is set to a value at which the negative conductance of the oscillation circuit portion becomes a critical value, it is possible to prevent the occurrence of overshoot or undershoot. In addition, since the digital code is increased by one, it is not necessary to perform a process of directly calculating the target value of the resistance value of the variable resistance unit from the oscillation amplitude. As the comparison unit, an AD conversion circuit, a CPU, etc. Compared to the complicated apparatus, an inexpensive comparator can be used, so that the cost can be reduced.

  According to a sixth aspect of the present invention, in the invention according to any one of the third to fifth aspects, the control unit includes a timing circuit that outputs a signal instructing a timing for outputting the digital code at a predetermined frequency, The predetermined frequency is lower than the oscillation frequency of the LC resonance circuit section.

  According to the sixth aspect of the present invention, it is possible to prevent the oscillation of the LC resonance circuit section resulting from the change of the resistance value of the variable resistance section, and to perform stable control.

  According to a seventh aspect of the invention, in the invention according to any one of the third to sixth aspects, the signal processing unit adds at least one of an offset and a gain to the value of the digital code output from the control unit. It is characterized by having.

  According to invention of Claim 7, the value of a detection signal can be made into the value within a desired range.

  The invention of claim 8 is characterized in that, in the invention of claim 7, the offset and gain in the output adjusting section can be changed.

  According to the invention of claim 8, even if there are variations in the characteristics of the detection coil, the relative position of the detection coil and the detected object, and the characteristics of the circuit such as the oscillation circuit section for each product, It is possible to prevent the range of the detection signal value from being different for each product, and the detection signal value can be set to a value within a desired range for any product.

  The invention according to claim 9 is the invention according to any one of claims 3 to 8, further comprising a temperature detection unit for detecting an ambient temperature, wherein the signal processing unit is a value of a digital code output by the control unit. And a temperature compensation unit that performs temperature compensation by multiplying the temperature by a correction temperature coefficient corresponding to the temperature detected by the temperature detection unit.

  According to the ninth aspect of the present invention, it is possible to prevent deterioration in detection accuracy due to temperature characteristics of circuits such as the detection coil, the detected object, and the oscillation circuit unit, and the detection accuracy can be improved.

  According to a tenth aspect of the present invention, in the ninth aspect of the invention, the correction temperature coefficient in the temperature compensation unit can be changed.

  According to the invention of claim 10, even if there are variations in the characteristics of the detection coil, the relative position between the detection coil and the object to be detected, and the temperature characteristics of the circuit such as the oscillation circuit section for each product, such variations Therefore, it is possible to prevent the value of the detection signal from being different for each product, and a desired detection signal can be obtained for any product.

  According to an eleventh aspect of the present invention, in the invention according to any one of the first to tenth aspects, the oscillation circuit unit, the monitor unit, the control unit, and the signal processing unit are integrated as a monolithic IC. It is characterized by.

  According to the eleventh aspect of the present invention, the oscillation circuit unit, the oscillation detection unit, the control unit, and the position information generation unit can be reduced in size as compared with the case where they are configured by separate ICs. Costs can be reduced and noise resistance can be improved.

  The present invention has an effect of being able to detect the distance to the detected object in addition to detecting the presence of the detected object.

(Embodiment 1)
As shown in FIG. 1, the proximity sensor according to the present embodiment includes an LC resonance circuit unit 1 including a parallel circuit of a detection coil 10 and a capacitor 11 used for detection of a detection target (not shown), and an LC resonance circuit unit. The oscillation circuit unit 2 that oscillates 1, the monitor unit 3 that detects the oscillation amplitude of the LC resonance circuit unit 1, and the negative conductance of the oscillation circuit unit 2 based on the oscillation amplitude detected by the monitor unit 3 1 includes a control unit 4 that sets a critical value that can oscillate, and a signal processing unit 5 that generates a detection signal indicating the distance between the detected object and the detection coil 10.

  The detection coil 10 used in the LC resonance circuit unit 1 is, for example, a coil that is wound around the outer peripheral surface of a cylindrical coil bobbin (not shown) in the form in which the winding axis direction is aligned with the axial direction of the coil bobbin. Yes. The detected body is formed in a pipe shape from a metal body (conductor), for example, and is arranged so as to pass immediately outside the detection coil 10 along the winding axis direction of the detection coil 10. Yes. The frequency of the oscillation voltage of the LC resonance circuit unit 1 is determined by the inductance of the detection coil 10 and the capacitance of the capacitor 11. Note that the configuration of the detection coil 10 and the detected body as described above is merely an example. For example, the detected body may be formed of a magnetic body and can be changed without departing from the spirit of the present invention. it can.

  In the proximity sensor of the present embodiment, the oscillation circuit unit 2, the monitor unit 3, the control unit 4, and the signal processing unit 5 are integrated as a monolithic IC.

  The oscillation circuit unit 2 includes a bias circuit 20 that is a constant current source that supplies a constant bias current to the LC resonance circuit 1 from the internal power supply Vcc, and an oscillation voltage of the LC resonance circuit unit 1 (a voltage across the LC resonance circuit unit 1). A level shift circuit 21 for level shifting, an amplifier circuit 22 for outputting a current corresponding to the oscillation voltage of the LC resonance circuit section 1, and a feedback current corresponding to the magnitude of the current output from the amplifier circuit 22 for the LC resonance circuit section. 1, a current feedback circuit 23 for maintaining oscillation and a variable resistance unit 24 that is inserted between the output terminal of the amplifier circuit 22 and the ground, and adjusts the magnitude of the current output from the amplifier circuit 22; have.

  Here, the level shift circuit 21 is composed of an npn transistor Q1, the collector of the transistor Q1 is connected to the output terminal of the bias circuit 20, and the emitter is the other end of the LC resonance circuit section 1 with one end grounded. (In the illustrated example, a parallel circuit including the detection coil 10 and the capacitor 11 is inserted between the emitter of the transistor Q1 and the ground). In the transistor Q1, a collector and a base are connected. Therefore, the potential of the emitter of the transistor Q1 is equal to the oscillation voltage of the LC resonance circuit unit 1. The level shift circuit 21 oscillates between the emitter of the transistor Q2 and the ground by level-shifting the oscillation voltage by the base-emitter voltage of an npn transistor Q2 (described later) of the amplifier circuit 22. A voltage equal to the oscillation voltage of the LC resonance circuit unit 1 is applied only during the positive half cycle.

  The amplifier circuit 22 is constituted by the transistor Q2. The emitter of the transistor Q2 is connected (grounded) to the ground via the variable resistor section 24. That is, the amplifier circuit 22 is a so-called emitter follower (emitter follower) circuit. The base of the transistor Q2 is connected to the base of the transistor Q1, and the potential of the emitter of the transistor Q1 level-shifted by the level shift circuit 21, that is, the level shift voltage generated by the level shift circuit 21 is input to the base of the transistor Q2. The Therefore, a current corresponding to the oscillation voltage of the LC resonance circuit unit 1 is output from the amplifier circuit 22.

  The current feedback circuit 23 is a current mirror circuit composed of pnp transistors Q3 and Q4, and positively feeds back current to the LC resonance circuit unit 1 in order to maintain the oscillation of the LC resonance circuit unit 1. The transistor Q3 is inserted between the transistor Q2 of the amplifier circuit 22 and the internal power supply Vcc so that the collector is connected to the collector of the transistor Q2 and the emitter is connected to the internal power supply Vcc. The base of the transistor Q3 is connected to the base of the transistor Q4, the emitter of the transistor Q4 is connected to the internal power supply Vcc, and the collector is connected to the emitter of the transistor Q1.

  Here, when a current is output from the amplifier circuit 22, a collector current of the transistor Q2 equal to this current flows between the emitter and collector of the transistor Q3. A current equal to the current flowing between the emitter and collector of the transistor Q3 flows between the emitter and collector of the transistor Q4, and this current becomes a feedback current supplied to the LC resonance circuit section 1. That is, the current feedback circuit 23 in the present embodiment supplies a feedback current equal to the current output from the amplifier circuit 22 to the LC resonance circuit unit 1.

  The variable resistor unit 24 is for adjusting the magnitude of the current output from the amplifier circuit 22, that is, for adjusting the feedback current, and is composed of a series circuit of a variable resistor 24a and a fixed resistor 24b. The variable resistor 24a is formed by, for example, connecting a series circuit including a fixed resistor and a semiconductor switching element such as a transistor in parallel. The variable resistor 24a is variable by selecting a fixed resistor to be used by turning on / off each semiconductor switching element. The resistance value of the resistor 24a can be changed. The LC resonance circuit unit 1 is supplied with a feedback current adjusted by a combined resistance value of the resistance value of the variable resistance unit 24a and the resistance value of the fixed resistor 24b.

  The monitor unit 3 includes a detection circuit including an npn-type transistor 30, a resistor 31, and a capacitor 32. Transistor 30 has a collector connected to internal power supply Vcc, a base connected to the collector of transistor Q1, and an emitter connected to resistor 31 and capacitor 32. A current corresponding to the voltage input to the base (the oscillation voltage of the LC resonance circuit unit 1 level-shifted by the level shift circuit 21) flows between the collector and the emitter of the transistor 30, and the capacitor 32 is caused by this current. Charged. The monitor unit 3 in this embodiment detects the voltage across the capacitor 32 as a value indicating the oscillation amplitude of the LC resonance circuit unit 1.

  The control unit 4 is composed of a CPU, for example, and sets the negative conductance of the oscillation circuit unit 2 to a critical value that allows the LC resonance circuit unit 1 to oscillate based on the oscillation amplitude detected by the monitor unit 3 as described above. . Such an operation of the control unit 4 is realized by a logic circuit or a program. The control unit 4 in the present embodiment sets the negative conductance of the oscillation circuit unit 2 to the critical value by adjusting the resistance value of the variable resistance unit 24. Here, when the negative conductance of the oscillation circuit unit 2 is Gosc (where Gosc is a negative value), the oscillation amplitude of the LC resonance circuit unit 1 is VT, and the feedback current is Ifb, the negative conductance Gosc is It can be represented by (1).

  Therefore, the negative conductance Gosc can be adjusted depending on how much feedback current Ifb is applied to the oscillation amplitude VT. When the current output from the amplifier circuit 22 is I0 and the resistance value of the variable resistor unit 24 is Re, the current I0 output from the amplifier circuit 22 can be expressed by the following equation (2). A voltage equal to the oscillation voltage of the LC resonance circuit unit 1 is applied between the emitter of the transistor Q2 and the ground only during the positive half cycle of oscillation. The sex conductance Gosc can be expressed by the following equation (3).

  As apparent from the above equation, the value of the negative conductance Gosc of the oscillation circuit section 2 can be adjusted by the resistance value Re of the variable resistance section 24.

  Here, the condition that the LC resonance circuit unit 1 oscillates is that the absolute value of the negative conductance Gosc of the oscillation circuit unit 2 is equal to or larger than the absolute value of the conductance of the detection coil 10, that is, the conductance of the detection coil 10 is set to Gcoil. Then, when the negative conductance Gosc and the conductance Gcoil are in a relationship of Gcoil ≦ | Gosc |, and when the negative conductance Gosc of the oscillation circuit unit 2 is Gcoil = | Gosc | The conductance Gosc is the maximum value that the LC resonance circuit unit 1 can oscillate. That is, −Gcoil, which is a negative value of the conductance Gcoil of the detection coil, is a critical value of the negative conductance Gosc of the oscillation circuit unit 2.

  Therefore, when the negative conductance Gosc is a critical value, the conductance Gcoil can be expressed by the following equation (4).

  The conductance Gcoil of the detection coil 10 changes in accordance with a change in eddy current loss caused by the distance between the detection object and the detection coil 10, that is, the distance between the detection coil 10 and the detection object. As long as Gosc is the critical value, the conductance Gcoil of the detection coil 10 is inversely proportional to the resistance value Re of the variable resistance unit 24. Therefore, the resistance value Re can be used as a value indicating the distance between the detection coil 10 and the detection target.

  The control unit 4 in the present embodiment determines whether or not the negative conductance Gosc is the critical value based on the oscillation amplitude VT obtained from the monitor unit 3, and determines the negative of the oscillation circuit unit 2 according to the determination result. Adjust conductance Gosc. For example, the control unit 4 changes the variable resistance so that the oscillation amplitude VT obtained from the monitor unit 3 becomes a predetermined value (the value of the oscillation amplitude VT when the absolute value of the negative conductance Gosc and the absolute value of the conductance Gcoil are equal). A control signal for turning on / off the semiconductor switching element of each series circuit of the device 24a is output, and the resistance value Re of the variable resistor section 24 is adjusted so that the negative conductance Gosc becomes the above critical value. The absolute value of the negative conductance Gosc and the absolute value of the conductance Gcoil are preferably matched. If it is (for example, a value slightly smaller than the above critical value), there is no problem. Therefore, in this embodiment, the control unit 4 has a value within a predetermined range of the oscillation amplitude VT obtained from the monitor unit 3 (in a range where the absolute value of the negative conductance Gosc and the absolute value of the conductance Gcoil can be regarded as approximately equal). The resistance value Re of the variable resistor section 24 is set so that the oscillation amplitude VT).

  The signal processing unit 5 includes a detection variable resistance unit (not shown). The detection variable resistor section is the same as the variable resistor section 24. The detection variable resistor (not shown) having the same configuration as the variable resistor 24a and the detection variable resistance section having the same configuration as the fixed resistor 24b are used. It consists of a series circuit consisting of a fixed resistor (not shown). The control signal of the control unit 4 is input to the detection variable resistance unit, and thereby the resistance value of the detection variable resistance unit is interlocked with the resistance value Re of the variable resistance unit 24. (Which is equal to the resistance value Re of the variable resistor section 24). Then, the signal processing unit 5 measures the resistance value of the detection variable resistance unit equal to the resistance value Re of the variable resistance unit 24, and analog format based on the measured resistance value (resistance value Re of the variable resistance unit 24). A detection signal (for example, a signal whose value increases in proportion to the distance between the detected object and the detection coil 10) is generated and output to the outside.

  Next, the operation of the proximity sensor of this embodiment will be described. An oscillation voltage having a frequency determined by the inductance of the detection coil 10 and the capacitance of the capacitor 11 is generated at both ends of the LC resonance circuit unit 1, and the LC resonance is obtained by positively feeding back the feedback current from the transistor Q4 of the current feedback circuit 23. The oscillation of the circuit unit 1 is maintained. The oscillation amplitude VT of the LC resonance circuit unit 1 is detected by the monitor unit 3, and the control unit 4 makes the negative conductance Gosc of the oscillation circuit unit 2 become the above critical value based on the oscillation amplitude VT detected by the monitor unit 3. Then, the control signal is output to the variable resistor 24 to set the resistance value Re. The control signal of the control unit 4 is also input to the signal processing unit 5, and the resistance value of the detection variable resistance unit of the signal processing unit 5 is set to the same value as the resistance value Re of the variable resistance unit 24. Generates and outputs the detection signal based on the resistance value of the variable resistance section for detection.

  Here, when the object to be detected approaches the detection coil 10 and the eddy current loss in the detection coil 10 increases, the absolute value of the conductance Gcoil of the detection coil 10 becomes larger than the absolute value of the negative conductance Gosc. Since the condition is not satisfied, the oscillation of the LC resonance circuit unit 1 tries to stop, and the oscillation amplitude VT decreases. When the oscillation amplitude VT detected by the monitor unit 3 no longer falls within the predetermined range, the control unit 4 determines the resistance value of the variable resistance unit 24 so that the oscillation amplitude VT becomes a value within the predetermined range. Since Re is set (the negative conductance Gosc is adjusted), in the above case, the resistance value Re is decreased in order to increase the absolute value of the negative conductance Gosc. The signal processing unit 5 creates and outputs the detection signal based on the resistance value Re set by the control unit 4 as described above.

  Conversely, when the object to be detected moves away from the detection coil 10 and the eddy current loss in the detection coil 10 becomes small, the absolute value of the conductance Gcoil of the detection coil 10 becomes smaller than the absolute value of the negative conductance Gosc, and the LC resonance circuit unit. The oscillation amplitude VT of 1 increases. When the oscillation amplitude VT detected by the monitor unit 3 no longer falls within the predetermined range, the control unit 4 determines the resistance value of the variable resistance unit 24 so that the oscillation amplitude VT becomes a value within the predetermined range. Since Re is set (the negative conductance Gosc is adjusted), in the above case, the resistance value Re is increased in order to reduce the absolute value of the negative conductance Gosc. The signal processing unit 5 creates and outputs the detection signal based on the resistance value Re set by the control unit 4 as described above.

  In this way, the detection signal based on the resistance value Re is output from the signal processing unit 5, and the distance, that is, the positional relationship between the detected object and the detection coil 10 is known by monitoring the detection signal. Can do.

  According to the proximity sensor of the present embodiment described above, the condition for the LC resonance circuit unit 1 to oscillate is that the absolute value of the negative conductance Gosc of the oscillation circuit unit 2 is greater than or equal to the absolute value of the conductance Gcoil of the detection coil 10. Therefore, when the negative conductance Gosc of the oscillation circuit unit 2 is a critical value that allows the LC resonance circuit unit 1 to oscillate, the absolute value of the negative conductance Gosc is the absolute value of the conductance Gcoil of the detection coil 10. Here, the conductance Gcoil of the detection coil 10 depends on the change in eddy current loss caused by the distance between the detection object and the detection coil 10, that is, the distance between the detection coil 10 and the detection object. The negative conductance Gosc of the oscillation circuit unit 2 equal to the conductance Gcoil of the detection coil 10 is L The feedback current Ifb supplied to the resonance circuit unit 1 is determined by the feedback current Ifb supplied to the resonance circuit unit 1 and the oscillation amplitude VT of the LC resonance circuit unit 1, and the feedback current Ifb supplied to the LC resonance circuit unit 1 increases or decreases according to the current output from the amplifier circuit 22. Therefore, the resistance value Re of the variable resistor section 24 that adjusts the magnitude of the current output from the amplifier circuit 22 can be used as a value indicating the distance between the detection coil 10 and the detected object. By using the resistance value Re of 24, an analog detection signal indicating the distance between the detection object and the detection coil 10 is used instead of a digital detection signal indicating whether or not the detection object exists as in the conventional example. Obtainable.

  In addition, by performing threshold processing on such a detection signal, it is possible to obtain a digital detection signal indicating whether or not the detected object exists, so in addition to detecting the presence of the detected object, Distance can also be detected. In this case, for example, when the value of the detection signal (analog detection signal) is equal to or greater than a predetermined threshold, the signal processing unit 5 exists in the detection range of the detection coil 10 and is less than the predetermined threshold. In this case, the detection object may be determined to exist outside the detection range of the detection coil 10 and a detection signal (digital detection signal) indicating whether or not the detection object is present may be output.

  Further, since the signal processing unit 5 acquires the resistance value Re of the variable resistance unit 24 from the control unit 4 and creates a detection signal using the resistance value Re, for example, in order to obtain the magnitude of the negative conductance Gosc. Unlike the case where the feedback current Ifb or the like is used, a detection circuit or the like for detecting the magnitude of the feedback current Ifb is not necessary, so that the circuit configuration can be simplified, downsizing, and the manufacturing cost can be reduced. I can plan.

  In addition, since the variable resistor unit 24 includes a series circuit of the fixed resistor 24b and the variable resistor 24a, the resistance value of the fixed resistor 24b can be used as an offset of the resistance value of the variable resistor 24a. Compared to the case where only the variable resistor 24a is used, the resolution (positional accuracy) of the distance between the detected object and the detection coil 10 can be improved. The variable resistor unit 24 in the present embodiment includes a series circuit of one fixed resistor 24b and one variable resistor 24a. However, the variable resistor unit 24 includes one or more fixed resistors 24b and one or more variable resistors. It may consist of a series circuit with the resistor 24a, or the plurality of fixed resistors 24b and the plurality of variable resistors 24a may be connected in parallel. That is, if the variable resistor unit 24 is composed of a series or parallel circuit of one or more fixed resistors 24b and one or more variable resistors 24a, the resistance value of the fixed resistor 24b is changed to the variable resistor unit. This can be used as an offset of the resistance value 24a, and the resolution of the distance between the detected object and the detection coil 10 can be improved as compared with the case where only the variable resistor 24a is used. The variable resistor section 24 may be composed only of the variable resistor 24a, but for the above reason, it is preferable to use it together with the fixed resistor 24b because the resolution can be improved.

  Since the oscillation circuit unit 2, the monitor unit 3, the control unit 4, and the signal processing unit 5 are integrated as a monolithic IC, the oscillation circuit unit 2, the monitor unit 3, the control unit 4, Compared with the case where the signal processing unit 5 is configured by separate ICs, the size can be reduced, the cost can be reduced, and the noise resistance can be improved. By the way, when the variable resistor unit 24 is constituted by the series circuit of the fixed resistor 24b and the variable resistor 24a as described above, the variable resistor 24a of the variable resistor unit 24 is incorporated in the monolithic IC, and the fixed resistor The device 24b may not be incorporated in the monolithic IC. In this way, the variable range of the resistance value Re of the variable resistor section 24 can be adjusted by exchanging the fixed resistor 24b. Easy to change.

  Note that the circuit configurations of the oscillation circuit unit 2 and the monitor unit 3 in the present embodiment are merely examples, and can be changed without departing from the gist of the present invention. For example, the monitor unit 3 has a configuration for detecting the peak value of the oscillation voltage as shown in FIG. 1 as a value indicating the oscillation amplitude, a configuration for detecting the integrated value of the oscillation voltage, and an effective value for the oscillation voltage. The structure which detects this may be sufficient. As described above, when only the AC component is detected and controlled so as to be constant, the influence of the bias current and the DC resistance of the detection coil 10 (of course, their temperature characteristics) can be eliminated. There is an advantage that it becomes possible.

  In addition, the proximity sensor according to the present embodiment uses a proximity sensor in which the LC resonance circuit unit 1 always oscillates and the oscillation of the LC resonance circuit unit 1 stops due to the approach of the detected object. It is also possible to use a proximity sensor in which the oscillation of the LC resonance circuit unit 1 is stopped and the LC resonance circuit unit 1 oscillates when the object to be detected approaches.

  By the way, the conductance of the detection coil 10 in the present embodiment varies depending on the oscillation frequency of the LC resonance circuit unit 1 in addition to the distance to the detected object. That is, even when the capacitance of the capacitor 11 changes, the conductance of the detection coil 10 changes. Therefore, the proximity sensor of the present embodiment can be used as a capacitance sensor, and even in this case, the resistance value Re of the variable resistance unit 24 can be used as an output.

(Embodiment 2)
The proximity sensor of the present embodiment is different from the first embodiment in the configuration of the variable resistor 24a of the variable resistor unit 24, the monitor unit 3, the control unit 4, and the signal processing unit 5, and other configurations are as follows. Since it is the same as that of Embodiment 1, the same code | symbol is attached | subjected about the same structure and description is abbreviate | omitted.

  The variable resistor 24a in the present embodiment is composed of a digital potentiometer whose resistance value can be set by a digital code (that is, a bit string such as '00000111'). That is, the variable resistor unit 24 in the present embodiment has a variable resistor 24a composed of a digital potentiometer whose resistance value can be set with a digital code, and the resistance value of a series circuit of the variable resistor 24a and the fixed resistor 24b. The combined resistance of the resistance value of the variable resistor 24 a and the resistance value of the fixed resistor 24 b is the resistance value Re of the variable resistor unit 24. In addition, since such a digital potentiometer is a conventionally well-known article, description is abbreviate | omitted.

  The monitor unit 3 in this embodiment includes an npn transistor 30, a resistor 31, a capacitor 32, and an A / D converter (analog / digital converter) 33. Here, the transistor 30, the resistor 31, and the capacitor 32 constitute a detection unit that detects the oscillation amplitude VT of the LC resonance circuit unit 1, and the detection unit outputs an analog signal indicating the oscillation amplitude VT as A. / Output to D converter. The A / D converter 33 converts the analog signal output from the detection unit into a digital signal with a predetermined quantization width and outputs the digital signal to the control unit 4. Since such an A / D converter 33 can be realized by a conventionally known configuration, detailed description thereof will be omitted.

  The control unit 4 in the present embodiment creates a digital code for setting the resistance value Re of the variable resistance unit 24 so that the negative conductance Gosc of the oscillation circuit unit 2 becomes the critical value, and the digital code is used as the variable resistance unit. 24 and the signal processing unit 5. Here, when creating the digital code, the control unit 4 creates a digital code from a digital signal indicating the oscillation amplitude VT output from the A / D converter 33. For example, the control unit 4 compares the digital signal obtained from the A / D converter 33 with the digital signal of the oscillation amplitude VT when the absolute value of the negative conductance Gosc and the absolute value of the conductance Gcoil are equal to each other. Create a digital code accordingly.

  The signal processing unit 5 in the present embodiment has a function of acquiring the resistance value Re of the variable resistance unit 24 from the digital code output from the control unit 4 instead of including the detection variable resistance unit described above. In creating the detection signal indicating the distance to the object to be detected, the detection signal is created using the digital code output by the control unit 4.

  According to the proximity sensor of the present embodiment described above, the same effect as that of the first embodiment is obtained, and the signal processing unit 5 performs arithmetic processing on the digital code output from the control unit 4 to perform the variable resistance unit 24. The resistance value Re of the variable resistor unit 24 can be easily realized by a program or the like. For example, the variable resistor 24a of the variable resistor unit 24 is a fixed resistor and a semiconductor as in the first embodiment. Compared to the case where a series circuit of switching elements is connected in parallel and the resistance value Re is set by turning on / off the semiconductor switching element (switch) by a control signal output from the control unit 4, a variable resistance The variable resistance part for detection having the same configuration as the part 24 is not necessary, and the resistance value Re of the variable resistance part 24 can be easily obtained.

  The monitor unit 3 detects the oscillation amplitude VT of the LC resonance circuit unit 1 and outputs an analog signal indicating the oscillation amplitude VT, and converts the analog signal indicating the oscillation amplitude VT output by the detection unit into a digital signal. An A / D converter 33 that converts and outputs the digital code from the digital signal indicating the oscillation amplitude VT output from the A / D converter 33. For example, a comparator or the like Is used to determine whether the oscillation amplitude VT obtained from the monitor unit 3 exceeds or falls below a predetermined value, and the resistance value Re is changed according to the result (that is, by analog calculation). A value at which the absolute value of the negative conductance Gosc immediately becomes equal to the absolute value of the conductance Gcoil, as compared to the case of setting the resistance value Re). Since it can be set, the processing speed (responsiveness and followability of the change in the resistance value Re of the variable resistor section 24) can be improved. For example, even when the proximity sensor is activated or the moving speed of the detected object is high The negative conductance of the oscillation circuit unit 2 can be quickly set to a critical value that allows the LC resonance circuit unit 1 to oscillate, and the occurrence of a delay can be suppressed.

(Embodiment 3)
In the proximity sensor of the present embodiment, the configuration of the variable resistor 24a of the variable resistance unit 24, the configuration of the control unit 4, and the configuration of the signal processing unit 5 are different from those of the first embodiment, and other configurations are implemented. Since it is the same as that of the first embodiment, the same components are denoted by the same reference numerals and the description thereof is omitted.

  The variable resistor 24a in the present embodiment is composed of the same digital potentiometer as in the second embodiment. That is, also in the present embodiment, the variable resistor section 24 has a variable resistor 24a composed of a digital potentiometer whose resistance value can be set by a digital code, and the resistance of a series circuit of the variable resistor 24a and the fixed resistor 24b. The value becomes the resistance value Re of the variable resistor section 24. In addition, since the signal processing unit 5 in the present embodiment is the same as that in the second embodiment, detailed description thereof is omitted.

  In the present embodiment, the control unit 4 compares the oscillation amplitude VT detected by the monitor unit 3 with a predetermined threshold at a predetermined cycle, a timing circuit 41, and a digital code based on the comparison result of the comparison unit 40. Is formed by a variable resistance unit 24 and an arithmetic processing unit 42 that outputs the signal processing unit 4 to the signal processing unit 4.

  The comparison unit 40 includes a voltage dividing circuit including a series circuit of resistors R1 to R3 inserted between the internal power supply Vcc and the ground, a first comparator COMP1, a second comparator COMP2, and a first negative gate. (A NOT gate or an inverter) 40a, an AND gate 40b, and a second negation gate 40c.

  The voltage dividing circuit is for giving predetermined threshold values V1 and V2 to the first comparator COMP1 and the second comparator COMP2, and the threshold value V1 composed of the potential at the connection point of the resistors R1 and R2 has an oscillation amplitude. VT is the upper limit value, and the threshold value V2 consisting of the potential at the connection point of the resistors R2 and R3 is the lower limit value of the oscillation amplitude VT.

  The non-inverting input terminal of the first comparator COMP1 is connected to the connection point of the resistors R1 and R2, and the inverting input terminal is connected to the output terminal of the monitor unit 3. Therefore, the first comparator COMP1 outputs a high level signal if the oscillation amplitude VT detected by the monitor unit 3 is equal to or less than the threshold value V1, and outputs a low level signal if the oscillation amplitude VT exceeds the threshold value V1. Output. The output terminal of the first comparator COMP1 is connected to the first negative gate 40a and the logical product gate 40b.

  The inverting input terminal of the second comparator COMP2 is connected to the connection point of the resistors R2 and R3, and the non-inverting input terminal is connected to the output terminal of the monitor unit 3. Therefore, the second comparator COMP2 outputs a high level signal if the oscillation amplitude VT detected by the monitor unit 3 exceeds the threshold value V2, and outputs a low level signal if the oscillation amplitude VT is equal to or less than the threshold value V2. Output. The output terminal of the second comparator COMP2 is connected to the second negation gate 40c and the logical product gate 40b.

  The output terminals of the first negative gate 40a, the logical product gate 40b, and the second negative gate 40c are separately connected to the arithmetic processing unit 42.

  Therefore, in such a comparison unit 40, if the oscillation amplitude VT exceeds the threshold value V1, a high level signal is output from the first negative gate 40a, and the AND gate 40b and the second negative gate 40c When a low level signal is output and the oscillation amplitude VT is equal to or less than the threshold value V1 and exceeds the threshold value V2, a low level signal is output from the first negative gate 40a and the second negative gate 40c, and from the AND gate 40b. When a high level signal is output and the oscillation amplitude VT is equal to or lower than the threshold value V2, a low level signal is output from the first negative gate 40a and the logical product gate 40b, and a high level signal is output from the second negative gate 40c. A signal is output.

  The timing circuit 41 includes an oscillation circuit that outputs a pulse signal to the arithmetic processing unit 42 at a predetermined frequency. Here, the frequency of the timing circuit 41 is set lower than the oscillation frequency of the LC resonance circuit unit 1.

  When the arithmetic processing unit 42 receives a high level signal from the first negative gate 40a, the arithmetic processing unit 42 increases the resistance value Re of the variable resistance unit 24, and when the arithmetic processing unit 42 receives a high level signal from the AND gate 40b, the variable resistance unit 24. When the high-level signal is received from the second negative gate 40c, the resistance value Re of the variable resistor unit 24 is decreased. That is, the control unit 4 in the present embodiment determines whether or not to change the resistance value of the variable resistor 24a formed of a digital potentiometer, that is, the resistance value Re of the variable resistance unit 24, based on the comparison result of the comparison unit 40.

  In addition, the arithmetic processing unit 42 changes the digital code by one when changing the resistance value Re of the variable resistance unit 24. For example, when the digital code corresponding to the resistance value Re of the variable resistor unit 24 is '00100110', when a high level signal is received from the first negative gate 40a, the digital code at the present time is increased by 1, A digital code consisting of “00100111” is created and this digital code is output. On the other hand, when a high level signal is received from the second negative gate 40c, the digital code at the present time is decreased by 1, a digital code consisting of '001001001' is created, and this digital code is output.

  Further, when outputting the digital code, the arithmetic processing unit 43 outputs the digital code when the pulse signal is obtained from the timing circuit 41, and thereby the frequency is lower than the frequency of the timing circuit 41. The digital code is not output from the arithmetic processing unit 43.

  According to the proximity sensor of the present embodiment described above, the same effects as those of the first embodiment can be obtained. In addition, the control unit 4 increases or decreases the digital code by one (ie, the resistance Re) when setting the resistance value Re of the variable resistance unit 24 to a value at which the negative conductance Gosc of the oscillation circuit unit 2 becomes the critical value. Therefore, when the resistance value Re is increased, the resistance value Re can be prevented from exceeding the target value, so that overshooting can be prevented and the resistance value can be prevented. When Re is decreased, it is possible to prevent the resistance value Re from being lower than the target value, thereby preventing undershoot from occurring. In addition, since the digital code is increased by one, it is not necessary to perform a process of directly calculating the target value of the resistance value Re of the variable resistance unit 24 from the oscillation amplitude VT, and the AD converter circuit, CPU, and the like are complicated. Since the comparison unit 40 can be configured by an inexpensive comparator as compared with a simple device, the cost can be reduced.

  The control unit 4 outputs a digital code when a pulse signal is input from the timing circuit 43, and the frequency at which the pulse signal is output from the timing circuit 43 is the oscillation frequency of the LC resonance circuit unit 1. Since it is lower, the digital code is not given to the variable resistor 24 at a time interval shorter than the oscillation period of the LC resonance circuit unit 1, and the LC is caused by changing the resistance value Re of the variable resistor 24. Oscillation of the resonance circuit unit 1 can be prevented and stable control can be performed.

(Embodiment 4)
The proximity sensor according to the present embodiment is different from the third embodiment in that the temperature sensor 6 serving as a temperature detection unit and the configuration of the signal processing unit 5 are the same as those in the third embodiment. Therefore, the same components are denoted by the same reference numerals and description thereof is omitted.

  The temperature sensor 6 is configured using a conventionally known thermal element such as a thermistor. Such a temperature sensor 6 is disposed in the vicinity of the detection coil 10 in order to detect an ambient temperature, for example, a temperature around the detection coil 10. The temperature sensor 6 may be disposed at a suitable position depending on the situation.

  As shown in FIG. 4, the signal processing unit 5 in the present embodiment multiplies the digital code value output by the control unit 4 by a correction temperature coefficient (correction coefficient) corresponding to the temperature detected by the temperature sensor 6. A temperature compensation unit 50 that performs compensation, and an output adjustment unit that adjusts the value of the digital code by adding at least one of an offset and a gain to the value of the digital code output by the control unit 4 that has been subjected to temperature compensation by the temperature compensation unit 50 51, the resistance value Re of the variable resistance unit 24 is calculated from the digital code output from the control unit 4 subjected to temperature compensation by the temperature compensation unit 50 and adjusted by the output adjustment unit 51, and the detection coil 10 and the detected object And an output circuit unit 52 for generating the detection signal indicating the distance to the. The temperature compensation unit 50 and the output adjustment unit 51 are realized by a program or the like.

  Further, the signal processing unit 5 in the present embodiment includes a storage unit 53 made of a rewritable nonvolatile memory such as an EEPROM, and the storage unit 53 includes a data table of a correction temperature coefficient used in the temperature compensation unit 50. The offset and gain used by the output adjustment unit 51 are stored. The correction temperature coefficient data table, offset, and gain stored in the storage unit 53 can be changed.

  When the digital code is input, the temperature compensation unit 50 acquires a correction temperature coefficient corresponding to the detected temperature of the temperature sensor 6 from the correction temperature coefficient data table stored in the storage unit 53, and acquires the acquired correction temperature. Multiply the coefficient by the value of the digital code and output the result as a new digital code. Note that the correction temperature coefficient used in the temperature compensation unit 50 is a value set in consideration of the temperature characteristics of the detection coil 10, the detected object, the oscillation circuit unit 2, etc., and uses a reference or the like. It can be obtained from the result of temperature measurement.

  When the digital code is input, the output adjustment unit 51 adds the offset and gain stored in the storage unit 53 to the value of the input digital code and outputs the result as a new digital code. Here, the offset value is a value that is added to or subtracted from the digital code value, and the gain value is a number other than 0, and is a value that is multiplied or divided by the digital code value. Therefore, if you want to shift the digital code value to the positive side, set the offset value to a positive value, and if you want to shift the digital code value to the negative side, set the offset value to a negative value. Should be set. Further, when it is desired to increase the difference in value between digital codes, the gain value may be set to a value larger than 1. When the difference between values between digital codes is desired to be reduced, the gain value is set to 0 or more. A value less than 1 may be set.

  The adjustment of the value of the digital code by the output adjustment unit 51 is performed for the purpose of setting a value that can be taken by the detection signal output from the output circuit unit 52 to a value within a desired range, for example. For example, depending on the usage status of the proximity sensor (for example, the type of material of the object to be detected), the magnitude of the detection signal obtained from the value of the digital code is larger than the magnitude that can be output by the output circuit unit 52, and the detection signal is saturated. (Saturation) may occur, and the distance between the detection coil 10 and the detected object may not be obtained. In such a case, the value of the detection signal is adjusted by adjusting the value of the digital code by the output adjustment unit 51 so that the size of the detection signal falls within the range of the size that can be output by the output circuit unit 52. Problems caused by saturation can be prevented.

  The output circuit unit 52 calculates the resistance value Re of the variable resistance unit 24 from the digital code that has passed through the temperature compensation unit 50 and the output adjustment unit 51, and uses the resistance value Re obtained from the digital code to detect the detection coil 10 and the object to be detected. The above-mentioned detection signal indicating the distance to is generated and output to the outside.

  According to the proximity sensor of the present embodiment described above, the same effect as that of the third embodiment is obtained, and the value of the digital code is corrected according to the temperature detected by the temperature sensor 6. It is possible to prevent deterioration in detection accuracy due to temperature characteristics of circuits such as the detection target and the oscillation circuit unit 2 and improve detection accuracy. Further, since an offset or gain can be arbitrarily added to the value of the digital code, the value of the detection signal can be set to a value within a desired range.

  Moreover, since the correction temperature coefficient in the temperature compensation unit 50 can be changed (rewritable), the characteristics of the detection coil 10, the relative position between the detection coil 10 and the detected object, the oscillation circuit unit 2, and the like for each product. Even if there is a variation in the temperature characteristics of the circuit, it is possible to prevent the value of the detection signal from being different for each product due to such variation, and it is possible to obtain a desired detection signal in any product.

  Furthermore, since the offset and gain in the output adjustment unit 51 can be changed (rewritable), the characteristics of the detection coil 10, the relative position between the detection coil 10 and the detected object, the oscillation circuit unit 2, etc., for each product. Even if there are variations in the circuit characteristics, it is possible to prevent the range of the detection signal value from being different for each product due to such variations, and the value of the detection signal is set to a value within the desired range for any product. It becomes possible to do.

  In addition, although the signal processing unit 5 in the present embodiment includes both the temperature compensation unit 50 and the output adjustment unit 51, it is not always necessary to include both, and the temperature compensation unit 50 and the output adjustment unit 51 are not necessarily provided. Or only one of them may be provided.

(Embodiment 5)
In the proximity sensor of the present embodiment, the configuration of the oscillation circuit unit 2 is different from that of the first embodiment, and the other configurations are the same as those of the first embodiment. Omitted.

  As shown in FIG. 5, the oscillation circuit unit 2 according to the present embodiment is different from the first embodiment in that the level shift circuit 21 is not provided and the configuration of the amplifier circuit 22.

  The amplifier circuit 22 in this embodiment includes an operational amplifier 22a and an npn transistor 22b. The collector of the transistor 22b is connected to the collector of the transistor Q3 of the current feedback circuit 23, and the variable resistor section 24 is inserted between the emitter of the transistor 22b and the ground. The output terminal of the operational amplifier 22a is connected to the base of the transistor 22b, the inverting input terminal of the operational amplifier 22a is connected to the emitter of the transistor 22b, and the non-inverting input terminal of the operational amplifier 22a is connected to the LC resonance circuit unit 1. ing.

  Since a voltage equal to the base-emitter voltage of the transistor 22b is applied between the inverting input terminal and the output terminal of the operational amplifier 22a, the transistor is connected to the oscillation voltage of the LC resonance circuit unit 1 from the output terminal of the operational amplifier 22a. A voltage obtained by adding a voltage equal to the base-emitter voltage of 22b is output. That is, the amplifier circuit 22 in this embodiment also has a function as a level shift circuit.

  Therefore, the amplifier circuit 22 in the present embodiment outputs a current corresponding to the oscillation amplitude of the LC resonance circuit unit 1 as in the first embodiment.

  According to the proximity sensor of the present embodiment described above, the same effects as those of the first embodiment can be obtained. The configuration of the oscillation circuit unit 2 in the present embodiment can also be applied to the second to fourth embodiments.

FIG. 3 is a circuit block diagram of the proximity sensor according to the first embodiment. 6 is a circuit block diagram of a proximity sensor according to Embodiment 2. FIG. It is a circuit block diagram of the proximity sensor of Embodiment 3. It is a circuit block diagram of the proximity sensor of Embodiment 4. FIG. 10 is a circuit block diagram of a proximity sensor according to a fifth embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 LC resonance circuit 2 Oscillation circuit part 3 Monitor part 4 Control part 5 Signal processing part 6 Temperature sensor (temperature detection part)
DESCRIPTION OF SYMBOLS 10 Detection coil 11 Capacitor 22 Amplifier circuit 23 Current feedback circuit 24 Variable resistance part 24a Variable resistor 24b Fixed resistor 33 A / D converter 40 Comparison part 41 Timing circuit 50 Temperature compensation part 51 Output adjustment part

Claims (11)

An LC resonance circuit unit composed of a detection coil and a capacitor used for detection of the object to be detected;
An oscillation circuit unit for oscillating the LC resonance circuit unit;
A monitor unit for detecting the oscillation amplitude of the LC resonance circuit unit;
A control unit for setting the negative conductance of the oscillation circuit unit to a critical value at which the LC resonance circuit unit can oscillate based on the oscillation amplitude detected by the monitor unit;
A signal processing unit that creates a detection signal indicating the distance between the detected object and the detection coil;
The oscillation circuit unit includes an amplification circuit that outputs a current corresponding to the oscillation voltage of the LC resonance circuit unit, a current feedback circuit that supplies a feedback current corresponding to the magnitude of the current output from the amplification circuit to the LC resonance circuit unit, A variable resistance unit that is inserted between the output terminal of the amplifier circuit and the ground and adjusts the magnitude of the current output from the amplifier circuit;
The control unit sets the resistance value of the variable resistance unit so that the negative conductance of the oscillation circuit unit becomes the critical value,
The signal processing unit creates the detection signal based on a resistance value of the variable resistance unit.   The proximity sensor according to claim 1, wherein the variable resistance unit includes a series or parallel circuit of one or more fixed resistors and one or more variable resistors. The variable resistor section has a variable resistor composed of a digital potentiometer capable of setting a resistance value with a digital code,
The control unit outputs a digital code that sets a resistance value of the variable resistance unit to the variable resistance unit and the signal processing unit so that the negative conductance of the oscillation circuit unit becomes the critical value,
3. The proximity sensor according to claim 1, wherein the signal processing unit creates the detection signal based on the digital code output from the control unit. The monitor unit detects an oscillation amplitude of the LC resonance circuit unit and outputs an analog signal indicating the oscillation amplitude, and converts the analog signal indicating the oscillation amplitude output by the detection unit into a digital signal and performs the control. An A / D converter that outputs to the unit,
4. The proximity sensor according to claim 3, wherein the control unit creates the digital code from a digital signal indicating the oscillation amplitude output from the A / D converter.   The control unit has a comparison unit that compares the oscillation amplitude detected by the monitor unit with a predetermined threshold, and determines whether to change the resistance value of the digital potentiometer according to a comparison result of the comparison unit, 4. The proximity sensor according to claim 3, wherein the digital code is changed one by one when the resistance value of the digital potentiometer is changed.   4. The control unit according to claim 3, further comprising a timing circuit that outputs a signal instructing a timing of outputting the digital code at a predetermined frequency, wherein the predetermined frequency is lower than an oscillation frequency of the LC resonance circuit unit. The proximity sensor according to any one of?   The said signal processing part has an output adjustment part which adds at least one of an offset and a gain to the value of the said digital code which the said control part output, The any one of Claims 3-6 characterized by the above-mentioned. The proximity sensor described.   The proximity sensor according to claim 7, wherein the offset and the gain in the output adjustment unit can be changed. It has a temperature detector that detects the ambient temperature,
The signal processing unit includes a temperature compensation unit that performs temperature compensation by multiplying the value of the digital code output by the control unit by a correction temperature coefficient corresponding to the temperature detected by the temperature detection unit. The proximity sensor according to any one of claims 3 to 8.   The proximity sensor according to claim 9, wherein the correction temperature coefficient in the temperature compensation unit is changeable.   11. The proximity sensor according to claim 1, wherein the oscillation circuit unit, the monitor unit, the control unit, and the signal processing unit are integrated as a monolithic IC. .

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