CN104792252A - Displacement sensor - Google Patents

Abstract

A displacement sensor is provided. A parallel resonant circuit (4) including a coil (6) and capacitors (8, 10) is arranged on the output side of the NPN transistor (12), and the output of the parallel resonant circuit 4 is fed back to the input side of the transistor (12), thereby forming a Continuously oscillating Colpitts oscillator circuit (2). The output level of the transistor (12) varies according to the position of the object to be measured relative to the coil (6). The constant current source (26) continuously provides DC current to the coil (6). The RC low-pass filter (30) detects a change in the DC resistance value of the coil (6) based on a voltage drop caused by a DC current flowing through the coil (6). The microprocessor (38) adjusts the output level of the transistor (12) based on the output of the RC low pass filter (30).

Description

Motion detector

technical field

The invention relates to a displacement sensor which uses a coil to detect the position change of the object to be measured, in particular to compensating the error caused by the change of the resistance value of the coil with temperature.

Background technique

As a displacement sensor using a coil, for example, there is an eddy current type displacement sensor. The eddy current displacement sensor utilizes the following principle: When a conductor as an object to be measured approaches a coil through which an alternating current flows, an eddy current flows through the conductor to generate an alternating magnetic field, thereby changing the impedance of the coil. In this eddy current displacement sensor, the impedance of the coil also changes according to the temperature change of the use environment, so it is necessary to compensate for the impedance change caused by the temperature change. An example of this compensation technique is disclosed in JP-A-60-67819 (Patent Document 1).

In the displacement sensor described above, when an alternating current from the oscillator is supplied to the coil to generate an alternating magnetic field from the coil, eddy currents whose magnitudes vary depending on the position of the object to be measured are induced in the coil. The displacement of the object to be measured is detected based on the output voltage of the coil which changes according to the magnitude of the eddy current. In a displacement sensor that utilizes the fact that the output voltage of the coil changes with the change in the position of the object to be measured, when the output voltage of the coil changes with the change in the ambient temperature, the position of the object to be measured cannot be accurately detected. displacement, temperature compensation is required. A DC current is supplied to the coil from a DC current supply unit instead of an AC current every time a predetermined sampling period passes, and a DC voltage output from the coil due to the DC current is detected by a DC voltage detector. Based on the detected DC voltage, the AC current supplied to the coil is controlled by the correction unit to correct a change in output voltage of the coil due to a change in resistance value of the coil due to temperature change.

In the displacement sensor described above, it is necessary to supply the coil with an alternating current from the oscillator, and therefore it is necessary to provide an oscillator in addition to the coil, and the circuit configuration becomes complicated. Moreover, a switching switch and a switching control circuit for switching between alternating current and direct current are required, and the circuit structure becomes more complicated, so it is not suitable to install the displacement sensor in a place with a small installation space. In addition, since temperature compensation is performed by supplying a direct current to the coil every predetermined sampling period, displacement cannot be detected during this temperature compensation period. For example, when detecting the displacement of a valve of an internal combustion engine, it is necessary to continuously detect the displacement, and the displacement sensor of Patent Document 1 cannot be used.

An object of the present invention is to provide a displacement sensor capable of simplifying the circuit configuration and continuously detecting displacement while performing temperature compensation.

Contents of the invention

A displacement sensor according to one aspect of the present invention has a self-excited oscillation unit that oscillates continuously. The self-excited oscillation unit includes a parallel resonance unit including a coil and a capacitor, and an amplification unit. The parallel resonant unit is arranged on the output side of the amplifying unit, and the output of the parallel resonant unit is fed back to the input side of the amplifying unit. In this self-excited oscillation unit, the output level of the amplifying unit changes according to a change in the position of the object to be measured with respect to the coil. In the case of an eddy current displacement sensor, the output level of the amplification unit changes as the position of the object to be measured changes relative to the coil. As the oscillation method of the oscillation unit, a so-called LC oscillation, such as a Colpitts type oscillation, a Hartley type oscillation, a Clapp type oscillation, a collector-tuned type anti-coupling oscillation, a fundamental Pole-tuned anti-coupling oscillations and their variants, and various other well-known oscillation modes. The DC supply unit continuously supplies a DC signal to the coil. The resistance value detection unit detects a change in the DC resistance value of the coil based on a voltage drop caused by the DC signal flowing through the coil. The DC resistance value of the above-mentioned coil changes with the temperature of the surrounding environment. The control unit adjusts the output level of the amplifying unit based on the output of the resistance value detecting unit.

In the displacement sensor configured as described above, since a self-excited oscillation unit is used, the circuit configuration can be simplified. In addition, a self-excited oscillation unit that continuously oscillates is used, and a DC signal is continuously supplied to the coil of the self-excited oscillation unit. Therefore, it is not necessary to stop the oscillation of the self-excited oscillation unit in order to detect the resistance value, and it is possible to continuously detect displacement.

The direct current supply unit can be used as a constant current source that supplies a constant current to the coil. In this case, the resistance value detection unit is a filter unit that detects a DC voltage generated in the coil.

With such a configuration, only the DC component is detected by the filter unit, and the resistance value can be detected without being affected by the AC component.

In the above mode, the amplifying unit can be provided with an active element having first to third electrodes, and the distance between the first electrode and the third electrode is changed according to a signal between the first electrode and the second electrode. conduction state between them. In this case, the current adjustment unit provided between the first electrode and the second electrode is controlled by the control unit. As active elements, for example, bipolar transistors or field effect transistors can be used.

When configured in this way, the gain of the oscillation unit can be adjusted by controlling the current flowing between the first electrode and the third electrode of the active element. As a result, the temperature compensation of the coil can be performed, and the self-excited The oscillation unit maintains a stable oscillation state.

Description of drawings

FIG. 1 is a circuit diagram of a displacement sensor according to one embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between an object to be measured and a coil in the displacement sensor of FIG. 1 .

FIG. 3 is a graph showing changes in resistance values of coils of the displacement sensor shown in FIG. 1 .

Detailed ways

The displacement sensor according to one embodiment of the present invention is, for example, an eddy current type displacement sensor, and has a self-excited oscillation unit such as a Colpitts oscillation circuit 2 as shown in FIG. 1 . The Colpitts oscillator circuit 2 has a parallel resonant unit, for example, a parallel resonant circuit 4 . This parallel resonant circuit 4 is formed by connecting a series circuit of two capacitors 8 and 10 to the coil 6 in parallel. For the parallel resonance circuit 4 , the values of the coil 6 and the capacitors 8 and 10 are selected so that the parallel resonance circuit 4 resonates in parallel at a predetermined frequency. Furthermore, the Colpitts oscillating circuit 2 further has an active element such as a bipolar transistor, specifically an NPN transistor 12 . The NPN transistor 12 has: a first electrode such as a base; a second electrode such as an emitter; and a third electrode such as a collector. An appropriate bias voltage is given to the NPN transistor 12 by a bias circuit (not shown). Furthermore, the NPN transistor 12 is connected to a reference potential, for example, a ground potential via a capacitor not shown, and operates, for example, as a base ground circuit.

One end of the parallel resonance circuit 4 is connected to the ground potential, and the other end of the parallel resonance circuit 4 is connected to the collector of the NPN transistor 12 via the DC blocking capacitor 14 . The emitter of transistor 12 is connected to the mutual connection point of capacitor 8 and capacitor 10 . Therefore, a part of the output generated between the base and the collector, which is connected at high frequency through a capacitor not shown in the figure, is fed back to the emitter side. The collector is connected to a power supply terminal, for example, a positive power supply terminal 18 via a load 16 including an impedance element, and the collector is also connected to an output terminal 20 . In addition, the emitter is connected to the ground potential via a current adjustment unit, for example, a variable current source 22 forming part of the bias circuit.

The Colpitts oscillation circuit 2 continuously oscillates at an oscillation frequency determined by the parallel resonance frequency of the parallel resonance circuit 4 , and the oscillation output thereof is taken out from an output terminal 20 . As shown in FIG. 2, when the position of the object to be measured, such as the valve 24 of the engine of the ship, changes relative to the coil 6, for example, when it approaches, as described in connection with the prior art, the impedance of the coil 6 changes, thereby The level of the oscillation output generated at the output terminal 20 changes. This level change is detected by a detection unit not shown, thereby detecting the displacement of the valve 24 .

As shown in FIG. 3 , the resistance value of the coil 6 and the intrinsic resistance value of the valve 24 change with the temperature of the surrounding environment. Therefore, if this is ignored, the level of the oscillation output from the output terminal 20 will change and cannot The displacement of the valve 24 is correctly detected. In particular, in an environment where the ambient temperature changes from below zero degrees Celsius to a temperature above 100 degrees Celsius, the level of the oscillation output greatly changes. Therefore, in this embodiment, a direct current supply unit, for example, a constant current source 26 is connected to one end of the coil 6 . One end of the constant current source 26 is connected to a positive power supply terminal, and the other end of the constant current source 26 is connected to one end of the coil 6 . A DC signal, for example, a DC current from the constant current source 26 continuously flows to the ground potential via the coil 6 . The DC blocking capacitor 14 is provided for the purpose of preventing the DC current from flowing to the collector of the NPN transistor 12 or the like.

When there is a temperature change in the surrounding environment, the resistance value of the coil 6 changes, and the value of the DC voltage generated between both ends of the coil 6 due to the DC current from the constant current source 26 changes. In order to detect this DC voltage, a resistance value detection unit such as a low-pass filter 30 is connected between both ends of the coil 6 . The low-pass filter 30 is, for example, an RC low-pass filter including a resistor 32 and a capacitor 34 . The low-pass filter 30 is used to prevent the detection of the oscillation signal of the Colpitts oscillation circuit 2 . The output signal of the low-pass filter 30 is amplified by a DC amplifier 36 and then provided to a control unit such as a microprocessor 38 . In the microprocessor 38, the output signal of the DC amplifier 36 is digitized, and the digital output signal is corrected so as to have a linear relationship with temperature. The corrected digital output signal is compared with a predetermined reference value, for example, a correction value at a reference temperature to generate a control signal. The control signal is provided to the variable current source 22 provided at the emitter of the NPN transistor 12 to control the current drawn from the emitter of the NPN transistor 12 and further control the current drawn into the collector. Thereby, displacement measurement can be performed in a state where the level of the oscillation output generated at the output terminal 20 is kept linear without being affected by the ambient temperature. For example, when a current that doubles the level of the oscillation output is supplied to the NPN transistor 12, the level of the oscillation output is also approximately 1.2 times over the entire stroke of the valve 24 to be measured, maintaining a linear relationship.

For example, when the temperature of the surrounding environment is higher than the reference temperature and the resistance value of the coil 6 becomes large, the oscillation output level becomes small, so the oscillation energy is increased by increasing the current of the variable current source 22, thereby making The oscillation output level becomes larger. Conversely, when the temperature of the surrounding environment is lower than the reference temperature and the resistance value of the coil decreases to increase the oscillation output, the current of the variable current source 22 is decreased to decrease the oscillation output level. Moreover, there is a linear relationship between the current value of the variable current source 22 and the oscillation output level, and the control of the variable current source 22 by the microprocessor becomes easy.

In this way, even if there is a temperature change in the surrounding environment, it is possible to perform temperature compensation on the output level of the displacement sensor. Furthermore, since this temperature compensation is not performed every predetermined time, but is continuously performed, precise temperature compensation can be performed, and there is no need to stop the oscillation of the oscillation circuit 2 for temperature compensation. Therefore, the displacement can be detected continuously, and the displacement can be detected precisely. For example, it is possible to continuously detect displacement while performing temperature compensation using a thermistor or the like. However, in this case, the number of parts increases and the structure of the displacement sensor becomes complicated. In addition, the failure rate of the displacement sensor and the number of parts The increase of the corresponding increase, so the use of thermistor etc. is not ideal. In addition, the change in the oscillation level due to temperature can also be corrected not by increasing or decreasing the oscillation level of the oscillator, but by inputting the output of the oscillator to a multiplication circuit as a multiplication One term of the operation, set the other term as a temperature-based coefficient. However, not only does the circuit become complicated, but the cost also increases. In contrast, in this displacement sensor, the emitter current is changed by controlling the variable current source 22 to change the collector current, so the output level of the oscillation signal can be changed while the oscillation state is stabilized, so the circuit structure is simple , is also advantageous in terms of cost.

In the above-described embodiments, the present invention is implemented for the eddy current type displacement sensor, but it is also possible to implement the present invention for other types, such as a differential transformer type displacement sensor. In the above embodiment, the displacement of the valve 24 is detected, but the invention is not limited thereto, and the displacement sensor of the present invention can be used to detect the displacement of other objects to be measured. In addition, although the Colpitts oscillation circuit 2 was used in the above-mentioned embodiment, it is not limited thereto, and a known self-excited oscillation circuit such as a Clapp oscillation circuit or a Hartley oscillation circuit can also be used. In addition, although the NPN transistor 12 was used in the above-mentioned embodiment, it is also possible to use a PNP transistor, or to use an FET. In addition, the NPN transistor 12 is operated with the base grounded, but it can also be operated with the emitter grounded. In the above embodiment, the RC low-pass filter 30 is used, but an active low-pass filter using an operational amplifier, a resistor, and a capacitor can also be used, and an LC low-pass filter can also be used. In the above embodiment, the variable current source 22 is used as the current adjusting means, but it is also possible to use, as the current adjusting means, a configuration in which one end of the resistor is connected to the emitter of the NPN transistor, and the other end of the resistor is connected to the emitter of the NPN transistor. One end is grounded via a variable voltage source. Alternatively, the current adjustment unit can be configured by grounding the emitter of the NPN transistor 12 via an emitter resistor, and adjusting the current by changing the base voltage. In the above embodiment, the microprocessor 38 is used to digitize the output of the DC amplifier 36 to control the variable current source 22, but it is also possible to provide the output of the DC amplifier 36 as it is to the linearity correction circuit configured in analog, The output of this linearity correction circuit is supplied to a comparator configured in an analog form, and the output of this comparator is supplied to a variable current source 22 .

Claims (3)

1. a displacement transducer, possesses:

The auto-excitation type oscillating unit vibrated constantly, it comprises parallel resonance unit and amplifying unit, this parallel resonance unit comprises coil and capacitor, this parallel resonance unit is arranged at the outgoing side of this amplifying unit, the output of above-mentioned parallel resonance unit is fed back to the input side of this amplifying unit, and the output level of above-mentioned amplifying unit changes relative to the change of the position of above-mentioned coil according to measured object;

Direct current providing unit, it provides direct current signal constantly to above-mentioned coil;

Resistance value detecting unit, it detects the change of the DC resistance of above-mentioned coil according to the pressure drop that the above-mentioned direct current signal because flowing through above-mentioned coil causes; And

Control module, its output based on above-mentioned resistance value detecting unit adjusts the output level of above-mentioned amplifying unit.

2. displacement transducer according to claim 1, is characterized in that,

Above-mentioned direct current providing unit is the constant current source above-mentioned coil being provided to constant current, and above-mentioned resistance value detecting unit is the filter unit detecting the DC voltage produced in above-mentioned coil.

3. displacement transducer according to claim 1 and 2, is characterized in that,

Above-mentioned amplifying unit has active component, and this active component has the first electrode to the 3rd electrode, changes the conduction state between the first electrode and the 3rd electrode according to the signal between the first electrode and the second electrode,

By above-mentioned control module, the current-variable unit be arranged between the first electrode and the second electrode is controlled.