What is an Inductive Sensor?

Catalog

Ⅰ Introduction

Inductive sensors use the principle of electromagnetic induction to convert the measured non-electric quantity, such as displacement, pressure, flow, vibration, etc., into the change of coil self-inductance L or mutual inductance M and then convert the change of inductance to the change of voltage or current output.

Inductive sensors have a series of advantages such as simple structure, reliable operation, high measurement accuracy, stable zero point, large output power, etc. Its main disadvantage is that sensitivity, linearity, and measurement range are mutually restricted, and the frequency response of the sensor is low, so it is not suitable for rapid dynamic measurement.

There are many types of inductive sensors, including self-inductance sensors, mutual inductance sensors, and eddy current sensors.

Ⅱ Classification of the inductive sensor

1 Self-inductance sensor

1) Structure of self-inductance sensor

The self-inductance sensor consists of a coil, a core, and an armature. The core and armature are made of silicon steel and other magnetic materials.

Structure of self-inductance sensor

2) Working principle of self-inductance sensor

The self-inductance sensor converts measured change into self-inductance L change and converts it into voltage or current output through a certain conversion circuit.

When the sensor is in use, the moving part of the sensor is connected with the moving core (armature). When the moving core moves, the thickness of the air gap between the core and the armature will change, which will cause the change of magnetic circuit reluctance and the change of coil inductance value. As long as the change of inductance is measured, the magnitude and direction of the displacement of the moving core can be determined.

Working principle of self-inductance sensor

When the coil turns N is constant, the inductance L is only a function of the reluctance in the magnetic circuit, as long as changing δ; or S can cause the inductance to change. Therefore, the variable reluctance sensor can be divided into a variable air gap δ; thickness sensor, and a variable air gap area S sensor.

If S remains unchanged, L is a single-valued function of δ, which can constitute a variable air gap type self-inductance sensor; if δ remains unchanged, S changes with displacement, so it can constitute a variable cross-section type self-inductance sensor; if a cylindrical armature is placed in the circle, and then moves up and down, the self-inductance will change accordingly, forming a solenoid-type self-inductance sensor.

Variable air gap type self-inductance sensor

 Variable air gap type self-inductance sensor structure

Variable area type self-inductance sensor

Variable area type self-inductance sensor structure

Solenoid type inductive sensor

When the sensor works, the variation of armature length in the coil will cause the variation of the inductance of the coil.

For long threaded armature coil l>>r, when the armature is working in the middle of the armature tube, the magnetic field strength in the coil can be considered to be uniform, and the coil inductance amount L is roughly proportional to the armature insertion depth l.

Solenoid type inductive sensor

The sensor is simple in structure, easy to make and low in sensitivity, which is suitable for measuring large displacement.

3) Differential self-inductance sensor

Because there is an AC excitation current in the coil, the armature is always subjected to electromagnetic suction, which will cause vibration and additional error. The output error will be caused by external interference, the change of power supply voltage frequency, and temperature.

In practice, two identical sensor coils often share one armature to form a differential self-inductance sensor, and the electrical parameters and geometric dimensions of the two coils are exactly the same.

This structure can not only improve linearity and sensitivity but also compensate for the influence of temperature change and power supply frequency change, to reduce the error caused by external influence.

A. Structure of differential self-inductance sensor

(a) Variable air gap type; (b) Variable-area type; (c) Solenoid type differential self-inductance sensor

B. Features of differential self-inductance sensor

The differential air gap inductance sensor is composed of two identical inductance coils 1, 2, and magnetic circuits.

During measurement, the armature is connected to the measured displacement through the measuring rod. When the measured body moves up and down, the guide rod drives the armature to move up and down with the same displacement, so that the magnetic resistance in the two magnetic circuits is equal and the direction changes in the opposite direction. Then the inductance of one coil to increase and the inductance of the other coil to decrease, forming a differential form.

The characteristic curve of the self-inductance coefficient is shown in the figure.

Self-inductance characteristic curve

2 Differential transformer type sensor

The sensor that converts the measured non-electric quantity change into the mutual inductance change of the coil is called the mutual inductance sensor. The sensor is made according to the basic principle of the transformer, which converts the measured displacement into the change of mutual inductance between primary and secondary coils.

When the primary coil is connected with the excitation power, the secondary coil will generate the induced electromotive force. When the mutual inductance between the two changes, the induced electromotive force will also change accordingly. Because two secondary coils use a different connection method, it is called a differential transformer type sensor, referred to as a differential transformer.

1) Structure of the differential transformer

There are many types of differential transformers, such as variable gap type, variable area type, and spiral pipeline type.

The differential transformers of A and B structures are all plate-shaped with high sensitivity and narrow measuring range, which are generally used to measure the mechanical displacement of several microns to several hundred microns.

(a) and (b) Variable gap differential transformer

For the measurement of displacement between 1mm and hundreds of mm, cylindrical armature solenoid type differential transformers are often used, such as C and D structures.

(c) and (d) Solenoid differential transformers

The e and F structures are differential transformers that measure the angle of rotation, and the tiny displacement of a few seconds can usually be measured. During Ø non-electricity measurement, the most used application is the spiral type differential transformer. It can be measured within the scope of the mechanical displacement and has high measurement precision, high sensitivity, simple structure, reliable performance, etc.

(e), (f) Variable-section differential transformer

2) Working principle of the differential transformer

The structure of the differential transformer consists of an iron core, armature, and coil. Its structure has many forms, but its principle of operation is basically the same.

There is a primary coil 1 and a secondary coil 2 in the upper and lower iron cores of the differential transformer. The upper and lower primary coils are connected in series with an ac excitation voltage, and the two secondary coils are connected in series according to the potential.

Schematic diagram of three-stage solenoid differential transformer

Two secondary windings with the same number of turns are connected in the reverse series. When the primary windings are applied with an excitation voltage, the induction potential will be generated in the two secondary windings according to the principle of transformer action.

When the active armature is in the initial equilibrium position, the output voltage is zero if the structure of the transformer is guaranteed to be completely symmetrical.

When the active armature moves towards a secondary coil, the magnetic flux in the secondary coil increases, making its induction potential increase. The differential transformer has an output voltage, and its value reflects the displacement of the active armature.

The output voltage curve of the three-stage solenoid differential transformer is shown in the figure.

Output voltage curve of the differential transformer

3 Eddy current sensor

1) Structure of eddy current sensor

The structure of the eddy current sensor is simple and mainly consists of a flat circular coil arranged in the probe shell.

The internal structure of the eddy current sensor

2) Working principle of eddy current sensor

According to the Principle of Faraday electromagnetic induction, a large metal conductor placed in a changing magnetic field will generate an induced current in the shape of a vortex, called an eddy current. This phenomenon is called the eddy current effect.

The eddy current sensor uses the eddy current effect to convert the non-electric quantity, such as displacement and temperature, into the change of impedance or inductance so as to measure the non-electric quantity.

Schematic diagram of eddy current sensor

The block metal conductor is placed in the magnetic field of the sensor coil with an alternating current. According to the principle of Faraday electromagnetic induction, due to the changes of electric current, an alternating magnetic field produced around the coil. When the conductor under test is placed within the range of the magnetic field, an eddy current is generated in the conductor under test. The eddy current will produce a new magnetic field. The new magnetic field is in the opposite direction which is to offset part of the original magnetic field, which leads to the change of coil inductance, resistance, and quality factor.

III Features of the inductive sensor

1. Advantages of the inductive sensor

(1) The structure is simple and reliable; there is no movable electrical contact and can last for a long time.

(2) High sensitivity, strong output signal, voltage sensitivity can reach hundreds of millivolts per millimeter. The highest resolution is 0.1μm; 

(3) High measurement accuracy, output linearity can reach ±0.1%;

(4) The output power is relatively large, in some cases it can be directly connected to the secondary meter without amplification.

(5) Large resolution: It can feel the small mechanical displacement and the small angle change.

(6) Good repeatability and linearity: within a certain displacement range, the linearity of the output characteristic is good, and the output is stable.

2. Disadvantages of the inductive sensor

(1) The frequency response of the sensor itself is not high, and it is not suitable for fast dynamic measurement;

(2) Higher requirements for the stability of the frequency and amplitude of the excitation power supply;

(3) The resolution of the sensor is related to the measurement range. The measurement range is large, the resolution is low, and vice versa.

(4) There is an AC zero signal, which is not suitable for high-frequency dynamic measurement.

IV Application of inductive sensor

As a tool to collect and obtain information, the sensor plays an important role in the automatic detection and quality monitoring of the system. Inductive sensors can convert geometric changes of non-electrical physical quantities such as length, inner diameter, and outer diameter caused by displacement, vibration, and pressure into tiny changes in electrical signals. And electrical signals are converted to electrical parameters measurement. It is a kind of high sensitivity sensor that has a simple structure and reliable, big output power, strong ability to resist impedance, good stability, and a series of advantages, and therefore is widely used in various kinds of engineering quantity detection and automatic control system.

For example 1. Using an inductive displacement sensor to improve the precision of bearing manufacturing; The change of micro-precision size was measured by inductance micrometer. Accurate measurement of hydraulic valve opening position; 2. Flexible sensors for intelligent textile design; 3. Aperture taper error measuring instrument with inductance sensor principle; 4. The inductive sensors were used to detect abrasive particles in the lubricating oil; 5. Using an inductive sensor to monitor the spreader guide wheel and so on.

Inductance sensor can also be used as a magnetic speed switch, gear age bar speed measurement, etc. This kind of sensor is widely used in textile, chemical fiber, machine tools, machinery, metallurgy, locomotive, and automobile industries. It is widely used for machine detection such as sprocket speed detection, chain conveyor belt speed, and distance detection, gear age counting tachometer, and automobile protection system control. Also, this kind of sensor can be used for small and medium-sized object detection, object ejecting control, wire breaking monitoring, small parts area division, thickness detection, and position control in the feeding tube system.

An inductive displacement sensor makes use of the wire to make a specific winding. According to the change of its displacement, the winding coil white inductance or mutual inductance changes to carry out displacement measurement. So according to its conversion principle, the inductive displacement sensor can be divided into two categories: self-inductance type and mutual inductance type.

The inductive displacement sensor is a kind of electromechanical conversion device, which is widely used in modern industrial production science and technology, especially in the white motion control system, machining, and measurement industry.

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