Working principle of inductive sensors
Inductive sensors operate on the principle of electromagnetic induction. They convert a measured quantity, such as displacement, into a change in inductance. Based on the conversion method, they can be divided into two types: self-inductance (including variable reluctance and eddy current types) and mutual inductance (differential transformer type).
1. Variable magnetoresistive sensor
When the current *i* in a coil changes, the magnetic flux *Φ* generated by that current also changes, thus inducing an electromotive force *e* in the coil itself. This phenomenon is called self-induction. The induced electromotive force is called the self-induced electromotive force. The structure of a variable reluctance sensor is shown in Figure 1. It consists of three parts: a coil, an iron core, and an armature. The iron core and armature are made of magnetically conductive materials such as silicon steel sheets or permalloy. There is an air gap between the iron core and the armature, with a thickness of δ. The moving part of the sensor is connected to the armature. When the armature moves, the air gap thickness δ changes, causing a change in the magnetic reluctance in the magnetic circuit, which in turn leads to a change in the inductance of the inductor coil. Therefore, as long as this change in inductance can be measured, the magnitude and direction of the armature displacement can be determined.
Features: Variable magnetoresistive sensors have high sensitivity, thus requiring low amplification of the measured signal. However, due to the influence of the air gap width δ, the measurement range of this type of sensor is very small.
2. Differential transformer type sensor
The working principle of a mutual inductance sensor is to utilize the mutual inductance phenomenon in electromagnetic induction to convert the measured displacement into a change in the mutual inductance of the coils. Because it often uses two secondary coils to form a differential type, it is also called a differential transformer sensor.
The output voltage of a differential transformer sensor is an AC quantity. If indicated by an AC voltmeter, the output value can only reflect the magnitude of the core displacement, not the polarity of the movement. Furthermore, the AC voltage output has a certain residual voltage at the zero point, meaning the output is not zero even when the movable armature is in the intermediate position. Therefore, the downstream circuit of a differential transformer sensor should use a differential DC output circuit that can both reflect the polarity of the core displacement and compensate for the residual voltage at the zero point.
A sensor that converts changes in a non-electrical quantity being measured into changes in the mutual inductance of coils is called a mutual inductance sensor. This type of sensor is based on the fundamental principles of a transformer, and its secondary windings are connected in a differential configuration; hence, it is called a differential transformer sensor. Differential transformers come in various structural forms, including variable gap, variable area, and solenoid types.
