Inductive sensors have a remarkable ability to convert a variety of physical parameters—such as displacement, pressure, and vibration—into variations in self-inductance or mutual inductance. This transformation leads to changes in voltage or current. Known for their straightforward design and reliable functionality, these sensors provide high measurement precision and powerful output. They are employed in a wide range of applications. However, achieving a balance among sensitivity, linearity, and measurement range can be quite challenging. Additionally, their relatively slow response speed can limit their performance in fast-changing conditions, restricting their use in dynamic environments. The primary types of these sensors include self-inductance, mutual inductance, and eddy current sensors.
The self-inductance sensor comprises a coil, a core, and an armature. The core and armature are constructed from silicon steel and other magnetic materials.
Structure of Self-Inductance Sensor
The self-inductance sensor transforms variations in the measured parameter into changes in self-inductance (L), which it then converts into voltage or current output via a particular conversion circuit.
During operation, the moving part of the sensor connects with the mobile core (armature). Movement of the core alters the air gap thickness between the core and armature, thereby changing the magnetic circuit reluctance and coil inductance value. By measuring this inductance change, one can determine both the magnitude and direction of the core's displacement.
If coil turns N remains constant, inductance L is solely a function of the magnetic circuit's reluctance. Modifying δ or S causes the inductance to vary. Consequently, the variable reluctance sensor can be categorized into a variable air gap δ thickness sensor and a variable air gap area S sensor.
Working Principle of Self-Inductance Sensor
When S is kept constant, L becomes a singular function of δ, forming a variable air gap type self-inductance sensor. Conversely, if δ remains unchanged, changes in S with displacement create a variable cross-section type self-inductance sensor. With a cylindrical armature placed in a circular configuration that moves vertically, the self-inductance alters, creating a solenoid-type self-inductance sensor.
The structure of this sensor alters with changes in the air gap thickness between core and armature, affecting the magnetic resistance and coil inductance.
Variable Air Gap Type Self-Inductance Sensor Structure
In this variation, the sensor's structure involves a constant air gap thickness, with the inductive area changing with displacement.
Variable Area Type Self-Inductance Sensor Structure
The solenoid type inductive sensor operates by varying the armature length within the coil, which changes the coil's inductance.
For a long threaded armature, when functioning in the middle of the armature tube, the magnetic field strength within the coil is uniform. Consequently, the coil's inductance L is proportional to the depth of armature insertion l.
This type of sensor has a simple structure, is easy to manufacture, and has low sensitivity. It is suitable for measuring large displacements.
Solenoid Type Inductive Sensor
Due to an AC excitation current in the coil, the armature experiences electromagnetic suction, resulting in vibration and additional error. The output error is also influenced by external interference, changes in power supply voltage frequency, and temperature.
In practice, two identical sensor coils often share one armature, forming a differential self-inductance sensor with exactly the same electrical parameters and geometric dimensions.
This configuration enhances linearity and sensitivity while compensating for temperature and frequency variations, thereby reducing errors from external influences.
This sensor can be variable air gap type, variable-area type, or solenoid type.
(a) Variable Air Gap Type; (b) Variable-Area type; (c) Solenoid Type Differential Self-Inductance Sensor
The differential air gap inductance sensor comprises two identical inductance coils and magnetic circuits. During measurement, the armature connects to the measured displacement via a measuring rod. Movement of the measured body drives the armature, altering magnetic resistance equally but oppositely in the two circuits. Consequently, the inductance of one coil increases while the other decreases, forming a differential configuration.
The differential transformer sensor converts the measured non-electric quantity change into coil mutual inductance changes. Based on transformer principles, this sensor translates displacement into a mutual inductance change between primary and secondary coils.
When the primary coil receives excitation power, the secondary coil generates an induced electromotive force, which varies with the mutual inductance. Known as a differential transformer sensor, it connects the secondary coils differently.
Self-Inductance Characteristic Curve
Differential transformers come in various types, such as variable gap, variable area, and spiral pipeline types.
Plate-shaped differential transformers (A and B) are highly sensitive yet have a narrow measuring range, suitable for displacements of a few microns to several hundred microns.
(a) and (b) Variable Gap Differential Transformer
Cylindrical armature solenoid type differential transformers (C and D) measure displacements between 1mm and hundreds of mm.
(c) and (d) Solenoid Differential Transformers
Rotational angle measurement differential transformers (E and F) capture minute displacements of a few seconds. The spiral type differential transformer is often used for Ø non-electricity measurement, boasting high precision, sensitivity, simple structure, and reliable performance.
(e), (f) Variable-Section Differential Transformer
This transformer's structure includes an iron core, armature, and coil, with various forms sharing the same operational principle.
The differential transformer consists of an upper and lower primary coil connected in series with an AC excitation voltage and two secondary coils connected in series by potential.
Schematic Diagram of Three-Stage Solenoid Differential Transformer
Two secondary windings, with the same number of turns, connect in reverse series. When the primary windings receive excitation voltage, induction potential is generated in the two secondary windings as per transformer principles.
In the initial equilibrium position of the active armature, the output voltage is zero if the transformer structure is perfectly symmetrical. Movement of the active armature towards a secondary coil increases its magnetic flux, thus raising the induction potential and reflecting the active armature's displacement.
Output Voltage Curve of the Differential Transformer
The eddy current sensor's structure is simple, primarily consisting of a flat circular coil within the probe shell.
The Internal Structure of the Eddy Current Sensor
According to Faraday's electromagnetic induction principle, a large metal conductor placed in a varying magnetic field generates an eddy current. This phenomenon, known as the eddy current effect, is utilized in the eddy current sensor to convert non-electric quantities, such as displacement and temperature, into impedance or inductance changes for measurement.
Schematic Diagram of Eddy Current Sensor
When the block metal conductor is placed in the magnetic field of the sensor coil with an alternating current, the changing electrical current induces an alternating magnetic field around the coil. When the conductor under test is within this magnetic field, it generates an eddy current that produces a new, oppositely directed magnetic field. This new magnetic field partially offsets the original field, resulting in changes to the coil's inductance, resistance, and quality factor.
Inductive sensors are celebrated for their resilience and simplicity in design. They deliver impressive sensitivity, with resolutions achieving up to 0.1 μm, making them highly suitable for applications demanding meticulous accuracy. These characteristics make them particularly valuable in quality control processes within the manufacturing sector. Alongside their robust power output, they smoothly integrate with secondary metering devices, streamlining data collection and analysis processes.
A notable quality of inductive sensors is their linear response over specific measurement spans, significantly boosting accuracy. In real-world deployments, this capability ensures dependable results, especially in settings where precision is critical, like tooling and machining operations. Furthermore, their straightforward design simplifies maintenance, thus minimizing downtime—a matter of concern in environments where continuity of operations holds substantial value.
Inductive sensors, while beneficial, do present certain challenges. Their relatively modest frequency response renders them less effective for applications demanding swift measurement updates. In dynamic scenarios, such as automotive testing environments where quick feedback is crucial, this can be a considerable drawback. Additionally, their performance heavily depends on stable power supply conditions; fluctuations can result in erratic readings, a significant issue in regions with unstable electrical infrastructure. Moreover, there's an inherent trade-off between resolution and measurement range, necessitating a thoughtful balance tailored to the application’s specific needs.
Inductive sensors are skilled at transforming physical changes such as displacement, vibration, and pressure into precise electrical signals. Their advanced capabilities serve automatic systems in a variety of sectors, including manufacturing, metrology, and automotive industries, showcasing their extensive and adaptable applications.
Inductive sensors elevate the precision of bearing production, which supports the smooth functionality of mechanical operations. Their ability to detect minute changes ensures the highest quality and longevity of manufactured parts.
In practical settings, manufacturers employ these sensors to pinpoint even the slightest defects early in the production line, preserving the integrity of the final product and reducing waste.
In hydraulic systems, inductive sensors are key in monitoring valve positions and adjustments, promoting operational efficiency and safety. Their constant vigilance helps prevent malfunctions and breakdowns.
Designing effective hydraulic systems utilizes this technology to sustain system integrity under various pressures, making operations smoother.
Flexible sensors developed for smart textiles are an innovative application of inductive technology. These sensors are crucial in creating wearables that react dynamically to environmental changes or user movements.
Progress in this field extends to health monitoring and adaptive clothing, providing practical solutions and improving user experiences.
Inductive sensors are employed to detect fine particles in lubricating oil, which helps prevent machinery wear and degradation. Their use in oil analysis ensures machinery longevity and optimal performance.
Integrating these sensors into maintenance routines exemplifies a proactive stance on industrial health, reducing downtime and maintenance costs.
The inherent reliability and immunity to interference of inductive sensors make them ideal for speed and motion control in automation systems. These traits help ensure that automated processes are smooth and precise.
By incorporating these sensors into automation frameworks, they become a vital component of modern industrial operations, balancing technological advancements with operational needs.
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