Inductive sensors are devices that utilize the law of electromagnetic induction to measure non-electrical quantities through changes in the self-inductance and mutual inductance of coils. Their key feature is the use of the physical properties of inductance, allowing them to measure signals such as displacement, vibration, pressure, and flow rate without direct contact with the object being measured.
Inductive sensors operate on the principle of electromagnetic induction, converting a measured quantity, such as displacement, into a change in inductance. Inductive sensors offer advantages such as simple structure, reliable operation, high measurement accuracy, zero-point stability, and relatively high output power. Their main disadvantages are the mutual constraints between sensitivity, linearity, and measurement range; low frequency response; and unsuitability for rapid dynamic measurements.
I. Characteristics of Inductive Sensors
Inductive sensors enable long-distance data transmission, recording, display, and control, and are widely used in industrial automatic control systems. They primarily possess the following characteristics:
(1) The structure is simple and the sensor has no moving electrical contacts, so it is reliable and has a long service life.
(2) It has high sensitivity and resolution, and can measure displacement changes of 0.01 micrometers. Its output signal is strong, and its voltage sensitivity can generally reach hundreds of millivolts per millimeter of displacement.
(3) The linearity and repeatability are both good. Within a certain displacement range, the nonlinear error of the sensor can reach 0.05%~0.1%.
II. Classification of Inductive Sensor Devices
Inductive sensors are classified into three types: self-inductance sensors that change the air gap thickness δ, i.e., variable gap inductive sensors; self-inductance sensors that change the air gap cross-section S, i.e., variable cross-section inductive sensors; and self-inductance sensors that change both the air gap thickness δ and the air gap cross-section S, i.e., solenoid inductive sensors.
Variable gap type inductive sensor
The air gap δ of this sensor changes with the measurement, thus altering the magnetoresistance. Both its sensitivity and nonlinearity decrease as the air gap increases, so a balance must often be struck. δ is typically chosen between 0.1 and 0.5 mm.
Change area type inductor sensor
The relative coverage area (i.e., magnetic flux cross-section) between the core and armature of this type of sensor changes with the measurement, thus altering the magnetic reluctance. Its sensitivity is constant, and its linearity is excellent. This is a solenoid-type inductive sensor. It consists of a solenoid coil and a cylindrical armature connected to the object being measured. Its working principle is based on the change in magnetic reluctance along the leakage path of the coil's magnetic field lines. As the armature moves with the object being measured, it changes the inductance of the coil. This type of sensor has a large measuring range, low sensitivity, simple structure, and is easy to manufacture.
Solenoid-type inductive sensor
It consists of a solenoid coil and a cylindrical armature connected to the object being measured. Its working principle is based on the change in magnetic reluctance along the leakage path of the coil's magnetic field lines. As the armature moves with the object being measured, it changes the inductance of the coil. This sensor has a large measuring range, low sensitivity, simple structure, and is easy to manufacture.
III. Applications of Inductive Sensors
Sensors, as tools for collecting and acquiring information, play a crucial role in the automated detection and quality monitoring of systems. Inductive sensors, a type of mutual inductance sensor, can convert minute geometric changes in non-electrical physical quantities such as displacement, vibration, and pressure (e.g., length, inner diameter, outer diameter, non-parallelism, non-perpendicularity, eccentricity, ellipticity) into minute changes in electrical signals. These changes are then converted into electrical parameters for measurement. As a highly sensitive sensor, it possesses advantages such as simple and reliable structure, high output power, strong impedance resistance, low requirements for the working environment, and good stability. Therefore, it is widely used in various engineering physical quantity detection and automatic control systems.
Inductive sensors, as a type of position feedback element, are now widely used in almost all industrial fields of automation control, playing a crucial role in the reliable operation of detection and automatic control systems.
Machining
(1) Inductive sensors can be used to measure physical quantities such as displacement, size, and pressure.
(2) Using inductive displacement sensors can improve the precision of bearing manufacturing.
(3) Use an inductive micrometer to measure minute and precise dimensional changes.
(4) To achieve accurate measurement of the opening position of the hydraulic valve.
(5) Flexible sensors for designing smart textiles,
(6) Aperture taper error measuring instrument based on the principle of inductive sensor
(7) Use an inductive sensor to detect abrasive particles in the lubricating oil;
(8) Use inductive sensors to monitor the guide wheels of the lifting device, etc.
(9) Inductive sensors can be used to achieve high-precision machining of workpieces, automate the workpiece machining process, and detect flaws in workpieces.
robot
Inductive sensors can also be used for magnetic speed switches, gear speed measurement, sprocket tooth speed detection, etc. For example, if the motion trajectory of a robotic arm needs strict control, inductive sensors can be used to limit the position of the robotic arm, monitor the rotation speed and position of gears within the robotic arm, and ensure stable operation of the robotic arm. Other applications include sprocket tooth speed detection, chain conveyor belt speed and distance detection, gear speed counters, tachometers, and control of automotive safety systems. Furthermore, these sensors can be used in feed pipe systems for small object detection, object ejection control, wire breakage monitoring, small part differentiation, thickness detection, and position control.
intelligent machinery
Inductive sensors can also be used to detect the speed and distance of conveyor belts, gear age counters, tachometers, and control automotive safety systems. For example, when unloading cargo from a truck, the position of the cargo box needs to be limited, and the lifting range of the cargo box can only be controlled within a certain limit. Proximity sensors can be used to detect the lifting height of the cargo box and thus control the hydraulic system to perform limit movements.
Other applications
Inductive sensors can also be used for active measurement in grinding processes, measuring length displacement, and manufacturing electronic micrometers.
IV. Advantages and disadvantages of inductive sensors
The main advantages of inductive sensors are:
1. Simple and reliable structure;
2. High sensitivity, with a maximum resolution of 0.1μm;
3. High measurement accuracy, with output linearity reaching ±0.1%;
4. It has a large output power, and in some cases it can be directly connected to a secondary instrument without amplification.
The disadvantages of inductive sensors are:
1. The sensor itself has a low frequency response, making it unsuitable for rapid dynamic measurements;
2. High stability requirements are placed on the frequency and amplitude of the excitation power supply;
3. The resolution of a sensor is related to its measurement range. A larger measurement range results in lower resolution, and vice versa.
V. Requirements for Inductive Sensor Products
1. Attenuation of detection distance. The slip is made of iron, which is suitable for inductive sensors; however, the size of the slip being measured is slightly smaller than the size of the standard object being measured (the standard object size is 3 times the rated detection distance, and in this application, the standard size should be 120*120mm), which will result in some attenuation.
2. On-site anti-interference capability. This is a crucial issue. Ordinary inductive sensors are easily interfered with by motors or frequency converters. Many technicians only select sensors with strong electromagnetic interference resistance for applications in such locations. However, in automotive manufacturing workshops, the factory is large, and on-site technicians are accustomed to communicating via walkie-talkies. Especially when talking on walkie-talkies while walking, they may inadvertently approach the sensor, causing temporary malfunction.
3. Installation: With the widespread adoption of inductive sensors, their electrical performance has improved, and their mechanical design has become increasingly user-friendly. This aims to maximize ease of installation, reducing the stock of numerous similar products and minimizing installation and maintenance time.
4. Ensuring Stable Operation. During use in the vehicle factory, any corrosion from oil or dust must be prevented. Furthermore, vibration is a constant feature as the skid passes over the track; therefore, excellent vibration resistance is also crucial.
Article source: EBITE IoT Applications, Electronics World, OFweek.com
