A magnetic sensor is an electronic component that converts magnetic signals into electrical signals. It is often used in automated machinery and equipment to provide information feedback for the control system to detect the linear position of the cylinder piston. According to different usage environments, select the appropriate magnetic sensor technology to maximize equipment performance, space and reliability. The main realization technologies of magnetic sensors include Hall technology (Hall), mechanical reed switch, anisotropic magnetoresistance technology (AMR), giant magnetoresistance technology (GMR) and so on. In the future, magnetic sensors for cylinders will develop in the direction of miniaturization, integration, intelligence, low power consumption, anti-interference, and high sensitivity.
1. Reed switch
A reed switch is a mechanical switch with a dry reed switch as its core component. The fully sealed structure of the reed switch enables it to work in a variety of environments. When the external magnetic field approaches, the reeds on the reed switch attract each other after being magnetized by the magnetic field, and the circuit is turned on at this time, providing information feedback for the control system.
Magnetic reed sensor is cost-effective and can connect alternating current and direct current; the contacts in the reed switch are sealed in a glass tube with an inert gas, and are less affected by the external environment; the contacts have excellent corrosion and wear resistance, which improves the Lifetime of reed switching times. Due to the physics of reed switches, reed switches have certain limitations. First of all, the conduction of the reed switch requires that the reeds contact each other, which is a mechanical process. Therefore, the switching cycle of the reed contact is limited, and maintenance is required during the use of the machine; the magnetic reed switch is not suitable for equipment exposed to large vibration or shock, and severe vibration will cause the contact of the reed to deviate. move, resulting in an unstable or ineffective reed connection. The switching characteristics of reed switches can also lead to accidental double-switching. Double switching means that the sensor output switches "ON" and "OFF" twice, while the cylindrical magnet goes through the reed switch once. The false double-switching of the sensor output is caused by the uneven strength of the magnetic field. The strength of the magnetic field is strongest at each pole of the magnet and weakest at the centre between each pole. If the piston magnet is not strong enough, it may cause the switch output to double toggle as it passes through the sensor. Finally, compared to solid-state sensors, reed switches are relatively slow to activate, which makes them unsuitable for applications that require fast response times.
In practical applications, reed switches are widely used on cylinders. This benefits from a relatively low price and does not require an additional switch-on load to operate under DC or AC conditions
2. Hall sensor
Hall sensor is a magnetic reed proximity sensor. There is a conductive plate in the Hall sensor through which the charge carriers flow from A to B and through the conductive plate. At this time, an external magnetic field is introduced. Under the action of the Lorentz force, the linear motion of the charge carriers is disturbed, and the electrons will shift to one side of the conductive plate, resulting in a potential difference between the two ends of the conductive plate. This voltage is called is the Hall voltage. The Hall voltage varies with the strength of the magnetic field. The stronger the magnetic field, the higher the voltage; the weaker the magnetic field, the lower the voltage. The Hall voltage is usually only a few microvolts per gauss, and the device needs a built-in high-efficiency amplifier to amplify the voltage enough to output a stronger signal. There are two types of Hall sensors, which provide analog signal output and digital signal output respectively. The former consists of a voltage regulator Hall element and an amplifier, which is suitable for measuring distances; the latter only provides two output states of "on" or "off". This type of Hall sensor requires an additional Summit trigger to provide delay or two different threshold voltages. Therefore, the output potential can only be "high" or "low". Unlike reed switches, Hall sensors do not include moving components and require less installation space. The solid-state design extends sensor life and is also resistant to shock and vibration. Compared with the mechanical contact of the reed switch, it overcomes the inertia caused by the contact of the reed switch, so it is more suitable for application equipment that needs to switch signals quickly. In addition, the Hall sensor has low sensitivity. Depending on the diameter and thickness of the cylinder block, the switch output may not give the correct command in time. Hall-effect air cylinder reed switch is often used as limit switches on 3D printers and CNC machine tools, as well as detection and positioning in industrial automation systems. Hall sensors are also used as wheel speed sensors and crankshaft or camshaft position sensors.
3. Anisotropic magnetoresistive sensor (AMR)
The phenomenon that the resistivity of a substance changes in a magnetic field is called the magnetoresistive effect, and the magnetoresistive sensor is made of the magnetoresistive effect. The basic structure of the AMR magnetic sensor is composed of four magnetoresistive Wheatstone bridges. The power supply is Vb, and the current flows through the resistor. When a bias magnetic field H is applied to the bridge, the magnetization directions of the two oppositely placed resistors will rotate toward the current direction, and the resistance of the two resistors will increase; while the magnetization of the other two oppositely placed resistors The direction turns in the opposite direction to the current, and the resistance of the two resistors decreases. The magnetic field value of the casing can be obtained by outputting the differential voltage signal at the two output terminals of the tested bridge. AMR magnetoresistive sensor can well sense the weak magnetic field measurement in the range of geomagnetic field, and make various displacement, angle, rotational speed sensors, various proximity switches, isolation switches, and are used to detect some ferromagnetic objects such as airplanes, trains, car. Other applications include compasses in various navigation systems, disk drives in computers, various magnetic card machines, rotational position sensing, current sensing, drilling orientation, line position measurement, yaw rate sensors, and head tracking in virtual reality track.
① The magnetic range of the best performance of anisotropic magnetoresistive technology is centered on the earth's magnetic field, and has a wide operating space for sensor applications that take the earth's magnetic field as the basic operating space.
②The anisotropic magnetoresistance technology is the only semiconductor process technology that has been verified and can measure the direction with an accuracy of one degree in the earth's magnetic field. Other technologies that can achieve the same precision are processes that cannot be integrated with semiconductors. Therefore, AMR can be integrated with CMOS or MEMS on the same silicon wafer and provide sufficient accuracy.
③AMR technology only needs one layer of magnetic film, which is convenient for production.
④AMR technology has the characteristics of high frequency and low noise, and is suitable for various applications.
⑤ Similar to Hall-effect sensors, AMR sensors act quickly, wear-free, and are resilient to shock and vibration.
⑥ Since it is able to detect weaker magnetic fields, it can improve piston detection at greater distances. Due to the higher sensitivity, the possibility of double switch points is eliminated.
4. Giant magnetoresistive sensor (GMR)
A giant magnetoresistive sensor (GMR) is a solid-state magnetic proximity sensor. The magnetoresistance effect is a phenomenon in which the resistance value of a conductor or semiconductor changes under the action of a magnetic field. In magnetic multilayer films such as Fe/Cr and Co/Cu, the ferromagnetic layers are separated by nanoscale thicknesses of nonmagnetic materials. Under certain conditions, the magnitude of the decrease in resistivity is quite large, which is about 10 times higher than the magnetoresistance value of common magnetic metals and alloy materials. This phenomenon is called "giant magnetoresistance effect".
The giant magnetoresistance effect can be explained by quantum mechanics, every electron can spin, and the scattering rate of electrons depends on the spin direction and the magnetization direction of the magnetic material. The spin direction is the same as the magnetization direction of the magnetic material, the electron scattering rate is low, and the electrons passing through the magnetic layer are more, thus showing low impedance. On the contrary, when the spin direction is opposite to the magnetization direction of the magnetic material, the electron scattering rate is high, so fewer electrons pass through the magnetic layer, and high impedance is present at this time. Under certain conditions, the resistivity of a substance will change greatly with the magnetic field, which is called the giant magnetoresistance effect (GMR). GMR can be an order of magnitude more sensitive than AMR. The giant magnetoresistance effect is a phenomenon of quantum mechanics and condensed matter physics, which refers to the phenomenon that the resistivity of magnetic materials changes greatly when there is an external magnetic field compared to when there is no external magnetic field. Sensors based on this effect are giant magnetoresistive sensors.
GMR sensors offer similar advantages to AMR sensors, but they are more sensitive to the presence of magnetic fields. The high sensitivity also allows for a highly compact sensor, suitable for smaller and shorter cylinders. While high sensitivity is a benefit for applications that require immediate sensor feedback, it can cause unexpected output signals if disturbed by surrounding magnetic fields. For example, nearby environments with high power (AC motors or AC input power) can interfere with sensor signals and cause unexpected errors.
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