The working principle and application field analysis of digital magnetic Hall effect sensor

“A digital magnetic sensor is a device in which, due to the presence of an external magnetic field, the output switch is switched between ON and OFF states. This type of device based on the physical principle of the Hall effect is widely used as proximity, positioning, speed and current detection sensors. Unlike mechanical switches, they are a long-lasting solution because they have no mechanical wear and can work even under particularly critical environmental conditions.Due to non-contact operation, maintenance-free, sturdy and durable, and immunity to vibration, dust and liquids, digital magnetic sensors are becoming more and more common, especially in automobiles

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A digital magnetic sensor is a device in which, due to the presence of an external magnetic field, the output switch is switched between ON and OFF states. This type of device based on the physical principle of the Hall effect is widely used as proximity, positioning, speed and current detection sensors. Unlike mechanical switches, they are a long-lasting solution because they have no mechanical wear and can work even under particularly critical environmental conditions. Due to its non-contact operation, maintenance-free, ruggedness and immunity to vibration, dust and liquids, digital magnetic sensors are becoming more and more common, especially in the automotive and consumer electronics fields.

For example, in the automotive field, these sensors are used to detect position, distance and speed. Inside the engine, they are used to identify the position of the crankshaft, in the passenger compartment, to detect the position of the seat and seat belt (basic information for operating the airbag control system), on the wheel, they are used to detect the position of the wheel . Detect the rotation speed required for ABS.

working principle

The heart of each magnetic sensor is represented by a Hall element, and the output voltage of the Hall element (also called Hall voltage, denoted by VH) is proportional to the strength of the magnetic field passing through the semiconductor material. Since this voltage is very low, about a few microvolts, other components must be included in the design, such as operational amplifiers, voltage comparators, voltage regulators, and output drivers. According to the output type, magnetic sensors are divided into linear and digital. The linear output and linear digital sensors have a linear relationship. In the linear output, the analog output voltage changes linearly with the intensity of the magnetic field. In the digital output, the sensor can only present two states. In both cases, the VH voltage satisfies the following formula:

VH= RH・((B・I)/t)

Among them: VH is the Hall voltage in volts, RH is the Hall effect coefficient, I is the current flowing through the sensor in amperes, t is the thickness of the sensor in millimeters, and B is expressed in Tesla Magnetic flux density. Figure 1 shows a block diagram of a general-purpose linear Hall-effect sensor, while the block diagram of Figure 2 refers to a digital sensor. The Hall element passes through the box with an “X” as shown in Figure 1, and depending on the type, the sensor may include multiple units of the same type (two are needed to detect the differential magnetic field, and three are used to detect the direction or movement). To increase the flexibility of the interface, analog sensors usually include an open emitter, open collector or push-pull transistor, which is connected to the output of the differential amplifier. The main difference between the two solutions is that the sensor with digital output includes a Schmitt trigger with built-in hysteresis, which is connected to an operational amplifier.

Figure 1: Block diagram of a linear (analog output) Hall-effect sensor

When the magnetic flux passing through the sensor exceeds a certain threshold, the output will switch from OFF to ON. Hysteresis is used to eliminate any oscillation of the output signal when the sensor enters and leaves the magnetic field. Devices based on the Hall effect are divided into unipolar and bipolar sensors. Bipolar sensors require a positive magnetic field (South Pole) to operate, while a negative magnetic field (North Pole) needs to be released. Unipolar sensors require a magnetic pole (south pole) to operate and release. In addition, the sensor is usually designed to produce an output in the OFF state (open circuit) without an electromagnetic field, and to output in the ON state (closed circuit) under the condition of receiving a sufficient strength and correct magnetic field. polarity.

Figure 2: Block diagram of a digital Hall-effect sensor

Application area

Regardless of the specific application type, the basic requirement for the correct operation of a Hall-effect sensor is that the magnetic flux lines are always perpendicular to the sensor surface and have the correct polarity. There are many applications for digital magnetic sensors, including automobiles, consumer electronics, Electronic medical systems, telecommunications, and industrial process control. The position sensor is used to detect the sliding movement between the magnet and the sensor, and the distance between the two elements is very short. The relative movement between the magnet and the sensor generates a positive magnetic field when the sensor moves to the south, and a negative magnetic field when the sensor moves to the north pole.

Several techniques can be used to determine the position: for example, if the application requires a limited and discrete position, a simple switch can be used, and for applications that require higher precision, a linear device can be used in conjunction with a microprocessor. Position or proximity sensors can also be used to monitor the level of liquids and are used in household appliances such as washing machines or dishwashers. In this case, several Hall switches are used in combination with a magnet placed on a float.

When the float rises in the pipe, it will activate the corresponding discrete switch located on the outside of the housing to indicate the water level digitally. Another important application involves DC brushless motors whose speed is controlled by electrical commutation rather than mechanical commutation. In this regard, three digital magnetic sensors are located on the motor stator, while the permanent magnets are located on the rotor shaft. The automotive industry has become a leader in the global magnetic field sensor market, occupying more than 40% of the market share. The increasing demand for integrating multiple safety functions into automobiles has created opportunities for Hall sensors, which have been used in several safety-related applications such as electronic stability control (ESC) and anti-lock braking systems (ABS). Used in the application.

An example of a digital magnetic sensor for position detection is the Allegro MicroSystems A1210-A1214 device series. The A121x series sensors have obtained the AEC-Q100 certification for automotive applications, have high reliability, and maintain stable, continuous operation over an extended temperature range, strong EMC performance, and high ESD ratings. The A1210-A1214 Hall effect latch includes the following components on a single silicon chip: voltage regulator, Hall voltage generator, small signal amplifier, Schmitt trigger and NMOS output transistor.

When the magnetic field perpendicular to the Hall element exceeds the operating point threshold, the output of these devices is switched to a low level (on). The sensor has a latching behavior, that is, a sufficiently strong South Pole will open the device and will remain open after the South Pole is removed. When the magnetic field is reduced below the release point, the sensor output becomes high (closed). The difference between the magnetic operating point and the release point is the hysteresis of the device.

Magnetic sensors are also suitable for accurately detecting angular positions. An example is the AMS AS5048A / AS5048B magnetic rotary encoder, a sensor that provides 14-bit high-resolution output for 360° angular position detection. Figure 3 shows the main functional modules of the device: Hall sensor, analog-to-digital converter and digital signal processing. The absolute position of the magnet can be directly accessed through the PWM output and can be obtained through a standard SPI or high-speed I²C interface (depending on the model). The zero position can be programmed via SPI or I²C commands, because there is no need to align the magnets mechanically, which simplifies the entire system. The sensor can withstand the effects of misalignment, air gap changes, temperature and external magnetic field changes. Reliability, robustness and wide temperature range make it an ideal choice for rotation angle sensing in harsh industrial and medical environments.

Figure 3: The main functional blocks of AS5048A[来源：AMS]

in conclusion

Digital magnetic Hall effect sensors are widely known among designers for their ruggedness, durability, and reliable operation for any position sensing application. Whether it is simply detecting the closure of the laptop cover, or performing complex motor commutation and precise position measurement, the Hall-effect sensor will sense the position with extremely high accuracy even under the harshest environmental conditions.