A digital magnetic sensor is a device in which the up and down states of the output switch are influenced by the presence of an external magnetic field. This type of device, based on the Hall effect, is widely used in proximity, positioning, speed, and current sensing sensors. Unlike mechanical switches, they are a durable solution because they have no mechanical wear and can operate even in particularly harsh environmental conditions. Digital magnetic sensors are becoming increasingly prevalent, especially in the automotive and consumer electronics sectors, due to their characteristics such as non-contact operation, lack of maintenance, robustness, and immunity to vibration, dust, and liquids.
For example, in the automotive sector, these sensors are used to detect position, distance, and speed. Inside the engine, they are used to determine the position of the crankshaft; in the passenger compartment, they are used to detect the position of the seats and seat belts (basic information for operating airbag control systems); and on the wheels, they are used to detect rotational speed, which is required by the Australian Bureau of Statistics.
Operating principles
The heart of each magnetic sensor is represented by a Hall element, whose output voltage (also known as the Hall voltage, denoted by V, H) is proportional to the strength of the magnetic field passing through the semiconductor material. Because this voltage is extremely low, only a few microvolts, it is necessary to incorporate it into the design of other components such as operational amplifiers, voltage comparators, voltage regulators, and output drivers. Depending on the type of output, magnetic sensors are classified as linear, where the analog output voltage changes linearly with the strength of the magnetic field; and digital, where the output can only have two states. In both cases, the H voltage satisfies the following formula:
VH = RH · ((B · I) / t)
Where: VH is the Hall voltage, RH is the Hall effect coefficient, I is the current passing through the sensor in amperes, T is the sensor thickness in millimeters, and B is the magnetic flux density in Tesla. Consisting of a box marked with an "x" and depending on the type, sensors may include multiple units of the same type (two units are needed to detect differential magnetic fields, three units to detect direction or motion). To increase interface flexibility, analog sensors typically include an open emitter, an open collector, or a push-pull transistor connected to the output of a differential amplifier. The main difference between these two approaches is that sensors with digital outputs include a Schmitt trigger with built-in hysteresis connected to light.
When the magnetic flux through the sensor exceeds a certain threshold, the output switches on and off from the start. Any oscillations in the output signal are eliminated by hysteresis as the sensor enters and leaves the magnetic field. Devices based on the Hall effect are classified as monopolar and bipolar sensors. Bipolar sensors require a positive magnetic field (south pole) for operation and a negative magnetic field (north pole) for release. Monopolar sensors require a single magnetic pole (south pole) for operation and release. Furthermore, sensors are typically designed to produce a disengaged output (open-circuit output) in the absence of an electromagnetic field and a disengaged output (closed-circuit output) when subjected to a sufficiently strong magnetic field and the correct polarity.
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Regardless of the specific application, a fundamental requirement for the correct operation of a sensor is that the magnetic flux lines are always perpendicular to the sensor surface and have the correct polarity. Digital magnetic sensors have numerous applications, including automotive, consumer electronics, electronic medical systems, telecommunications, and industrial process control. Position sensors detect sliding motion between a magnet and a sensor, with the two elements placed at a very short distance. The relative motion between the magnet and the sensor generates a positive magnetic field when the sensor moves south and a negative magnetic field when it moves north.
Several techniques can be used to determine position: for example, if the application requires finite and discrete positions, a simple switch can be used, while for applications requiring higher precision, a linear device combined with a microprocessor can be used. Position or proximity sensors can also be used to monitor liquid levels, applied in household appliances such as washing machines or dishwashers. In this case, several Hall switches are used in conjunction with a magnet placed on a float.
When the float rises into the pipe, it activates a corresponding discrete switch located outside the casing, providing a digital indication of the water level. Another important application involves brushless DC motors, where speed control is electrical rather than mechanically commutated. In this case, three digital magnetic sensors are positioned on the motor stator, while a permanent magnet is placed on the rotor shaft. The automotive industry has become the global leader in the magnetic field sensor market, holding over 40% of the market share. The increasing demand for integrating multiple safety features into automobiles has created an opportunity for Hall effect sensors to be utilized in several safety-related applications, such as Electronic Stability Control (ESC) and Anti-lock Braking System (ABS).
An example of a digital magnetic sensor used for position detection is the Fast Microsystems A1210-A1214 family of devices. With AEC-Q100 automotive application certification, the A121X series sensors offer high reliability, stable and continuous operation over a wide temperature range, robust EMC performance, and a high ESD rating. The time-effect lock of the A1210-A1214 comprises the following on a single silicon chip: a voltage regulator, a high-voltage generator, a small-signal amplifier, a Schmitt trigger, and an NMOS output transistor.
When the magnetic field perpendicular to the Hall element exceeds the operating point threshold, the output of these devices will switch low (on). The sensors exhibit a latch-up behavior, meaning that a sufficiently strong south pole activates the device, but it is also activated after the south pole is removed. When the magnetic field decreases below the release point, the sensor output will become high (off). The difference between the magnetic operation and release points lies in the device's hysteresis.
Magnetic sensors are also suitable for precise angular position detection. For example, the AMS AS5048A/AS5048B magnetic rotary encoder is a sensor that provides a 14-bit high-resolution output for 360-degree angular position detection. The main functional modules of the device are shown: Hall effect sensor, analog-to-digital converter, and digital signal processing. The absolute position of the magnet is directly accessible via a pressure wave M output and can be obtained via a standard SPI or high-speed plasma interface, depending on the version. The zero position can be programmed via SPI or IVOMC commands, simplifying the entire system as the magnet does not require mechanical calibration. The sensor is tolerant of misalignment, air gap variations, temperature, and external magnetic field variations. Its reliability, robustness, and wide temperature range make it ideal for rotary angle sensing in harsh industrial and medical environments.
in conclusion
Digital magnetic Hall effect sensors are renowned among designers for their robustness, durability, and reliable operation. Whether simply detecting the closing of a laptop lid or performing complex motor commutation and precise position measurements, Hall effect sensors will sense position with extremely high accuracy, even under the harshest environmental conditions.
