Magnetic sensors convert magnetic or magnetically encoded information into electrical signals for processing by electronic circuits.
Magnetic sensors are solid-state devices that are becoming increasingly popular because they can be used in many different types of applications, such as sensing position, speed, or directional motion. They are also a popular sensor choice for electronic designers because of their non-contact, wear-free operation, low maintenance, robust design, and sealed Hall effect devices that are unaffected by vibration, dust, and water.
One of the main applications of magnetic sensors is in automotive systems for sensing position, distance, and speed. For example, the angular position of the crankshaft is used for the ignition angle of spark plugs, the position of car seats and seat belts used for airbag control, or wheel speed detection in anti-lock braking systems (ABS).
Magnetic sensors are designed to respond to various positive and negative magnetic fields in a variety of applications. One type of magnetic sensor whose output signal is a function of the magnetic field density around it is called a Hall effect sensor.
A Hall effect sensor is a device activated by an external magnetic field. We know that a magnetic field has two important properties: magnetic flux density (B) and polarity (north and south poles). The output signal of a Hall effect sensor is a function of the magnetic field density around the device. When the magnetic flux density around the sensor exceeds a certain preset threshold, the sensor detects it and generates an output voltage called the Hall voltage VH. See the figure below.
Hall effect sensor principle
Hall effect sensors are essentially composed of a thin, rectangular sheet of p-type semiconductor material, such as gallium arsenide (GaAs), indium antimonide (InSb), or indium arsenide (InAs), through which a continuous current flows. When the device is placed in a magnetic field, the magnetic flux lines exert a force on the semiconductor material, deflecting charge carriers, electrons, and holes to either side of the semiconductor plate. This movement of charge carriers is a result of the magnetic force they experience as they pass through the semiconductor material.
As these electrons and holes move along the sides, a potential difference is generated between the two sides of the semiconductor material through the accumulation of these charge carriers. The electrons are then affected by an external magnetic field perpendicular to the semiconductor material, and this effect is greater in flat, rectangular materials.
The effect of generating a measurable voltage using a magnetic field is known as the Howard Hall effect, discovered after the 1770s. The basic physical principle of the Hall effect is the Lorentz force. For a magnetic flux line to produce a potential difference across the device, it must be perpendicular (90ø to the current flow) and of the correct polarity, typically one south pole.
The Hall effect provides information about the type and magnitude of magnetic poles. For example, the south pole will cause the device to produce a voltage output, while the north pole will have no effect. Typically, Hall effect sensors and switches are designed to be in an "off" state (open circuit) when no magnetic field is present. They only become "on" (closed circuit) when subjected to a magnetic field of sufficient strength and polarity.
Hall effect magnetic sensor
The output voltage of a basic Hall element (called the Hall voltage, (VH)) is proportional to the strength of the magnetic field passing through the semiconductor material (output αH). This output voltage can be very small, only a few microvolts even under strong magnetic fields. Therefore, most commercial Hall effect devices are manufactured with built-in DC amplifiers, logic switching circuits, and voltage regulators to improve the sensor's sensitivity, hysteresis, and output voltage. This also enables Hall effect sensors to operate under a wider range of power and magnetic field conditions.
Hall effect sensor
Hall effect sensors provide linear or digital outputs. The output signal of a linear (analog) sensor is directly taken from the output of an operational amplifier, and the output voltage is proportional to the magnetic field passing through the Hall sensor. The output Hall voltage is as follows:
Linear or analog sensors provide a continuous voltage output that increases with a strong magnetic field and decreases with a weak magnetic field. In a linear output Hall effect sensor, the amplifier's output signal increases with increasing magnetic field strength until it begins to saturate the limit imposed by the power supply. Any additional increase in magnetic field will not affect the output but will make it more saturated.
On the other hand, the digital output sensor features a Schmitt trigger with a built-in hysteresis connected to an operational amplifier. When the magnetic flux through the Hall sensor exceeds a preset value, the device's output rapidly switches between its "off" and "on" states without any type of contact bounce. This built-in hysteresis eliminates any oscillations in the output signal as the sensor moves into and out of the magnetic field. The digital output sensor then has only two states, "ON" and "OFF".
There are two basic types of digital Hall effect sensors: bipolar and monopolar. Bipolar sensors require a positive magnetic field (south pole) to operate them and a negative magnetic field (north pole) to release them, while monopolar sensors only require a magnetic south pole to operate and release them in and out of the magnetic field.
Most Hall effect devices cannot directly switch large electrical loads because their output drive capability is very small, approximately 10 to 20 mA. For high current loads, an open-collector (current sink) NPN transistor is added to the output.
The transistor operates as an NPN absorption switch in its saturation region, and will short-circuit the output to ground as long as the applied magnetic flux density is higher than the "ON" preset point.
The output switching transistor can be an open emitter transistor, an open collector transistor configuration, or both, in a push-pull output configuration. It can sink enough current to directly drive many loads, including relays, motors, LEDs, and lamps.
Hall effect applications
Hall effect sensors are activated by a magnetic field, and in many applications, the device can be operated by a single permanent magnet attached to a moving axis or the device itself. There are many different types of magnet movements, such as "front," "side," "push-pull," or "push-push" sensing movements. For each type of configuration, to ensure maximum sensitivity, the magnetic flux lines must always be perpendicular to the sensing area of the device and must have the correct polarity.
Similarly, to ensure linearity, a magnet with a high magnetic field strength is required, which produces a large field strength change in response to the desired motion. There are several possible paths for motion to detect the magnetic field, and below are two more common sensing configurations using a single magnet: head-on detection and side-on detection.
Front detection
As the name suggests, "frontal detection" requires the magnetic field to be perpendicular to the Hall effect sensor, and for detection to occur, it must be directly facing the moving surface of the sensor. One "frontal" method.
This frontal proximity method generates an output signal VH, which in a linear device represents the magnetic field strength and flux density as a function of the distance from the Hall effect sensor. The closer the magnetic field, and therefore the stronger it is, the larger the output voltage, and vice versa.
Linear devices can also distinguish between positive and negative magnetic fields. Nonlinear devices can be triggered to output "ON" at a preset air gap distance away from the magnet to indicate position detection.
Lateral detection
The second sensing configuration is "lateral detection." This requires moving a magnet across the surface of the Hall element in a lateral motion.
Lateral or sliding pass detection is used to detect the presence of a magnetic field because it moves across the surface of the Hall element, for example, within a fixed air gap distance. It is useful for counting the rotational speed of a rotating magnet or motor.
Depending on the position of the magnetic field as it passes through the sensor's zero-field centerline, a linear output voltage representing positive and negative outputs can be generated. This allows for directional motion detection, which can be vertical or horizontal.
Hall effect sensors have many different applications, especially proximity sensors. They can replace optical and light sensors in environments with conditions including water, vibration, dirt, or oil, such as in automotive applications. Hall effect devices can also be used for current detection.
When an electric current passes through a conductor, it generates a circular electromagnetic field around it. By placing a Hall sensor next to the conductor, currents ranging from a few milliamperes to thousands of amperes can be measured from the generated magnetic field without the need for large or expensive transformers and coils.
In addition to detecting the presence or absence of magnets and magnetic fields, Hall effect sensors can also detect ferromagnetic materials, such as iron and steel, by placing a small, permanent "bias" magnet behind the effective area of the device. The sensor is now in a permanent and static magnetic field, and any changes or disturbances to this magnetic field introduced by ferrous materials will be detected, with sensitivity potentially as low as mV/G.
Depending on the type of device, whether digital or linear, there are many different ways to connect a Hall effect sensor to electrical and electronic circuits. A very simple and easy-to-construct example is using a light-emitting diode (LED) as shown below.
Position detector
When no magnetic field is present, the frontal position detector will be "off" (0 Gauss). When the south pole of the permanent magnet (positive Gauss) moves vertically into the effective region of the Hall effect sensor, the device becomes "on" and the LED lights up. Once switched to "on", the Hall effect sensor remains in the "on" state.
To "turn off" the device and LED, the magnetic field must be lowered below the release point of the monopole sensor or exposed to the magnetic north pole (negative Gaussian) of the bipole sensor. If the output of the Hall effect sensor is required to switch larger current loads, a larger power transistor can be used instead of the LED.
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