Hall effect flow sensor

Magnetic sensors convert magnetic or magnetically encoded information into electrical signals for processing in electronic circuits. These sensors are a type of solid state components that are becoming more and more popular. Because they can be made in different shapes and for different applications; Such as location detection sensor, speed detection or directional motion detection. These sensors are also a good choice for electronic designers. They have low maintenance costs and reliable design. These sensors are insulated from the surrounding space. Therefore, they are not sensitive to vibration, dust and water.

One of the basic applications of this type of sensor is to measure location, distance and speed in vehicle systems. For example, a car spark plug needs information about the angular location of the engine crankshaft to ignite. A Hall effect sensor calculates this information. The location of the car seat and airbag is controlled by the Hall effect sensor. As another example, this sensor is responsible for detecting the speed of the car to activate the "anti-lock system" (ABS).

Magnetic sensors are used to detect magnetic fields in a variety of applications. One type of sensor, in which the output signal is a function of the density of the surrounding magnetic field, is called the Hall Effect Sensor.

The Hall effect sensor is a device that is activated by applying a magnetic field. We know that a magnetic field has two important characteristics. One is the magnetic flux density (B) and the other is the polarity (north and south poles). The output signal of the Hall effect sensor is a function of the magnetic field density applied to the sensor. When the density of the magnetic flux around the sensor exceeds a certain threshold, the sensor detects this and generates an output voltage called the "Hall Voltage".

Principles of operation of Hall effect sensor

The Hall effect sensor consists of a p-type rectangular semiconductor material such as gallium arsenide (GaAs), indium antimonide (InSb) or indium arsenide (InAs). A stream flows continuously through this material. When the sensor is placed in a magnetic field, the magnetic flux lines apply a force to the semiconductor. This force directs the charge carriers in the semiconductor (ie, electrons and holes) to either side of the blade. The motion of charge carriers is the result of the magnetic force applied to them.

Due to the accumulation of load carriers on both sides of the blade, a potential difference occurs between the two sides of the semiconductor. The phenomenon of voltage generation on both sides of a semiconductor (caused by a magnetic field) is called the "Hall Effect". The physical principle that causes this phenomenon is Lorentz force. To generate this potential difference in matter, the magnetic flux lines must be appropriate to the direction of orthogonal current and its polarity. In general, the south pole of the magnet should be perpendicular to the direction of flow.

The Hall effect depends on the magnetic pole of the magnet and the intensity of the applied magnetic field. For example, using Antarctica to activate a sensor causes a voltage difference. This voltage difference is measurable. But using the North Pole does not cause any voltage difference (ie it does not move the load carriers). In general, Hall effect sensors and switches are made for "off" mode (open circuit conditions). In this case, there is no magnetic field. Only if the magnetic field is strong enough and its polarization is correct, the sensor is activated and the so-called switch changes to the on state (closed circuit condition).

Hall effect magnetic sensor

The output voltage, known as the Hall voltage (VH), is directly related to the magnitude of the orthogonal magnetic field on the semiconductor. When the sensor is exposed to a suitable magnetic field, its output voltage usually does not exceed a few microvolts. For this reason, most Hall effect sensors have internal DC amplifiers, switching logic circuits, and voltage regulators to enhance sensor sensitivity, reduce error due to hysteresis losses, and obtain the desired output voltage. In this way, the sensitivity of the sensor can be adjusted for different magnitudes of the magnetic field and different feeding conditions.

The output of the Hall effect sensor can be digital or linear. In linear (analog) sensors, the sensor output is connected directly to an amplifier. The signal is also taken from the output of the amplifier. This issue is shown in the figure below.

The output voltage in this case is directly proportional to the magnetic field passing through the sensor. The output voltage of the hall is given as follows: VH = RH (It × B)

where in:
VH is the voltage of the hall in volts.
RH is the Hall effect factor.
I is the current flowing through the sensor in amperes.
t is the thickness of the sensor in millimeters.
B is the magnetic flux density with a Tesla unit.

The output voltage of linear or analog sensors is continuous. This voltage increases with increasing magnetic field strength. If the applied magnetic field decreases, the output voltage decreases. In linear or analog Hall effect sensors, as the magnetic field intensity increases, the output of the amplifier increases to a certain point and reaches its saturation point. From this point on, any increase in the magnetic field has no effect on the output voltage. If we increase the magnetic field, the sensor becomes more saturated.

On the other hand, digital sensors use the "Schmitt trigger". Schmidt trigger is a comparator circuit that uses positive feedback. The Schmidt trigger is an active circuit that converts the analog input signal into a digital output signal. In this circuit, when the input voltage exceeds a certain threshold, the output voltage will be one volt. That is, the key is in the "ON" state. When the input voltage is less than the threshold value, the output voltage of this circuit will be zero volts. That is, the key is in the "OFF" state. When this sensor is exposed to a magnetic field or the magnetic field is removed, due to the Schmidt trigger circuit, no oscillation is observed in the output circuit of the sensor.

There are two main types of digital Hall effect sensors. One is "bipolar" and the other is "unipolar". Bipolar digital sensors require a positive magnetic field (magnet pole) to activate and a negative magnetic field (magnet pole) to deactivate. Unipolar sensors, on the other hand, are activated by exposure to the South Pole magnet. If we move the magnet away from the sensor from the Antarctic side, the sensor will be deactivated and there is no need for the north pole of the magnet.

Switches that work with the Hall effect sensor do not have the ability to switch to a large electrical output because their capacity is very limited. The output current of these circuits is very small and about 10 to 20 mA. An amplifier (an open collector NPN transistor) is used to have large electrical currents at the output.

This transistor acts like an NPN switch. Whenever the density of the magnetic flux applied to the sensor exceeds the threshold value, the switch drops the output. This transistor can be used as an open collector or as an open emitter. In this way, we have a cover-money amplifier circuit. Now this sensor can be connected to high current loads such as relays, motors, LEDs and lamps.

Hall effect sensor applications

As mentioned, the Hall effect sensor is activated by a magnetic field. In many applications, the sensor is mounted on a moving cylinder with a permanent magnet. The motion of the sensor and the magnet relative to each other can have different states. Forward, sideways, push-pull, and push-push modes are different types of sensor and magnet movement relative to each other. In all these cases, in order to ensure maximum sensitivity of the sensor, the magnetic flux lines must be perpendicular to the sensor and its polarization must be correct (ie use the south pole of the magnet to activate the sensor).

A strong magnet is also used to ensure the linear behavior of the sensor. In this way, with the movement of the magnet, we will have a sharp change in the intensity of the magnetic field passing through the sensor. To detect a magnetic field, a sensor and a magnet can have different positions relative to each other. The two most common types of Hall effect sensors are forward motion and lateral motion.

Move forward

In this case, the Hall effect sensor and the magnetic field are perpendicular to each other. To activate the sensor, the magnet moves directly towards the sensor.

In this case, a signal is generated at the sensor output (VH). This voltage in linear sensors indicates that the strength of the magnetic field and the density of the magnetic flux are a function of the distance of the magnet from the sensor. The closer the magnet is to the sensor, the stronger the magnetic field. So the output voltage of the sensor will be bigger.

Lateral movement

In this case, the magnet moves laterally relative to the sensor. This position is suitable for the case where we want to keep the distance between the magnet and the sensor constant. For example, in the case of a rotating magnet, this method is used for detection; Such as: detection of engine rotation speed.

The center line of the Hall effect sensor is its zero point. By moving the magnet sideways, the output voltage of the sensor can be positive or negative.

Hall effect sensors are also specifically used as "proximity sensors". In situations where we have a specific environment, such as exposure to water, dust, vibration or oil, these sensors are used instead of optical sensors. A good example of this particular environment is inside the car system. The Hall effect sensor can also be used to measure current.

We know from previous teachings that when a current passes through a conductor, a circular electromagnetic field is created around the conductor. In this case, without the need for transformers and windings, by placing the Hall effect sensor near the conductor, electrical currents can be measured from a few milliamperes to several thousand amps.

We know that Hall effect sensors are used to detect the presence or absence of a magnetic field or magnet. They can also be used to detect ferromagnetic materials such as iron or steel. To do this, a permanent magnet is placed near the active area of ​​the sensor so that the sensor becomes "biased". In this case, the sensor is placed in a permanent and stationary magnetic field. Now, if we place a piece of iron next to the sensor, the magnetic field that hits the sensor will be changed and disturbed, and the sensor will detect the presence of this piece of iron. The sensitivity of the sensor can be increased up to even a few millivolts.

Light emitting diode (LED)

A common use of the Hall effect sensor is the "Light-Emitting Diode" (LED). This application is shown in the figure below. When there is no magnetic field (zero Gauss) the key is in the "off" position. When the south pole of the magnet approaches the active area of ​​the sensor vertically, the switch is in the "on" position and the diode turns on. As a result, the Hall effect sensor remains in the "on" state.

To turn off the diode of the unipolar sensors, we have to move the magnet away from it. For bipolar sensors, we have to place the north pole of the magnet next to the sensor. In devices that have a larger electrical charge and require a larger current, we must use a high power transistor at the sensor output.

As another example of this sensor, we can mention the car speedometer. The sensor is mounted on a rod next to a permanent magnet. Next to this bar is a gear. The air gap between the gear and the Hall sensor is very small. As a result, as each tooth passes close to the sensor, the magnetic field around the sensor changes. This causes the sensor output signal to be zero or one. Therefore, the output signal of the sensor is a square wave.The periodicity of this square wave can be used to calculate velocity.

Other applications of this sensor are used in atmospheric and "magnetometer" (Magnetometer). On a larger scale, the Hall Effect is used for the Hall Effect Thruster, which is used to send spacecraft into space.