- Magnetic Sensors
- Hall Effect Sensors
- What is Hall voltage (VH)?
- Hall Coefficient (RH)
- Construction of Hall Effect Sensors
- Symbol of Hall Effect Sensor
- Working Principle of Hall Effect sensors
- Hall Effect experiment
- Analog and Digital Hall Effect Sensor
- Type of Hall Effect Sensors
- Applications of Hall Effect Sensors
What is magnetic sensor?
Magnetic sensors are devices which are able to detect and analyze magnetic fields generated by magnet or current. They can be used for different kinds of applications such as to sense the change in the position and angle of a magnetic field, to sense the change in strength or the direction of the applied magnetic field, etc.
There are various types of magnetic sensors like Hall Effect sensor (Hall switches, linear Hall sensors, etc.) used for detecting a change in the strength of the magnetic field, Magneto Resistive sensor used for detecting a change in the direction of the magnetic field, angle sensors used for detecting a change in the angle of a magnetic field, 3D Hall sensors and as well as magnetic speed sensors. Hall Effect sensors are employed in an extensive range of applications such as proximity sensor, position and speed measurement etc. They are even used in computer printer, pneumatic cylinder, computer keyboards etc.
Magnetic sensors are generally a solid-state device which is in high demand now a day due to its high precision and accuracy, contact less operation, comparatively low maintenance cost, compact design, etc. Now a day coreless magnetic sensors dedicated for different kinds of industrial applications are available for example, sealed Hall Effect devices are water-proof and are made in such a way to resist any vibration too.
Magnetic sensors are extensively used in automotive systems especially for analysing the position of the car seats, seat belts and for controlling air-bag system and also for detection of wheel rotation speed for the anti-lock braking system (ABS).
Hall Effect Sensors
Hall Effect sensors are magnetic sensors whose output is dependent on the magnetic field or magnetic flux density around the magnetic sensor.
- The word “Hall” came from Dr Edwin Hall, who discovered this Hall Effect for the first time.
- If there is an external magnetic field vertical to the object through which current is passing, an electromotive force will generates in the direction perpendicular to the magnetic field and to the current.
What is Hall voltage (VH)?
If an external magnetic field is applied in the magnetic sensor, it gets activated. Hall Effect sensor’s output voltage is proportionate to the strength of the applied magnetic field passing by. After a particular threshold of magnetic flux density is exceeded by the external field, an output voltage is generated, which is commonly known as Hall voltage (VH).
Hall Coefficient (RH)
The quantity of the potential difference per unit thickness of metal stripe in the Hall Effect distributed by the product of the magnetic intensity and the longitudinal current density.
The units of Hall coefficient RH are in general conveyed as m3/C, or Ω·cm/G.
Construction of Hall Effect Sensors:
Hall Sensor Design
Hall Effect sensors generally consist of a rectangular piece of semiconductor such as indium antimonite (InSb) or gallium arsenide (GaAs) known as a Hall probe mounted on an aluminum plate and covered altogether inside the probe head. A probe handle made up of a non-magnetic material is connected with the probe head such that the plane of the rectangular plate of semiconductor is perpendicular to the probe handle.
When the device is activated, a continuous flow of current occurs through the semiconductor. If the external magnetic field lines are at right angles to the probe head such that the filed lines are passing through the right angles through the sensor of the probe, a voltage originates which known as the “Hall effect” voltage and the device provides a reading of magnetic flux density (B) of the external field.
Symbol of Hall Effect Sensor:
What is Hall Effect Transducer ?
Working Principle of Hall Effect sensors
- The Hall Effect sensor primarily works due to the effect of Lorentz Force (it is the force experienced by a charged particle due to an electric field or a magnetic field, i.e. simply an electromagnetic field).
- In the presence of an existing external magnetic field of sufficient magnitude, the electrons in the semiconductor slab are deflected toward one edge of the slab, i.e. the holes and the electrons shift towards either side of the slab due to the Lorentz force acting on them.
- For this, one side of the semiconductor is negative charged, and the opposite side turn out to be positive charged. This produces a voltage gradient across the two opposite sides of the rectangular slab due to the accumulation of opposite charges at these two sides. This voltage is known as Hall voltage (VH), and the effect of generating this measurable Hall voltage by using an external magnetic field is known as the Hall Effect.
- To generate a potential difference such that a measurable voltage is produced, the external magnetic field lines must be at a right angle to the plane where the current flows through the slab. Also, a correct polarity should be provided for the Hall Effect sensors to work.
- As the electrons and holes shift apart from each other, a potential gradient is generated, and the separation increases until the force due to the electric field balances the force produced by the magnetic field. When both the forces balance each other, the current is not changing, and the Hall voltage that is detected at this point and from this magnetic flux density (B) has been calculated.
- If the output voltage depends linearly on magnetic flux density, then we call it as linear Hall Effect sensors, and if there is a sharp decrease of the output voltage at different magnetic flux density, then it is called as threshold Hall Effect sensor.
- We have heard about Inductive sensors which respond to a changing magnetic field as it induces a current in a coil of wire and hence generates a voltage in its output. Therefore inductive sensors can detect only static (non-changing) magnetic fields whereas the Hall Effect sensors can detect both changing and non-changing magnetic field.
- The Hall Effect sensor can give information regarding the type of magnetic pole used to generate the voltage and also the magnitude of the external magnetic flux density (B). Using a group of sensors, we can find the relative position of the external magnet used.
- The output voltage of the Hall Effect sensor is generally of a quite small magnitude, like a few micro-volts even when a strong external magnetic field is applied across the sensor. Hence, most commercially available Hall Effect sensors are constructed with a built-in DC amplifier and voltage regulators to improve the sensor’s sensitivity and magnitude of the output voltage.
Hall Effect Experiment
Analog and Digital Hall Effect Sensor
Hall Effect Sensor’s output can either be linear (analog) or digital. The output of the linear Hall Effect sensor is directly proportionate to the external magnetic field, i.e. magnetic flux density passing through the sensor and the output is taken from the output of the differential OP-AMP. Hall Effect Linear (analog) sensors have a continuous voltage output that changes as per the strength of the external magnetic field changes.
Formula of Hall effects Sensor:
The output of the linear Hall Effect sensor can be expressed as:
- VH is the Hall Voltage
- RH is the Hall Effect co-efficient
- I is the current flowing through the sensor (semiconductor slab)
- t is the thickness of the sensor
- B is the external magnetic flux density
In the case of the Hall Effect digital sensor’s output is taken from the output of the OPAMP, which in turn is connected with a Schmitt-trigger with built-in hysteresis which reduces oscillations in the output voltage. In this case, only when the external field strength is higher than a specific value in the device, the device switches to the ON condition from OFF condition.
Type of Hall Effect Sensors:
Depending on the type of the external magnetic pole required for them to operate, Hall Effect sensors are of two types.
The two of the most common sensing configurations in a Hall Effect sensor using a single magnet are Head-on Detection and Sideways Detection. In sideways detection, it is required to move the magnet in a sideways motion in front of the face of the Hall Effect element. While in head-on detection the magnet moves towards and away from the hall element in a way perpendicular to the plane of the element.
Applications of Hall Effect Sensors:
- Position sensor: When operating in the on/off mode, i.e. having a digital output, detecting the occurrence of magnetic materials is one of the important industrial applications of Hall Effect sensors.
- DC transformers: The Hall Effect sensor is used to measure the DC magnetic flux, and as a result, the DC current can be calculated.
- Keyboard switch: For some computer keyboards Hall Effect switches are being used. But due to its comparatively high cost, it is limited to the field of aerospace and military for their high reliability.
- Fuel level indicator: The Hall Effect sensor senses the position of a floating element using position sensing and employed as automotive fuel level indicator.
- Electric treadmill: Hall sensors are used here for speed sensors and also for emergency stop due to any accidental fall. The waistband of the user in the treadmill is attached to a pull cord which is in turn attached to a magnet. If accidentally the user falls, the magnet gets detached, and there is an interruption to the power supply which stops the machine.
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