• general concept - guc sensor...f= q v b • the effect is based on the interaction 14 dr.-eng....

Hall Effect Sensor – Transducer

Outline

• General Concept• Theory of Operation.• Magnetic Force General over view.• Principle Summary.• Electrical Model• Sensor Types

1Dr.-Eng. Hisham El-SherifElectronics and Electrical Engineering Department

ELCT903, Sensor Technology

• Sensor Types • Analog• Digital

• Sensing Techniques• Applications

Hall Effect Transducer

-Used to measure position, displacement, level and flow.

-can be used as an analog motion device as well as digital device



2

General Concept

-If a semiconductor element subjected to a dc voltage, while a magnetic field is applied perpendicular to the direction of current flow.

-An output voltage will be generated



3

The Hall effect is a voltage induced in a semiconductor material as it passes through a magnetic field.

-If a thin plate of a conductive material (copper), with current I supplied by a battery E.

- If a voltmeter is applied along the sides of this plate, the measured voltage is zero

Theory of Operation

- When a magnetic field is applied perpendicular to the current flow into the plate.



to the current flow into the plate.

-A small voltage appears across the plate which can be measured by the voltmeter.

-If the direction of the magnetic field is reversed, the polarity of the voltage is also reversed.

This phenomenon discovered by Edwin hall in 1879 is called the hall effect

If a current-carrying conductor is placed in a magnetic field, a potential difference is generated in a direction perpendicular to both the current and the magnetic field.It arises from the deflection of charge carriers to one side of the conductoras a result of the magnetic force they experience.

An upward magnetic force



FB = q vd B

are deflected upward, and accumulate at the upper edge of the flat conductor,

This accumulation of charge at the edges establishes an electric field in the conductor

FE = qEH

The electric force increases until the electric force on carriers remaining in the bulk of the conductor balances the magnetic force acting on the carriers.



qvdB = qEH

EH = vd B

v

d

+++++

_____

�VH = EH d = vd B d

As shown the measured Hall voltage gives a value for the drift speed of the charge carriersif d and B are known.

Also we can express the drift speed as charge carrier density n.

nqAI

vd =



vEH

nqAvd =

where A is the cross-sectional area of the conductor.A = tdwhere t is the thickness of the conductor,

tIB

HnqtIB

nqtdIBd

nqAIBd

v cH ====∆t

IBHv cH =∆

Where is the hall coefficient nq1

Magnetic force General over view

-The current flowing in a conductor experienced a mechanical force when placed in a magnetic field.

- The magnetic fields case the charged partials to move in circular or helical paths.

- The force exerted on a charged partial due to electro magnetic field.

BvqEqF .+=



BvqEqF oo .+=F = resultant force

E = electric field.

v = velocity of charge.

B = magnetic field

qo = magnitude of the charge All these variables are vector quantities, they contain independent x, y and z

Response of a charge to an electric field

The response of a moving charge to a magnetic field

-In case of electric field, a charge will experience a force in the direction



-In case of electric field, a charge will experience a force in the direction of the field proportional to the magnitude of the charge and the strength of the field .

-Electric field is develop by the difference in potential at different points.

-This effect causes an electric current to flow.

- In case of magnetic field, a charged particle doesn't experience any force unless it is moving.

-When it moves, a force is experienced by a charged particle is a function of its charge, the direction in which it is moving and the orientation of the magnetic field it is moving through.

- In simple case, where the velocity is at right angle to the magnetic field, the force exerted is at right angle to both the velocity and the magnetic field.

-The cross product for each axis (x,y,z)

( )yzzyox BvBvqF −=

( )xzzzoy BvBvqF −=

( )xyyxoz BvBvqF −=



-In case the charge carries moving through a hall transducer.

-The charge carrier velocity is considered in one direction along the length of the device.

-Constraining the carrier velocity to the x axis (vy=0, vz=0, Bx=0, Bz=0).

-The charge to the z axis is

yxoz BvqF =



yzyz Bv

q

Bqv

qF

E −=−

==

Where E is the hall electric field

The relationship between the induced voltage, the current , and the magnetic field strength is a vector relation ship



When a sheet of semiconductor (gallium-arsenide,) or conducting material has a current passing through it, and placed in a magnetic field, a voltage is induced in the direction perpendicular to both current and magnetic field vector

-The induced voltage magnitude is also a function of the material type.

It is very small for conductors, but large enough for semiconductors such that this effect is widely used in sensor designs.

In as sensor application, the current ( i ) is fixed by the sensor power supply and the resistor (RS).

The hall effect sensor requires an external power supply for the current I and the magnetic field density (B ) for it to work.

The material of the sensor is also fixed.



The material of the sensor is also fixed.

Then the output voltage varies as function of magnetic field strength for a given hall sensor.

The measured variable then must be arranged such that it changes the magnetic field strength over the hall sensor.

Principle Summary -When a strip of conducting material carries current in the presence of a transverse magnetic field.-The hall effect results in the production of an electric field perpendicular to the directions of both the magnetic field and the current.-The magnitude is proportional to the product of the magnetic field strength, the current, and various properties of the conductor

The force is represented by

F= q v B • The effect is based on the interaction



F= q v B

Whereq =1.6�10−19 C an electronic charge, v = the speed of an electron, B = the magnetic field.

• The effect is based on the interaction between moving electric carriers and an external magnetic field. • In metals, these carriers are electrons. • When an electron moves through a magnetic field, a sideways force acts upon it:



• The current is passed through leads 1 and 2 of the element.• The output leads are connected to the element faces 3 and 4. • These output ends are at the same potential when there is no transverse magnetic field passing through the element.• When there is a magnetic flux passing through the element, a voltage V appears between output leads.• The voltage is proportional to the current and the field strength. • The output voltage is represented in terms of element thickness, flux density of the field, the current through the element

dIB

HVH =

H

I

= the Hall-constant depending on the semiconductor material. which can be defined as transverse electric potential/unit magnetic field /unit current density, V-m per A- Wb/m2

= current the element, A

Hall voltage VH



IBd

= current the element, A

= flux density of magnetic field, Wb/m2

= thickness of the element, m

the sensitivity of the transducer depends on the hall coefficient. the hall effect may by either –ve or +ve, depending on the material crystalline structure and is present in metals and semiconductor based on the material characteristic

Rotational Measurement

The basic operation principle of the hall effect, produces an output voltage

proportional to a small rotary displacement



-Assume that the electrons move inside a flat conductive strip which is placed in a magnetic field B

-The strip has two additional contacts at its left and right sides which are connected to a voltmeter.

-Two other contacts are placed at the upper and lower ends of the strip. These are connected to a source of electric current.

-Due to the magnetic field, the deflecting force shifts moving electrons toward the right side of the strip, which becomes more negative than the left side; that is, the magnetic field and the electric current produce the so-called transverse Hall



magnetic field and the electric current produce the so-called transverse Hall potential difference VH .

Where

� = the angle between the magnetic field vector and the Hall plate

H = the coefficient of overall sensitivity whose value depends on the plate material.

dHIBVH

αsin=

Hall effect sensor



A magnetic field deflects movement of electric charges.



The output signal of a Hall sensor depends on the angle between the magnetic field vector and the plate

Operational Example



-The hall sensor is suspended between the poles of a permanent magnet which is connected to the shaft.

-The probe is stationary, and the permanent magnet connected to the shaft rotates.

-With a constant control current applied to the electrical contacts at the end of the probe.

-The hall voltage generated across the probe is directly proportional to the sine of the angular displacement of the shaft

-Small rotations up to six degrees can be precisely measured with such probes.

-The main advantage of such devices is that they are non-contact with small size and good resolution



-The output voltage generated for a rotation of degrees

dHIBVH

αsin=

α

α = angle between the magnetic field and the hall plate



-Depending on the material crystalline structure, charges may be either electrons (negative) or holes (positive). As a result, the Hall effect may be either negative or positive.

-The cross [ X ] indicates the direction of the magnetic field from the viewer to the symbol plane.

The Electrical Model



• The four resistors describe the input and output resistance of the sensor.• in case the sensor with four way symmetry, all of the resistors are equal.•The voltage source model the sensor sensitivity, which is a linear function of the bias voltage and the applied magnetic field.

Types of Hall Effect Sensors

1- Analog output Sensor - linear Hall Effect

•Analog sensors provide an output voltage that is proportional to the magneticfield to which it is exposed.

• The sensed magnetic field can be either positive or negative. • As a result, the output of the amplifier will be driven either positive or negative, thus requiring both plus and minus power supplies.

• To avoid the requirement for two power supplies, a fixed offset or bias is



• To avoid the requirement for two power supplies, a fixed offset or bias is introduced into the differential amplifier.

•The bias value appears on the output when no magnetic field is present and is referred to as a null voltage.

• When a positive magnetic field is sensed, the output increases above the null voltage.

• Conversely, when a negative magnetic field is sensed, the output decreases below the null voltage, but remains positive.

• The output of the amplifier cannot exceed the limits imposed by the power supply.

• In fact, the amplifier will begin to saturate before the limits of the power supply are reached.

• The saturation takes place in the amplifier and not in the Hall element. Thus, large magnetic fields will not damage the Hall effect sensors, butrather drive them into saturation.

• An open emitter, open collector, or push-pull transistor is added to



• An open emitter, open collector, or push-pull transistor is added tothe output of the differential amplifier.

- These sensors are not quite linear with respect to magnetic field density and, therefore, the precision measurements require a calibration



2- Digital output sensors - Threshold Hall Effect

• This sensor has an output that is just one of two states: ON or OFF. •The basic analog output device illustrated above can be converted into a digital output sensor with the addition of a Schmitt trigger circuit.



• The Schmitt trigger compares the output of the differential amplifier with a preset reference.

• When the amplifier output exceeds the reference, the Schmitt trigger turns ON.

• Conversely, when the output of the amplifier falls



output of the amplifier falls below the reference point, the output of the Schmitt trigger turns OFF.

• Hysteresis is included in the Schmitt trigger circuit for jitter-free switching. Hysteresis results from two distinct reference values which depend on whether the sensor is being turned ON or OFF.

• The principal input/output characteristics are the operate point, release point and the difference between the two or differential.

• As the magnetic field is increased, no change in the sensor output will occur until the operate point is reached.

• Once the operate point is reached, the sensor will change state. Furtherincreases in magnetic input beyond the operate point will have no effect.

• If magnetic field is decreased to below the operate point, the output will remain



• If magnetic field is decreased to below the operate point, the output will remain the same until the release point is reached. At this point, the sensor’s output will return to its original state (OFF).

•The purpose of the differential between the operate and release point (hysteresis) is to eliminate false triggering which can be caused by minorvariations in input.



• An output transistor is added to increase application flexibility. Thisoutput transistor is typically NPN (current sinking).

Sensing Techniques

1- Head on Sensing

- The most common methods for activating the sensor in a binary application.

-The sensitive axis of the sensor and the axis of the magnetization are co-linear.

-The magnetic flux density that the sensor sees as a result of being approached is highly nonlinear with respect to magnet sensor air gap. It decreases rapidly as air gap increases.

-To have a binary switch operation as one approaches the magnet, the field



-To have a binary switch operation as one approaches the magnet, the field increases as inverse function of the distance.

- When the field exceeds the operate point (Bop) of the sensor it will activate.

-As one moves away from the magnet, the sensor will deactivate when the field drops below its release point (BRP).



Mechanical operate and release points can be determined from the superimposing the magnetic BOP and BRP on a flux map.

2- Slide By Sensing

-The magnetic axis of the magnetization and the sensitive axis of the hall sensor are both parallel.

- The magnet moves perpendicularly to the axis of magnetization.

- In the case of a head ON



37

sensing configuration, over travel could damage either the sensor or the magnet.

- The magnetic can travel past the sensor, then a separate mechanical stop must be provided to limit the magnetic movement.

X=0 as for sensing the field returning back the rear pole

3- Magnetic Null Point Sensing

• This technique used to detect the null-points in the field around a magnet, or places where the net field in a particular axis is zero.

•These techniques are useful where a high degree of switching accuracy over a small amount of total travel is needed.

•Using positive flux to denote the ON region and negative flux to denote the OFF region provides several advantages:

1. The position of the magnetic null points tend to be stable over temperature



temperature

2. Highly sensitive symmetric latch type Hall ICs can be used.

3. It is possible to create very sharp transitions between negative and positive fields with the proper magnetic , allowing for fine control.

4 - Rotary Position Sensing

• Hall sensors can be used to measure the as opposed to the linear position of an object .

• The hall sensor is placed at the center of rotation, it always exposed to the same field, but from a different direction.

• As the hall sensor is responsive to field components in a single axis, this results in response that is of the form of



in response that is of the form of

αsinkvo =

Applications of Hall Effect Transducers

Are widely used as proximity sensors, limit switch, liquid level and flow measurement

Also used for sensing deflections in biomedical implants.

They are constructed in various configurations depending on the application.

Position Sensing



-The sensor is used for sensing sliding motion.

-A tightly controlled gap is maintained between the magnet and the hall element.

-As the magnet moves back and forth at that fixed gap.

-The magnetic field induced by the element becomes -ve as it approaches the north pole, and positive as its approaches the south pole.

-The output characteristic of the sensor has a fairly large linear range



It is necessary to maintain rigidity in linear motion and prevent any orthogonal movements of the magnet when the sensor is used for measuring sliding motion

Liquid Level Measurement

Determining the height of a float is one method of measuring the level of a liquid in the tank.



- As the liquid level goes down, the magnet moves closer to the sensor.

Causing an increase in the output voltage

Position Sensor



-For robot positioning, as the wheel rotates, a pulse train is generated.

-Counting the number of pulses gives the angle of rotation.

Flow Measurement Using the Hall Effect



-The chamber has fluid in and fluid out.

-As the flow rat through the chamber increases, a spring loaded paddle turns a threaded shaft.

-As the shaft turns, it raises a magnetic assembly that energizes the transducer.

-When the flow rate decreases, the coil spring causes the assembly to go down, which reduces the transducer output.

-The calibration of the mechanize is according to a reference standards to provide a linear relation ship between the measured voltage and the flow rate





Hall Effect Flow sensor

Hall Effect Switches



-Hall switches have wide operating temperature ranges and are often used in automobile engine compartments.

-Another advantage is that they are not susceptible to most of the fouling mechanisms of optical or mechanical switches, such as liquids or dirt.

-While often the moving part that is detected is a magnet, it can also be arranged that a stationary “bias” magnet is intensified in its effect on the hall switch by the approach of a ferrous part, such as a gear tooth, thus allowing non magnetized objects to be detected



Interrupter Switching



-The Hall sensors can be used for interrupter switching with a moving object.

- The activating magnet and the Hall sensor are mounted on a single ruggedassembly with a small air gap between them.

-The sensor is held in the ON position by the activating magnet.

-If a ferromagnetic plate, or vane, is placed between the magnet and the Hall sensor, the vane forms a magnetic shunt that distorts the magnetic flux away from the sensor. This causes the sensor to flip to the OFF position.



Automobile Distributor.



-The Hall sensor and the magnet could be molded into a common housing, thus eliminating the alignment problem.

-The ferrous vanes which interrupt the magnetic flux

Three hall effect sensors used to sense commutation position in a brushless DC motor



-Hall effect sensors in position sensing of the rotor of a brushless DC motor.

-Brushless DC motors need current commutation, the shaping of the desired current in each phase as function of rotor position, for proper operation.

-In trapezoidal current commutated brushless DC motors, we need to know six ranges of rotor position in order to properly commutate the current.

-A set of three hall effect sensors each operating in ON/OFF mode, is used to provide rotor position information for current commutation.

-The sensor heads and permanent magnets are arranged such that at any given position, one or two of the sensors are ON.



Reed Sensor (switch)• A reed switch consists of a pair of contacts hermetically sealed which are activated by an external magnetic flux (typically • when a magnet approaches the sensor by about 5 mm). • The reed switches have a

Zones of ON and OFF state when permanent magnet moves close to the reed switch



• The reed switches have a substantial amount of hysteresis which makes them immune to small fluctuations in the magnetic field.• When a perpendicularly-oriented magnet moves close to a reed switch, several zones of ON and OFF states could occur

• general concept - guc sensor...f= q v b • the effect is based on the interaction 14 dr.-eng....

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