Hall Effect Sensors - What Makes Them Tick

Hall Effect Sensors - What Makes Them Tick
By Rosa Telip Ten

Perhaps you have heard of magnetic speed sensors by now and are wondering just how they work? How in the heck can a magnet function to determine the speed of something? If it does, what on earth does the magnet focus on to work, because after all magnets respond to ferrous metals such as iron and steel.

Magnetic Speed Sensors

When someone is speaking about a magnetic speed sensor, what they really are referring to is a hall effect sensor. While they are commonly used in such systems as anti-lock braking systems in cars, they are now in common use in any number of high tech systems and machines that require the use of electronic transmission of speed or RPM data and information.

Say What!?

They get their name for the Hall effect which was discovered by a man by the name of Edwin Hall in 1879. In short, is refers to an electronic phenomena that is created on the opposite sides of an electronic conductor when an electronic current is flowing through it while a magnetic field is applied perpendicular to the current.

Still Need Ferrous Metal

While that is a mouthful to comprehend, in layman's terms it allows for mechanisms to be used to actually calculate the speed of something using electricity rather than a cable and gears. However; there has to be ferrous metal components of the system for the magnets in the sensors to focus on.

Counting Gear Teeth Real Fast

For instance, a gear tooth hall effect speed sensor, such as is in use in anti-lock braking systems uses a gear for the sensor to focus on and tracks the speed of the passing gear teeth to generate data that is sent to the main component that regulates the entire anti-lock braking system.

Article written by Rosa Telipten. Here you will get all the details you need on Hall Effect Sensors and you can also find the best info on Hydraulic Motor Speed Sensor.
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Gear Tooth Hall Effect Speed Sensor

Everybody Knows What a Gear Tooth Hall Effect Speed Sensor is - So Why Don't You?
By Rosa Telip Ten

So what on earth is a gear tooth hall effect speed sensor anyway and do you really need to know? The fact is that if you drive a car with anti-lock brakes then you make use of them each and every time you drive your car. This is because they are the very latest technology in electronic speed sensors that are in use in automobiles today.

Magnetic Speed Sensors

Gear tooth hall effect speed sensors is a big mouthful of words to say, so these types of speed sensors are commonly referred to as magnetic speed sensors. They do in fact rely on the use of magnets in their function but it is the effect that the magnets have on electricity in sensor that powers their function.

Anti-Lock Braking Systems

Without electronic anti-lock braking sensors anti-lock braking systems would not be possible. It is the speed and frequency of the actual processes that transpires when anti lock braking systems are activated that requires the use of a high tech electronic sensor of this type.

The Focus on the Teeth in a Gear

Gear tooth hall effect sensors focus on and gage the number of gear teeth that pass by them to function. They use magnets to accomplish this and the information is passed on to a small central computer in the braking system that regulates the brakes in an anti lock braking system.

A Mouthful of Words

This process takes place up to twelve times every second and without this level of speed and accuracy anti-lock braking systems simply couldn't function correctly. So, the next time that you hear magnetic sensors being mentioned, you will now know that what the person is really talking about is gear tooth hall effect speed sensors.

Article by Rosa Telipten. Find the latest details on Gear Tooth Hall Effect Speed Sensor plus the best on [http://motionsensors.com/magnetic_speed_sensors.html]Magnetic Speed Sensors.

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Digital Hall Effect Sensor Connection

The output of a digital Hall effect sensor is NPN (current sinking, open
collector), as shown in Figure 4-1. The illustration shows the outputs
in the actuated (ON) state.

Current sinking derives its name from the fact that it “sinks current from a
load.” The current flows from the load into the sensor. Current sinking
devices contain NPN integrated circuit chips. The physics of chip
architecture and doping are beyond the scope of this book.
Like a mechanical switch, the digital sensor allows current to flow when
turned ON, and blocks current flow when turned OFF. Unlike an ideal
switch, a solid state sensor has a voltage drop when turned ON, and a
small current (leakage) when turned OFF. The sensor will only
switch low level DC voltage (30 VDC max.) at currents of
20 mA or less. In some applications, an output interface may
be current sinking output, NPN.
Figure 4-2 represents an NPN (current sinking) sensor. In
this circuit configuration, the load is generally connected
between the supply voltage and the output terminal
(collector) of the sensor. When the sensor is actuated, turned
ON by a magnetic field, current flows through the load into
the output transistor to ground. The sensor’s supply voltage
(VS) need not be the same value as the load supply (VLS);
however, it is usually convenient to use a single supply. The
sensor’s output voltage is measured between the output terminal
(collector) and ground (-). When the sensor is not
actuated, current will not flow through the output transistor
(except for the small leakage current). The output voltage, in this
condition, will be equal to VLS (neglecting the leakage
current). When the sensor is actuated, the output voltage will drop
to ground potential if the saturation voltage of the output
transistor is neglected. In terms of the output voltage, an NPN
sensor in the OFF condition is considered to be normally
high.

Source pdf
http://www.honeywell-sensor.com.cn/prodinfo/

magnetic_position/technical/chapter4.pdf


ELECTRICAL INTERFACE FOR DIGITAL HALL DEVICES
The output stage of a digital Hall switch is simply an open-collector
npn transistor. The rules for use are the same as those for any similar
switching transistor.
When the transistor is OFF, there is a small output leakage current
(typically a few nanoamperes) that usually can be ignored, and a
maximum (breakdown) output voltage (usually 24 V), which must not
be exceeded.
When the transistor is ON, the output is shorted to the circuit
common. The current flowing through the switch must be externally
limited to less than a maximum value (usually 20 mA) to prevent
damage. The voltage drop across the switch (VCE(sat)) will increase for
higher values of output current. You must make certain this voltage is
compatible with the OFF, or “logic zero,” voltage of the circuit you wish
to control.
Hall devices switch very rapidly, with typical rise and fall times in
the 400 ns range. This is rarely significant, because switching times
are almost universally controlled by much slower mechanical parts.

COMMON INTERFACE CIRCUITS

Figure 17 illustrates a simplified schematic symbol for Hall digital
switches. It will make further explanation easier to follow.
Interface for digital logic integrated circuits usually requires only an
appropriate power supply and pull-up resistor.
With current-sinking logic families, such as DTL or the popular
7400 TTL series (figure 18A), the Hall switch has only to sink one unitload
of current to the circuit common when it turns ON (1.6 mA maximum
for TTL). In the case of CMOS gates (figure 18B), with the
exception of switching transients, the only current that flows is through
the pull-up resistor (about 0.2 mA in this case).
Loads that require sinking currents up to 20 mA can be drivendirectly

by the Hall switch

A good example is a light-emitting diode (LED) indicator that
requires only a resistor to limit current to an appropriate value. If the
LED drops 1.4 V at a current of 20 mA, the resistor required for use
with a 12 V power supply can be calculated as:

The nearest standard value is 560 , resulting in the circuit of
figure 19.

Source pdf
http://www.allegromicro.com/en/Products/Design/an/an27701.pdf

HALL-EFFECT SWITCHES

Hall Effect Sensor Direction Sensor

Two digital output Hall effect devices may be used in combination

to determine the direction of rotation of a ring magnet, as shown

in Figure 4-20. The sensors are located close together along the

circumference of the ring magnet. If the magnet is rotating in the

direction shown (counter-clockwise) the time for the south pole of

the magnet to pass from sensor T2 to T1 will be shorter than the

time to complete one revolution. If the ring magnet’s direction is

reversed, the time it takes the south pole to pass from T2 to T1 will

be almost as long as the time for an entire revolution. By comparing

the time between actuations of sensors T2 and T1 with the time for

an entire revolution (successive actuations of T2), the direction can

be determined. A method by which these two times can be compared

is also shown in Figure 4- 20. An oscillator is used to generate

timing pulses. The counter adds these pulses (counts up) starting

when sensor T2 is actuated and stopping when sensor T1 is actuated.

The counter then subtracts pulses (counts down) for the remainder

of the revolution. The shorter time interval between T2 and T1

actuation will result in fewer pulses being added than subtracted,

thus actuating the counter’s BR (borrow) output. When the time

between T2 and T1 is longer, more pulses are added than subtracted

and the BR output is not actuated. For the configuration shown, there

will be no output for clockwise motion and a pulse output for each

revolution for counterclockwise motion. In addition to the interface

design concepts covered in this section, there are many other possible

ways to utilize the output of digital Hall effect sensors. For example,

the output could be coupled to a tone encoder in speed detection

applications or a one-shot in current sensing applications. To a large

extent, the interface used is dependent on the application and the

number of possible interface circuits is as large as the number of

applications.

Figure 4-20 Digital output sensor direction sensor

Source pdf

http://www.honeywell-sensor.com.cn/prodinfo/magnetic_position/

technical/chapter4.pdf

ROTARY ACTIVATORS FOR HALL SWITCHES

A frequent application involves the use of Hall switches to generate

a digital output proportional to velocity, displacement, or position of a

rotating shaft. The activating magnetic field for rotary applications can

be supplied in either of two ways:

MAGNETIC ROTOR ASSEMBLY

The activating magnet(s) are fixed on the shaft and the stationary

Hall switch is activated with each pass of a magnetic south pole

(figure 22A). If several activations per revolution are required, rotors

can sometimes be made inexpensively by molding or cutting plastic or

rubber magnetic material. Ring magnets can also be used. Ring

magnets are commercially available disc-shaped magnets with poles

spaced around the circumference. They will operate Hall switches

dependably and at reasonable costs.

Ring magnets do have limitations:

The accuracy of pole placement (usually within 2 or 3 degrees).

Uniformity of pole strength ( 5%, or worse).

These limitations must be considered in applications requiring

precision switching.

FERROUS VANE ROTOR ASSEMBLY

Both the Hall switch and the magnet are stationary (figure 22B); the

rotor interrupts and shunts the flux with the passing of each ferrous

vane.

Vane switches tend to be a little more expensive than ring magnets,

but because the dimensions and configuration of the ferrous vanes can

be carefully controlled, they are often used in applications requiring

precise switching or duty cycle control.

Properly designed vane switches can have very steep flux density

curves, yielding precise and stable switching action over a wide

temperature range.

Source pdf

http://www.allegromicro.com/en/Products/Design/an/an27701.pdf

Hall Effect Sensor

Hall Sensors Device

Hall effect sensor Transfer function

The transfer function for a digital output Hall effect sensor incorporating
hysteresis is shown in Figure 2-11.

Figure 2-11 Transfer function hysteresis Digital output sensor


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. Further
increases 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 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 minor
variations in input.

As with analog output Hall effect sensors, an output transistor
is added to increase application flexibility. This
output transistor is typically NPN (current sinking). See
Figure 2-12. The features and benefits are examined in detail
in Chapter 4.

Figure 2-12 NPN (Current sinking) . . . Digital output sensor

The fundamental characteristics relating to digital output
sensors have been presented. The specifications and the
effect these specifications have on product selection follows.

Source ( pdf )
Honeywel

http://content.honeywell.com/sensing/prodinfo/solidstate/technical/hallbook.pdf

Hall Sensors Device - High Linearity

Bulk Indium BH-900 Series
High Linearity

Bell Technologies Inc.,

Description
F.W. Bell 900 Series Hall Sensors are high-performance units
providing high linearity and broad field and temperatures ranges
for a wide variety of magnetic field measurements. All units in the
series are encapsulated in rugged, epoxy, sealed cases.

A room temperature linearity error curve from -30 to +30 kG is
supplied, indicating optimum operating conditions for each device.

The models 900 and 921 are not calibrated above 30 kG.

Models
a. BH-910 High Linearity
b. BH-921 Cryogenic Operation (1.5 to 350° k)
c. BH 921 & 900 Wide Dynamic Range

Hall Sensors Device - Three Axis

Bulk Indium Arsenide BH-703
Three Axis

Bell Technologies Inc.,

Description
The BH-703 multi-axis Hall sensor consists of three individual
Hall elements oriented in mutually perpendicular planes and
encapsulated in a small epoxy package. This enables the BH-703
to produce voltages proportional to the three orthogonal
components (Bx, By, Bz) of a magnetic flux in any direction.
Thus the BH-703 may be permanently mounted or arbitrarily
oriented to sense fields in any direction.

The magnitude of the flux vector, B, can be found using the following
relation:


Features
• Three Axis, simultaneous measurement
• Instrumentation Quality

Hall Sensors Device - Single Axis

Bulk Indium Arsenide BH-700 Series

Single Axis

Bell Technologies Inc.,

Description
Designed to meet the requirements of a wide range of magnetic
field measurement applications, the BH-700 Series are small,
solid-state devices that provide an output voltage proportional to
the product of control current and ambient flux density. Five
single-axis models are available to measure axial and transverse
magnetic field components with sensitivities from 7.5 to 50
mV/kG and input and output resistance of several ohms.

Electrical Specifications

BH-702
a. Air gap: between concentrator and substrate, 0.0025" nominal
and 0.003" maximum.

b. Sensitivity: Basic sensitivity of Hall element .15 V/A-kG min.
With the unit suspended in a free field of 100 oersteds and
Ic=200 mA, the open circuit Hall voltage is 8.0 mV min. In a
closed magnetic circuit with Ic=200 mA, VH is 3.mV/Ampere
turn min.

c. Polarity: With the magnetic field vector as shown and Ic entering
the red lead, the positive Hall voltage will appear at the blue lead.

BH-701
a. Linearity: VH vs. B, –10 to +10 kG: ±0.25% of reading, max.

BH-704
VH vs. B, –30 to +30 kG: ±1.0% of reading, max.
VH vs. Ic, 0 to 100 mA: ±0.1% of reading, max.
VH vs. Ic, 0 to 300 mA: ±1.0% of reading, max.

b. Encapsulation:
The BH-701 and the BH-704 are encapsulated in a rugged aluminum
oxide ceramic and epoxy case for excellent heat transfer and strength.

Models
1. BH-700 Low cost, Transverse, General Purpose
2. BH-701 Rugged, High-Linearity, Transverse, Instrumentation Quality
3. BH-702 Low Field (ferrite-embedded), Transverse
4. BH-704 Rugged, High Linearity, Axial, Instrumentation Quality
5. BH-705 General Purpose, Transverse

Basic Hall effect sensors

The Hall element is the basic magnetic field sensor.
It requires signal conditioning to make the output
usable for most applications. The signal conditioning
electronics needed are an amplifier stage and temperature
compensation. Voltage regulation is needed
when operating from an unregulated supply. Figure
2-4 illustrates a basic Hall effect sensor.
If the Hall voltage is measured when no magnetic
field is present, the output is zero (see Figure 2-1).
However, if voltage at each output terminal is measured
with respect to ground, a non-zero voltage will
appear. This is the common mode voltage (CMV),
and is the same at each output terminal. It is the potential
difference that is zero. The amplifier shown in
Figure 2-4 must be a differential amplifier so as to
amplify only the potential difference – the Hall voltage.
The Hall voltage is a low-level signal on the order of
30 microvolts in the presence of a one gauss magnetic
field. This low-level output requires an amplifier with
low noise, high input impedance and moderate gain.

A differential amplifier with these characteristics can be readily
integrated with the Hall element using standard bipolar transistor
technology. Temperature compensation is also easily integrated.

As was shown by equation 2-1, the Hall voltage is a function of the
input current. The purpose of the regulator in Figure 2-4 is to hold
this current constant so that the output of the sensor only reflects
the intensity of the magnetic field. As many systems have a
regulated supply available, some Hall effect sensors may not
include an internal regulator.

Source ( pdf )
http://content.honeywell.com/sensing/prodinfo/solidstate/

technical/hallbook.pdf