Saturday, March 28, 2009

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


Figure 4-20 Digital output sensor direction sensor

Source pdf



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:


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.


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 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

Hall Effect Sensor

Hall Sensors Device

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