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.
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
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