3046 Datasheet, PDF (4/8 Pages) Allegro MicroSystems – HALL EFFECT GEAR-TOOTH SENSORS ZERO SPEED

3046, 3056, AND 3058
HALL EFFECT
GEAR-TOOTH SENSORS
–ZERO SPEED
Figure 1
TYPICAL GEAR-TOOTH SENSING
APPLICATION
BACK-BIASING
MAGNET
S
OPTIONAL POLE PIECE
SENSOR IC
A
N
S
TARGET
GEAR
APPLICATIONS INFORMATION
A gear-tooth sensing system consists of the sensor IC, a back-
biasing magnet, an optional pole piece, and a target (Figure 1). The
system requirements are usually specified in terms of the effective
working air gap between the package and the target (gear teeth), the
number of switching events per rotation of the target, temperature and
speed ranges, minimum pulse duration or duty cycle, and switch point
accuracy. Careful choice of the sensor IC, magnet material and
shape, target material and shape, and assembly techniques enables
large working air gaps and high switch-point accuracy over the system
operating temperature range.
Naming Conventions. With a south pole in front of the branded
surface of the sensor, a north pole behind the sensor, the field at the
sensor is defined as positive. As used here, negative flux densities are
defined as less than zero (algebraic convention), e.g., -100 G is less
than -50 G.
Magnet Biasing. In order to sense moving non-magnetized
ferrous targets, these devices must be back-biased by mounting the
unbranded side on a small permanent magnet. Either magnetic pole
(north or south) can be used.
The devices can also be used without a back-biasing magnet.
In this configuration, the sensor can be used to detect a rotating ring
magnet such as those found in brushless dc motors or in speed
sensing applications. Here, the sensor detects the magnetic field
gradient created by the magnetic poles.
Figure 2
TYPICAL TRANSFER CHARACTERISTIC
Dwg. AH-003
24 V
MAX
B _OP
0
-B
B _RP
V _OUT(SAT)
0
DIFFERENTIAL FLUX DENSITY, BE1 – BE2
+B
Dwg. GH-034
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 [phone]