3046 Datasheet, PDF (6/8 Pages) Allegro MicroSystems – HALL EFFECT GEAR-TOOTH SENSORS ZERO SPEED

3046, 3056, AND 3058
HALL EFFECT
GEAR-TOOTH SENSORS
–ZERO SPEED
SYSTEM ISSUES
Optimal performance of a gear-tooth
sensing system strongly depends on four
factors: the IC magnetic parameters, the
magnet, the pole piece configuration, and
the target.
Sensor Specifications. Shown in
Figure 5 are graphs of the differential field as
a function of air gap. A 48-tooth, 2.5”
(63.5 mm) diameter, uniform wheel similar to
that used in ABS applications is used. The
samarium cobalt magnet is 0.32” diameter by
0.20” long (8.13 x 5.08 mm). The maximum
functioning air gap with this typical gear/
magnet combination can be determined
using the graphs and the specifications for
the sensor IC.
In this case, if an A3056EU/LU sensor
with a B_OP of +25 G and a B_RP of -25 G is
used, the maximum allowable air gap would
be 0.110” (2.79 mm). If the switch points
change +75 G with temperature (B_OP = + 100
G, B_RP = +50 G), the maximum air gap will be
approximately 0.077” (1.96 mm).
All system issues should be translated
back to such a profile to aid the prediction of
system performance.
Magnet Selection. These devices can
be used with a wide variety of commercially
available permanent magnets. The selection
of the magnet depends on the operational
and environmental requirements of the
sensing system. For systems that require
high accuracy and large working airgaps or
an extended temperature range, the usual
magnet material of choice is rare earth
samarium cobalt (SmCo). This magnet
material has a high energy product and can
operate over an extended temperature range.
For systems that require low-cost solutions
for an extended temperature range, Alnico-8
can be used. Due to its relatively low energy
product, smaller operational airgaps can be
expected. At this time, neodymium iron
boron (NeFeB) is not a proven high-tempera-
ture performer; at temperatures above
Figure 4
POSITIVE AND NEGATIVE SWITCH OPERATION
150 G
B –B
E1 E2
B _OP= +100 G
B _RP= +50 G
(a)
-150 G
V
OUT
V
OUT(SAT)
OUTPUT DUTY CYCLE = 65%
150 G
B –B
E1 E2
(b)
B_OP= -50 G
B _RP= -100 G
-150 G
V
OUT
V
OUT(SAT)
OUTPUT DUTY CYCLE = 33%
Dwg. WH-004
+150°C it may irreversibly lose magnetic strength. Of these three
magnet materials, Alnico-8 is the least expensive by volume and
SmCo is the most expensive.
Either cylindrical- or cube-shaped magnets can be used, as long
as the magnet pole face at least equals the facing surface(s) of the IC
package and the pole piece. Choose the length of the magnet to
obtain a high length-to-width ratio, up to 0.75:1 for rare earths, or 1.5:1
for Alnico-8. Any added magnet length may incrementally improve the
allowable maximum air gap.
Magnets, in general, have a non-uniform magnetic surface profile.
The flux across the face of a magnet can vary by as much as 5% of the
average field over a 0.10” (2.5 mm) region. If a Hall sensor is placed
directly on a magnet face, the non-uniformity can appear to shift the
operating parameters of the sensor. For example, if a device is placed
on a 3000 G magnet with ±2% face offsets, each of the operating
points might be shifted by ±60 G. When offsets are present, the
operating characteristics may be greatly altered.
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