3046 Datasheet, PDF (5/8 Pages) Allegro MicroSystems – HALL EFFECT GEAR-TOOTH SENSORS ZERO SPEED

3046, 3056, AND 3058
HALL EFFECT
GEAR-TOOTH SENSORS
–ZERO SPEED
Sensor Operation. The A3046EU/LU,
A3056EU/LU, and A3058EU/LU sensor ICs
each contain two integrated Hall transducers
(E1 and E2) that are used to sense a mag-
netic field differential across the face of the
IC (see SENSOR LOCATION drawing). Referring
to Figure 2, the trigger switches the output
ON (output LOW) when B_E1 – B_E2 > B_OP and
switches the output OFF (output HIGH) when
B_E1 – B_E2 < B_RP. The difference between B_OP
and B_RP is the hysteresis of the device.
Figure 3 relates the output state of a
back-biased sensor IC, with switching
characteristics shown in Figure 2, to the
target gear profile and position. Assume a
north pole back-bias configuration (equivalent
to south pole at the face of the device). The
motion of the gear produces a phase-shifted
field at E1 and E2 (Figure 3 (a)); internal
conditioning circuitry subtracts the field at the
two elements (Figure 3 (b)); and the Schmitt
trigger at the output of the conditioning
circuitry switches at the pre-determined
thresholds (B_OP and B_RP). As shown (Figure
3 (c)), the IC output is LOW whenever sensor
E1 sees a (ferrous) gear tooth and sensor E2
faces air. The output is HIGH when sensor
E1 sees air and sensor E2 sees the ferrous
target.
A gear-tooth sensor can be configured
(see ASSEMBLY TECHNIQUES) to operate as a
latch, a (positive) switch, or a negative
switch. Note the change in duty cycle in
each of the cases (Figure 4).
A latch is a device where the operate
point is greater than zero gauss and the
release point is less than zero gauss. With
the configuration shown in Figure 3, such a
device will switch ON on the leading edge
and OFF on the trailing edge of the target
tooth.
A (positive) switch is a device where
both the operate and release points are
greater than zero gauss (positive values).
Figure 3
GEAR-TOOTH SENSOR OPERATION
LEADING
EDGE
TRAILING
EDGE
GEAR
DIRECTION
OF ROTATION
E2
E1
NORTH
SOUTH
4300 G
B &B
E1 E2
(a)
4130 G
150 G
B _OP= +25 G
B –B
E1 E2
0G
B RP= –25 G
-150 G
V
OUT
V
OUT(SAT)
(b)
OUTPUT DUTY CYCLE ≈ 50%
(c)
Dwg. WH-003
In the configuration shown in Figure 3, such a device will switch ON
and then switch OFF on the leading or rising edge of the target tooth
(Figure 4 (a)).
A negative switch is a device where both the operate and release
points are less than zero gauss (negative values). In the configuration
shown in Figure 3, such a device will switch OFF and then switch ON
on the trailing or falling edge of the target tooth (Figure 4 (b)).
Speed sensors can use any of the three sensor configurations
described. Timing sensors, however, must use a latch to guarantee
dual-edge detection. Latches are most easily made using the
A3058EU or A3058LU device types.
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