3059 Datasheet, PDF (6/8 Pages) Allegro MicroSystems – HALL-EFFECT GEAR-TOOTH SENSORS -AC COUPLED

3059 AND 3060
HALL-EFFECT
GEAR-TOOTH SENSORS
—AC COUPLED
Low-Frequency Operation. Low-frequency opera-
tion of the sensor is set by the value of an external
capacitor. Figure 3 provides the low-frequency cut-off (-3
dB point) of the filter as a function of capacitance value.
This information should be used with care. The graph
assumes a perfect sinusoidal magnetic signal input.
In reality, when used with gear teeth, the teeth create
transitions in the magnetic field that have a much higher
frequency content than the basic rotational speed of the
target. This allows the device to sense speeds much
lower than those indicated by the graph for a given
capacitor value.
Figure 3
1k
100
10
1.0
0.1
0.01
0.1
1.0
CAPACITANCE IN µF
10
Dwg. GH-025
Capacitor Characteristics. The major requirement
for the external capacitor is its ability to operate in a
bipolar (non-polarized) mode. Another important require-
ment is the low leakage current of the capacitor (equiva-
lent parallel resistance should be greater than 500kΩ). To
maintain proper operation with frequency, capacitor
values should be held to within ±30% over the operating
temperature range. Available non polarized capacitors
include ceramic, polyester, and some tantalum types. For
low-cost operation, ceramic capacitors with temperature
codes Z5S, Y5S, X5S, or X7S (depending on operating
temperature range) or better are recommended. The
commonly available Z5U temperature code should not be
used in this application.
Magnet Selection. The UGx3059KA or UGx3060KA
can be used with a wide variety of commercially available
permanent magnets. The selection of the magnet de-
pends on the operational and environmental requirements
of the sensing system. For systems that require high
accuracy and large working air gaps or
an extended temperature range, the usual magnet mate-
rial 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 sys-
tems that require low-cost solutions for an extended
temperature range, AlNiCo 8 can be used. Due to its
relatively low energy product, smaller operational air gaps
can be expected. Neodymium iron boron (NeFeB) can be
used over moderate temperature ranges when large
working air gaps are required. Of these three magnet
materials, AlNiCo 8 is the least expensive by volume and
SmCo is the most expensive.
System Issues. Optimal performance of a gear-tooth
sensing system strongly depends on four factors: the IC
magnetic parameters, the magnet, the pole piece configu-
ration, and the target.
Sensor Specifications. Shown in Figure 4 are
graphs of the differential field as a function of air gap.
A 48-tooth, 2.5” (63.5 mm) diameter, uniform target 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 specifications for the sensor IC.
In this case, if a UGx3060KA sensor with a typical B_OP
of 15 G and a B_RP of -15 G is used, the maximum allow-
able air gap would be approximately 0.120”. If the worst
case switch points of ±35 G for the UGx3060KA are used,
the maximum air gap is approximately 0.105”.
All system issues should be translated back to such a
profile to aid the prediction of system performance.
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