Self-Calibrating, Zero-Speed Differential
Gear Tooth Sensor IC with Continuous Update
ATS643LSH
9
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Hall Technology. The ATS643 contains a single-chip differ-
ential Hall effect sensor IC, a samarium cobalt pellet, and a flat
ferrous pole piece (concentrator). As shown in figure 1, the Hall
IC supports two Hall elements, which sense the magnetic profile
of the ferrous gear target simultaneously, but at different points
(spaced at a 2.2 mm pitch), generating a differential internal
analog voltage (V
PROC
) that is processed for precise switching of
the digital output signal.
The Hall IC is self-calibrating and also possesses a tempera-
ture compensated amplifier and offset cancellation circuitry. Its
voltage regulator provides supply noise rejection throughout the
operating voltage range. Changes in temperature do not greatly
affect this device due to the stable amplifier design and the offset
rejection circuitry. The Hall transducers and signal processing
electronics are integrated on the same silicon substrate, using a
proprietary BiCMOS process.
Target Profiling During Operation. When proper power is
applied to the IC, it is capable of providing digital information
that is representative of the mechanical features of a rotating
gear. The waveform diagram in figure 3 presents the automatic
translation of the mechanical profile, through the magnetic
profile that it induces, to the digital output signal of the ATS643.
No additional optimization is needed and minimal processing
circuitry is required. This ease of use reduces design time and
Functional Description
Target (Gear)
Back-biasing
Rare-earth Pellet
South Pole
North Pole
Case
(Pin 1 Side)(Pin n >1 Side)
Hall IC
Pole Piece
Element Pitch
(Concentrator)
Dual-Element
Hall Effect Device
Hall Element 1
Hall Element 2
of Package
Rotating Target
Branded Face
1
4
incremental assembly costs for most applications.
Determining Output Signal Polarity. In figure 3, the top
panel, labeled Mechanical Position, represents the mechanical
features of the target gear and orientation to the device. The bot-
tom panel, labeled IC Output Signal, displays the square wave-
form corresponding to the digital output signal that results from
a rotating gear configured as shown in figure 2. That direction of
rotation (of the gear side adjacent to the package face) is: perpen-
dicular to the leads, across the face of the device, from the pin 1
side to the pin 4 side. This results in the IC output switching from
low, I
CC(Low)
, to high, I
CC(High)
, as the leading edge of a tooth (a
rising mechanical edge, as detected by the IC) passes the package
face. In this configuration, the device output current switches to
its high polarity when a tooth is the target feature nearest to the
package. If the direction of rotation is reversed, so that the gear
rotates from the pin 4 side to the pin 1 side, then the output polar-
ity inverts. That is, the output signal goes high when a falling
edge is detected, and a valley is the nearest to the package. Note,
however, that the polarity of I
OUT
depends on the position of the
sense resistor, R
SENSE
(see Operating Characteristics table).
Continuous Update of Switchpoints. Switchpoints are the
threshold levels of the differential internal analog signal, V
PROC
,
at which the device changes output signal polarity. The value of
V
PROC
is directly proportional to the magnetic flux density, B,
Figure 1. Relative motion of the target is detected by the dual Hall ele-
ments mounted on the Hall IC.
Figure 2. This left-to-right (pin 1 to pin 4) direction of target rotation
results in a high output signal when a tooth of the target gear is nearest
the face of the package (see figure 3). A right-to-left (pin 4 to pin 1) rota-
tion inverts the output signal polarity.
Figure 3. The magnetic profile reflects the geometry of the target, allow-
ing the ATS643 to present an accurate digital output response.
B
OP(#1)
B
RP(#1)
B
OP(#2)
On OffOff On
IC Internal Switch State
Package Orientation to Target
IC Internal Differential Analog Signal, V
PROC
Mechanical Position (Target movement pin 1 to pin 4)
IC Output Signal, I
OUT
Target
(Gear)
Package
Package Branded Face
Pin 1
Side
Pin 4
Side
+t
Target Magnetic Profile
+B
+t
+t
This tooth
sensed
earlier
This tooth
sensed
later
