True Zero Speed Miniature Differential Peak-

Detecting Gear Tooth Sensor

ATS645LSH

8

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

www.allegromicro.com

Sensing Technology

The gear tooth sensor subassembly contains a single-chip dif-

ferential Hall effect sensor IC, an optimized samarium cobalt

magnet, and a flat ferrous pole piece. The Hall IC supports two

Hall elements, which sense the magnetic profile of the ferrous

target simultaneously, but at different points (spaced at a 1.5 mm

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

An operating device is capable of providing digital information

that is representative of the mechanical features on a rotating tar-

get. The waveform diagram shown in figure 3 presents the auto-

matic translation of the mechanical profile, through the magnetic

profile that it induces, to the digital output signal of the sensor.

Output Polarity

Figure 3 shows the output polarity for the orientation of target

and sensor shown in figure 2. The target direction of rotation

shown is: perpendicular to the leads, across the face of the

device, from the pin 1 side to the pin 4 side. This results in the

the leading edge of a tooth (a rising mechanical edge, as detected

by the sensor) passes the sensor face. In this configuration, the

device output current switches to its low polarity when a tooth is

the target feature nearest to the sensor. If the direction of rota-

tion is reversed, then the output polarity inverts.

Note that output voltage polarity is dependent on the position of

Target (Gear)

Back-biasing Magnet

South Pole

North Pole

Case

(Pin 1 Side)

(Pin 4 Side)

Hall IC

Pole Piece

Element Pitch

(Concentrator)

Dual-Element

Hall Effect Device

Hall Element 1

Hall Element 2

of Sensor

Rotating Target

Branded Face

1

4

Functional Description

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 low output signal when a tooth of the target gear is nearest

the face of the sensor (see figure 3). A right-to-left (pin 4 to pin 1) rota-

tion inverts the output signal polarity.

Figure 4: Voltages profiles for high side and low side two-wire sensing.

Figure 3. Output Profile of a ferrous target for the polarity indicated in

figure 2.

ATS645

VCC

GND

V

CC

I

CC

1

4

ATS645

VCC

GND

V

CC

1

4

I

CC

R

SENSE

R

SENSE

V+

V+

I+

Representative

Differential

Magnetic Profile

Target

Mechanical Profile

Sensor Electrical