True Zero-Speed, High Accuracy Gear Tooth Sensor IC

ATS667LSG

8

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Functional Description

Hall Technology

The ATS667 contains a single-chip differential Hall-effect sensor

IC, a samarium cobalt pellet, and a flat ferrous pole piece (con-

centrator). 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

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

compensation circuitry. The Hall transducers and signal process-

ing electronics are integrated on the same silicon substrate, using

a proprietary BiCMOS process.

Target Profiling During Operation

An operating device 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 transla-

tion of the mechanical profile, through the magnetic profile that

it induces, to the digital output signal of the ATS667. No addi-

tional optimization is needed and minimal processing circuitry is

required. This ease of use reduces design time and 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 bottom panel, labeled IC Output Signal, displays the

square waveform corresponding to the digital output signal that

results from a rotating gear configured as shown in figure 2, and

electrically connected as in figure 6. That direction of rotation (of

the gear side adjacent to the package face) 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 IC output switching from low state

to high state as the leading edge of a tooth (a rising mechanical

edge, as detected by the IC) passes the package face. In this con-

figuration, the device output 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 polarity inverts. That is, the out-

put signal goes high when a falling edge is detected, and a valley

is nearest to the package.

Target (Gear)

Back-biasing

Rare-earth Pellet

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 Package

Rotating Target

Branded Face

1

4

B

OP(#1)

B

RP(#1)

B

RP(#2)

B

OP(#2)

On

Off

Off

On

IC Internal Switch State

Package Orientation to Target

Mechanical Position (Target movement pin 1 to pin 4)

Target

(Gear)

(Package Top View)

Sensor Branded Face

Pin 1

Side

Pin 4

Side

Branded Face

Hall Element Pitch

Target Magnetic Profile

+B

This tooth

sensed earlier

This tooth

sensed later

Back-Biasing

Pellet

IC

Figure 1. Relative motion of the target is detected by the dual Hall

elements 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 state when a tooth of the target gear is nearest the

package face (see figure 3). A right-to-left (pin 4 to pin 1) rotation inverts

the output signal polarity.

Figure 3. The magnetic profile reflects the geometry of the target, allowing

the ATS667 to present an accurate digital output response.