8

30Dec03, Rev. 1.42

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

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Allegro MicroSystems, Inc.

Self-Calibrating Gear Tooth Sensor with 9-Bit Signal Capture

ATS672LSB

Target

Sensor

Application Information

Target Design

The Hall-effect sensor can detect variations in magnetic flux

density generated by features of a target. To distinguish between

two features of a target, such as between a tooth and a valley of a

spur gear, there must be a minimum differential of 120 G in the

magnetic flux densities corresponding to the features, as measured

at the sensor.

The target must be mounted with an air gap in the range AG, the

distance between the target object and the plane of the Hall-

effect sensor as installed. In general, the nearer a target feature

is to the active area of the sensor, the greater the magnetic flux

density at the sensor.

The following figures and table specify a design that can be used

to construct a reference target. The target represents a ferrous

spur gear, with uniform tooth and valley widths. The target

would be mounted so that its axis of rotation is parallel to the

plane of the Hall-effect sensor element, and centered on the ele-

ment. When the target is produced to the specifications listed in

the Reference Target column of the table, the required differen-

tial (tooth peak to valley) in magnetic flux densities is generated.

Also in the table, the Minimum Required for TPOS column

provides specifications for a similar application. These values are

the minimum required for the TPOS function to operate accu-

rately, as defined in the Operating Characteristics table.

Reference Target Characteristics

Characteristic

Symbol

Description

Reference

Target 8X

Only

Minimum

Required for

TPOS

Units

Material

Target has uniform composition

CRS1018

–

–

Diameter

Diameter of target, to valley

120

–

mm

Tooth Thickness

F

Breadth of tooth, with respect to sensor

6

5

mm

Tooth Height

5

mm

Tooth Width

T

Width of tooth

22.5*

5

deg.

Valley Width

22.5*

13

deg.

8X Reference Target

For additional general application information, visit the Allegro

MicroSystems Web site at www. allegromicro.com.