HALL EFFECT

GEAR-TOOTH SENSORS

–ZERO SPEED

115 Northeast Cutoff, Box 15036

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

24 V

MAX

0+B

0

Dwg. GH-034

-B

OP

RP

B

B

APPLICATIONS INFORMATION

A gear-tooth sensing system consists of the sensor IC, a back-

biasing magnet, an optional pole piece, and a target (Figure 1). The

system requirements are usually specified in terms of the effective

working air gap between the package and the target (gear teeth), the

number of switching events per rotation of the target, temperature and

speed ranges, minimum pulse duration or duty cycle, and switch point

accuracy. Careful choice of the sensor IC, magnet material and

shape, target material and shape, and assembly techniques enables

large working air gaps and high switch-point accuracy over the system

operating temperature range.

Naming Conventions. With a south pole in front of the branded

surface of the sensor, a north pole behind the sensor, the field at the

sensor is defined as positive. As used here, negative flux densities are

defined as less than zero (algebraic convention), e.g., -100 G is less

than -50 G.

Magnet Biasing. In order to sense moving non-magnetized

ferrous targets, these devices must be back-biased by mounting the

unbranded side on a small permanent magnet. Either magnetic pole

(north or south) can be used.

The devices can also be used without a back-biasing magnet.

In this configuration, the sensor can be used to detect a rotating ring

magnet such as those found in brushless dc motors or in speed

sensing applications. Here, the sensor detects the magnetic field

gradient created by the magnetic poles.

Figure 2

TYPICAL TRANSFER CHARACTERISTIC

Figure 1

TYPICAL GEAR-TOOTH SENSING

APPLICATION

BACK-BIASING

MAGNET

OPTIONAL POLE PIECE

SENSOR IC

NS

TARGET

GEAR

A

Dwg. AH-003

S