ATS612LSG

DYNAMIC, SELF-CALIBRATING, PEAK-DETECTING,

DIFFERENTIAL HALL-EFFECT GEAR TOOTH SENSOR

Page 7 of 15

115 Northeast Cutoff, Box 15036

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

Copyright © 2001, 2003 Allegro MicroSystems, Inc.

DEVICE DESCRIPTION

Assembly Description.

The ATS612 gear-tooth sensor

is a Hall IC/magnet configuration that is fully optimized to

provide digital detection of gear tooth edges. This

sensor is packaged in a molded miniature plastic body

that has been optimized for size, ease of assembly, and

manufacturability. High operating temperature materials

are used in all aspects of construction.

The use of this sensor is simple. After proper power is

applied to the component the sensor is then capable of

instantly providing digital information that is

representative of the profile of a rotating gear. No

additional optimization or processing circuitry is required.

This ease of use should reduce design time and

incremental assembly costs for most applications.

Sensing Technology.

The gear tooth sensor module

contains a single-chip differential Hall effect sensor IC, a

Samarium Cobalt magnet, and a flat ferrous pole piece

(Figure 2). The Hall IC consists of 2 Hall elements

(spaced 2.2 mm apart) located so as to measure the

magnetic gradient created by the passing of a ferrous

object. The two elements measure the magnetic

gradient and convert it to an analog voltage that is then

processed in order to provide a digital output signal.

Magnetic Circuit

Figure 2

Internal Electronics.

The processing circuit uses a

patented peak detection scheme to eliminate magnet

and system offsets. This technique allows dynamic

coupling and filtering of offsets without the power-up and

settling time disadvantages of classical high-pass

filtering schemes. The peak signal of every tooth and

valley is detected by the filter and is used to provide an

instant reference for the operate and release point

comparator. In this manner, the thresholds are adapted

and referenced to individual signal peaks and valleys,

providing immunity to zero line variation from installation

inaccuracies (tilt, rotation, and off center placement), as

well as for variations caused by target and shaft

eccentricities. The peak detection concept also allows

extremely low speed operation for small value filter

capacitors.

The ATS612 also includes self-calibration circuitry that is

engaged at power on. The signal amplitude is measured

and the device gain is normalized. In this manner,

switch-point drift versus air gap is minimized and

excellent timing accuracy can be achieved. The AGC

(Automatic Gain Control) circuitry, in conjunction with a

unique hysteresis circuit, also eliminates the effect of

gear edge overshoot as well as increases the immunity

to false switching caused by gear tooth anomalies at

close air gap. The AGC circuit sets the gain of the

device after power on. Up to 0.25 mm air gap change

can occur after calibration is complete without significant

performance impact.

Superior Performance.

The ATS612 peak-detecting

differential gear-tooth sensor module has several

advantages over conventional Hall-effect gear-tooth

sensors. The signal-processing techniques used in the

ATS612 solve the catastrophic issues that affect the

functionality of conventional digital gear-tooth sensors.

•

Temperature drift.

Changes in temperature do not

greatly affect this device due to the stable amplifier

design and the offset rejection circuitry.

•

Timing accuracy variation due to air gap.

The

accuracy variation caused by air gap changes is

minimized by the self-calibration circuitry. A 2x-to-3x

improvement can be seen.