ATS612LSB

DYNAMIC, SELF-CALIBRATING, PEAK-DETECTING,

DIFFERENTIAL HALL-EFFECT GEAR-TOOTH SENSOR

9

www.allegromicro.com

DEVICE DESCRIPTION — Continued

•

Tilted or off-center installation. Traditional differen-

tial sensors will switch incorrectly due to baseline

changes versus air gap caused by tilted or off center

installation. The peak detector circuitry references the

switch point from the peak and is immune to this failure

mode. There may be a timing accuracy shift caused by

this condition.

•

Large operating air gaps. Operating air gaps greater

than 2.5 mm are easily achievable (dependent on target

dimensions, material, and speed) with this device due

to the sensitive switch points after start up.

•

Immunity to magnetic overshoot. The air gap-

independent hysteresis minimizes the impact of over-

shoot on the switching of device output.

•

Response to surface defects in the target. The gain-

adjust circuitry reduces the effect of minor gear anoma-

lies that would normally cause false switching.

•

Immunity to vibration and backlash. The gain-adjust

circuitry keeps the hysteresis of the device roughly

proportional to the peak to peak signal. This allows the

device to have good immunity to vibration even when

operating at close air gaps.

•

Immunity to gear run out. The differential sensor

configuration eliminates the base line variations caused

by gear run out.

Differential vs. Single-Element Sensing. The differential

Hall-element configuration is superior in most applications

to the classical single-element gear-tooth sensor. As

shown in the flux maps on this page, the

single-element configuration commonly used (Hall-effect

sensor mounted on the face of a simple permanent

magnet) requires the detection of a small signal (often

<100G) that is superimposed on a large back-biased field,

often 1500G to 3500G. For most gear/target configura-

tions, the back-biased field values change due to concen-

tration effects, resulting in a varying baseline with air gap,

with valley widths, with eccentricities, and with vibration.

The differential configuration cancels the effects of the

back-biased field and avoids many of the issues presented

by the single Hall element.

NOTE — 10 G = 1 mT, exactly.

Single-element flux maps

showing the impact of varying valley widths

Differential flux maps vs. air gaps

10

20

30

60

-2000

-5000

ANGLE OF TARGET ROTATION IN DEGREES

-2500

-4000

-4500

0

Dwg. GH-061-1

-3500

-3000

T

= 25

°C

A

50

40

TARGET

AIR GAP = 0.5 mm

AIR GAP = 1.0 mm

AIR GAP = 1.5 mm

AIR GAP = 2.5 mm

AIR GAP = 2.0 mm

10

20

30

60

1500

-1500

ANGLE OF TARGET ROTATION IN DEGREES

1000

-500

-1000

0

Dwg. GH-061

0

500

T

= 25

°C

A

50

40

TARGET

AIR GAP = 2.5 mm

AIR GAP = 2.0 mm

AIR GAP = 1.5 mm

AIR GAP = 0.5 mm

AIR GAP = 1.0 mm