DYNAMIC, PEAK-DETECTING,

DIFFERENTIAL HALL-EFFECT

GEAR-TOOTH SENSORS

11

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APPLICATIONS INFORMATION

Gear Diameter and Pitch. Signal frequency is a direct

function of gear pitch and rotational speed (RPM). The

width of the magnetic signal in degrees and, hence, the

signal slope created by the tooth is directly proportional to

the circumference of the gear (

πDo). Smaller diameters

limit the low-speed operation due to the slower rate of

change of the magnetic signal per degree of gear rotation

(here the limitation is the droop of the capacitor versus the

signal change). Larger diameters limit high-speed opera-

tion due to the higher rate of change of magnetic signal

per degree of rotation (here the limitation is the maximum

charge rate of the capacitor versus the rate of signal

change). These devices are optimized for a 50 mm gear

diameter (signal not limited by tooth width), 0.33

µF

capacitor, and speeds of 10 RPM to 8000 RPM. For very

large diameter gears (diameter >200 mm), the devices

must be configured with a lower value capacitor, but not

less than 0.1

µF. This allows for a range of 5:1 in gear

diameters.

NOTE — In application, the terms “gear” and “target” are

often interchanged. However, “gear” is preferred when

motion is transferred.

Air Gap and Tooth Geometry. Operating specifications

are impacted by tooth width (T), valley width (pc - T) and

depth (ht), gear material, and gear face thickness (F). The

target can be a gear or a specially cut shaft-mounted tone

wheel made of stamped ferrous metal. In general, the

following gear or target guidelines must be followed to

achieve greater than 2 mm air gap from the face of unit:

Tooth width, T .............................. >2 mm

Valley width, pc - T ...................... >2 mm

(Whole) depth, ht ......................... >3 mm

Gear material ............................... low-carbon steel

Gear face width (thickness), F .... >3 mm

Deviation from these guidelines will result in a reduc-

tion of air gap and a deterioration in timing accuracy. For

applications that require the sensing of large-tooth targets,

the optimal sensor choice is the ATS610LSA. Here, the

higher switching thresholds provide increased immunity to

false switching caused by magnetic overshoot and other

non-uniformities in the gear or target. For applications

that require the sensing of a target with a repetitive target

structure (valley width less than 5 mm), the optimal sensor

choice is the ATS611LSB. Here, the lower switching

thresholds make the device more sensitive to magnetic

field changes and will provide larger operating air gaps.

Operation with Fine-Pitch Gears. For targets with a

circular pitch of less than 4 mm, a performance improve-

ment can be observed by rotating the front face of the

sensor module. This sensor rotation decreases the

effective sensor-to-sensor spacing and increases the

capability of detecting fine tooth or valley configurations,

provided that the Hall elements are not rotated beyond the

width of the target.

Dwg. MH-018-1 mm

A

2.235

α

TARGET FACE WIDTH, F

>2.235 SIN

α

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Signal Timing Accuracy. The magnetic field profile

width is defined by the sensor element spacing and

narrows in degrees as the target diameter increases. This

results in improved timing accuracy performance for larger

gear diameters (for the same number of gear teeth). The

slope of this magnetic profile also changes with air gap,

resulting in timing accuracy shift with air gap (refer to

typical operating characteristic curves). Valley-to-tooth

transitions will generally provide better accuracy than

tooth-to-valley transitions for large-tooth or large-valley

configurations. For highest accuracy, targets greater than

100 mm in diameter should be used.