Dynamic, Self-Calibrating, Peak-Detecting, Differential
Hall Effect Gear Tooth Sensor IC
ATS617LSG
12
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Superior Performance The ATS617 peak-detecting differen-
tial design has several advantages over conventional Hall-effect
gear-tooth sensors. The signal-processing techniques used in the
ATS617 solve the catastrophic issues that affect the functionality
of conventional digital gear-tooth sensors, such as the following:
" 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 varia-
tion caused by air gap changes is minimized by the self-calibra-
tion circuitry. A 2?to-3?improvement can be seen.
" Dual edge detection. Because this device switches based on the
positive and negative peaks of the signal, dual edge detection is
guaranteed.
" Tilted or off-center installation. Traditional differential sensors
can switch incorrectly due to baseline changes versus air gap
caused by tilted or off-center installation. The peak detector cir-
cuitry references the switchpoint 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. Large operating air gaps are achiev-
able with this device due to the sensitive switchpoints after
power-on (dependent on target dimensions, material, and
speed).
" Immunity to magnetic overshoot. The patented adjustable
hysteresis circuit makes the ATS617 immune to switching on
magnetic overshoot within the specified air gap range.
" Response to surface defects in the target. The gain-adjust
circuitry reduces the effect of minor gear anomalies 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 im-
munity to vibration even when operating at close air gaps.
" Immunity to gear run out. The differential chip configuration
eliminates the baseline variations caused by gear run out
Differential vs. Single-Element Design The differential
chip is superior in most applications to the classical single-ele-
ment design. The single-element configuration commonly used
(Hall-effect element mounted on the face of a simple permanent
magnet) requires the detection of a small signal (often <100 G)
that is superimposed on a large back-biased field, often 1500 G to
3500 G. For most gear/target configurations, the back-biased field
values change due to concentration effects, resulting in a varying
baseline with air gap, valley widths, eccentricities, and vibration
(figure 4). The differential configuration (figure 5) cancels the
effects of the back-biased field and avoids many of the issues
presented by the single Hall element design.
Figure 4. Affect of varying valley widths on single-element circuits.
Figure 5. Affect of varying air gaps on differential circuits.