Two-Wire Self-Calibrating Differential Speed and Direction Sensor with Vibration Immunity

ATS651LSH

10

651LSH-DS, Rev. 3

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

115 Northeast Cutoff, Box 15036

www.allegromicro.com

Allegro MicroSystems, Inc.

Applications Information

Data Protocol Description

When a ferrous target passes in front of the sensor (the branded

face of the sensor case), each tooth of the target generates a pulse

at the output pin of the sensor. Each pulse provides target speed

and direction data: speed is provided by the pulse rate, while

direction of target rotation is provided by the pulse width.

The ATS651 can sense target movement in both the forward and

reverse directions. The maximum allowable target rotational

speed is limited by the width of the output pulse and the shortest

Low-State Duration the system controller can resolve.

Forward Rotation

(See panel a in figure 1) When the target is

rotating such that a tooth near the sensor passes from pin 4 to

pin 1, this is referred to as forward rotation. This is diagrammed

below. Forward rotation is indicated on the output pin by a 45 μs

pulse width.

Reverse Rotation

(See panel b in figure 1) When the target

is rotating such that target teeth pass from pin 1 to pin 4, it is

referred to as reverse rotation. Reverse rotation is indicated on

the output pin by a 90 μs pulse width, twice as long as the pulse

generated by forward rotation.

Timing

. As shown in figure 2, the pulse appears at the output pin

slightly before the sensed mechanical edge traverses the sensor.

For targets in forward rotation, this shift, Δfwd, results in the

pulse corresponding to the valley with the sensed mechanical

edge, and for targets in reverse rotation, the shift, Δrev, results in

the pulse corresponding to the tooth with the sensed edge. The

sensed mechanical edge that stimulates output pulses is kept the

same for both forward and reverse rotation by using only channel

A for switching.

The overall range between the forward and reverse pulse occur-

rences is determined by the 1.5 mm spacing between the Hall

elements of the corresponding differential channel. In either

direction, the pulses appear close to the sensed mechanical edge.

The length of the target features, however, can slightly bias the

occurrence of the pulses.

Figure 1. Target rotation

Output

(Forward

Rotation)

Output

(Reverse

Rotation)

Forward

Reverse

45 μs

Δfwd

Δrev

90 μs

Tooth

Valley

(a) Forward Rotation

(b) Reverse Rotation

Rotating Target

Branded Face

of Sensor

Pin 1

Pin 4

Pin 1

Pin 4

Rotating Target

Branded Face

of Sensor

Figure 2. Output pulse timing