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Direction-Detection Prototype

by Christine Graham

Purpose

Determine a target definition that would allow proper speed and direction detection with two open-collector, differential Hall-effect sensors.

Specifications

The system is to detect the direction and speed of each wheel of the vehicle and indicate the direction and speed by outputting the equivalent of a dual channel logic output capable of being processed for the required information.

Design

The design of the sensor consists of two back-biased differential Hall effect devices joined at an offset of 1.1 mm. This assumes that the differential Hall element spacing is 2.2 mm. Since a tooth or valley will affect one sensor before the other with such an offset, this joined sensor assembly can determine direction of a spinning target. The diagram below indicates the offset of the sensor assembly.

The signal processing system will need to consist of three subsystems: Direction processing, position processing, and output processing. Direction processing takes in both sensor outputs, detects the direction, and outputs the direction to both the output processing section and position processing. Position processing takes one sensor output and the direction signal and then sends the position to output processing. It needs the direction signal because typical back-biased differential sensors detect the edges of teeth on a ferrous target not the teeth themselves. Output processing takes in the direction signal and position signal, and adjusts the output accordingly. The block diagram and timing diagram below depicts the configuration.

Target

With respect to the target, the orientation of the sensor assembly is critical. The sensor assembly consists of two sensors. When the sensor assembly is placed over a gear, the thickness of the sensed teeth must accommodate both sensor-to-sensor package dimensions as well as ensure the Hall elements within each Hall sensor package are not off the edge of the tooth. The diagram below indicates nominal dimensions for the assembly.

The ideal tooth width needs to be greater than or equal to the element spacing of the two Hall elements in the differential device. For examples if the element spacing in a particular device is 2.2 mm, the tooth width should be at least 2.2 mm. The valley width depends on the duty cycle required for the application. For ease of explanation, assume that a 50/50 duty cycle mechanical target design is required. Therefore, the tooth and valley widths will be 2.2 mm each, for a total cycle of 4.4 mm. To obtain a 90-degree phase difference between elements of a device with 2.2 mm sensor-to-sensor spacing, they would need to be offset by half the tooth width. Thus the offset should be 1.1 mm as shown in the above drawing.

The setup as described will provide two periodic digital signals 90-degrees out of phase from each other. Thus, the two periodic digital outputs can then be processed for direction and speed.

Summary

• The two sensors placed side-by-side will require at least 8.8 mm of metal (tooth) to guarantee the Hall elements within a device, sensor spacing of 2.2 mm, are not over an edge.
• The two back-biased packages will need to be offset, as shown in the figure 4., by half the tooth width. In this particular example the offset is equivalent to 1.1 mm.
• If it were too difficult to overcome the force of the two magnets, two solutions would be as follows.
  1. Make the thickness greater than 8.8 mm and separate the two sensors. As long as they are offset by half a tooth width, their close proximity doesn't matter.
  2. Instead of separating the two differential devices on a single tooth, place the sensors on separate teeth, still offset by the half tooth width. For example, place them 180 degrees out of phase on the target with one sensor one half a tooth width offset from the other.
• Example calculations:
  1. Tooth/valley width = 2.2 mm (total cycle = 4.4 mm)
  2. Tooth count = 69 cycles (Tooth/valley)
  3. Target Circumference = pi * diameter = 69 * 4.4 mm
  4. Diameter = Circumference / pi = 69 * 4.4 mm / pi = 96.64 mm diameter.
• Coming Soon: a single IC solution for speed and direction detection of a ferrous target. Please visit Allegro's website, www.allegromicro.com., over the coming months for more information. Both two-wire and open-collector styles will be available in a one-shot over-molded, back-biased package.

Suggested Devices

Allegro Part Number	Temperature Range(s)	Package Type(s)	Tape & Reel Available
ATS665LSG	L	SG	No
ATS645LSH	L	SH	No

Possible Applications

• Transmission
• Alternator / Generator
• Treadmill
• Industrial Machinery
• ABS
• Conveyor Belt systems
• Fan motor
• Any ferrous target speed and direction sensing

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