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P0098

APPLICATION INFORMATION

TRANSMISSION GEAR TOOTH SENSING
By Kevin Maffei
Allegro MicroSystems

Introduction
Transmission applications have diverse conditions to consider for optimal speed sensing design-in and performance.
Sensed targets are primarily made of ferromagnetic material,
but magnetic encoders are used in a small percentage of theapplication sockets. Solutions include both two- and threewire output interface protocols and may include rotational
direction and fault reporting for ASIL compliance, in addition
to standard speed information.

Key Parameters
Design-in of an Allegro speed or speed and direction sensor
into a transmission is simple. The two key mechanical parametersthat must be given close attention are listed below:
1. Alignment of the sensor branded package face withthe target teeth.
2. Distance between the sensor branded package face
and the target tooth.
A. This distance is frequently referred to as the sensor
to target “air gap.”
The radial sensing image (Figure 1) depicts a good mechanical sensor-to-target alignment. Four mechanical factors work
together to form good alignment:
1. Sensor branded face points to target teeth.
2. Center of the sensor branded face is aligned with thecenter of the target.
3. During target rotation, the teeth pass over the branded
face of the sensor such that they move in a pin 1 topin 2 or pin 2 to pin 1 direction.
4. Air gap between sensor and target teeth is within
datasheet limits.

Air Gap
Sen sorFigure 1: System mechanics

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Environmental Factors
Transmissions are subject to constantly changing conditions, which create challenges that a magnetic sensor must overcome inorder to continue providing reliable speed and/or speed and direction information. Temperature fluctuations cause materials
to change in size, impacting the sensor-to-target air gap. This may happen while the target is in rotation or while stationary, andboth situations must be handled without interruption. With a relatively high vehicle weight, large forces are exerted to create
acceleration and deceleration. These forces can cause sudden movements inside a transmission, which can change the air gap in
a sudden or gradual manner with respect to the speed of the target being sensed. For example, cold starts may cause a shaft to lift
as it starts to spin, and transitions from one gear to another can create undesirable target movements. This undesired movementof targets being sensed can confuse a transmission control unit (TCU), and it’s vital to consider how these movements are handled.

Air Gap Range
Understanding the range of air gaps that a sensor may encounter is a key parameter that must be considered during systemdesign. This range is impacted by many factors related to the design of the target to be sensed, and as such, a reference target
design is included in all pertinent device datasheets. Allegro sensors are designed to maintain an equal air gap ability over thetemperature extremes normally found within vehicles.

Figure 2: Air Gap Range (click images for animation)

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Gradual Air GapTransmission sensors are primarily dynamic threshold detectors. This method of signal processing allows for sensor learning,which provides the ability to update output switching thresholds as the target teeth pass by. Updating thresholds based on new
information allows the sensor to provide uninterrupted output signal during gradual air gap movements that may occur on thesystem level. A gradual air gap movement may be repetitive; for example, it may be present in a target rotating on- and off-center
axially, or radially in the case of runout, as depicted in the example shown.

Figure 3: Gradual Air Gap (click image for animation)

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Sudden Air Gap
Sudden air gap refers to changes in air gap that occur within the rotational span of a single tooth-valley pair. In cases wheremovement is large enough in magnitude, the detection thresholds may not be crossed. This extreme event would ordinarily
cause a permanent loss of speed and direction information; however, Allegro transmission sensors include advanced eventbased algorithms to recover the speed and direction output quickly, only causing a momentary output signal disruption to anextreme system-level condition.

Figure 4: Sudden Air Gap (click image for animation)

Direction Change
Target direction of rotation detection is possible due to the physical location of sensing elements. This physical position of thesensitive elements facilitates phase relationship between sensor channels. A phase relationship can be interpreted as a verticalseparation between channels at any given time. This separation information is used to determine target direction of rotation.
However, a target direction of rotation change may be angular target vibration and not a true direction change. To verify adirection change, the sensor must observe a certain amount of consistent same direction rotation. This amount of consistentsame-direction rotation can be flexible to provide information as soon as possible, or can be delayed based on the high andlow vibration detection option.

Figure 5: Direction Change (click image for animation)

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Vibration
Vibration of a transmission gear can present as air gap vibration, angular vibration, or a combination of both. Under the correct conditions, a transmission sensor vibration signal can look like a normal rotational signal. Allegro transmission sensors use
industry leading low and high vibration detection algorithms to distinguish vibration from normal rotation and suppress output
during vibration.

Figure 6: Vibration (click image for animation)

Conclusion
All of the above factors can occur, often within a single application socket. Therefore, the industry-leading, time-proven algorithms
included in Allegro transmission ICs have been designed to handle the most stringent application conditions.

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Revision History
NumberDate

Description

Responsibility–April 22, 2021

Initial releaseK. Maffei

1July 6, 2021

Minor editorial updates

M. WestCopyright 2021, Allegro MicroSystems.

The information contained in this document does not constitute any representation, warranty, assurance, guaranty, or inducement by Allegro to the customer with respect to the subject matter of this document. The information being provided does notguarantee that a process based on this information will be reliable, or that Allegro has explored all of the possible failure modes.
It is the customer’s responsibility to do sufficient qualification testing of the final product to ensure that it is reliable and meetsall design requirements.
Copies of this document are considered uncontrolled documents.

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