SPEED SENSORSBasic Operating Principles and Applications

Service Training Update 2008

Speed Sensors – General Information• Speed sensors are divided into two classes

• Passive (no power required)• VR – Variable Reluctance

• Active (requires a power source)• Hall Effect

• Single (3 wires)• Dual (4 wires)• Current Loop (2 wires)

• AMR – Anisotropic Magneto Resistance• Packaging differentiate product families

• Passive• Thru-mold

• Adjustable• Fixed Gap

• Overmold• Fixed Gap

• Active• Single Hall Effect

• Adjustable• Fixed Gap• Slip Head (obsolete)

• Dual Hall Effect• Fixed Gap

• AMR• Fixed Gap

Speed Sensors Product FamiliesPassive

Active

Thru-mold Adjustable Gap Thru-mold Fixed Gap Over-mold Fixed Gap

Current Loop Hall EffectSingle Hall Effect Dual Hall Effect

AMR

Passive Sensors

Also known as:VR (Variable Reluctance)

Mag PickupPulse Generator

Timing ProbeCrankshaft Position Sensor

General - Passive• Passive sensors do not require any external electrical power

supply.• Output signal is an alternating current.• Wave form is function of the actuator.

• Generally, sinusoidal in nature• Voltage and frequency are both proportional to surface speed of

the actuator as it passes the sensor’s pole piece.

Product Description - Passive• Non-contact transducer that converts mechanical

motion into electrical signal• Actuator must be ferro-magnetic material

• Carbon steel, magnetic stainless steel, or iron• Commonly used actuators include

• Gears, slotted discs, shafts with keyways• No moving parts

Shell

Lead Wires

Molding Material

Magnet

Pole Piece

Coil

Principle of Operation - Passive• Permanent magnetic field applied through coil of wire.• Figures below illustrate how the magnetic field changes by the

approach and passing of a gear tooth, e.g. flux discontinuity• Change in the magnetic field produces a voltage across the coil

• Just like on an electric generator• Voltage and frequency are directly proportional to target speed.

Low Reluctance Position High Reluctance Position

Principle of Operation – Passive (cont.)• All physical space exhibits a degree of opposition to the passage of

magnetic flux. This is called Reluctance.• Ferrous materials provide low reluctance path• Air provides high reluctance path

• In a Variable Reluctance (VR) system, the reluctance of the magnetic flux is varied.

• The path loops through a coil of wire, which generates a voltage at the terminals of the coil that is exactly proportional to the rate of change of magnetic flux.

• Relationship:

dtdNe φ

=

e = voltage generated N = number of turns of wire in the coil φ = magnetic flux

dtdφ = time rate of change of the flux

Principle of Operation – Passive (cont.)• The flux is provided by a permanent magnet.• The flux is directed through the coil by a ferrous core called a

pole piece.• Most targets are spur gears.• As a tooth of a gear comes in alignment with the pole piece, the

reluctance decreases so the magnetic flux increases.• There are many factors which affect the voltage level, wave

shape, and frequency:• Strength of magnet• Shape of target• Number of turns of wire in the coil• Speed of target• Air gap between pole piece and target• Permeability of target• Load impedance

Timing and Position Sensing - Passive

X0

TARGET

X = 0X(-) X(+)

VRSENSOR(PASSIVE)

GEARTOOTH

φ

X = 0

+

-

VOLTS

Zero crossing is in the center of

the tooth!

Active Sensors

Hall Effect• Single (3-wires)• Dual (4-wires)

• Current Loop (2-wires)

AMR

General - Active• Active sensors require an external electrical power supply.• Output signal is an alternating voltage.• Wave form is function of the actuator.

• Square wave output• Frequency is both proportional to surface speed of the actuator as it passes

the sensor’s tip.• Zero speed detection

Product Description – Active Hall Effect• Single Hall Effect sensors have 1 Hall element.• Dual Hall Effect sensors have 2 Hall elements.

• Each element is spaced to provide two signals that are 90° apart.• Rotor has to be designed to fit hall cell spacing• This allow direction detection

• Signal A leads Signal B or vice-versa• Current Loop

• Draws 4-8 mA in one state• Draws 12-16 mA in other state• State depends on direction of rotation of target

• In one direction, draws low current over tooth & high over valley• In reverse direction, draws high current over tooth & low over valley

• Allows diagnostics to detect if sensor is connected

Principle of Operation – Active Hall Effect• A Hall element is semiconductor that outputs a voltage

proportional to magnetic flux density.• Cat’s Hall effect sensors have a permanent magnet.

• This provides a magnetic flux field.• A bias voltage is applied across the Hall element.• Current through the Hall element varies with changes in flux

density• When a gear tooth passes in front of the sensor, the flux density from the

permanent magnet changes similar to that of a VR sensor.• This creates a differential voltage across the semiconductor.• The differential voltage is directly proportional to the rate of change of

magnetic flux.• The differential voltage is amplified, filter, and then various peak detection

schemes are used to determine a tooth edge. All of this is done with an integrated circuit (IC).

Principle of Operation – Active/Hall Effect

Vs+

Vo+ Vo-

Vs-

H

Hall Element

I

Vo

H

ZBv

I

HV

Current in silicon sensor

Perpendicular Magnetic Field

Timing and Position Sensing - Active

Vsupply - 1V max

0 - 1V max

SIGNAL A

SIGNAL B

DUTY CYCLE & PHASEANGLE DEPENDANT ONTARGET PROFILE

TARGET

Signal edge is at the

tooth edge

Current Loop Hall Effect Speed Sensor• 2 – wires

• Fewer wires means better harness reliability• Diagnostics - ability to detect if the sensor is connected or not• Detects speed and direction• Lowest cost speed sensor in our product line• Requires ECM input circuitry currently available on A4E2

2-Wire Current Loop System

ECM

SENSOR

Ion

OUTPUT

VOLTAGESIGNAL

ILO = IOFFIOFF

IHL = IOFF - ION

Product Families - Passive• Thru-mold

• Sensor has an external metal housing.• During manufacturing, nylon is injected through the housing to mold over the magnet and

coil; hence, “thru-mold”.• High Output

• Lower accuracy• 5/8-18, ¾-16, M18x1.5 threads

• Low Output• Higher accuracy• 5/8-18, ¾-16, M16x1.5threads

• Pigtail• Fixed gap• Adjustable w/ jam nut – NOT RECOMMENDED FOR NEW DESIGNS

• Overmold• Injection molded nylon directly over the magnet and coil; hence, “overmold”.• High Gain Output

• Lower accuracy• Low Gain Output

• Higher accuracy• Bolt-n-go

Applications - Passive• Low Gain Output Speed Sensors

• High position accuracy• Lower output voltage• Typical applications

• Crank• Timing applications• TC, TIS, and intermediate transmission speed sensors

• High Gain Output Speed Sensors• Less accuracy in position• Higher output voltage, i.e. lower RPM detection• Typical applications

• CAM• TOS

Key Characteristics - Passive

• Output voltage decreases with decrease of RPM

• Output voltage decreases with increase of air gap

2.52.01.51.00.5

10

8

6

4

2

0

air gap

Pea

k to

Pea

k

2

50100150

set speed

25C

Product Families – Active/Hall Effect• Single Hall Effect

• One Hall cell• One output• 3-wires

• Dual Hall Effect• Two Hall cells• Quadrature output• Phase shift indicates direction• 4-wires (power, return, two signals)

• Current loop dual Hall effect• One Hall cell• Requires current source be provided by ECM• Polarity change indicates direction• 2-wires

Applications - Active

• Used where zero speed or near-zero speed detection is required.

• Transmission Output Speed• Traction Control Systems• Steering• Speed / Timing

• Perkins compact common rail• LEC engines

• Century Propulsion Motor

Key Characteristics - Active

• Tooth profile

• Engines – edge accuracy

• Transmissions – duty cycle

B C

A

A = 3 MIN TOOTH HEIGHTB = 2.5 OR 3 MIN TOOTH WIDTHC =B TOOTH SPACINGD = 6 OR 10 MIN GEAR THICKNESS

ACTUAL VALUES ARE SENSOR ANDAPPLICATION DEPENDENT

D

Active/Hall Effect vs Passive• Passive

• Speed Range• Low Output 200 Hz – 45 kHz typ• High Output 50 Hz – 15 kHz typ• Application dependent

• Air Gap• 0.5mm min• Application dependent

• Seal• Thru-mold – not sealed• Overmold – 5 psi

• Direction• Requires 2 sensors located 90 degrees

electrically apart• Sensor’s Housing material

• Thru-mold• Aluminum• SST

• Overmold• Nylon

• Active – Hall Effect• Speed Range

• 0 – 15 kHz• Application dependent

• Air Gap• 0.5mm min. • Application dependent

• Seal• 100-150 psi

• Direction• Dual Hall Effect• 2-wire• Phase shift is application dependent

• Sensor’s Housing material• Brass• SST• Nylon

Anisotropic Magneto Resistance - AMR

• AMR presents a new opportunity to achieve higher speed resolution than previously available at Cat

• Two levels of resolution• 1x: 96 ppr (pulses per revolution)

• 8x: 768 ppr• Excellent duty cycle & phase shift accuracy• Air gap performance >2x hall effect• Zero speed detection

Anisotropic Magneto Resistance - AMR

• Permalloy thin film technology• 2-Part Encoder

• Hi/Lo Resolution Sensor• Magnetic Ring

AMR Functional Description

• AMR (Anisotropic Magneto Resistance) occurs in thin, ferrous films.• Preferred axis of magnetization is in the long direction.• External field (Hy) applied perpendicular to the long axis causes the

magnetization vector (M) to rotate through the angle (θ). • Resistance of the strip of material changes with the angle of the

magnetic field.

Mθ

Hy

Ix

ΔR = (1 - COS2 θ)ΔRmax

AMR measures magnetic angle

Ring Magnet

• Acts similar to teeth on a rotor

VALLEYTOOTH

Rotor

N S NN SS

Cross Sections

Ring magnet emulates

traditional rotor

AMR Ring Magnet Encoder Design

• Cat’s hall effect sensors contain a magnet to back bias the hall cell• Ferrous rotor passes in front of sensor interrupting the magnetic field

• AMR senses a magnetic ring• Magnetic poles rotate in front of the sensor

Magnetic poles emulate rotor’s teeth

DHP w/ Rotor DHP Rotor vs AMR RingAMR w/ Magnetic Ring

AMR Advantage

• With hall effect, you get one pulse for each tooth/valley or pole pair.

• AMR outputs 2 Pulses for every pole pair• Due to cosine square function of

angle to sensor• Allows bigger magnetic poles for

same resolution of ppr• Bigger magnets means stronger

magnets• Increases air gap performance

ΔR = (1 - COS2 θ)ΔRmax

Sensor

Magnet Rotation

0 1 2 3 4 5 6

Flux Density resistance

Wider air gap performance than

hall effect