Service Training Update 2008
SPEED SENSORS Basic Operating Principles and Applications
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 • •
•
Overmold •
•
Adjustable Fixed Gap Fixed Gap
Active •
Single Hall Effect • • •
•
Dual Hall Effect •
•
Adjustable Fixed Gap Slip Head (obsolete) Fixed Gap
AMR •
Fixed Gap
Speed Sensors Product Families Thru-mold Adjustable Gap
Passive Thru-mold Fixed Gap
Active Single Hall Effect
Dual Hall Effect
Over-mold Fixed Gap
Current Loop Hall Effect
AMR
Passive Sensors Also known as:
VR (Variable Reluctance) Mag Pickup Pulse Generator Timing Probe Crankshaft 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 Coil
Magnet
Shell Molding Material
Pole Piece
Lead Wires
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: e=N
dφ dt
e = voltage generated N = number of turns of wire in the coil φ = magnetic flux dφ = time rate of change of the flux dt
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 VR SENSOR (PASSIVE) GEAR TOOTH
TARGET
X 0
φ X(-)
X=0
+
VOLTS
-
X=0
X(+)
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 v BZ
Perpendicular Magnetic Field
Current in silicon sensor
I
VH
Vo Vs+ Vo+
H
Hall Element I
Vs-
Vo-
H
Timing and Position Sensing - Active DUTY CYCLE & PHASE ANGLE DEPENDANT ON TARGET PROFILE
SIGNAL B
Vsupply - 1V max SIGNAL A 0 - 1V max
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
IHL = IOFF - ION IOFF Ion SENSOR
ILO = IOFF VOLTAGE SIGNAL
OUTPUT
ECM
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 •
•
Low Gain Output •
•
Lower accuracy 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 25C 10
set speed 50 100 150
Peak to Peak
8
6
4
2
0
2
0.5
1.0
1.5
air gap
2.0
2.5
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
C
A
B
• Transmissions – duty cycle
A = 3 MIN B = 2.5 OR 3 MIN C =B D = 6 OR 10 MIN
TOOTH HEIGHT TOOTH WIDTH TOOTH SPACING GEAR THICKNESS
ACTUAL VALUES ARE SENSOR AND APPLICATION DEPENDENT
D
Active/Hall Effect vs Passive •
Active – Hall Effect •
Speed Range • 0 – 15 kHz • Application dependent
•
Air Gap • 0.5mm min. • Application dependent
•
Seal •
•
Direction • • •
•
100-150 psi Dual Hall Effect 2-wire Phase shift is application dependent
Sensor’s Housing material • • •
Brass SST Nylon
•
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
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 measures magnetic angle
• 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. Ix
M θ Hy
ΔR = (1 - COS2 θ)ΔRmax
Ring Magnet • Acts similar to teeth on a rotor
Ring magnet emulates traditional rotor
TOOTH VALLEY Rotor
N S
S N Cross Sections
N S
AMR Ring Magnet Encoder Design Magnetic poles emulate rotor’s teeth
• 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 DHP w/ Rotor
AMR w/ Magnetic Ring
DHP Rotor vs AMR Ring
AMR Advantage
Wider air gap performance than hall effect
ΔR = (1 - COS2 θ)ΔRmax
• With hall effect, you get one pulse Sensor for each tooth/valley or pole pair. • AMR outputs 2 Pulses for every pole pair Magnet Rotation • 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
0
1
2
3
Flux Density
4
resistance
5
6
