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Mark LaCroix A John Santos
Dr. Lei Wang 8 FEB 13 • Orlando
A Comparison of Performance Characteristics of On and Off Axis High Resolution Hall Effect Encoder ICs
Sensor Products
Originally Presented at the Motor and Drive Systems 2013 Conference
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PRESENTATION OUTLINE
1. DESCRIPTION OF ON & OFF AXIS SENSORS 2. THEORY OF OPERATION 3. CALCULATIONS & THEORETICAL MODELING 4. TESTING RESULTS 5. SELECTION GUIDELINES
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15 years in sensor business Supplier in integrated Hall encoder market serving industrial customers & critical vehicle systems
Sensor Products
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MAGNETIC ENCODER MARKET • The Magnetic Encoder Market is Growing
• Reliability of magnetic technology • Environmental capabilities • Accurate feedback from system-on-a-chip designs • Excellent value
• Two primary sensor configurations
On Axis Designs --- Off Axis Designs
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ON AXIS SENSORS • Aligned with the centerline of the
rotating shaft
• Rare earth button magnet: 4 to 10 mm Dia. mounted off the end of the shaft
• Sensor IC has typically 4 to 8 Hall elements in a circular array
• Produces a once per turn Sine & Cosine signal that is converted to a quadrature or serial position signal
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OFF AXIS SENSORS • Sensor is offset from the
center of the rotating shaft
• Multi-pole magnet: Typically 20 to 300 mm diameter axial or radial
• Sensor IC has typically 8 to 16 Hall element array
• Produces a Sine & Cosine signal for each pole pair and signal is converted to a quadrature or serial position signal
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THEORY OF OPERATION FOR BOTH SENSOR TYPES
On-Axis 1 pole pair
Off-Axis 32 pole pairs
4096 counts 1 Revolution
128 counts 1/32 of Revolution 4096 total / Rev
Deep interpolator required to get full resolution from 1 revolution
Shallow interpolator used to get 1/32 of the resolution from 1 pole pair
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TYPICAL ON-AXIS SENSOR CHARACTERISTICS
• Incremental (quadrature) and absolute position signals available
• U, V, W commutation
• Index/marker pulse
• Interpolator must extract full
resolution from sine/cosine (1 Rev.)
• DSP/Digital Signal Processor interpolator (Deep but with slower response time than hardware interpolators)
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• Incremental (quadrature) position • U, V, W commutation with external Halls. • Hardware based interpolator is very fast (real-time) • Shallow interpolator produces resolution for each pole pair • Index/marker pulse - one per turn or once per pole pair • One chip works with a wide range of pole sizes & diameters
OFF-AXIS PERFORMANCE CHARACTERISTICS
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• Diameter & mechanical leverage effect. • Number of poles – leveraging interpolator accuracy
Detailed on the following two slides:
KEY CHARACTERISTICS AFFECTING ACCURACY
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Small magnet imperfection
Large output error
On-Axis Off-Axis
Small magnet imperfection
DIAMETER & MECHANICAL LEVERAGE EFFECT
Magnet errors: Larger error for on-axis sensors - small diameter. Smaller error for off-axis sensors - large diameter.
Small output error
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NUMBER OF POLES – LEVERAGING INTERPOLATOR ACCURACY
Example shows interpolator with 4% error (From sine wave purity and interpolator errors)
The interpolator error shows up directly (1:1 ratio) for on-axis The interpolator error is divided by the pole pair count for off-axis
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CALCULATIONS & THEORETICAL MODELING
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MAGNETIC TARGETS: MAGNETIC FIELD AND RESULTING SINE WAVE WHICH AFFECT THE FINAL ACCURACY
On Axis (4mm OD) Off Axis (2mm pole pitch)
Mag
netic
fie
ld
Resu
lting
sig
nal Error Error
Ideal magnetic field
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CHIP/SENSOR TOLERANCE PLACEMENT
Very small error with perfect IC placement for both types
Large error over sensor placement tolerances
Small error over sensor placement tolerances
0
-100
400
On axis
Off axis
16.7 13.3 10 6.7 3.3 0
0.5mm X & Y Position error of sensor
Erro
r (pp
deg
rees
)
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TEST RESULTS
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18
Accuracy versus radial position at 1mm gap Er
ror (
pp d
egre
es)
Radial position of sensor chip
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
-1 -0.5 0 0.5 1
"Off Axis Supplier #1"
"Off Axis Supplier #2"
"On Axis Supplier #1"
On-Axis Supplier #2
"On-Axis Supplier #3"
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Real world testing correlated well with theoretical predictions • Errors were larger over all conditions for on-axis sensors • Errors were larger for non-ideal chip placement for on-axis sensors
MEASURED ERRORS OVER RADIAL POSITION AND AIR GAP
0.04ᵒ (0-p)
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20
1.3 1 0.67 0.33 0 0.33 0.67 1 1.3
Erro
r (pp
deg
rees
)
over 1 Revolution
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ERROR FREQUENCY COMPONENT EFFECT ON SYSTEM PERFORMANCE Errors on both sensors are linked to pole pair count
• Once/rev for on-axis • 20-50/revolution for off axis
Low frequency errors generally cause more system problems such as vibration (Typical of on-axis sensors) Higher frequency errors, system performance is generally tolerant
• Motor controllers are less sensitive to higher frequency errors on encoder feedback signals
• Mechanical system filters high frequency feedback errors
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22
(1mm gap)
Common Mode Bias Field (gauss)
Erro
r (pp
deg
rees
) ERROR: EXTERNAL COMMON-MODE FIELDS From magnetized shafts, motor windings and motor magnets
0
0.5
1
1.5
2
2.5
3
3.5
4
-150 -100 -50 0 50 100 150
"Off Axis Supplier #1"
Off Axis Supplier #2
"On Axis Supplier #1"
"On-Axis Supplier #2"
"On axis Supplier #3"
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SELECTION GUIDELINES
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SELECTION GUIDELINES • Both on-axis and off-axis sensor continue to grow
as viable alternatives to optical encoders
• Performance & physical constraints drive selection
Key Characteristics of on-axis and off-axis sensors Characteristics
Maximum Resolution
Position Accuracy
IC & PCB Mounting Tolerance
Absolute Position
Stray Field Rejection
Target Inertia
Propagation Delay / Real Time Sensing
On Axis - - - + +/- + -
Off Axis + + + - + +/- +
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High resolution or high accuracy
required?
S
Real time position needed?
Access to shaft end?
N N Y
Y N Y
SELECTION GUIDELINES SENSOR SELECTION PROCESS FLOW
On Axis Design
Off Axis Design
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