Download Sensores Magnéticos

Sensores Magnéticos
José Augusto
EPUSP 2010
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The Types of Magnetic Sensors
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Reed Switches
Variable Reluctance
Flux-gate Magnetometers
Magneto-Inductor
Hall solid state devices
Anisotropic Magnetoresistive (AMR)
Giant Magnetostrictive (GMR)
New
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Magnetic Sensor Technology Field Ranges
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Hall Effect Magnetic Sensors
• Magnetic Sensors based in Hall effect uses Bipolar/FET IC
technology in combination with the Hall effect sensor to
produce two type detectors: Linear and Threshold.
• The Linear sensors detects the motion, position, or change in
field strength of an electromagnet, a permanent magnet, or a
ferromagnetic material. The output is almost linear in respect to
a magnetic field.
• The Threshold devices gives digital output capabilities.
• When the applied magnetic flux density exceeds a certain limit,
the trigger provides fast and clean transition from Off to On.
• Hall sensors are reffered as “High Field Sensor”
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The Hall Effect
• The basic Hall element is a small sheet of semiconductor
material, referred as the Hall element, (or active area).
• A external constant voltage source forces a constant bias
current, to flow in the semiconductor sheet.
• The output takes the form of a voltage, measured across
the width of the sheet proportional to the magnetic field.
E. H. Hall in 1879
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Linear Hall Effect Sensors
• The output voltage of the basic Hall element is quite small. The addition
of a high-quality dc amplifier and a voltage regulator to the circuit
improves the transducer’s output and allows it to operate over a wide
range of supply voltages.
• The sensor’s frequency response is flat-up on the range 25 kHz ~100 kHz.
Vout= 1 – 25 mV/G
Tr= 3 µS ≈ 116 kHz
R$ 7,00 - US$ 20,00
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Ratiometric Linear Hall Effect Sensor
• These devices has a BiCMOS monolithic circuit improved
temperature-compensating circuitry, small-signal amplifier,
filter and a rail-to-rail output.
• Sensitivity: 2.5 mV/G to 5 mV/G.
• Bandwidth: 30kHz
• Ideal for use in the rigorous environments found in
automotive and industrial linear sensing systems.
• New programmable devices has both the quiescent voltage
output and sensitivity adjustable between 2 and 9 mVG
A1321 R$ 4,50
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Digital Hall-Effect Switches
• The addition of a Schmitt-trigger threshold detector,
with built-in hysteresys gives digital output capabilities
• An open-collector NPN output transistor added to the
circuit gives the switch digital logic compatibility.
• Output is clean, fast and typically operates at up to a
100 kHz repetition rate.
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Digital Hall Sensor Operation
• In the absence of any magnetic field, most Hall-effect
digital switches are designed to be OFF (open circuit).
• When the applied magnetic flux density exceeds a certain
limit, the trigger provides a transition from Off to On.
• Hall switches have an active area that is closer to one face.
To operate the switch, the magnetic flux lines must be
perpendicular to this face, and must have the correct
polarity.
Built-in hysteresis
eliminates oscillation
by introducing a
magnetic dead zone
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Categories of Digital Hall Effect Sensors
• Unipolar- referred to as “switches” require a single polarity
magnetic field switchpoints (north or south pole) for operation (A).
• Latching- require both positive and negative magnetic fields for
operation. Are designed for symmetrical operation (B).
• Bipolar- have switchpoints that occur in either relatively more
positive or more negative ranges.
A
B
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Switchpoint for a Bipolar Switch
Possible switchpoint ranges for a bipolar switch
Possible switchpoint ranges for a bipolar switch
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Magnetoresistor
• It is possible to measure the increased resistance of special
resistors due to the deflected electrons. In this case, the sensor is
called a Magnetoresistor. Two types are available:
• AMR- Anisotropic Magneto Resistive
• GMR- Giant Magneto Resistive
• Magneto-Resistive sensors (“low-field magnetic sensor”) are
usually made of a nickel-iron (Permalloy) thin film deposited on a
silicon wafer and patterned as a resistive strip.
• The Magnetoresistor AMR Bandwidth 1-5 MHz
Sensivity 2,5 - 4 mV/V/Gauss
The Giant Magnetoresistance Effect
(GMR) has an important application for
miniature magnetic sensors in the dataread head for disk drives.
Wheatstone Bridge
Patterned Array, Philips
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AMR Sensors Characteristics
• AMR sensors are directional and provide only an amplitude
response to magnetic fields in their sensitive axis.
• By combining two AMR sensors, the part becomes a 2-axis
sensor and when mounted horizontally, is able to detect any
horizontal magnetic fields into X and Y directions.
HMC 1001 (1 axis) US$ 21,00
HMC1002 (2 axis) US$ 25,00
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Magnetoresistive Sensor Exemple
• Hall effect sensor: measures the magnet field “strength”
• Magnetoresistive sensor: measures the “angle direction” of magnetic field
• The output voltage from the AMR sensor is a function of the sensor sensitivity
equation:
• VOut+ - VOut- = S * Vb * Bs
• For Honeywell HMC1001
Vout= 2,5 mV
• S = Sensitivity (nominally 1mV/V/gauss)*
• Vb = Bridge Supply Voltage in volts (5 V)
• Bs = Bridge Applied Magnetic Flux (0.5 Gauss)
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Magnetoresistive Sensor Circuit Amplifier
• Sometimes the magnetoresistive sensors require
follow-on amplification to make induced field
changes easier to detect.
• The (diffential) output of the sensors requires a
differential amplifier.
• For a AmpOp gain of 200 and Vcc=3 V
• Vout ~ 300 mV
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Some Vehicle Detection Circuits
HMC1052= R$ 200,00
Simple Vehicle Detection Circuit
Microcontroller-Based Vehicle Detection Circuit
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Linear Position Sensing
Output PP
Bridge
Ohms
Sensitivity
Range
120 mV
5000
2.1 mV/Deg
+/- 45 Deg
Multiple sensor bridges lined up
Combined Bridge Waveforms
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Compatibility With Microprocessors
The analog output voltage of the amplifier is typically fed to an Analog-to-Digital
Converter (ADC) stage stand-alone or within a microprocessor integrated circuit.
A recommendation of 10-bit ADC circuits or higher is expected.
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Sensor Solutions for Automotive Applications
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FIM
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