Sensores Magnéticos

1. Sensores Magnéticos José Augusto EPUSP 2010

2. The Types of Magnetic Sensors • Reed Switches • Variable Reluctance • Flux-gate Magnetometers • Magneto-Inductor • Hall solid state devices • Anisotropic Magnetoresistive (AMR) • Giant Magnetostrictive (GMR) New

3. Magnetic Sensor Technology Field Ranges

4. 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. • TheLinear 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”

5. 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

6. 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

7. 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

8. 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.

9. 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

10. Categories of Digital Hall Effect Sensors • Unipolar- referred to as “switches” require a single polarity magnetic fieldswitchpoints (north or south pole) for operation (A). • Latching- require both positive and negative magnetic fieldsfor operation. Are designed for symmetrical operation (B). • Bipolar- have switchpoints that occur in either relatively more positive or more negative ranges. A B

11. Switchpoint for a Bipolar Switch Possible switchpoint ranges for a bipolar switch Possible switchpoint ranges for a bipolar switch

12. 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 magneticsensor”) 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 data-read head for disk drives. WheatstoneBridge Patterned Array, Philips

13. 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

14. 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 • S = Sensitivity (nominally 1mV/V/gauss)* • Vb = Bridge Supply Voltage in volts (5 V) • Bs = Bridge Applied Magnetic Flux (0.5 Gauss) Vout= 2,5 mV

15. 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

16. Some Vehicle Detection Circuits HMC1052= R$ 200,00 Simple Vehicle Detection Circuit Microcontroller-Based Vehicle Detection Circuit

17. Linear Position Sensing Combined Bridge Waveforms Multiple sensor bridges lined up

18. 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.

19. Sensor Solutions for Automotive Applications

20. FIM