Hall-Effect Sensors

Theory and Application

Edward Ramsden

• Publisher: Newnes
• Year: 2011
• Print ISBN: 9780750679343
• eBook ISBN: 9780080523743
• Edition: 2
• Topic: Computer Science
• Subtopic: Hardware
• Index: Computer Science

Without sensors most electronic applications would not exist—sensors perform a vital function, namely providing an interface to the real world. Hall effect sensors, based on a magnetic phenomena, are one of the most commonly used sensing technologies today. In the 1970s it became possible to build Hall effect sensors on integrated circuits with onboard signal processing circuitry, vastly reducing the cost and enabling widespread practical use. One of the first major applications was in computer keyboards, replacing mechanical contacts. Hundreds of millions of these devices are now manufactured each year for use in a great variety of applications, including automobiles, computers, industrial control systems, cell phones, and many others. The importance of these sensors, however, contrasts with the limited information available. Many recent advances in miniaturization, smart sensor configurations, and networkable sensor technology have led to design changes and a need for reliable information. Most of the technical information on Hall effect sensors is supplied by sensor manufacturers and is slanted toward a particular product line. System design and control engineers need an independent, readable source of practical design information and technical details that is not product- or manufacturer-specific and that shows how Hall effect sensors work, how to interface to them, and how to apply them in a variety of uses. This book covers:•the physics behind Hall effect sensors•Hall effect transducers•transducer interfacing•integrated Hall effect sensors and how to interface to them•sensing techniques using Hall effect sensors•application-specific sensor ICs•relevant development and design toolsThis second edition is expanded and updated to reflect the latest advances in Hall effect devices and applications! Information about various sensor technologies is scarce, scattered and hard to locate. Most of it is either too theoretical for working engineers, or is manufacturer literature that can't be entirely trusted. Engineers and engineering managers need a comprehensive, up-to-date, and accurate reference to use when scoping out their designs incorporating Hall effect sensors.* A comprehensive, up-to-date reference to use when crafting all kinds of designs with Hall effect sensors*Replaces other information about sensors that is too theoretical, too biased toward one particular manufacturer, or too difficult to locate*Highly respected and influential author in the burgeoning sensors community

Chapter 1

Hall-Effect Physics

Conceptually, a demonstration of the Hall effect is simple to set up and is illustrated in Figure 1-1. Figure 1-1 a shows a thin plate of conductive material, such as copper, that is carrying a current (I), in this case supplied by a battery. One can position a pair of probes connected to a voltmeter opposite each other along the sides of this plate such that the measured voltage is zero.

Figure 1-1 The Hall effect in a conductive sheet.

When a magnetic field is applied to the plate so that it is at right angles to the current flow, as shown in Figure 1-lb, a small voltage appears across the plate, which can be measured by the probes. If you reverse the direction (polarity) of the magnetic field, the polarity of this induced voltage will also reverse. This phenomenon is called the Hall effect, named after Edwin Hall.

What made the Hall effect a surprising discovery for its time (1879) is that it occurs under steady-state conditions, meaning that the voltage across the plate persists even when the current and magnetic field are constant over time. When a magnetic field varies with time, voltages are established by the mechanism of induction, and induction was well understood in the late 19th century. Observing a short voltage pulse across the plate when a magnet was brought up to it, and another one when the magnetic field was removed, would not have surprised a physicist of that era. The continuous behavior of the Hall-effect, however, presented a genuinely new phenomenon.

Under most conditions the Hall-effect voltage in metals is extremely small and difficult to measure and is not something that would likely have been discovered by accident. The initial observation that led to discovery of the Hall effect occurred in the 1820s, when Andre A. Ampere discovered that current-carrying wires experienced mechanical force when placed in a magnetic field (Figure 1-2). Hall’s question was whether it was the wires or the current in the wires that was experiencing the force. Hall reasoned that if the force was acting on the current itself, it should crowd the current to one side of the wire. In addition to producing a force, this crowding of the current should also cause a slight, but measurable, voltage across the wire.

Figure 1-2 A magnetic field exerts mechanical force on a current-carrying wire.

Hall’s hypothesis was substantially correct; current flowing down a wire in a magnetic field does slightly crowd to one side, as illustrated in Figure 1 – lb, the degree of crowding being highly exaggerated. This phenomenon would occur whether or not the current consists of large numbers of discrete particles, as is now known, or whether it is a continuous fluid, as was commonly believed in Hall’s time.

1.1 A Quantitative Examination

Enough is presently known about both electromagnetics and the properties of various materials to enable one to analyze and design practical magnetic transducers based on the Hall effect. Where the previous section described the Hall effect qualitatively, this section will attempt to provide a more quantitative description of the effect and to relate it to fundamental electromagnetic theory.

In order to understand the Hall effect, one must understand how charged particles, such as electrons, move in response to electric and magnetic fields. The force exerted on a charged particle by an electromagnetic field is described by:

(Equation 1-1)

where

is the resultant force,

is the electric field,

is the velocity of the charge,

is the magnetic field, and

is the magnitude of the charge. This relationship is commonly referred to as the Lorentz force equation. Note that, except for

, all of these variables are vector quantities, meaning that they contain independent x, y, and z components. This equation represents two separate effects: the response of a charge to an electric field, and the response of a moving charge to a magnetic field.

In the case of the electric field, a charge will experience a force in the direction of the field, proportional both to the magnitude of the charge and the strength of the field. This effect is what causes an electric current to flow. Electrons in a conductor are pulled along by the electric field developed by differences in potential (voltage) at different points.

In the case of the magnetic field, a charged particle doesn’t experience any force unless it is moving. When it is moving, the force experienced by a charged particle is a function of its charge, the direction in which it is moving, and the orientation of the magnetic field it is moving through. Note that particles with opposite charges will experience force in opposite directions; the signs of all variables are significant. In the simple case where the velocity is at right angles to the magnetic field, the force exerted is at right angles to both the velocity and the magnetic field. The cross-product operator (×) describes this relationship exactly. Expanded out, the force in each axis (x,y,z) is related to the velocity and magnetic field components in the various axes by:

(Equation 1-2)

The forces a moving charge experiences in a magnetic field ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Introduction
• Chapter 1: Hall-Effect Physics
• Chapter 2: Practical Transducers
• Chapter 3: Transducer Interfacing
• Chapter 4: Integrated Sensors: Linear and Digital Devices
• Chapter 5: Interfacing to Integrated Hall-Effect Devices
• Chapter 6: Proximity-Sensing Techniques
• Chapter 7: Current-Sensing Techniques
• Chapter 8: Speed and Timing Sensors
• Chapter 9: Application-Specific Sensors
• Chapter 10: Development Tools
• Appendix A: A Brief Introduction to Magnetics
• Appendix B: Supplier List
• Appendix C: Glossary of Common Terms
• Appendix D: References and Bibliography
• About the Author
• Index