The measurement of magnetic fields for their own sake is a relatively uncommon goal outside the laboratory. It is more typical (in industrial, consumer, and automotive applications, for example) to see a magnetic field used as an indicator of some other phenomenon such as position or speed. Over the past 20 years, inexpensive, high-quality integrated Hall effect and magnetoresistive sensors have been making many applications both technically and economically feasible. This article presents a small sampling of what these sensors can do and how to make them do it.

Is It There?
Determining whether something is present is one of the simplest and most widespread sensor applications. Sensors that perform this function are called proximity detectors.

  Magnet Proximity. Figure 1 shows two types of magnet proximity detector. In the head-on detection mode, the sensor actuates when the magnet is brought up to it and deactuates when the magnet is removed. This arrangment is suitable for situations where the magnet will be stopped before it comes into contact with the sensor, as sensors do not tend to make good end-stop bumpers. For applications where there is a significant chance that the magnet will overshoot its mark, the slide-by detection mode is often a better choice. This configuration also offers the advantage of somewhat more consistent actuation and deactuation points.

  Precision Magnet Proximity. Simple magnet proximity does not always provide sufficient accuracy or unit-to-unit repeatability for a given application. If improved definition of actuate and deactuate points is required, the slide-by detection mode may be modified as shown in Figure 2. Joining two magnets with their opposite poles side by side creates a very steep transition between north and south as the poles slide by the sensor. This transition gives rise to very well controlled and repeatable actuation points. With modern Hall effect sensors and rare-earth magnets, actuation points can easily be controlled to a few thousandths of an inch.

  Ferrous Object Proximity. Attaching a magnet to the object to be detected is often either impractical or just a nuisance. The sensor in Figure 3 can detect the presence of a ferrous (iron or steel) object. A ferrous object brought up to the face of the sensor will concentrate and increase the magnetic field passing through the sensor element. Although conceptually simple, this sensor is very difficult to implement well (from performance, manufacturing, and stability standpoints) because of manufacturing variations in both magnets and commonly available magnetic sensors. Nevertheless, there are situations in which it is the best solution.

  Ferrous Vane Interrupter. While it can be difficult to detect the presence of a ferrous object with a sensor placed between a magnet and that object, it is quite easy to detect a ferrous object when it is placed between a magnet and a sensor (see Figure 4). This configuration, commonly called a magnetic vane interrupter, works in a manner similar to that of an optointerrupter. The difference is that the vane in an optointerrupter interrupts the passage of light from an LED to a photodiode, and the vane in a magnetic vane interrupter interrupts the flow of a magnetic field from the magnet to the sensor. Magnetic vane interrupters often can be used in the same types of application served by optical devices, and even come in similar packages (see Photo 2).

Magnetic devices have one important advantage over their optical counterparts-near-total immunity to spurious operation and failure caused by dirt and other contamination. Two caveats apply, however. The first is that nonferrous vanes do not actuate magnetic vane interrupters. The second is that the magnet in the magnetic vane interrupter exerts significant mechanical force on the vane (the photons from the LED in the optointerrupter also exert a force on the vane, but I have yet to see an application where anybody really cared about this force or was interested in measuring it). For the most part this is not a problem, but in low-force applications such as a paper-path sensor in a printer it becomes a major issue.

  Magnetic Pushbutton. Magnetic sensors can also be used to make updated versions of electromechanical sensors, such as pushbutton switches. In the device shown in Figure 5, not only do the sensor and magnets provide a replacement for the contacts, but they also replace the spring and other mechanical components. The opposing pole faces of the two magnets serve two purposes: they provide the mechanical spring-return force for the switch; and they provide a null point exactly halfway between them, which has a very steep magnetic gradient. As the plunger is depressed, this null point moves exactly half that distance. The steep gradient surrounding the null point ensures accurate and repeatable mechanical operate points. This represents an improvement over a single magnet used with a wire spring to provide return force.

Where Is It?
While simply knowing whether something is there or not is adequate for many applications, it is occasionally useful to know exactly where that something is. Linear and rotary position are two quantities that simple magnetic sensors can be constructed to measure.

  Linear Position. For short distances (on the order of a few hundred mils), the position sensor in Figure 6 can provide an economical solution to linear measurement problems. This configuration partially solves two of the more annoying problems commonly encountered in magnetic distance sensing schemes-lack of linearity and lack of a well-defined reference point. A pair of opposing magnets mounted on a movable, nonmagnetic yoke creates a magnetic null point exactly halfway between the two magnets. This is convenient as a stable reference point. Another benefit offered by opposed magnets is that the sensed field vs. displacement is a nearly linear function over a significant range of travel.

  Rotary Position. Figure 7 shows a sensor that is commonly used for measuring rotary position or angle. This device uses a pair of magnets to create a moderately uniform field over the space occupied by the sensor in the center, which is stationary. When the magnets are rotated around the sensor, it sees a field that is a sinusoidal function of the angle of rotation.

In the version shown, the simplest form of this type of sensor, usable levels of linearity are obtainable over a span of about ±30º of rotation. Advanced versions can use multiple sensor devices (oriented at various angles) and linearization circuitry or a microcontroller to provide an output that is linear over a full 360º of angular rotation. Most applications for this type of device are in high-use or dirty environments, where the wipers on traditional potentiometers would create a reliability problem.

How Fast Is It Going?
Magnetic sensors have long been used to measure velocity, especially that of rotating objects. Here are a couple of simple ways to do this.

  Ring Magnet Speed Sensor. A ring magnet (see Figure 8) is formed in the shape of a ring or torus. While these magnets can be magnetized in a variety of patterns, the pattern of interest for use in a speed sensor is one in which alternating poles (north and south) are set up along the circumference. When a ring magnet is affixed to a rotating shaft, and a magnetic sensor is placed so as to detect the poles as they pass by, the sensor delivers a pulse train whose frequency is proportional to the shaft speed. This sensor is extremely easy to implement, and offers good performance over a wide range of operating conditions and manufacturing tolerances. When ferrite ring magnets are used, the cost is also very low. For speed sensing applications, this should probably be the first magnetic scheme to consider.

  Speed and Direction (Magnetic Encoder). By adding another sensing element and a bit of ancillary electronics to the previously discussed speed sensor, it becomes possible to sense direction of rotation as well. Figure 9 shows how this is done.

Orienting one sensor half a pole's distance ahead of the other causes the first sensor's output to either lead or lag the second sensor's by 90º (with respect to the electrical waveforms output from the sensors). This lead or lag is then detected by the flip-flop, whose output is 0 for rotation in one direction and 1 for rotation in the other. Many of you may recognize this as the basic scheme used by optical encoders. The primary advantage of the magnetic version is immunity to contamination; the primary disadvantage is difficulty in obtaining comparable resolution. While it is easy to find optical encoders that provide 10,000 pulses per revolution, it can be hard to make magnetic encoders that provide more than a few hundred.

  Linear Speed and Direction. By "unwinding" the ring magnet used in the previous application, we get a device that can measure both linear speed and direction of travel, as shown in Figure 10. While a rigid rod magnet could be used to provide the required alternating pole pattern, the new flexible magnetic materials have made this sensor much easier to implement, especially for long linear runs. For low-resolution applications where the sensor can be very close to the magnetic strip, flexible ferrite materials offer up to 16 poles per linear inch. These materials can be purchased in rolls of varying thickness and width, often with pressure-sensitive adhesive backing for easy installation. For more demanding applications, either in terms of number of poles per inch or working distance from sensor to magnet strip, rare-earth materials (neodymium-iron-boron or samarium-cobalt) mixed with plastic binders are also available.

Summary
This article has presented 10 applications for inexpensive magnetic sensors. While there are many more, these provide a starting point for thinking about how the devices can be used in your designs.

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