Hall Effect Magnetic Design: Head-on and Slide-by Configurations
Learn about how the Hall effect is used in position sensing applications.
Position sensing is one of the most important applications of Hall effect sensors. A Hall effect device senses the strength of the applied magnetic field. To detect the position of an object, we can affix a small permanent magnet to the object. As the object moves the magnet in relation to the Hall device, the strength of the magnetic field changes. These changes can be processed by the system to detect the object position.
There are several different sensor-magnet configurations that can be used in Hall effect-based position sensing applications. With each sensor-magnet configuration, the magnet is moved in relation to the sensor in a different way. This affects the magnetic field sensed by the Hall device and changes the system characteristics.
In this article, we’ll take a look at some of the basic magnetic configurations that are used in Hall effect-based position sensing and discuss their advantages and disadvantages.
The simplest magnetic configuration is the head-on sensing shown in Figure 1.
In this case, the south pole of the magnet is moved directly toward or away from the sensor. When the magnet is very close to the sensor, a larger number of the magnetic lines of flux pass through the sensing face of the sensor. However, as the south pole of the magnet moves away from the sensor, the magnetic field intensity decreases rapidly as shown in Figure 1(b).
Note that the Gauss versus distance curve is sometimes referred to as the flux map of the structure. The magnetic flux density of the head-on configuration is inversely proportional to the square of the distance between the magnet and the sensor. The flux values given in Figure 1(b) can be produced by a magnet that is approximately 30 mm long and has a diameter of about 6 mm.
Application: Detecting the Presence of an Object
Head-on configuration can be used with a digital (ON/OFF) Hall effect sensor to detect the presence of an object. Assume that the magnetic operate and release points of the sensor are as shown in Figure 1(b). The Magnetic operate point specifies the level of a strengthening magnetic field at which a Hall device switches on. The magnetic release point corresponds to the weakening magnetic field at which the Hall device switches off.
As the magnet approaches the sensor, the magnetic field intensity becomes increasingly larger. At a distance of about 3.1 mm, the sensed magnetic field is equal to the magnetic operate point (about 620 Gauss in our example) which turns the sensor on. Bringing the magnet closer to the sensor leads to an even larger magnetic field and keeps the sensor ON. When the sensor moves away from the magnet, the magnetic field decreases.
However, as long as the magnetic field is not smaller than the release point (about 420 Gauss in our example), the sensor remains ON. Only when the magnetic field drops below the release point does the device turn off. In this way, we can detect the presence of an object.
Limitations of Head-On Sensing
This configuration is less precise especially in terms of the distance that makes the sensor turn off. This is due to the fact that the slope of the flux map (the Gauss versus distance curve) is smaller around the release point. A given variation in the value of the magnetic release point can lead to a relatively large variation in the value of the distance at which the sensor switches off. The same variation in the magnetic operate point leads to a smaller distance variation. This is illustrated in Figure 2.
Assume that the unit-to-unit variation of the operate and release points are ΔB for a hypothetical sensor as shown in Figure 2. Since the curve has a larger slope at the magnetic operate point, Δd1 is much smaller than Δd2. Hence, the distance corresponding to the magnetic operate point will be more consistent between different boards.
Another disadvantage is that head-on sensing cannot be used for detecting large displacement ranges because the magnetic field lines decay very rapidly. Besides, with head-on configuration, the relationship between the sensed field and distance is non-linear. This makes detection of long stroke movement challenging when linear position measurement is required. Due to these limitations, head-on sensing is usually used as a proximity detector in applications where accuracy requirements are not very demanding.
Unipolar Slide-By Sensing
In this arrangement, a single pole of the magnet moves sideways past the sensing face of the sensor. This is shown in Figure 3(a).
The magnet pole is at a constant perpendicular distance (shown by “air gap” in the figure) as it moves past the sensor. At the center position (distance=0), the sensed magnetic field is at its maximum. As the magnet moves away from the sensor, the magnetic field decreases. Since the magnetic field produced by the magnet is symmetric, the flux map is symmetrical around the origin as shown in Figure 3(b).
The peak value of the flux map changes with the “air gap” value as shown in Figure 4.
Figure 4. Image courtesy of Allegro.
Head-On or Slide-By Configuration?
It’s important to note that the choice of magnetic configuration depends on the type of motion being detected. Each system might have a different set of mechanical limitations and features. For example, with the head-on configuration, the object cannot move past the sensor. This arrangement suits applications where the object being detected has a definite end position and we are interested in detecting the presence/absence of the object at this end point.
For example, head-on sensing might be a good choice for detecting whether the lid of a smartphone cover is open or closed. The slide-by sensing doesn’t have this limitation; it can be used no matter if the object moves past the sensor or not.
One interesting feature about the slide-by configuration is its symmetry. Since the Gauss versus distance curve of this structure is symmetrical around the origin, the operate and release points do not depend on whether we are moving toward or away from the sensor. This can be useful when detecting deviation from a center line.
Note that there are other slide-by configurations some of which do not provide a symmetric response. In the next article, we’ll take a look at more sophisticated magnetic configurations that are commonly used in Hall effect position sensing applications.
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