Sensor Applications for Magnetic Materials

The successful design of commodity magnetic sensors requires an understanding of the fundamental magnetic properties of the materials used in their fabrication. This article describes a few of the types of magnetic materials commonly encountered, their properties, and the manner in which they are used.

While most, if not all materials have some measurable electromagnetic interactions, we are looking at those that fall into the following categories:

  • Hard magnetic materials
  • Soft magnetic materials
  • Electrical conductors
Hard Magnetic Materials
Hard magnetic materials, for the purposes of this discussion, are simply those out of which you can make useful permanent magnets. Examples include:

  Ferrites: Low magnetic flux; low cost; commonly mixed with plastic binder to make "refrigerator magnet" strips

  Alnico: Moderate cost; low to moderate flux, depending on grade; can be used at high (>200ºC) temperataures; wide variety of magnetic properties in various grades makes it very useful in sensor applications

  NdFeB (neodymium-iron-boron): Moderate cost; high flux; limited temperature operation (<150ºC); corrodes very easily

  SmCo (samarium-cobalt): High cost;

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Figure 1. Magnetic sensors configured in a linear mode are used to determine how close an object is to the magnet (A); devices configured in a rotatary mode measure the rotational rate of the object of interest (B).
high flux; high-temperature operation; very brittle

Hard magnetic materials are typically used in either of two ways in sensor applications. The first is as an actuator magnet. In this operating mode, the actuator magnet itself is the sensor's target of interest. This approach to sensing (proximity of a magnet) is useful mainly because strong magnetic fields (>200-300 G) do not commonly occur in nature nor are they typically produced by accident. Figure 1 shows these materials being used in linear and rotary modes.

The linear actuation mode is commonly used in proximity devices to detect whether the magnet is sufficiently close to the sensor. The rotary actuation mode is typically used to measure the speed of the object to which the ring magnet is affixed.

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Figure 2. A simple proximity sensor can be made to detect ferrous objects by measuring changes in a bias field caused by the presence of the target.

The second way in which hard magnetic materials are used is to provide a bias field in the sensor assembly. This bias field interacts with the object being sensed (typically made of a soft magnetic material such as steel), and a sensor element detects the changes in the bias field caused by this interaction.

Figure 2 shows a simple ferrous article proximity detector made in this way. The flux measured at the pole face of the bias magnet

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Figure 3. The flux at the pole face of the bias magnet in Figure 2 varies inversely with the distance of the target from the pole face.
varies inversely with the distance of a ferrous object from that pole face (see Figure 3). One major problem with realizing this type of sensor is the necessity of setting an appropriate trigger point that is effective over the normal manufacturing distribution of magnet strengths and sensor errors.

Another common bias-magnet sensor architecture is the vane interrupter (see Figure 4). Here, a ferrous vane interrupts the flux path between

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Figure 4. The vane interrupter style of bias-magnet sensor interrupts the flux path between the magnet and the sensor element.
the bias magnet and the sensor element. This leads to a very sharp rate of change of sensed flux vs. position of the vane (see Figure 5, below), much higher than that of the previously described proximity detector. This provides the ability to set very well defined turn-on and turn-off positions for the vane. An additional advantage of this architecture is that the large contrast between the interrupted and noninterrupted states makes it a very easy sensor to manufacture.
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Figure 5. The response of the sensor shown in Figure 4 exhibits a sharp rate of change of the sensed flux vs. the vane position.
Soft Magnetic Materials
Soft magnetic materials are those with a magnetic permeability significantly higher than that of free-space (µr > µ0), and which cannot be permanently magnetized to a significant degree. These properties allow soft magnetic materials to conduct magnetic flux in much the same way as copper wires are used to conduct electric currents. Some common examples are pure iron and cold rolled steel, and nickel-iron steels such as Permalloy. The principal sensor applications of soft magnetic materials are:
  • Flux guides
  • Shields
  • Sensor elements using nonlinear effects

Flux guides are useful in magnetic sensors because they allow the designer to channel magnetic flux in a more arbitrarty manner than that provided by free space. This provides two major benefits. The first is that the designer

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Figure 6. The vane interrupter type of magnetic sensor can be enhanced by the addition of a flux guide that increases the available flux. This technique reduces the overall cost of the sensor assembly.
can more closely specify the points of interaction between the sensor and the object to be sensed. This improves spatial resolution and directionality of the sensor, making it less sensitive to unintended stimuli. The other benefit is that flux guides can be used to concentrate and increase the detectable flux levels, relaxing requirements on the sensor element.

Figure 6 shows how a flux guide can be used in the design of a vane interruptor. In this example, the addition of a flux guide increases the available flux (as measured by the sensor) over that which would be

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Figure 7. Soft magnetic materials can be used as flux shields that shunt the flux lines around a region where they are not desired.
present without the flux guide. This permits the use of smaller, cheaper magnets as well as cheaper sensor elements. Such sensors are widely used in automobile ignition timing systems.

Shields are another common application of soft magnetic materials. While magnetic flux lines can't be stopped dead (they form closed loops), they can be shunted around a region in which they are not desired (see Figure 7). Multiple layers of shielding can be arranged in a nested pattern to provide even more effective shielding.

Finally, nonlinear effects of soft magnetic materials can be exploited

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Figure 8. The flux-variable inductive sensor operates on the principle that the permeability of a soft magnetic material is not constant as a function of the applied field.
for actually sensing magnetic fields. One simple sensor of this type is the flux-variable inductive sensor. This device works on the principle that the permeability of a soft magnetic material is not constant as a function of the applied field. Permeability reaches a peak at zero flux and drops off to approach that of free space as the magnitude of the flux increases. An inductor wound on a core of such a material (see Figure 8) will have an inductance vs. flux response such as that shown in Figure 9 (below). Devices of this type can be made quite sensitive and have been used as the basis for magnetic compassing applications.
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Figure 9. The sensor design in Figure 8 will have the inductance vs. flux response shown here.
Conductors, the "Nonmagnetic" Magnetic Materials
Highly conductive nonferrous materials such as aluminum, brass, and copper are not normally viewed as having significant magnetic properties. While this assumption is largely true for DC or steady-state fields, it becomes less accurate when describing these materials' interaction with AC or time-varying magnetic fields. The reason is that exposure to a time-varying field sets up induced currents (often called eddy currents) in these materials (see
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Figure 10. Eddy current sensors are based on the way highly conductive nonferrous materials interact with time-varying magnetic fields.
Figure 10). Eddy currents form in such a way as to oppose the change of the magnetic field. They work to prevent a magnetic field from entering a conductor, and from a field established within a conductor from exiting. In the case of a field's turning on, the field will largely go around a conductive body at the instant it forms, and take a certain amount of time to enter that body (see Figure 11, below). For the case of continuously varying, or AC, fields, the field will penetrate only to a certain depth, called the skin depth. Skin depth is a function of both field frequency and conductivity of the material in question.
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Figure 11. Eddy currents work to prevent a magnetic field from entering a conductor, and to prevent a field in the conductor from exiting.

The applicability of eddy current effects to the design of transformers and RF systems has long been recognized, but is often ignored in the design of commodity sensors where magnetic fields are often assumed to behave "instantaneously." Knowledge of these effects, however, becomes more and more important as dynamic performance requirements increase, such as in automobile ignition timing systems, where frequency response in the 10 kHz range is now becoming necessary.

Summary
Designing a cost-effective magnetic sensor system requires knowledge of the properties of the materials surrounding the sensor elements. Some of these properties are obvious, while others, such as eddy current effects, are not. An awareness of these issues can make the difference between a viable sensor assembly and one that meets neither cost nor performance goals.

Adapted from a paper presented at Materials Week, sponsored by ASM International-TMS, 7-10 October 1996, Cincinnati, Ohio.