Sensor Applications for Magnetic Materials

he 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

  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;

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).

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.

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

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.

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

Figure 4. The vane interrupter style of bias-magnet sensor interrupts the flux path between the magnet and the sensor element.

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.

• 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

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.

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

Figure 7. Soft magnetic materials can be used as flux shields that shunt the flux lines around a region where they are not desired.

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

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.

Figure 9. The sensor design in Figure 8 will have the inductance vs. flux response shown here.

Figure 10. Eddy current sensors are based on the way highly conductive nonferrous materials interact with time-varying magnetic fields.

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.

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