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Understanding the Magneto Resistance Element

Tuesday, August 1, 2017 - 06:00
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Recently a local shop was working on a vehicle with a wheel speed sensor problem. During the diagnoses of the problem, they had removed the wheel speed sensor from the wheel bearing hub. Upon looking through the sensor-mounting hole, they could not see a trigger wheel or tone ring present. So it was determined that this missing trigger wheel was the problem with the wheel speed sensor. However, this is the correct design for this type of sensor. This design does not have a conventional trigger wheel present. Sometimes the electrical world is strange and does not work the way one thinks it should. There are many different ways in which to use electric sensing to measure the physical world. Sensors make measurements of physical quantities and convert these into electrical outputs. These electrical outputs are then used by an electrical circuit or microprocessor. One such sensor that measures the physical world in quite a different way is the Magneto Resistance Element (MRE).

What is Magneto Resistance?

Magnetoresistance was discovered in 1856 by Lord Kelvin, a prominent mathematical physicist of the time period. However, this principle was not used widely until the 1960s when it was discovered that it could be used for computer memory storage. Magnetoresistance is the ability of a material to change its electrical resistance when exposed to an external magnetic field.  When the force of a magnetic field is parallel to the current flow, the resistance of the conductor increases and when the magnetic field moves at a 90-degree angle to current flow, the resistance of the conductor decreases as seen in Figure 1. This effect is referred to as the “Anisotropic Magnetoresistance.” Most conductors have some degree of Anisotropic Magnetoresistance, the cause of which is based on the “Lorentz force.” The Lorentz force acts on a moving charge in the presence of a magnetic field. This force causes the charge carriers, electrons that are carrying the current, to move in curved paths, which increases the distance and changes the speed of the carriers across the conductor. This increased distance adds resistance to the current flowing through the conductor. There is also a crowding effect that occurs as a result of the carriers being forced sideways as well as forward. This sideways movement of the carrier crowds the conductor and decreases the effective area of the conductor thus adding resistance to the conductor as well.

Figure 1

The material that is used for the conductor varies depending on the application of the sensor. Ferromagnetic materials are used widely due to their magnetic properties and their ability to work in high operating temperatures. One widely used magnetic material is a nickel-iron known as Permalloys. Permalloys consist of a blend of approximately 80 percent nickel and 20 percent iron; slight traces of other magnetic metals can also be used in the alloy. Permalloys electrical resistivity generally varies within about 5 percent, depending on the strength and the direction of the applied magnetic field. Newer thin film technology uses a combination of layered materials of indium antimonide (InSb) or metallic n-doped indium antimonide (n-InSb) to increase the magnetoresistive ability of the conductor. Thin film electrical resistivity generally ranges from 5 percent to 20 percent depending on the combination of layered materials and their orientation. 

With Permalloy or thin film, the sensor element is made into very thin rectangular strips. The resistivity of the element will be based on the material and the way that the element is constructed, the thickness of the element or the cross-sectional area of the element, the magnetic strength applied to the element, the magnetic angular position of the element, and the distance of the magnetic force from the element. So if a stronger magnet is used, the air gap can be significantly larger, up to 3mm or 118 thousandths, while still consistently producing accurate high-resolution signals. These traits make the Magneto Resistive Element a very good choice to be used in the design of automotive sensors.

Advantages over alternatives

The Magneto Resistive Element has been used in the design and construction of all types of sensing devices from pressure sensing and rotational sensing to sensing the earth’s magnetic field. In automotive industries, use of MRE is primarily for rotational sensing.   

When designing a system that will incorporate proximity sensing (non-contact sensing) for rotational sensors, there are several methods that can be utilized. Passive systems such as Variable Reluctance (VR) sensors, active systems such as Hall Effect and Magneto Resistive sensors, or optical sensors. The difference between these systems is that in a passive system the sensor produces its own output whereas in semi-active or fully active systems, the electronic control module supplies current to the circuit so the sensor can produce its own output. In the past, Variable Reluctance sensors have been used predominately for sensing rotational angular position and angular speed. However, the VR sensor has many drawbacks in the design requirements of modern systems. The main drawback of the VR sensor is it cannot be used to sense slow Rotations Per Minute (RPM). The VR sensor has a very small output at slow RPM and ultimately there is a minimum speed that can be detected. This is due to the magnetic field movement following the target wheel. If the target is moving slowly, so too is the magnetic field.  This field moving across the turns of the windings in the VR sensor determines the output. The faster or quicker the magnetic field movement across the windings the greater the output of the VR sensor.

In modern designs the size of this sensor is also a problem. The magnet in the VR sensor will need to be large in order for the sensor to work properly. This is reflected in the size of the sensor (Figure 2). In active systems, such as Hall-effect or MRE, these sensing devices have an output when stationary so the rotational speed can be tracked to zero RPM. The sensitivity of the active sensor to a magnetic field is substantially greater as well. This means that the active sensors can be packaged in a much smaller device, enabling changes to the over all design and placement of these sensors. The difference between a Hall-element and a magnetoresistive element is that the magnetoresistive element operating in a low magnetic field is 10 times more sensitive than that of the Hall-element.

Figure 2
Figure 3

It will be important to recognize this increased sensitivity that is produced from the MRE sensor. If a shaft has a small scratch in the trigger wheel area the sensor can pick this up and make a voltage output change based on this scratch. This in turn creates problems in the control system. Additionally, the Electromagnetic Interference (EMI) such as voltage spikes and reverse voltage tolerance are better with the MRE sensor over that of the Hall-element. The operating working temperature of the MRE is much higher than that of that Hall-element as well. The benefits of MRE over all other types of sensing devices are clear and make this sensor one that you will encounter in your shop more frequently, as seen Figure 3.  

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