Magnetic field sensors are popular choices for position sensors for a variety of reasons. They are non-contact sensors, which means there are no parts to wear out. The reduction in mechanisms also means there are fewer parts to assemble and fewer parts to fail in a design. Some magnetic field sensors can provide three-dimensional position information.
Magnetic field sensors use one of three physical phenomena to detect the intensity and direction of a nearby magnetic field: anisotropic magnetoresistance effect (AMR), giant magnetoresistance effect (GMR), and tunnel magnetoresistance effect (TMR).
Structure of AMR, GRM, and TMR elements courtesy of TDK
The anisotropic magnetoresistance effect in metals describes how the resistance of the material is related to the orientation between the current and the magnetic field—reaching a minimum when the current and magnetic field are at right angles to one another.
The giant magnetoresistance effect is a large change in resistance between two ferromagnetic layers that are separated by a non-magnetic conductor. The resistance decreases when the magnetic field in the two layers is parallel and increases when the magnetic fields are anti-parallel.
The trouble with these two approaches is that the changes in resistance are relatively small and require a Wheatstone bridge and a signal amplifier for detection. Thermal noise in the sensor and Wheatstone bridge is amplified and decreases the overall sensitivity of the device. Temperature changes also have a significant effect on the sensor over its operating range.
With this in mind, some sensor developers have focused on reducing noise by utilizing the third phenomena, tunnel magnetoresistance.
A timeline of research and development into TMR technology. Image courtesy of MDT
Let's take a look at one of the TMR sensors available for use.
In the case of the TAS 214x line of magnetic field sensors, released in June, two ferromagnetic layers are separated by a nanometer-thick insulator. Electrons use the physics of quantum mechanics to tunnel from one layer to another. If the magnetic fields of both layers are parallel, electrons are more likely to tunnel from one layer to another. When the magnetic fields are anti-parallel, tunneling decreases or ceases altogether.
The TMR output is 20 times greater than an AMR sensor, and six times greater than a GMR sensor. This means that the output can be read directly by an analog to digital converter, without first passing through an amplifier. This decreases noise and increases accuracy. Additionally, the tunneling effect is less sensitive to temperature variations than its counterparts.
AMR, GMR, and TMR output voltages from tdk.com
These sensors have two TMR elements inside the package that are oriented 90-degrees from one another. This simultaneously provides Sin and Cos output. This feature allows engineers to incorporate a very simple error correction routine that takes advantage of the trigonometric identity that $$Sin(\theta)^2+Cos(\theta)^2\equiv 1$$. Any deviation from 1 would indicate a sensor-fault and let a computer know that the sensor output cannot be trusted. An important feature for drive-by-wire and fly-by-wire control mechanisms, where an errant sensor reading might steer a vehicle into oncoming traffic.
TDK lists several applications for TMR sensors in automotive applications. On the list are acceleration pedal sensors, steering angle sensors, and magnetic encoders for BLDC motors.
Other TMR Sensors
TMR sensors in the market include entries from familiar names in the industry. Many of them were released in 2016 with some updates in more recent years.
The TMR2015, a TMR-based geartooth encoder from Chinese-founded MDT (MultiDimension Technology), was released in June of 2018. This is a continuation of MDT's work with the October 2017 MDT release of the TMR2105 magnetic field sensor.
The Crocus CT219 differential current sensor debuted in 2016. In May of last year, Crocus licensed its advanced TMR sensor technology to NXP.
A Crocus TMR differential current sensor. Image used courtesy of Crocus via Mouser.
The Coto RedRock series of magnetic sensors and switches are also based on TMR technology. Their most recent release, featured at June's Sensors Expo 2018, is the RedRock RR111 wafer-based TMR analog sensor. According to its documentation, this sensor is suited for automotive, medical, and industrial applications—especially level detection sensing and proximity sensing for all of the above. Coto claims that its TMR sensors leverage patented technologies that allow "seamless CMOS integration".
Littelfuse, for their part, offers TMR options for many of their sensors in the automotive sphere. According to their product catalog (PDF) for various automotive applications, their fluid level sensors, clutch position sensors, seat belt buckle sensors, and seat occupancy sensors are all available in TMR options. Other options include Reed and Hall sensors, and also Pulsed WaveGuideTM (PWG) sensors, which is a patent-pending Littelfuse technology.
A clutch position sensor available with TMR technology. Image used courtesy of Littelfuse.
Based on this sampling, it appears that TMR sensors have found ample use in the automotive industry, though there are many examples of TMR sensors in medical applications, as well.
Have you worked with TMR sensors in your designs? Which applications have you found them best suited for? Share your experiences in the comments below.
Featured image of the TAS2141 package used courtesy of TDK.