Magnetic Sensors: Growing in Use, Shrinking in Size

Magnetic sensors are traditionally used for speed, rotational speed, linear position, linear angle, and position measurement in automotive, industrial, and consumer applications. In particular, the automotive sector has emerged as the global market leader for magnetic field sensors, accounting, and according to some estimates, for more than 40 percent of the total magnetic-field-sensor market. Increasing demand to integrate more safety functions in automobiles has created opportunities for magnetic-field sensors, which are used in various safety applications, such as Electronic Stability Control (ESC) systems, steering-angle sensing, force and torque sensing, and Anti-Lock Braking Systems (ABS), among others.

There is also a high growth rate associated with magnetic sensors in the consumer electronics market as electronic compasses (or E-compasses) have become popular primarily due to their ability to enhance the user navigation experience in consumer electronic devices.

According to the market research company iSuppli, global revenue for silicon magnetic sensors will reach $10.8 billion by 2017 and has been running at a compound annual growth rate (CAGR) of about 12 percent.

Over the past two decades, the use of magnetic sensors has blossomed based on the lower cost and higher quality of Hall Effect and magnetoresistive (MR) sensors. Hall-Effect solutions are popular because they outperform potentiometers, reed sensors, and mechanical switches at higher performance levels thanks to their robustness and the elimination of mechanical wear, a common source of premature failure. MR sensors feature inherently low hysteresis and high linearity for improved measurement accuracy. (As the magnet moves closer to the magnetic-field sensor, the value becomes high enough for the circuitry and the output is turned on.  As the magnet is moved away from the sensor the field is weakened and the output is turned off.  Hysteresis is the differential between when the output is turned on and when the output is switched off.)

Microelectromechanical (MEMS)-based magnetic-field sensors are small devices used to detect and measure magnetic fields. They detect changes in force so that voltage or resonant frequency is easily measured electronically or, alternately, the mechanical displacement can be measured with optics. A MEMS-based magnetic field sensor can be placed close to the measurement location and thereby achieve higher spatial resolution. A MEMS-based magnetometer can be combined on-chip with an accelerometer, further expanding its range of application. Magnetometers are instruments used to measure the magnetization of a material like a ferromagnet, or to measure the strength and in some cases the direction of the magnetic field at a particular point. Finally, since constructing a MEMS magnetic-field sensor does not involve the microfabrication of magnetic material, the cost of the sensor can be reduced.

Depending on the application at hand, magnetic sensors run the gamut from reasonable to extreme sensitivity. Magnetic sensing can be accomplished with many devices, including an AMR magnetometer, GMR magnetometer, Hall-Effect sensor, Lorentz force-based MEMS sensor, MEMS compass, and magnetic-field sensor, to name a few.  

Taken together these advances have made many applications technically and economically feasible, opening up the number and types of designs where magnetic sensors can be put to use.

Let’s now look at some representative magnetic sensors available to design engineers.

The Honeywell HMC5983 (Figure1) is a temperature-compensated three-axis integrated circuit magnetometer. This surface-mount, multi-chip module is designed for low-field magnetic sensing for applications such as automotive and personal navigation, and vehicle detection. The HMC5983 includes high-resolution HMC 118X series magnetoresistive sensors and an ASIC that contains amplification, automatic degaussing strap drivers, offset cancellation, and a 12-bit ADC that enables 1° to 2° compass-heading accuracy. The I²C or SPI serial bus allows for easy interface.

Figure 1: Schematic diagram of the HMC5983.  Besides keeping all components that may contain ferrous materials (nickel, etc.) away from the sensor on both sides of the PCB, it is also recommended that there is no conducting copper under/near the sensor in any of the PCB layers.

The HMC5983 is based on Honeywell’s Anisotropic Magnetoresistive (AMR) technology that is said to provide excellent linearity, low hysteresis, null output, and scale-factor stability over temperature, and with very low cross-axis sensitivity. The sensor construction is designed to measure direction and magnitude of magnetic fields.

The 3-Axis magnetoresistive sensors and ASIC are housed in a 3.0 x 3.0 x 0.9 mm LCC surface-mount package and its small size is ideal for highly integrated products. Engineers working on high-volume, cost-sensitive OEM designs just need to add a microcontroller interface and two external SMT capacitors to achieve a solution. Other features include easy assembly and compatibility with high-speed SMT assembly, temperature-compensated data output and temperature output, automatic sensitivity maintenance under a wide temperature range, and offset compensation that maximizes the sensor’s full dynamic range and resolution. The device is compatible with battery-powered applications and eliminates sensor calibration necessary for other magnetic-sensor technologies.

For applications requiring open/close or slide detection (i.e., in mobile phones), ROHM offers magnetic-switch Hall ICs and Hall ICs for wheel keys and trackballs. ROHM’s Hall-Effect sensor IC has the Hall element embedded, and it sends out the electric signal as the Hall element detects the magnetic field with Hall Effect. Its Omnipolar Detection BU52051NVX Hall ICs are magnetic switches that can operate both S- and N-pole, with output that goes from high to low. Features include low-power operation (it uses the intermittent operation method—current consumption maxes out only at the sensing operation and is in stand-by for all the rest) and is available in a variety of ultra-compact, thin wafer level, ultra-compact wafer level, ultra-small outline, and small outline packages, as well as with a variety of power supply voltages, from 1.8 V to 5.0 V. Applications include mobile phones, notebook computers, digital video cameras, digital still cameras, white goods, and more.

The A1468 three-wire true zero-speed differential peak-detecting sensor IC with continuous calibration by Allegro (Figure 2) is a Hall-Effect sensor that is optimized for digital ring-magnet sensing or, when coupled with a magnet, for ferromagnetic target sensing in three-wire applications. The IC incorporates dual Hall-Effect elements with 2.2 mm spacing and signal processing that switches when responding to differential magnetic signals created by ring-magnet poles. The circuit contains a digital circuit that reduces system offsets to calibrate the gain for air-gap-independent switch points and to arrive at true-zero speed operation. Running-mode calibration provides immunity to environmental effects, such as micro-oscillations of the target.

Figure 2:  Functional Block Diagram of the Allegro A1468.

The device is said to be ideal for obtaining speed- and duty-cycle information in ring-magnet-based speed, position, and timing applications such as in automotive speedometers (given the magnetic sensor’s ability to operate under extreme conditions that include dust, grease, dirt, humidity, and vibration it is a natural for automotive applications).

While it cannot be questioned that the contactless type of magnetic-position sensor has been a big success, there are questions and challenges that remain in that magnetic sensors can be sensitive to stray magnetic fields, impacting output accuracy should the fields sufficiently reduce the signal-to-noise ratio (SNR) and overwhelm the weak field generated by the target magnet with which a Hall-Effect sensor is paired. In applications where high electromagnetic interference is found, such as in electric cars or in industrial settings, this is a real concern. Fortunately, there are a number of countermeasures available to offset the SNR problem. These include magnetic shielding (although this can be expensive and space consuming), repositioning of magnets closer to the magnetic sensor (which may also increase cost) and higher sensor integration of IC circuits (making the device less susceptible to stray magnetic fields).

For more information about the parts discussed in this article, use the links provided to access product pages on the DigiKey website.