MEMS accelerometers have had a dramatic impact on cell phones, gaming consoles, and location-based devices. MEMS-based gyroscopes and geomagnetic sensors are about to take it to a whole new level.
The MEMS consumer market grew by 27 percent in 2010 to $1.6 billion, according to iSuppli, which predicts revenues for these devices to top $3.7 billion by 2014. The continued demands from consumer and mobile applications dominate this market’s growth, and these fields are expected to become the biggest MEMS segment by 2014.
MEMS sensors are well recognized as the key building blocks for implementing disruptive applications in consumer devices. From game consoles to mobile phones and from laptops to white goods, consumer devices have already benefited in recent years from the use of low-g accelerometers for the implementation of motion-activated user interfaces and enhanced protection systems. It is now the turn of MEMS gyroscopes and geomagnetic sensors, as use of these sensors is propelling a new wave of compelling applications.
Much has been written about the technology behind MEMS accelerometers, which are sensors capable of detecting linear accelerations. Accelerometers will thus be barely touched in this article, this will focus on gyroscopes, geomagnetic sensors, and other devices capable of providing multiple degrees of freedom to sensing applications.
Capable of measuring angular rates around one or more axes, these gyroscopes represent a fitting complement to MEMS accelerometers. Thanks to the combination of accelerometers and gyroscopes it is possible to track and to capture complete movements in a three-dimensional space. This gives system developers the ability to deliver more immersive user experiences, accurate navigation systems, and much more.
With the recent introduction of 30 different gyroscopes delivering high performances with reduced current consumption and compact packages, STMicroelectronics is continuing its rapid growth in the MEMS market. The heart of ST’s gyroscopes is represented by a micromachined, mechanical element designed to operate according to a tuning fork scheme, exploiting the Coriolis effect to transform an angular rate into a displacement of a specific sensing structure.
Let’s consider, for example, the simple case of a single-axis yaw gyroscope (see Figure 1). Two moving masses are kept in continuous movement in opposite directions, which are indicated by the blue arrows. As soon as an external angular rate is applied, a Coriolis force, which is shown with the orange arrows, will appear in the direction orthogonal to the movement and will cause a displacement of the sensing masses proportional to the magnitude of the rate applied. Since the moving electrodes (rotors) of the sensing portion of the sensor stand beside fixed electrodes (stators), the displacement will induce a change in the electrical capacitance among stators and rotors, thus transforming the angular rate applied at the input of the gyroscope into an electrical parameter detectable by means of a dedicated circuit.
|Figure 1: Single-axis MEMS yaw gyroscope.|
ST’s micromechanical gyroscope sensors exploit the same technology ST has already employed for the manufacturing of over 600 million accelerometers. This choice guarantees the customer state-of-the-art, reliable products for direct use in their final applications. Compared to other gyroscopes, the differential nature of the tuning fork approach adopted by ST makes the system intrinsically insensitive to undesired linear acceleration and spurious vibrations acting on the sensor. When such unwanted signals are applied to the gyroscope, both masses will be displaced along the same direction, causing the overall resulting capacitance variation to be null upon a differential measurement.
The conditioning circuit employed for a gyroscope can be schematized as the combination of a driving section for a motor and the sensing circuitry of an accelerometer (see Figure 2).
- The driving section excites the mechanical element, making it oscillate back and forth by means of an electrostatic actuation.
- The sensing circuitry measures the displacement of the sensing mass produced by Coriolis force by measuring variations in capacitance, a robust and reliable technique successfully used across ST’s MEMS product lines. The sensor circuitry provides either an analog or digital output signal that is proportional to the angular rate applied to the sensor.
|Figure 2: Block diagram of a single-yaw MEMS gyroscope.|
Advanced power-down features embedded in the control circuit allow a shut-down of the complete sensor when its function is not needed. Alternately, the sensor can enter a low-power sleep mode where the gyroscope’s overall power consumption is reduced significantly compared to the normal mode, while still being able to wake up immediately upon user command to measure the applied angular rates.
Much like its MEMS accelerometers, ST’s MEMS gyroscopes are built following a system-in-package (SIP) approach, with the mechanical sensing element and its conditioning ASIC placed inside the same package. Smart design techniques together with advanced packaging approaches have significantly shrunk the size of this family of devices. ST’s multiple-axis MEMS gyroscopes are available in packages with a footprint as small as 3 x 5 mm and a maximum thickness of only 1 mm (see Figure 3), while keeping sensor stability and ensuring a high level of performance over the life cycle of the final product.
|Figure 3: Multi-axis gyroscopes from ST in ultra-compact LGA packages.|
STMicroelectronics offers a wide selection of gyroscopes covering from one to three axes of sensitivity and full-scale ranges from 30 dps up to 6,000 dps. This gives system designers the possibility of addressing different applications ranging from image stabilization to gaming, pointing devices and robotics control.
Similar to what happened with three-axis accelerometers, the availability from STMicroelectronics of high-performance three-axis gyroscopes is propelling the implementation of advanced man-machine interfaces integrated inside mobile phones, game consoles and other applications.
Not only motion
A new class of devices is now finding its way into consumer devices: geomagnetic sensors. Capable of measuring the earth’s magnetic field along multiple axes, these devices enable the implementation of enhanced compassing and navigation functions for portable devices.
Just as with motion sensors, the fastest growing market for magnetic sensors is consumer electronics and mobile handsets, which experienced more than 10-fold increase in shipments of electronic compasses in 2009. According to iSuppli, this market is expected to accelerate rapidly, growing from eight million units in 2008 to over 540 million units in 2013, a CAGR of 129 percent.
Among the different technologies available for the manufacturing of silicon magnetic sensors, AMR (anisotropic magneto-resistive) sensor technology is gaining momentum. This is due to its ability to combine high spatial resolution and high accuracy with low-power consumption, which are crucial parameters in battery-operated portable devices. This type of sensor exploits the change in resistance that occurs in a thin strip of ferrous material when a magnetic field is applied perpendicular to the flow of current passing in the strip itself. The transducer is normally in the form of a Wheatstone bridge as shown in Figure 4, and is composed of magnetoresistors having, at rest, the same resistance, R. During the measurement, the bridge is supplied with a voltage, Vb that forces a current to flow through the resistors. Whenever a magnetic field, H, is applied, it causes the magnetization vector in two of the oppositely placed resistors to rotate toward the current, resulting in an increase in their resistance. In the remaining two oppositely placed resistors, the magnetization vector rotates away from the current, resulting in a decrease in their resistance. In the linear range, the output of the sensor is thus proportional to the applied field.
|Figure 4: Magnetoresistive transducer simplified diagram.|
With mobile phones representing the hottest area for sensor usage, magnetometers are becoming more and more interesting when combined together with accelerometers for implementing tilt compensated compasses. An example of such a device exhibiting six degrees of freedom (6 DOF) is the six-dimensional module LSM303DLH from STMicroelectronics. This device features a high-performance three-axis accelerometer integrated together with a high resolution three-axis magnetometer into a compact LGA package (see Figure 5). The magnetic sensing part also includes additional current straps that allow it to electrically set or reset the polarity of the output, and to apply an offset field to compensate for ambient magnetic fields.
The LSM303DLH provides three-dimensional heading accuracy within buildings, automobiles, and at high latitudes in such places as the northern US, Canada, and northern Europe, where the declination angle of the earth’s field is difficult to measure with Hall-type sensors. In combination with software drivers for heading, auto calibration, and soft-iron/hard-iron compensation—available for a variety of popular mobile phones’ operating systems—the LMS303DLH six-dimensional sensor gives system designers a powerful instrument for implementation of navigation functions.
More generally, the balanced combination of accelerometer, gyroscope, and magnetometer data integrated by means of specific software layers for sensor fusion fosters the implementation of advanced applications such as location-based services, pedestrian dead reckoning, and enhanced motion-controlled games. Sensor fusion refers to a set of adaptive algorithms capable of mixing the complementary information coming from the various sensors, and combining them together intelligently to achieve better system-level performance in terms of accuracy, resolution, stability, and response time. These advances enable platform makers to build, for example, robust GPS systems capable of providing navigation services even when signals from satellites are either weak (in urban canyons) or not available (indoors or in a subway). In the near future, merging the knowledge of a GPS unit’s exact position with additional metadata delivered by service providers and displayed on the screen of the user’s mobile phone will enable the user to benefit from location-based services. This will enable them, for example, to get information on the shops located in a shopping mall, directions to places where they can find desired goods, as well as special promotions and discounts customized to the user’s interests. Examples of such applications are already appearing, such as the mobile augmented-reality browser Layar, which is available in the Netherlands for Android mobile phones. The recent availability of open software platforms, where software development can easily encounter advanced sensing capabilities, is encouraging the collaborative work of developers to create applications that respond to their own needs, at the same time adding higher value to electronic equipment. The combination of MEMS accelerometers, gyroscopes, and geomagnetic sensors is also spreading into inexpensive toys, where motion capture allows interactive gaming experiences and web presence even for the youngest. Children will be soon able to create virtual dolls and characters, playing with them not with buttons and keyboards but with natural movements, and even share their gaming activity within online communities of friends involved in the same virtual worlds.
The recent introduction of tiny, reliable, and inexpensive MEMS gyroscopes and magnetometers, together with specific software for sensor fusion, makes enhanced motion tracking and more immersive user experience a reality in numerous consumer devices. STMicroelectronics includes all these devices in its sensors portfolio.
ST has recently expanded its MEMS portfolio with microphones and pressure sensors. MEMS microphones enable smaller, thinner, and lighter designs for mobile phones and other portable devices with better directionality, noise cancellation and fidelity. With the addition of ST’s micromachined pressure sensors, consumer devices will be able to measure atmospheric pressure and/or identify their precise location in all three dimensions.
|Figure 5: LSM303DLH 3x accelerometer and 3x magnetometer in compact LGA package.|