Electronic Design
MEMS Inertial Sensors Push Size, Performance Limits For Next-Gen Mobile Devices

MEMS Inertial Sensors Push Size, Performance Limits For Next-Gen Mobile Devices

Microelectromechanical-system (MEMS) motion technology ICs are moving toward higher levels of integration, smaller chip sizes, lower costs, and higher performance and reliability. These developments, reflected in the latest accelerometers, gyroscopes, and inertial measurement units (IMUs), are enabling MEMS devices to address a broad spectrum of applications, particularly in consumer electronics, that call for next-generation devices.

Semiconductor manufacturers, as well as leading-edge research facilities, are using the latest hardware and software advances to develop motion-sensing devices that deliver the key advantages of MEMS technology—namely low cost and small size—to existing as well as new markets. These technical advances are enabling the development of motion sensor ICs with unique characteristics that allow companies to develop cutting-edge products and gain production differentiation in an extremely competitive market.

MEMS accelerometers and gyroscopes are rapidly growing markets. Market research firm Yole Développement projects the combined market for MEMS accelerometers and gyroscopes (Fig. 1) to reach about $3 billion by 2013. This compares with $1.85 billion in 2008, when approximately 859 million MEMS accelerometers and gyroscopes were produced worldwide.

Another market research firm, iSuppli, forecasts the MEMS market to reach $8.3 billion by 2010, up from $5.6 billion in 2006. This projection includes many other functions besides motion sensors.

CONSUMER ELECTRONICS DRIVE THE INDUSTRY

The consumer electronics market is a key driving force. Although automotive electronics applications like airbags, brake pressure (electronic stability control or ESC), and tire-pressure monitoring systems (TPMSs) still dominate, the consumer sector is forecast to overtake the automotive market by 2012. Other growing market applications include medical home healthcare devices, military, and industrial applications.

Smaller, lower-power MEMS accelerometers are in demand for consumer electronics products including mobile phones, small appliances, video games, remote controls, smart books, mobile Internet devices, and personal navigation devices (PNDs). MEMS accelerometer chips for such products are in greater demand, as many of these products are being designed to use several accelerometers and must operate from batteries with longer battery lifetimes.

The consumer electronics sector stands to benefit from already low-cost multi-axis MEMS accelerometers whose prices are now less than a dollar per chip in large volumes. “Despite the deep cuts in consumer spending on major electronics and mobile products, the MEMS industry continues to thrive in mobile handsets and consumer devices,” says Jérémie Bouchaud, director and principal analyst at iSuppli. “A number of factors drive the success of MEMS sensors, including the desire for intuitive motion-based interfaces in mobile devices and providing rich and realistic experiences.”

The demand for such real-time motion detection features such as gesture recognition, orientation, single tap, double tap, freefall, and shaking, is insatiable. MEMS accelerometer manufacturers are responding with chips that contain more circuitry than just the sensing element and are making greater use of embedded software algorithms that give designers greater flexibility in implementing any combination of these features with ever greater accuracy than heretofore possible.

“Cell phones using accelerometers are now recognized as the primary driver of MEMS sensor sales,” states Richard Dixon, an analyst at iSuppli Corp.

“Only 3% of mobile phones today contained an accelerometer in 2007. But thanks to advances in MEMS technology and consumer demand for enhanced user interfaces, that number is expected to jump to 33% by 2010,” says Demetre Kondylis, vice president and general manager of Freescale Semiconductor’s sensors and actuators solutions division.

Market research company IDC supports these forward-looking statements. It says that despite the economic downturn, smart-phone shipments rose 39% in the fourth quarter of 2009 over 2008’s fourth quarter. For 2009, smart-phone shipments totaled 174.2 million units, up 15.1% from 151.4 million in 2008. This upward trend is expected to continue.

ENHANCING THE MOBILE-CONNECTION EXPERIENCE

Many of the recent three-axis MEMS accelerometers introduced have been aimed at users of smart consumer electronics products. The object is to enhance a user’s mobile-connection experience through embedded features that deliver faster and more deliberate motion detection.

One example is the MMA8450Q (Fig. 2) low-power 12-bit, three-axis MEMS accelerometer from Freescale Semiconductor. Freescale Semiconductor used a system-level approach to develop the sensor, leveraging its power-management, i.MX ARM-based processor, sensor, and software expertise.

“This chip represents a very high-end and efficient design,” says Michelle Kelsey, marketing manager for Freescale Semiconductor’s inertial sensors.

Sensing elements within the IC detect motion with embedded intelligence features like XYZ sample first-in/first-out (FIFO) memories, high-pass filters, and embedded algorithms. Embedded functions available include orientation, tap, double tab, jolt, freefall, and shake detection.

“The FIFO and configurability are keys to the MMA8450Q’s performance. They ensure that no data from any given sensing element is lost,” adds Kelsey.

All of this is accomplished within a compact 3- by 3- by 1-mm package with high performance. These include a 12-bit digital output to an I2C port, acceleration ranges of ±2, ±4 and ±8 g, just 2 µA of current drain in the off mode, operation from a 1.71- to 1.89-V dc supply, and two programmable pins for eight interrupt sources.

Power dissipation also stands out. In the standby mode, the MMA8450Q dissipates just 10 µA with an active I2C port. In the active mode, the sensor typically dissipates 27 µA (50-Hz output data rate) or 42 µA (100-Hz output data rate).

Kionix also embeds algorithms for orientation and activity monitoring in its three-axis, digital-output KXTE9 MEMS accelerometer. The accelerometer is available in a 3- by 3-by 0.98-mm leadless-grid array (LGA) package, operates from 1.8 to 3.6 V, and dissipates just 30 µA. The company also plans to introduce application developer tools that will assist in accessing the system-level functionality that is embedded in silicon.

SQUEEZING DOWN CHIP SIZES

MEMS IC accelerometer manufacturers like VTI Technologies, STMicroelectronics, and Bosch Sensortec are pushing MEMS technology chip-size limits by making sensors smaller and less power-hungry. Last year, VTI Technologies was the first to introduce a small-size, three-axis MEMS accelerometer, the CMA3000 (Fig. 3). This three-axis, low-power (10 µA at 1.8 V) accelerometer fits into a 2- by 2- by 0.98-mm package.

The devices in the STMicroelectonics LIS3Dx Femto family (Fig. 4) of three-axis digital accelerometers are available in a 2- by 2- by 0.98-mm LGA package. They consume just 10 µA in the full-function mode at a 100-Hz sampling rate while operating from just 1.8 V. Lower power consumption values are also possible at lower data rates, such as 4 µA at 25 Hz and 2 µA at a few Hz. This chip has programmable FIFO memories, a serial peripheral interface (SPI) and I2C interface, tap and double-tap motion detection/wakeup, 4D/6D orientation detection, and acceleration ranges of ±2, ±4 and ±8 g.

Bosch Sensortec, the leading MEMS sensor supplier according to iSuppli Corp., has also shrunk its MEMS accelerometers (Fig. 5). Its three-axis digital BMA220 sports the same small-footprint package of the STMicroelectronics unit (2 by 2 by 0.98 mm), but offers a wider sensitivity range of ±2, ±4, ±8 g, and ±16 g. It dissipates 250 µA (1.8-V supply) in the full-function mode and can consume as little as 10 µA, depending on the duty cycle. Fully programmable, it features tap, double tap, auto-wake, shake, interrupt, and high-g/low-g detection, step configurability, and I2C and SPI interfaces.

“This accelerometer can be used in a unique dedicated I/O mode we created as a standalone device, without requiring a microcontroller,” explains Leopold Beer, Bosch Sensortec’s global marketing director.

Analog Devices is the first to introduce a monolithic planar MEMS accelerometer, designed for automotive airbags. Its most recent device, the ADXL346 three-axis digital-output iMEMS accelerometer, features power dissipation of just 1 µA in the standby mode and 35 µA in the full-function mode. A distinguishing feature of this ±2-g, ±4-g, ±8-g, and ±16-g user-selectable 13-bit device is that resolution can be increased in all g ranges at 4 mg/least-significant bit (LSB). This device is available in Research In Motion’s Blackberry Storm 2 SmartPhone.

“We’re focused on those applications that require a premium performance level and the end user is willing to pay for it,” explains Wayne Meyer, MEMS marketing manager for Analog Devices. “Achieving higher levels of integration on a single chip depends on the application. We’re adopting both monolithic and hybrid approaches to address different market needs.”

MEMSIC makes use of a heat principle to measure acceleration in its MXP7205VW/VF MEMS accelerometer, which can withstand a 50,000-g shock. It is accurate to within ±30 mg of zero-g offset over the operating temperature range of –40°C to 105°C, an important feature for automotive electronic-stability control (ESC) applications.

Smaller size is also being used to develop more enhanced performance by employing novel inertial-sensing architectures. This will impact the development of smaller, lower-cost MEMS accelerometers, gyroscopes, and IMUs.

A significant MEMS development from Hewlett-Packard (HP) Labs is an ultrasensitive inertial MEMS accelerometer (Fig. 6) platform that’s 1000 times more sensitive than high-volume commercial accelerometers, yet is low in cost and small in size. It delivers noise-density performance in the sub-100 ng√Hz range and a 130-dB dynamic range to allow dramatic improvements in data quality, thanks to the use of a large proof mass.

The company says that the sensor is not only much more sensitive than other MEMS sensors, it can also deliver this performance at a size and cost equivalent to present-day accelerometers used in game controllers and automotive airbags, with equivalent power dissipation. The sensor is aimed at automotive, medical, and industrial infrastructure sensing and monitoring applications. It leverages HP’s own MEMS fluidic technology.

“This new sensing technology is a critical enabler of HP’s Central Nervous System for the Earth (CENSE),” says Peter Harwell, HP senior researcher. “We’re already working on the next generation of sensors for this vision, blending in a single planar chip a three-axis inertial sensor and gyroscope.”

HP is not selling this sensor as a commodity component. “We plan to piggyback this sensor with other sensing elements and electronics and partner with others to devise a wireless sensing system solution,” explains Grant Pease, business development manager at HP’s Technology Development Organization.

“A major application for our sensor is roadway monitoring as part of the Intelligent Transportation System laid out by the U.S. government’s Department of Transportation (DoT), which can provide large energy savings on roads and freeways,” Pease added. We can easily modify this sensor’s performance to fit specific application needs, like a greater number of axes and more bandwidth.”

GYROS ALSO BENEFIT

MEMS gyroscopes are benefiting from MEMS inertial-sensor improvements. Invensense, STMicroelectronics, and VTI Technologies are major MEMS gyroscope manufacturers for the consumer electronics market. Honeywell is another major MEMS gyroscope manufacturer. However, most of its products are used in industrial, aerospace, and military applications.

InvenSense Inc., ranked as the top supplier of MEMS gyroscopes for consumer electronics motion processing according to iSuppli, recently announced the MPU-3000 family of three-axis motion-processing-unit gyroscopes. These devices include an embedded digital motion processor hardware accelerator engine.

The gyroscopes allow complete motion processing in smart phones, including the widest range of motions from 250 to over 2000°/s, and feature built-in 16-bit analog-to-digital converters (ADCs), programmable digital filters, factory calibration to 1% sensitivity, built-in six-axis sensor fusion, and 13 mW of power consumption. They’re available in small-footprint (4- by 4- by 0.9-mm) packages and offer SPI and I2C interfaces.

VTI Technologies has also announced a MEMS gyro-accelerometer combo chip aimed at industrial applications. The SCC1300-D02 is a single-axis ±100°/s x-axis gyro and three-axis ±2-g accelerometer. A D04 version features a ±300°/s x-axis gyro and a ±6-g three-axis accelerometer. Each is housed in an 18.6- by 8.5- by 4.53-mm package. The gyroscope portion is temperature-compensated over the entire operating-temperature range of –40°C to 125°C. Bias stability (Allan variance) is less than 1°/h, and bias accuracy over the full operating-temperature range is ±0.6°/s.

Late last year, STMicroelectronics integrated a three-axis digital accelerometer with a two-axis analog gyroscope (Fig. 7a) in a compact module. The LSM320HAY30 combines a user-selectable full-scale acceleration range of ±2, ±4, and ±8 g with a gyroscope whose angular detection rate ranges from 30 to 600°/s along the pitch and yaw axes. It has two user-selectable and simultaneous outputs for each axis: an unamplified output for high accuracy of slow motion, and a 4x amplified output for very fast gesture movements. The module operates from a 2.7- to 3.6-V supply and includes self-test capability.

STMicroelectronics also unveiled the low-cost LYPR540AH three-axis MEMS gyroscope (Fig. 7b), a high-performance device aimed at consumer electronics. It accurately measures angular rates along three orthogonal axes. It provides 360° angular-rate detection for high-precision gesture and motion recognition in mobile phones, game controllers, PNDs, and other portable consumer electronics items. The gyroscope has two separate outputs, operating at the same time, for each of the three axes: a 400°/s full-scale value for high accuracy of slow motion, and a 1600°/s full-scale value to detect and measure very fast gestures and movements. The devices are available in a 4.4- by 7.5- by 1.1-mm package.

The STMicroelectronics L3G200D three-axis MEMS digital gyroscope breaks new ground in motion-control realism for mobile phones and gaming consoles. It employs a single sensing structure for motion measurement along three orthogonal axes. This increases accuracy and reliability over existing two- or three-structure gyroscopes, according to the company. Available in a 4- by 4- by 1-mm package that includes the ASIC interface, the gyroscope offers user-programmable full-scale ranges from ±250°/s to ±2000°/s and a 16-bit data output.

STMicroelectronics is one of six partners in the development of the EU-funded microscopic-sized gyroscope (Fig. 8) DAVID (Downscale Assembly of Interconnected Devices) project, whose main focus lies on inertial sensor systems. The project targets extremely high packaging density for the hybrid integration of MEMS and ASICs and is supported by €2.8 million from the European Commission under the Sixth Framework Programme. Other partners include Germany’s Fraunhofer Institute of Technology, Austria’s Datacon Technology, the Netherlands’ FICO B.V., Italy’s SAES Getters S.p.A., and Poland’s Wroclaw University of Technology.

MODULAR IMUs

STMicroelectronics is spearheading the development of relatively low-cost modular-packaged IMUs for consumer electronics and portable medical electronics products. Its iNEMO v2 integrates five STMicroelectronics sensors and a 32-bit microcontroller. The sensors include a two-axis roll-and-pitch gyroscope, a single-axis yaw gyroscope, a six-axis geomagnetic module, a pressure sensor, and a temperature sensor.

Analog Devices is also actively producing modular IMUs for industrial, aerospace, and military applications using its iMEMS technology. Products like the ADS16350/54/55 provide six-degree-of-freedom movement detection.

“IMUs are definitely very performance-focused devices for the industrial and military markets that require very high-performance levels,” explains Analog Devices’ Wayne Meyer. “So you can’t expect them to be in typical small IC packages and generally command a higher price.” 

All this activity points to the need for a system-level approach. “MEMS suppliers of inertial products, especially accelerometers, are trying very hard to differentiate their products, since these products have become mature and are now commoditized,” says Roger Grace, president of Roger Grace Associates, a marketing firm specializing in MEMS. “The adding of functions, especially software algorithms, is now becoming a strategy for product differentiation. The trend has been to add functionalities on separate chips, especially ASICs, which are typically bonded to the MEMS structure.” This systems solution approach vis-à-vis functionality enhancement, especially via advanced packaging, will be a major technique for product differentiation, he adds.

This systems-level approach is being addressed by Grace and others at upcoming major MEMS conferences like the Smart Systems Integration 2010 Conference, Sensors Expo, and the Microtech Conference

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