Electronic Design

MEMS On The Move: Motion Sensors For The Masses

Applications reap the benefits of steady advances in low-cost, low-power, and small-size advances in a maturing MEMS sensor technology.

The MEMS motion sensor is no longer the bastion of just the automotive and industrial markets. Thanks to the maturation of design and manufacturing methods, these accelerators and gyroscopes are meeting the price points for mass-market requirements. As a result, they've extended their reach into many consumer, computer, well-being, well-care, and security applications.

For instance, low-cost, small-form-factor, multi-axis MEMS sensors are being used in high-volume consumer and portable-computing applications. They protect these products from damage, improve wireless connectivity, or provide new user-input commands based on movement instead of keyboard entries or buttons.

Research company IC Insights believes inertial sensors (accelerometers and gyroscope devices) will become the largest product category in solid-state sensors in 2008. By next year, they'll overtake the pressure and magnetic-field sensors used in automotive, industrial, and other applications (Fig. 1).

ACCELEROMETERS DRIVE THE MARKET
The expansion of MEMS sensors in consumer electronics applications can be credited to MEMS accelerometers. Early single-axis models that sensed in only one direction along a line have given way to dual-axis units. Both Freescale Semiconductor and Analog Devices have pioneered the way toward the application of low-cost MEMS accelerometers in consumer applications. Another pioneer in 3-axis MEMS accelerometers is ST Microelectronics (see the sidebar).

Most MEMS accelerometers are manufactured on a bulk micromachining process, necessitating the use of an additional chip for signal processing. Here, the two chips are interconnected via wafer bonding or wafer-scale packaging. In bulk processing, a single crystal of silicon is anisotropically etched to form the 3D MEMS structure. This is a subtractive process in which the silicon on the wafer is selectively removed.

The other MEMS manufacturing technique is surface micromachining. Analog Devices was the first company to produce sensors on a surface-micromachined process by investing in and developing its own proprietary process. Its iMotion sensors are formed on top of the silicon using deposited thin-film materials. The deposited materials form the sensor and the sacrificial layer that define the gap between the structural layers. Signal-processing circuitry can be formed on the same wafer holding the MEMS sensor (Fig. 2).

The transduction mechanism for commercially available MEMS accelerometers employs either the piezoresistive or capacitive principles. In piezoresistive-implanted materials, the change in the stress experienced by a cantilever beam or a diaphragm causes a strain and a corresponding change in resistance. A Wheatstone bridge measures this change in resistance and then converts it to a voltage.

The capacitive approach uses interlocking fingers or elements, which alternate between a fixed position and a moving position caused by stress or strain of the inertial motion. The capacitance differences between these states, which are proportional to the acceleration changes, are then measured and used to produce potential (Fig. 3).

MEMSIC takes a different approach. The proof mass for its dual-axis accelerometers is based on heat transfer by natural convection using a gas (Fig. 4). A single heat source, centered on the silicon chip, is suspended across a cavity. Equally spaced aluminum/polysilicon thermopiles (groups of thermocouples) are located equidistantly on all four sides of the heat source.

Under zero acceleration, a temperature gradient is symmetrical about the heat source. That makes the temperature the same on all four thermopiles, causing them to produce the same voltage. Acceleration in any direction disturbs the temperature profile, due to free convection heat transfer, causing it to be asymmetrical. The temperature differential at the thermopiles is proportional to the acceleration.

 

APPLICATIONS GALORE
The list of MEMS accelerometer applications for relatively inexpensive consumer and computer electronics is endless. For example, drop detection is a major growth area. Accelerometers enable users who are reading a book on a PDA to turn the page by simply turning the PDA over and then back again.

Exciting new capabilities such as gesture recognition are now possible for mobile phones and video games thanks to the availability of multi-axis MEMS accelerometers. These help simplify the interface between a phone or a game and its user. Gesture recognition involves adding commands to a phone handset using movement.

Motion sensors may sense when the phone is dropped and shelter its hard-disk drive to prevent memory loss. Answering the phone could be easier, too, since the accelerometer can sense when the user picks it up and automatically connects to the call, instead of waiting for the user to activate the phone's "send" button.

Given the small size and density of modern mobile-phone keypads, it isn't uncommon for users to strike the wrong key. A MEMS motion sensor could use a simple gesture, such as a quick shake, to clear the last entered keystroke. Longer shakes could be used to clear the last complete typed string.

A phone's awareness of its surroundings could increase its usability. For instance, the sensor would let the phone know it's sitting on a table or desk and automatically switch from vibrate to ring mode. Or, if the phone is placed face-down, the sensor would activate its silent mode, deactivating the vibration and ring functions.

A mobile-phone accelerometer also could count the number of steps its user takes, just like a pedometer, by measuring the user's acceleration and estimating the distance traveled. Furthermore, accelerometers already have found success in video-game applications, directing the action in Nintendo's standalone Tilt and Tumble Kirby game and in the controllers for the company's popular Wii console.

Unlike the eight-position control standard that's on most handsets, an accelerometer can enable variable (analog) control, increasing cursor speed with tilt inclination. A third axis (Z) also can be incorporated for jumping actions. Since the handset's initial position could be in just about any orientation (the user may be lying down, for example), games typically start with a keystroke that sets the cursor's neutral position.

Finnish company VTI Technologies supplies its SCA3000 three-axis accelerometer to a Swiss manufacturer that produces a wristwatch-type biofeedback device called "ikcal." This unit measures an individual's calorie consumption through physical activity and compares it to the individual's personal energy uptake in the form of food consumption on a daily basis. In addition, the technology measures physical activity by measuring a person's heart rate and converting it into kilocalories.

INEXPENSIVE SOLUTIONS
Low-cost MEMS gyroscopes are making their bid for the vast consumer electronics market. Until now, these units have been used mostly in the automotive sector, where ruggedness, reliability, and performance are key.

The quartz-based Micro Gyro gyroscope from the Systron Donner Automotive Division has found success in automatic electronic-stability-control applications (Fig. 5). Generally, though, these devices are relatively more expensive (about $10 per axis) than what's needed in consumer electronics.

InvenSense, a fabless vendor of motion-sensing MEMS sensors and gyroscope manufacturer, announced volume manufacturing of a family of integrated dual-axis gyroscopes with a target price under $1 per axis late last year. This month, the company plans to introduce its latest MEMS gyroscope for an "air" mouse targeting the new generation of Internet Protocol TV applications, such as looking at video albums.

The company's design is based on a two-chip bulk-silicon process, using wafer-scale integration of electronics and wafer-scale packaging. The proprietary bonding process developed by InvenSense allows the company to develop the electronics on virtually any CMOS platform and integrate the finished product at the wafer level.

"We're the only company that offers a dual-axis integrated MEMS gyroscope," claims Steve Nasiri, CEO of InvenSense. "Unlike other single-axis MEMS gyroscope makers like Analog Devices with its iMEMS gyro and Bosch, our gyroscope makes use of out-of-plane sensing where the sensing element is driven up and down with respect to the XY plane."

MEMS gyroscopes will take motion sensing to a new level of performance. Current motion sensors use accelerometers (gravity sensing) or magnetic sensors (the Earth's magnetic-field sensing) that sense linear motion or rotational motion due to changes in gravity vectors.

"This dependency on external forces inserts possible sources of error and major inaccuracies," says Nasiri. "Gyroscopes are the only sensors that can provide absolute information on rotation without the need for any external force."

MEMS gyroscopes also can be used to cancel jitter in video and still cameras as well as motion-sensing game controllers. In fact, InvenSense says its technology has been designed into many digital video and digital camera products already on the market. The company employs optical image-stabilization circuitry with its IDG-1000 dual-axis MEMS gyroscope (Fig. 6).

Hewlett-Packard is currently developing a MEMS gyroscope that will be integrated with the company's MEMS accelerometer. The plan is to leverage HP's expertise in ink-jet printheads using microfluidics integrated with electronics. The company foresees a lucrative consumer electronics market for gyroscopes capable of "dead reckoning" location capabilities in mobile phones, video games, and smart GPS systems.

Then there's the totally integrated inertial measurement unit (IMU), which is sometimes confused with a gyroscope. An IMU isn't a gyroscope. In a car, an IMU system may consist of a six-degree-of-freedom IMU (x, y, and z linear-rate data and x, y, and z angular-rate gyroscope data), as well as a medium-g dual-axis sensor for crash detection. An IMU is part of a sensor cluster than can deliver information to various systems, including crash detection, vehicle dynamic control, navigation and driver information, and body/chassis control.

Though no one has produced a totally integrated IMU, work is in progress. Honeywell is working on a totally integrated unit for the military that's designed for armaments and aerospace applications. But getting all of an IMU's circuitry into one package at a reasonable cost and within a reasonable size is proving to be a formidable challenge.

A BRIGHT FUTURE
Yet research on motion sensors continues unabated, with developments pointing to a strong future. Freescale Semiconductor, which manufactures a range of multi-axis MEMS accelerometers, is teaming up with researchers at the University of Florida in Gainesville to produce an advanced process to manufacture high-performance, very low-cost MEMS accelerometers.

Results so far have shown the development of a monolithic CMOS MEMS three-axis accelerometer with low-noise and low-power performance using a dual-chopper amplifier. Measuring about 3 mm2, it consumes just 1 mW and has sensitivities of 560 mV/g and 320 mV/g in the lateral and z-axis planes, respectively. The overall noise floors are a low 12 mg/√Hz and 110 mg/√Hz, respectively.

The strong future of MEMS motion sensors can be seen in market reports and projections. According to research company IC Insights, motion MEMS sales are growing at about twice the rate of ICs.

Worldwide sales of solid-state sensors and actuator devices will grow 19% in 2007 to $6.3 billion after increasing nearly 18% in 2006 to $5.3 billion. Actuators will lead this charge with a compound annual growth rate (CAGR) of 22% per year, reaching $8.8 billion in 2001. Acceleration and yaw sensors will grow at a 15% CAGR to $1.4 billion.

"I see a $2 billion total available market for motion sensors, in which MEMS gyroscopes are a key element," says InvenSense's Nasiri.

NEED MORE INFORMATION?
Analog Devices Inc.
www.analog.com

Freescale Semiconductor Inc.
www.freescale.com

Hewlett-Packard Inc.
www.hp.com

Honeywell Inc.
www.honeywell.com

IC Insights Inc.
www.icinsights.com

InvenSense Inc.
www.invensense.com

MEMSIC Inc.
www.memsic.com

Systron Donner Inc.
www.systron.com

University of Florida at Gainesville
www.ufl.edu

VTI Technologies
www.vti.fi

STMicroelectronics
www.st.com

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