Accelerating mobility with 3-axis MEMS
While demanding to manufacture, 3-axis accelerometers are already available from Freescale, Kionix, Analog Devices, Fujitsu, Hitachi Metals, OKI Electric, MEMSIC, STMicroelectronics and more recently traditional automotive sensor suppliers like VTI and Robert Bosch. Robert Bosch has even set up a subsidiary called Bosch Sensortec dedicated to consumer applications for MEMS sensors.
The majority of designs employ either capacitive or piezoresistive sensing of a proof mass as its moves relative to a silicon micromachined frame. Leveraging high-volume silicon micromachining processes is key to meeting the demanding package size, power consumption and price points needed for mobile applications.
Typical requirements include sensor and signal conditioning in the same small-footprint package, an ability to handle leadfree soldering processes, analog or digital output and 1 mA or less at 2 to 3 V operation for low current consumption. Many devices feature sleep mode to reduce power requirements even further. All this should be available for $1.35 or less for three axes by 2009.
For example, Japanese manufacturer Hitachi Metals manufactures a piezoresistive 3-axis accelerometer with a package volume of just 12 mm3, including a height of just 0.92mm, important for those extra-slim clamshell mobile phones we see around on the streets. This diminutive dimension is achieved using a MEMS micromachined structure to form the frame and suspended proof mass and piezoresistive elements on four arms in a Wheatstone bridge arrangement.
Massachusetts-based Analog Devices, Inc. on the other hand uses a MEMS micromachined capacitive sensing element and has recently released a sensor consuming just 0.2 mA at 3V for as little $2 in high volume, with an aggressive roadmap targeting $1 over the next few years. At these low prices, 3-axis accelerometers will rapidly replace 2-axis devices in the consumer segment. And, unlike the automotive sector where stringent reliability issues rule, consumer applications benefit sensor makers with relaxed standards that aid in reaching these tough price targets. However, we note that despite high penetration, precipitously falling prices and fierce competition will limit margins.
Applications and market drivers
The big opportunity for multi-axis low g accelerometers is for portable consumer electronics, especially mobile phones, expected to grow from a small market to $120 million by 2009- or over half of all inertial sensors used in portable consumable electronics. The technology push is great, as the mobile phone market is predicted to exceed 1 billion units by the end of the decade and MEMS will have capitalized on just 10-20% of this potential opportunity.
Accelerometers are interesting because a small change in force of around 1 to 2 g occurs when a device like a mobile phone, MP3, or PDA is either tilted or moved in some fashion. The resultant signal has a signature that can be sensed using the MEMS sensor. This signature can be programmed as an interface command, thereby allowing gestures to control the functionality of the cell phone or allow games. If GPS is fitted to a notebook or other device, the accelerometer provides dead reckoning and acts as an antitheft device by calculating the new location from a previous GPS reading.
Power saving where a multi-axis sensor signals a wake up mode or tap-based "muting" are additional features that could further excite the market. These combined functions increase the return on investment per sensor and thereby the attractiveness to integrators looking for product definition. Another interesting application is "freefall" detection. Sensors embedded in IBM, Toshiba and Apple notebooks and also MP3 players and Toshiba digital camcorders use 2-axis or better 3-axis accelerometers to sense when the laptop is in a free-fall in a matter of ms.
Intelligent signal processing algorithms interrogate the signal to distinguish a drop condition from other movements such as running or dancing, and if necessary park the head to avoid damage. Strong interest in portable consumer items will continue to provide excellent opportunities for MEMS acceleration sensors, which meet the technical and price point challenges set in this price-competitive sector. WTC would like to acknowledge the support of NEXUS in the preparation of the data used in this article.
