Feature article Think Small! issue 2, Volume 1:
MEMS Inertial Sensors go Consumer
by Richard Dixon and Jérémie Bouchaud
The market for mems inertial sensors (accelerometers and gyroscopes)
is set to grow from $835 million in 2004 to over $1360 million
in 2009 —a cagr of 10%. Currently, the main applications
are in the automotive industry. These markets are well established
and growth rates range from a stagnant 1% for airbag acceleration
sensors up to 8% for gyroscopes used in esp units and gps navigation
assistance.
Exciting for mems inertial sensors is the market opportunity for
mobile applications and consumer electronics (see table on page
8). Over the next few years, we predict annual growth rates exceeding
30% for accelerometers. Mobile phones in particular will provide
multi-axis accelerometers with interesting opportunities in menu
navigation, gaming, image rotation, pedo-meters, gps navigation
and the like. Gyroscopes are largely servicing markets for image
stabilization and hdd protection in camcorders.
In contrast to the automotive sector, consumer applications feature
relaxed specifications. Failure rates for automotive electronic
control units (ecu) that house airbag accelerometers must be less
than 50 ppm, and down to a few ppm for asics. Car manufacturers
deploy reliable, high performance accelerometers that are relatively
expensive (up to $5 to measure lateral acceleration in esp units,
for example). Mobile phones manufacturers on the other hand tolerate
failure rates of 5000 ppm. Active control of the car motion
is critical to safety whereas failure in consumer applications
result more in an inconvenience. Sensor manufacturers can therefore
sacrifice some accuracy and reliability to lower the price.
Tri-axis accelerometers for mobile
applications
At least 10 companies are working on tri-axis mems accelerometers,
some of which were initially developed for the automotive industry.
Three axes sensing is needed to fully describe three-dimensional
motion, although certain applications benefit from 2-axis devices
if one axis (like the horizon) is assumed fixed or for a simple
2d (tilt) game in a cell phone.
With few exceptions accelerometers work by measuring the motion
of a proof mass versus a fixed frame or reference. The main sensing
approaches are capacitive (e.g. Bosch, Freescale, Kionix, oki
Electric, STMicroelectronics and Analog Devices), piezoresistive
(e.g. Hitachi Metals, Matsushita, Fujitsu, and Hokuriku) or less
common thermal accelerometers from sole proponent memsic (with
2-axis and 3-axis in development). Each has its advantages but
price is the bottom line.
Requirements for cell phones
Mobile phone integrators want the following attributes from accelerometers:
• Small packages, e.g. 3.5 × 3.5 mm² and
1 mm thick or less
• Sensor and signal conditioning asic in the same package
• Digital output
• Power consumption < 3 mW (1 mA @ 3V). Many devices
feature a microwatt-level sleep mode to reduce power consumption
further
• Compatibility with lead-free soldering processes
• Availability in volume for $1.35 or less in next 2–3
years.
These conditions have already been met. The smallest accelerometer
to date is made by Hitachi Metals, a 3-axis device in a 12 mm³
(3.4 × 3.7 × 0.92) package at
least 50% smaller than its nearest competitor.
The lowest cost 3-axis sensors come from Analog Devices or Freescale
($2 in volume). Memsic provides the lowest cost solution for 2-axis
sensors (now under $1 and $0.50 expected in 2007). Meanwhile,
the power consumption prize goes to Analog Devices’ adxl330
with power consumption well under 1 mW.
Gyroscopes improve picture quality
Today, gyroscopes are used in image stabilization, where two axes
of motion sensing are needed to remove vibration and shake. While
other approaches are also used to damp vibration, gyros are a
superior solution. (continued next page) C
Gyroscopes have been a feature of the top range of Panasonic camcorders
for some years. They are now also employed in some Pentax and
Panasonic digital still cameras costing as little as $250. Interestingly,
mems gyroscopes are also used for hdd free-fall protection in
recent camcorders from jvc and Toshiba (these camcorders retail
for over $1,000).
Most commercial gyros are vibratory and use the transfer of energy
between two vibration modes caused by Coriolis force. They are
challenging to manufacture (hence their higher cost compared to
accelerometers), being essentially two very high performing mems
devices in single device, usually a self tuned resonator for drive
and micro-g for the sensing. Compared to automotive applications,
packages in consumer electronics are about 1/10th the size (targeting
25 mm³). The resolution requirements are lower, i.e.
5–10 °/s compared to 0.1–1 °/s in cars.
A gyro is a larger device than an accelerometer, and can require
as much as 10 times more power due to the higher drive requirements
(5?V).
There are many different sensing approaches and materials employed
to make mems gyroscopess. Silicon micromachined capacitive gyros
are produced by Analog Devices and Kionix. Piezo-ceramic devices
are available from nec Tokin and Murata, thin-film resonators
deposited with pzt from Matsushita, quartz from Seiko Epson and
Microcomponents (part of Swatch). These are all single axis solutions.
Currently, Murata manufactures a piezoceramic single-axis solution
that costs $7–8. Sony and Invensense are examples of companies
with two-axis solutions estimated to cost under $10.
Today, the smallest packaged device is 21 mm³ —
a single axis quartz piezoelectric gyroscope from Seiko Epson.
At 6 mW, it is also the lowest power solution, in part due to
a lower drive voltage compared to 5?V drive capacitive gyroscopes.
The cell phone remains the dangling carrot for gyroscopes, but
they are still too expensive for this application. Cell phone
companies tell us the acceptable price is somewhere around $3–5
for 2-axis sensing for an application like GPS navigation.
Currently it is not clear which technology or solution will win.
While Sony is manufacturing a quartz 2-axis gyro, Murata is developing
silicon capacitive micromachined sensing elements (in parallel
to its ceramic gyros) to meet the consumer price targets. We believe
the route for low cost 2-axis gyros will likely be a monolithic
silicon capacitive solution. This is an important consideration
to reduce package size and for integration of the asic.
Inertial measurement units arrive
on consumer markets
There is a trend in consumer markets
to integrate accelerometers and gyroscopes into inertial measurement
unit (imu) modules. This follows the military muster and recent
automotive trends. In automotive, module are partitioned to provide
measurement in four degrees-of-freedom (dof) to be distributed
between navigation and vehicle dynamics (e.g. vehicles from Volvo
and Fiat).
imus are targeting navigation applications in pdas and mobile
phones. Akm already supplies an electronic compass, albeit with
a 3-axis magnetometer and mixed-signal processor in phones from
nec, Sanyo, SonyEricsson and lg. One application that has gained
traction in Japan is an electronic compass that allows the calculation
of the position of the constellations. Akm is working on compasses
that include a 3-axis accelerometer, due in 2007. The package
size is 4.8 × 4.8 × 1.3 mm³. A leading Asian
supplier is also developing a navigation module, first using a
3-axis accelerometer and magnetometers, and subsequently a 2-axis
gyro.
The first imus with mems sensors are estimated to cost $20, e.g.
for high-end pdas ($800). In 2-3 years time we believe $6-8 is
the goal, whereby a gyro will need to cost under $5 for 2-axes.
Prices for mobile phone compass modules will range from $5.5 to
$9 depending on the sensors that are included.
An important requirement is the signal processing algorithm —
which is non-trivial for five or six degrees-of-freedom. In contrast
to the automotive industry, consumer integrators prefer to plug and play.
Three ways to deal with tight
margins in consumer markets
Finally, let’s look at how to make money in the consumer
sensor market, where margins are tight.
1. Prices can be pressured downwards and better margins generated
when sensors are developed to be transparent to cmos processes.
An example is memsic that uses fabs to produce its low cost dual
axis accelerometers in high volume.
2. The margin for an imu module is much higher than for sensors
alone.
3. There are also opportunities for companies providing algorithms
for imus. These are required to harness the power of a 5- to 6-axis
functionality by turning the complex signal signatures into commands
and for signal recalibration in real time.
Summary
Accelerometers have already met the requirements in terms of size,
power and cost to open the markets for mobile consumer electronic.
However, we believe it is will be a considerable challenge to
reach the price target of $1 and below set by cell phone integrators
for 3-axis devices.
Gyroscopes remain challenging to manufacture. There is great interest
in a gyro costing under $5. Such a price would expand the applications
of gyroscopes from image stabilization in digital still cameras
and camcorders to cell phone cameras, navigation imus and even
further into protection of hard disc drive.
In the next issue of Think Small!: OLED displays