MEMS based motion sensing design
Measure motion, acceleration,
inclination, and vibration in 3-D using MEMS
By Vishal Goyal, Technical
Manager Marketing (INDIA) for APM, ST Microelectronics
Micro Electro-Mechanical Systems [MEMS] have become widely
popular sensors for measuring motion, acceleration, inclination,
and vibration. MEMS sensors are System-in-Package solutions,
delivering high resolution and low power consumption, in
an extremely small size.
MEMS are radically different technology from other silicon
ICs, which primarily exploit the electrical properties of
silicon. The heart of MEMS does not have gate-drain-source
but actually miniature mechanical structures made up entirely
of silicon. A typical MEMS structure contains moving masses,
springs, and dampeners, which work in the similar way as
the mass-spring model.
The sensors can add an intuitive man-machine interface
to a mobile phone, MP3/MP4 player, PDA, or game controller,
creating interaction by linking the user's movements to
applications, navigation, gaming, and much more.
MEMS accelerometers are also commonly integrated as vibration
detectors in today's electronic home appliances, such as
washing machines or dryers, to alert users to unbalanced
loads and to protect against the excessive wear of parts,
before a failure occurs.
One- and two-axis accelerometers are widely used in the
automotive market for passive safety systems, like frontal
and lateral air-bags. Accelerometers and gyroscopes are
also used in navigation systems and active safety systems,
like ABS and Vehicle Dynamic Control.
In the last few years MEMS "Consumerization" has
begun to emerge. MEMS have started to penetrate the consumer
market to address many new applications.
The consumer market is looking for tiny, inexpensive, low
voltage, and low power consumption devices. Mobile phones,
MP3 and MP4 players, and portable PCs are all battery operated
and are becoming smaller and thinner. Moreover, in the consumer
market, multi-axis solutions are mandatory since consumers
want to activate any function from any initial physical
position. In a handheld application, there is no constant
frame of reference.
Besides, the product life cycle of consumer devices is
getting shorter so product designers need MEMS to be integrated
into the end application quickly and seamlessly.
At the forefront of the MEMS technology development, ST
has started integrating multiple sensors - accelerometers,
gyroscopes, and magnetometers - into one package, leading
to leaps in functionality and performance in a wide variety
of applications besides motion monitoring. Integrated sensors
enable autonomous and automated systems, monitoring specific
conditions and turning them into actions with no or minimal
The development of a sensor which accurately measures angular
rates along three orthogonal axes has enabled 360° angular-rate
detection for high-precision 3D gesture and motion recognition
in mobile phones, game controllers, personal navigation
systems, and other portable devices. Besides, the combination
of a 3-axis accelerometer with a gyroscope has enabled the
creation of Inertial Measurement Units, devices that track
and deliver complete information on the type, rate, and
direction of motion of humans, vehicles, and other objects.
New accelerometers also embed a host of enhanced features,
including click and double-click recognition, motion-detection/wake-up,
and 4D/6D orientation detection. Other important features
include a programmable FIFO (first-in, first-out) and two
programmable interrupt signals that enable immediate notification
of motion detection, click/double-click events, and other
conditions also introduced in new MEMS devices.
A revolution in handsets has been triggered by the integration
of compass modules for advanced location-based services.
With compassing and GPS capabilities, consumers will be
able to identify and retrieve information on nearby points
of interest, such as restaurants or shops, by simply pointing
their mobile devices in the direction of the object concerned.
The combination of accelerometers and magnetic sensing
can enhance the mobile-user experience in a number of ways.
Application developers can improve the use of a phone or
PDA limited display size for maps by putting the current
user position at the bottom of the map with the rest of
the display in the forward ('up ahead') direction. The user
can get accurate heading information, to indicate the direction
in which a person or vehicle is moving, when the GPS signal
is not available. Dead-reckoning applications, including
pedestrian navigation in places with no or low GPS signals,
such as inside buildings and structures or in mountainous
and forested terrain, are also possible.
ST has integrated a 3-axis digital accelerometer with a
3-axis digital magnetic sensor in a single module - the
LSM303DLH. The digital compass module combines high accuracy
with a small form factor and low power consumption, meeting
the growing market demand for advanced navigation capabilities
and emerging smart location-based services. ST's high-performance
system-in-package digital compass uses magneto-resistive
technology from Honeywell and aims to accelerate the adoption
of enhanced electronic compassing in portable consumer applications,
including direction finding, map/display orientation, location-based
services and pedestrian dead reckoning. The device has a
linear acceleration full-scale of ±2/±4/±8g
and a magnetic field full-scale from ±1.3 up to ±8
gauss, both fully selectable by the user
Many technical and business experts believe that wireless
sensor networks, domestic robots, smart pills, and Labs-on-Chips
represent the next big commercial wave for appropriate MEMS
products. Silicon Labs-On-Chips for personalized drug development
and research purposes, and Tire Pressure Monitoring Systems,
simple 5-node wireless pressure sensor networks, are just
the first market examples.
MEMS experts are now focusing their efforts on the development
of 'smart sensors'. These are MEMS devices with integrated
processing capability, able to run the sensing-related algorithms
independently of the main processor unit, decreasing power
consumption at the system level, which is especially crucial
in battery-hungry portable devices.