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issue: May 2006 APPLIANCE Magazine

Technology Report
Sensitive Sensors

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Three highly sensitive accelerometers offer appliance engineers design flexibility and enhanced functionality for a range of appliance applications.

New accelerometers from Freescale (Austin, Texas, U.S.) can be used to detect an out-of-balance washing machine. “There is a specific rotational signature that is created during…operation,” explains Hiro Ueda, manager for inertial sensors. “The signature changes when the condition is out of balance. An accelerometer can detect that signature and how it changes to determine how the washing machine is out of balance in order for correction to occur.”

According to Freescale Semiconductor, its new MMA6270Q (XY-axis), MMA6280Q (XZ-axis) and MMA7261Q (XYZ-axis) accelerometers address the rapid shift in the micro-electromechanical
-systems (MEMS) industry toward greater adoption within the consumer market. The three low-gravity (low-g) sensors are ideal for electronic systems that require the detection of small changes in force resulting from fall, tilt, motion, positioning, shock, or vibration.
The devices, which provide two and three axes of sensitivity based on the application needs, are said to represent some industry firsts. The MMA6280Q is said to be the industry’s first XZ-axis accelerometer—it can sense in both lateral and perpendicular planes, eliminating the need for daughter cards and multiple sensors. In addition, the MMA7261Q features motion detection up to 10g, reportedly making it the first consumer-product accelerometer to detect motion higher than 6g.
One of the sensors’ key features is the g-range select option, which allows design engineers to select the appropriate sensitivity for a specific application and even allows for “on the fly” changes. Hiro Ueda, manager for inertial sensors, says Freescale’s two-chip solution made this option possible. “The g-range select feature was an upgrade to the ASIC to provide selectable amplification,” he explains. “Having a stacked die configuration of the two chips, the g-cell and the ASIC enabled us to reduce the package size while providing this additional functionality.”
With their wide sensitivity range of 1.5g to 10g and six sensing functions, the MEMs-based sensors can be applied to applications ranging from consumer electronics and fitness devices to traditional white goods: the XY sensor could be used in personal digital assistants (PDAs) for scrolling interfaces; the XZ device could be used in an advanced pedometer to measure a runner’s foot motion, distance and speed; and the XYZ accelerometer could be used in a washing machine to detect out-of-balance conditions.
The accelerometers also boast a low current consumption of
500 µA. Ueda says this was achieved by simplifying the circuit architecture of previous-generation sensors that were used in automotive applications. “There were some automotive-specific features that were deemed as unnecessary for the consumer market and were removed,” he explains. “One example is that the self-test feature was removed. This feature is extremely important in automotive safety applications, but is not necessary for consumer applications. The clock speed was also reduced. After many changes, the g-select sensors are able to achieve very high performance, but with a low current consumption.”
Other features include a sleep mode at 3 µA for extended battery life, fast power-up response time of 1 ms and integrated signal conditioning, amplification and temperature compensation with a low pass filter.

Suppliers mentioned in this article:
Freescale Semiconductor Inc.

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