10 May 12

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There has been an increasing need for an ultra-low power device that will simultaneously measure high accelerations along all three axes.

ST Microelectronics has introduced what is claimed to be the world's first motion sensor of this type. This device can make precise measurements of accelerations of up to 400g, making it ideal for applications such as car black boxes, medical monitoring devices, consumer electronics, and sports equipment, and other applications where a low power 3-axis accelerometer in a small space is needed. This form of shock detection would be very useful to monitor for concussions in helmets for sports like hockey and football. When used in consumer electronics, the device could activate a protection mode as it is dropped on the floor.

Most of the high-g shock-detection devices made are based on either a single- or dual-axis, such as the type used for the automotive airbag sensors, and many of these operate at relatively high power levels. ST’s new high-g accelerometers now combine three-dimensional precision with ultra-low current consumption, which makes them optimal for battery-operated applications.

The H3LIS331DL is a low power high performance 3-axis linear accelerometer belonging to the "nano" family, and boasts excellent stability over time and temperature. The full scale ranges are user-selectable, with choices of ±100 g, ±200, and ±400 g. The accelerations are measured and the data is output at a rate from 0.5 Hz to 1 kHz, depending on the power mode. The analog signal from the sensor is converted to a 12-bit digital bit stream that can be transmitted through an I2C/SPI interface. This data can be sent directly to a microcontroller through this serial interface.

The H3LIS331DL provides ultra low power operational modes for advanced power saving and features a "sleep to wake-up" function. The device has the ability to sense acceleration and generate interrupt requests even while in the sleep mode. The data is updated at a lower rate in this mode, and when the “sleep to wake-up” function is activated, the device will automatically wake up as soon as the interrupt event has been detected and go back to normal output data rates.

The interrupt events for positioning and acceleration are user-selectable for maximizing power saving and flexibility.

A proprietary process is used to create the surface micro-machined sensing element in the device. When acceleration is applied to the sensor, it causes an imbalance in the capacitive half bridge, and this is measured by charge integration as voltage pulses are applied to the capacitor.

The sensing element is in a steady-state when it is at rest on a horizontal surface, and it measures 0 g in X-axis, 0 g in Y-axis, and 1 g on the Z-axis. If the sensor is turned upside-down, it would read -1 g on the Z-axis. If the larger output value is subtracted from the smaller one, and then divided by 2, the result would be the actual sensitivity of the sensor. It has been found that this value changes very little overtime and temperature. The IC interface is factory calibrated for sensitivity, and these constant calibration values are used as measurements are made.

The new sensing-structure design of the H3LIS331DL is combined with the market-proven low-power interface of ST’s LIS331DLx acceleration sensors, the industry-standard devices that have shipped in several hundred million units to date. The H3LIS331DL is available in small thin plastic land grid array package (LGA), and operates over an extended temperature range from -40 °C to +85 °C.