Hitachi Metals Ltd developed a new triaxial acceleration sensor, which the company claims is the world's smallest. It is a piezoresistive-type sensor measuring 2.5 x 2.5 x 1mm and weighing 14mg and is sealed in a plastic package.
Compared with the company's existing product, the new sensor is 30% smaller in volume and 46% lighter. It is resistant to impacts of more than 20,000g. The maximum detectable acceleration is ±4 or ±8g.
It outputs signals as 12-bit data via the I2C interface. The current consumption is 0.27mA when operated at 3V. The company employed a structure that excludes mechanical resonance points so that the sensor can eliminate the effect on the acceleration detection due to external vibration, etc.
Hitachi Metals will start shipping samples in May 2008. The sales targets of triaxial acceleration sensors in total including the latest sensor are ¥2 billion (US$19.4 million) in fiscal 2008, ¥3 billion in fiscal 2009 and ¥4 billion in fiscal 2010.
In piezoresistive-type sensors such as this, the displacement caused by the acceleration of a weight based on the MEMS technology is detected as a deflection in the silicon beam supporting the weight. The deflection is measured based on the variation of resistance in a piezoresistor formed in the Si beam.
This method ensures a relatively high sensitivity, but it is difficult to seal the sensor in a plastic package, which is suited to reduce the cost. This is because, when the MEMS sensor unit is covered with plastic, the piezoresistance is varied by the stress due to the thermal change, etc in the plastic package, thus making it impossible to correctly detect the acceleration.
Therefore, the company reportedly overcame the problem by adopting a device structure where such a stress will not affect the MEMS unit.
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