MEMS rate sensor features high reliability for accurate dead-reckoning
November 25, 2010 // Christoph Hammerschmidt
With its MEV-50C-R MEMS rate sensor, Murata aims at automotive navigation markets: The device supports dead-reckoning for car navigation devices plus the high reliability and thermal stability required for automotive environments. The rate sensor nevertheless can be used in other applications as well, such as GPS modules or satellite antennas or even sophisticated toys — radio control helicopter, for example.
The sensor features the second-generation of Murata's technology - the first generation was used in the company's famous bicycle-riding robot, (Murata Boy), and the unicycle-riding robot (Murata Girl).
According to the vendor, the sensor's high temperature stability does away with the need to adjust for temperature drift in software. Temperature drift across the entire operating range, -40 to 85 °C, is just 0.6 degree/s. Materials used in the construction of the rate sensor help to ensure the stable temperature performance of the device. Single-crystal silicon is used for the core MEMS vibrator, while the MEMS element is composed of silicon and pyrex glass, which have very similar temperature expansion coefficients, increasing stability.
Another key factor for rate sensors used in car navigation systems is the start-up drift. It typically takes several minutes after the system is switched on for a GPS signal to be acquired, so dead-reckoning is commonly used during this time. The MEV-50C-R exhibits a low start-up drift of 0.1 degree/s between 0.5s and 5 minutes after start-up. Other features are high linearity of 0.5 percent across the operating range of angular velocity, as well as low cross-axis sensitivity (±5 percent).
The MEMS element uses anodic bonding creating a seal which achieves much better performance than the current industry standard of 10-9 Pa m3/s. The seal did not change its efficiency during air tank/heat shock tests with 2000 cycles of -55 to +125°C (30 min/30 min). This high level of seal allows Murata's MEMS technology to be applied under harsh conditions such as automotive.
The MEV-50C-R measures 10.0- x 6.2- x 2.9-mm and weighs just 0.3 g (maximum). Maximum angular velocity is ±70degree/s. The sensor's unique 'ladder' structure means the part is not influenced by external physical noise (shock or vibration), and thus exhibits more accurate angular-rate information than competing parts.
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