STMicroelectronics’ new L6671 Rotational Accelerometer System combines a MEMS capacitive rotational accelerometer plus an interface chip that integrates analog-to-digital conversion, filtering and interface functions in a single SO-24 package. The L6671 provides a digital output through a 3.3V (5V tolerant) three-wire serial bus and achieves a signal bandwidth of 800Hz, sensitivity of 2.5rad/s<sum>, a full scale sensitivity of 200rad/s<sum> and a signal-to-noise ratio of 37dB over 30-800Hz. Through the serial bus it is also possible to program device characteristics including gain, offset, filter performance and phase delay.
Inside the device, movement about the rotational axis – the axis orthogonal to the package – causes a displacement of a structure inside the MEMS sensor thus producing a capacitive change. These changes in capacitance are converted by the interface chip into a digital output for direct connection to a microprocessor. Capacitance changes as small as 0.05fF (0.05femtoFarad = 0.05×10-15F) can be measured by the device.
In hard disk drive applications, the L6671 sensor device allows drive designers to reduce the effects of mechanical vibration on head tracking and thus increase the number of tracks per inch and hence drive capacity.
In a typical hard disk drive, rotational vibrations act on the mass of the voice coil actuator used for head positioning, causing errors in the read/write head position. With the L6671 it is possible to detect and measure rotational vibrations so that the firmware can generate a feedforward correction signal for the voice coil drive circuit to keep the head in the correct position. This approach means that the track-to-track distance can be reduced significantly, increasing the density per unit area. In addition, the speed performance of the drive is enhanced because less time is wasted restoring the correct head position when vibration has caused the tracking to be temporarily lost.
The company claim that compared to solutions based on pairs of linear accelerometers the new solution is simpler, easier to use and more reliable since it is a single package and needs a minimum of external components.