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20 ST/Industrial Sensing Solutions The majority of currently available MEMS gyroscopes using capacitive technology find their basis in a tuning-fork configuration. The previous illustration shows the tuning-fork scheme of a MEMS gyroscope (Figure 3). Two masses oscil- late and continuously move in opposite directions. When angular velocity gets applied to the structure of the two masses, the Coriolis force generated on each mass acts in opposite directions, which then induces a capacitance change. This differential value change in capacitance is proportional to the angular velocity applied to the struc- ture and is then converted into output voltage for analog gyroscopes or LSBs for digital gyroscopes. The advantage of the tuning-fork configuration is that when linear acceleration gets applied to two masses, they move in the same direction. Therefore, there will be no capacitance difference detected. The gyroscope will out- put zero-rate level of voltage or LSBs which shows that the MEMS gyroscopes are not sensitive to linear acceleration such as tilt, shock, or vibration. There are many applications for MEMS-based capacitive gyroscopes. For example, digital cameras use gyroscopes to detect hand rotation for image stabilization solutions. In cars, a yaw rate gyro can be used to activate the electron- ic stability control (ESC) brake system to prevent accidents from happening when the vehicle is spinning too fast, and a roll gyro can be used to activate airbags when a rollover condition occurs. The angular velocity that is sensed by a capac- itive gyroscope can be integrated over time to provide the amount of angular displacement that the object has experienced. For example, in cars, a yaw rate gyroscope can be used to mea- sure the orientation to keep the vehicle moving on a map when GPS signal is lost, which is also known as car dead-reckoning backup system. Inertial Measurement Unit An Inertial Measurement Unit (IMU) or iNEMO™ consists of a three-axis accelerometer and a three-axis gyroscope. The mechanical structures of these two devices are inte- grated onto the same silicon die to reduce the size the overall size and to minimize the axis misalignments. This die is then housed in a tiny package together with a dedi- cated ASIC. Shown is the internal block diagram and the packaging of one of the latest IMUs (Figure 4). Those familiar with MEMS IMUs are aware that noise, bias stability, offset, sensitivity, non-linearity, and cross-axis sensi- tivity are critical to the overall performance and accuracy of an IMU. As the need for new industrial applications has emerged continuously, more demanding and tighter requirements for high performance, high accuracy, low power consumption, and better stability over temperature have significantly increased. These critical parameters are vital for applications such as dynamic and static tilt calcula- tions, robot arm movement, vibration monitoring of industrial equipment, navigation, autonomous driving, virtual & Aug- mented Reality (AR), and optical image stabilization (OIS). Figure 5: Key roles of an iNEMO™ in Applications. Figure 4: High-Level Internal Block Diagram of an ST IMU and the packaging. Figure 3: Tuning Fork Scheme of a MEMS Gyroscope

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