MEMS Accelerometers， Gyroscopes， and Geomagnetic Sensors -

　　Propelling Disruptive Consumer Applications

　　作者：Fabio Pasolini

　　MEMS accelerometers have had a dramatic impact on cell phones， gaming consoles， and location-based devices. MEMS-based gyroscopes and geomagnetic sensors are about to take it to a whole new level.

　　The MEMS consumer market grew by 27 percent in 2010 to $1.6 billion， according to iSuppli， which predicts revenues for these devices to top $3.7 billion by 2014. The continued demands from consumer and mobile applications dominate this market’s growth， and these fields are expected to become the biggest MEMS segment by 2014.

　　MEMS sensors are well recognized as the key building blocks for implementing disruptive applications in consumer devices. From game consoles to mobile phones and from laptops to white goods， consumer devices have already benefited in recent years from the use of low -g accelerometers for the implementation of motion-activated user interfaces and enhanced protection systems. It is now the turn of MEMSgyroscopes and geomagnetic sensors， as use of these sensors is propelling a new wave of compelling applications.

　　Much has been written about the technology behind MEMS accelerometers， which are sensors capable of detecting linear accelerations. Accelerometers will thus be barely touched in this article， this will focus on gyroscopes， geomagnetic sensors， and other devices capable of providing multiple degrees of freedom to sensing applications.

　　MEMS gyroscopes

　　Capable of measuring angular rates around one or more axes， these gyroscopes represent a fitting complement to MEMS accelerometers. Thanks to the combination of accelerometers and gyroscopes it is possible to track and to capture complete movements in a three-dimensional space. This gives system developers the ability to deliver more immersive user experiences， accurate navigation systems， and much more.

　　With the recent introduction of 30 different gyroscopes delivering high performances with reduced current consumption and compact packages， STMicroelectronics is continuing its rapid growth in the MEMS market. The heart of ST’s gyroscopes is represented by a micromachined， mechanical element designed to operate according to a tuning fork scheme， exploiting the Coriolis effect to transform an angular rate into a displacement of a specific sensing structure.

　　Let’s consider， for example， the simple case of a single-axis yaw gyroscope （see Figure 1）。 Two moving masses are kept in continuous movement in opposite directions， which are indicated by the blue arrows. As soon as an external angular rate is applied， a Coriolis force， which is shown with the orange arrows， will appear in the direction orthogonal to the movement and will cause a displacement of the sensing masses proportional to the magnitude of the rate applied. Since the moving electrodes （rotors） of the sensing portion of the sensor stand beside fixed electrodes （stators）， the displacement will induce a

　　change in the electrical capacitance among stators and rotors， thus transforming the angular rate applied at the input of the gyroscope into an electrical parameter detectable by means of a dedicated circuit.

　　Figure 1： Single-axis MEMS yaw gyroscope.

　　ST’s micromechanical gyroscope sensors exploit the same technology ST has already employed for the manufacturing of over 600 million accelerometers. This choice guarantees the customer state-of-the-art， reliable products for direct use in their final applications. Compared to other gyroscopes， the differential nature of the tuning fork approach adopted by ST makes the system intrinsically insensitive to undesired linear acceleration and spurious vibrations acting on the sensor. When such unwanted signals are applied to the gyroscope， both masses will be displaced along the same direction， causing the overall resulting capacitance variation to be null upon a differential measurement.

　　The conditioning circuit employed for a gyroscope can be schematized as the combination of a driving section for a motor and the sensing circuitry of an accelerometer （see Figure 2）。

　　· The driving section excites the mechanical element， making it oscillate back and forth by means of an electrostatic actuation.

　　· The sensing circuitry measures the displacement of the sensing mass produced by Coriolis force by measuring variations in capacitance， a robust and reliable technique successfully used across ST’s MEMS product lines. The sensor circuitry provides either an analog or digital output signal that is proportional to the angular rate applied to the sensor.