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Guide to developing high-performance MEMS sensors

Posted: 15 Dec 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Gyroscopes? accelerometers? MEMS? Sigma-delta? A/D converter?

The story began almost two centuries ago with French physicist Leon Foucault. He used his famous pendulum C a 28kg brass-coated lead bob with a 67m long wire from the dome of the Panthon, Paris C to demonstrate the rotational rate of the earth in 1851, and then went on to perfect the measurement using a gyro in 1852.

In order to grasp the underlying mechanics, one has to imagine that the plane of oscillation of the pendulum remains fixed relative to the far distant masses of the universe, while Earth rotates underneath it.


 Foucault Pendulum

Figure 1: Foucault Pendulum, Pantheon, Paris.

Gyroscopes and accelerometersalso called inertial sensorsremained scientific curiosities for almost a century but had a huge impact during the Second World War, as they were used in a large set of applications such as ship navigation, guided missiles, battery fire control, aircraft artificial horizon and flight controls.

 Mechanical gyros during World War II

Figure 2: Mechanical gyros used for German V2 rockets during World War II.

Since the end of the Second World War, these sensors have progressed from complex electromechanical devices assembled with more than 100 parts to the modern solid-state devices. In 1999, the first high performance inertial MEMS measurement unit (IMU) was launched: this first MEMS IMU and its evolutions have paved the way to well-established industry standards with up and running production. We will review in this article, the main standards of these high performance inertial MEMS sensors.

MEMS architecture
A lot of different MEMS architectures have been developed and studied since the 1980s. We will review hereafter the designs families that have emerged as the dominant ones for high performance inertial sensors.

For high performance accelerometers, the "in-plane force-rebalance" is the dominant design family. A symmetric silicon proof mass is suspended by pairs of opposing spring flexures on either side of the proof mass. An applied acceleration acts on the proof mass. This in-plane motion is counterbalanced by applying voltages that generate electrostatic forces to rebalance the proof mass (closed-loop operation). The applied voltage is directly proportional to the input acceleration [1].

 MEMS Accelerometer

Figure 3: MEMS Accelerometer design principle.

For high performance gyroscopes, the "tuning fork" design family is the dominant design implantation. This design family is based on a pair of proof-masses (this type of gyro is also called dual-mass) that are electrostatically driven to oscillate with equal amplitude but in opposite directions. When the device is rotated, the Coriolis force creates an orthogonal vibration that can be sensed by capacitive electrodes [2].

 MEMS gyroscope

Figure 4: MEMS gyroscope design principle.

Electronics architecture
The dominant interface electronics includes ultra-low noise capacitive to voltage converters (C2V) followed by high-resolution voltage digitisation (ADC). Excitation voltage required for capacitance sensing circuits is generated on the common electrode node. 1bit force feedback DACs are used for system actuation [3].

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