MEMS Inertial Sensor Laboratory

Michael Greene, Director
Victor Trent (Archangel Systems)
Bogdan Wilamowski

What are Inertial Sensors?

Inertial Sensors, such as accelerometers and gyroscopes, are widely used in many applications in the aerospace, military, automotive and marine industries. In the aerospace industry, these devices are used in the basic flight stabilization of aircraft and rockets as well as in navigation. Military applications include the same usages in air to air missiles, air to ground missiles, ground to air missiles, ground to ground missiles, barrage rounds and hypersonic projectiles. Automotive applications include vehicle stability systems and rollover prevention systems. Naval and marine applications include ship stabilization and navigation.

As inertial sensors shrink in size and cost, the number of applications increases exponentially. As the accuracy and stability of these miniature, low-cost devices increases, higher performance systems are being introduced into lower cost items and consumer goods such as automobiles, thereby enhancing safety and functionality.

Micro-Electro-Mechanical Systems (MEMS) Inertial Sensors consist of miniature devices that combine electrical and mechanical inertial sensing components. Typical of such devices are accelerometers containing miniature proof masses and sensing electronics and gyroscopic devices based on the coriolis effect using vibrating forks.

MEMS Accelerometers

The first technology to come to fruition using MEMS technology was in accelerometers. A typical MEMS accelerometer uses a silicon mass suspended by a silicon beam. This design is mechanically equivalent to a spring mass damper of a traditional mechanical accelerometer. Capacitive sensing is used to measure the motion of the mass.

MEMS Accelerometer

MEMS Gyroscopes

MEMS gyroscopes to date have not used the traditional spinning mass approach to sense rotational velocities. Rather these devices use a vibrating comb drive seen below. The comb is rapidly driven in the x axis (20 KHz). Rotation about the z axis induces motion in the comb about the y axis. This motion is detected using capacitive proximity and demodulated using the drive signal.

Since the deformation due to rotation velocities is a function of the material properties of the comb, much effort has been spent on improving the materials as well as post manufacturing calibration of the devices.

A new concept

MARS (MEMS Annular Rotating Sensor) is based on proven technology using a new paradigm developed by Archangel Systems, Inc. and funded by both the Missile Defense Agency and the Navy. The MARS system returns to a spinning mass gyroscope but without any bearings which can cause drift and inaccuracies. Furthermore, by electrostatically suspending the rotor, it is free to move in six dimensional space. This creates, in one sensor system, a combined three axis accelerometer and a three axis gyroscope. The rotor is servo-controlled to maintain position and the output of the servo loops are the measurements of accelerations and rotations.

Rotor

Functional Alpha prototypes of MARS are currently being produced using glass – silicon – glass bonding technology as seen above and below. External electronics are used for the servo-control and sensing. Beta prototypes will use silicon-silicon bonding technology with greater inclusion of electronic support into the device. The goal of the MARS project is a complete ASIC containing sensor, support electronics, signal conditioning and power conditioning circuitry.