Inertial sensors

Overview

Accelerometers – inertial sensors

MEMS inertial accelerometers consists of a mass-spring system, which reside in a vacuum. Exerting acceleration on the accelerometer results in a displacement of the mass in the spring system. The displacement of the mass depends on the mass-spring system, so a calibration is needed. Read-out can be via a capacitive system. MEMS accelerometers are available in 1D, 2D and 3D versions. As the size of the mass-spring system directly relates to the resolution (and accuracy) of the accelerometer, 3D accelerometer chip are not yet accurate enough to be used in high-accuracy MEMS AHRS’s, such as the MTi-10, the MTi-100 and MTi-G-700. Main manufacturers of MEMS accelerometers are Fairchild, Analog Devices, Kionix and Colibrys.

Gyroscopes – inertial sensors

Inertial gyroscopes can be found in various classes. Ring Laser Gyroscopes (RLG) and Fiber Optic Gyros (FOG) are very reliable, and very expensive. They rely on light to be sent through a set of mirrors or a fiberglass cable in opposite direction. A rotation of the gyroscope results in light in one direction to reach the other side of the set of mirrors/fiberglass cable earlier than light sent away in the opposite direction. Optical gyroscopes, such as the RLG and FOG are so accurate, that they can be used without reference sensors (see AHRS). This intrinsic accuracy makes them so expensive that they cannot be used in cost-sensitive applications.

MEMS Gyroscopes on the other hand are relatively inexpensive, and the reduced accuracy are compensated using reference sensors. Analog Devices is the major manufacturer of MEMS gyroscopes

MEMS gyroscopes

MEMS gyroscopes have a small vibrating mass that oscillates at e.g. 10’s of kHz. The mass is suspended in a spring system, readout is via a capacitive system as it is in accelerometers. When the gyroscope is rotated, the rotation exerts a perpendicular Coriolis-force on the mass that is larger when the mass is further away from the center of rotation. The oscillating mass thus gets a different read-out on either side of the oscillation, which is a measure for rate of turn. A typical error in gyroscopes is g-sensitivity, caused by the deformation of the spring system inside the gyroscope. Xsens also compensates for this error source. Details on the precise working of MEMS gyroscopes and accelerometers is available on the website of Sensors Magazine.

Typical applications

Typical applications for sensor systems vary from type to type. The graph shows the various grades of gyroscopes.

Strategic and tactical sensors can often be found in high-risk applications, such as navigation in defense and commercial aviation. Consumer gyroscopes with typical bias stabilities of >30 deg/h are often used in mobile applications, and can be found in the Nintendo Wii controller. For inertial measurement units, these gyroscopes give too much drift in dead reckoning, so they cannot be used for professional applications. Because of the increase in accuracy in inertial sensors, a new group of gyroscopes has emerged (an industrial grade), which are small and can be used in the stabilization of unmanned vehicles and the measurement and correction of other sensor systems. Unmanned vehicles can be underwater vehicles (Saab Seaeye Jaguar), aerial vehicles (Norut) or land vehicles (Wambot). In satcom and sensor correction, examples include EM Solutions’ Ka-band Satcom system and Sonardyne’s USBL systems.

A more comprehensive list of possible applications can be found here: Xsens customer cases