The battle between MEMS and FOGs for precision guidance
Fiber optic gyroscopes (FOGs), previously the low-cost equivalent to other technologies such as ring laser gyroscopes (RLG), have some fresh competition. Microelectromechanical system (MEMS) gyroscopes are beginning to take market share away from traditional FOG applications. Specifically, antenna array stabilization, agricultural machine control, and general vehicle navigation are the battlegrounds where MEMS and FOGs faceoff.
To determine the similarities between the two technologies for use in navigation applications, a comparison of select high-end MEMS gyroscopes to low-end FOG gyroscopes is explored. The navigation software and test cases are controls used in the analysis to determine whether MEMS are truly prepared to function at tactical navigation performance levels.
1. MEMS for Precision guidance
In the last few years the navigation industry has seen MEMS gaining traction due to improved error characteristics, environmental stability, increased bandwidth, better g-sensitivity, and the increasing availability of embedded computational power that can run advanced fusion and sensor error modeling algorithms.
New precision inertial navigation system (INS) markets are materializing and MEMS technology is also entering markets that were previously dominated by FOG technology. An apparent transition from FOG to MEMS technology is in antenna array stabilization applications.
Machine control applications could also benefit from the advancements in MEMS technology. Traditionally, users have gravitated towards FOG or RLG navigation systems costing $30,000+ because the performance has been 20 times more accurate and reliable than a representative $1,000 MEMS navigation system. Precision agriculture and UGV/UAV/USV are two examples of applications that would greatly benefit from improvements of low-cost MEMS navigation.
2. Real-time navigation hardware
The navigation system used in this work was designed to provide high rate attitude outputs to a motor, which then stabilized an antenna array on the roof of a vehicle. The antenna array’s purpose was to maintain communication with a geostationary satellite.
The navigation system was used as a strapped down INS/GNSS navigator, which provided high rate positions and velocities. Inertial measurement unit (IMU) data flowed to the navigation filter at 1,000 Hz and these data packets were used to predict the position, velocity, and attitude solution. GNSS positions, velocities, and headings derived from dual antennas were used as updates to the navigation filter. When GNSS was not available a magnetometer was used to help initialize the heading. A barometer was also used to aid altitude.
Special calibration routines occurred in parallel to the navigation filter. These routines calibrated the magnetometer, the dual-antenna mounting misalignment, the IMU mounting misalignment, and the level of vehicle vibrations for static period detection.
The system was designed to operate in two hardware configurations. The first configuration consisted of two FOGs (for heading and pitch angles), one MEMS gyroscope (for roll), a tri-axial MEMS accelerometer, a tri-axial MEMS magnetometer, and a MEMS barometer with a total sensor hardware bill-of-materials (BOM) cost of about $8,000 for low volumes.
The second configuration contained three MEMS gyroscopes (for all attitude angles), the same tri-axial MEMS accelerometer, tri-axial MEMS magnetometer, and MEMS barometer as the previous configuration with a total cost of about $1,000 for low volumes. The prices of these systems can fluctuate with market conditions and volume, but generally FOGs are eight to ten times more expensive than the MEMS.
The MEMS gyroscopes and accelerometers chosen for this design have very good bias stability, orthogonality, g-sensitivity, and bandwidth within their price class. The primary constraint of this system is the high bandwidth requirement. Many MEMS accelerometers offer a high bandwidth, but MEMS gyroscopes typically have 100 Hz bandwidth or less.This is fine for typical vehicle navigation, but the application for which this system was designed needed to accommodate high rate control. Moreover, several MEMS gyroscopes are available that provide good bias stability, but have reduced bandwidths or high noise. The MEMS gyroscopes chosen for this system balanced bandwidth with performance. The actual specifications of the MEMS
chosen are given in Table 1.