1998 - Draper Laboratory

Performance of Small,Low-Cost Rate Sensorsfor Military andCommercial ApplicationsAnthony Kourepenis, Jeffrey Borenstein, James Connelly, Paul Ward, Marc WeinbergThe Charles Stark Draper Laboratory, Inc.Based on the paper published in the Proceedings of the AIAA Guidance, Navigation, and Control ConferenceAbstractDraper Laboratory and Boeing North American (BNA) haveformed an alliance to develop small, low-cost rate sensors forcommercial and military applications. Advanced siliconmicromachining techniques produce sensors of highperformance, ruggedness, and inherent symmetry. Whenintegrated with Applications Specific Integrated Circuits (ASICs),the rate sensor will fit in a 3-cm per side flat pack and operatefrom a single 5-Vdc supply.Gyroscopes are fabricated using a dissolved wafer process thatfeatures single-crystal silicon anodically bonded to a glasssubstrate, resulting in a sensor die size of approximately 1 mm.Uncompensated bias and scale-factor performance of 0.5 deg/sand 1.0 percent are nominally demonstrated over the automotivetemperature range of -40ºC to +85ºC. Bias stability over smallertemperature ranges of 0.5ºC has surpassed 10 deg/h (0.003deg/s) in overnight (6-h) tests. Nominal resolution is 150 deg/hin a 60-Hz bandwidth, yielding an angle random walk of 0.25deg/√h. Best to date resolution and angle random walkperformance is 25 deg/h in 60 Hz and 0.04 deg/√h, respectively.The robustness of the design with regard to environment hasbeen demonstrated in the ability to survive air gun tests in excessof 60,000 g.This paper discusses the principle of operation, fabricationtechniques, and measured and projected performance, with aprincipal focus on recent Tuning-Fork Gyro (TFG) test results.Associated electronics, controls, and applications issues will alsobe addressed.IntroductionDraper has invented and developed inertial guidance systems forearth and space applications for over 50 years. Draper has beendeveloping miniature micromachined gyroscopes for over 10years and was the first to demonstrate rate sensing with amicromachined silicon sensor (Ref. [1]). Micromachiningborrows processes from the semiconductor industry to fabricatetiny sensors and actuators on silicon chips. Many newapplications are enabled by the reduced size and lower costinherent to sensors developed with this process.Draper has made great strides in simplifying the devicefabrication process and realizing rate sensors of increasinglyhigher performance (Refs. [2], [3], [4], [5]). Initial sensorsinvented at Draper were fabricated using a 17-step process andhad resolution performance (integrated PSD) of 1,000,000 deg/hin a 60-Hz bandwidth. Combining the TFG architecture anddissolved wafer fabrication process, the same performancestandard has improved to 150 deg/h nominal with a best score todate of 25 deg/h in the same 60-Hz bandwidth. Sensors are nowfabricated with a simpler three-step process, increasing yield andlowering unit production costs.Consistent with Draper’s mission of transferring technology toindustry, an Alliance with Rockwell Autonetics (now BNA) wasconsummated in 1993 for the purpose of transitioning Draperdevelopedinertial micromechanical technology for productionand commercialization. The intended initial application for thesensors is for use in automated skid-control/antilock brakingsystems for automotive applications. Additionally, the higherperformance of recent instruments enables insertion into militarysystems, the first demonstration being low-end, unguidedartillery shells and rockets.Theory of OperationThe TFG (Figure 1) consists of a silicon structure suspendedabove a glass substrate containing metallization deposited forsensor interfacing. The silicon structure contains two massessuspended by a sequence of beams that are anchored to thesubstrate at specific points. By applying voltages to the outermotor drives, the two masses are electrostatically forced togenerate lateral, in-plane oscillatory motion. An angular rate, Ω,applied about the input axis, perpendicular to the velocity vectorof the masses, generates a Coriolis Force that acts to push themasses in and out of the plane of oscillation (labeled F1 and F2in Figure 2). Because the instantaneous velocity vectors of therelative masses are equal and opposite, antiparallel motion isinduced in response to the Coriolis force. This resultant motionis measured by capacitor plates under each of the two masses,providing a signal proportional to the rate input. Closed-loopPerformance of Small, Low-Cost Rate Sensors for Military and Commercial Applications2