Concord, CA--To find their way around the sky, modern aircraft depend on accurate velocity and positional data generated by an inertial navigation system (INS). A typical INS includes several gyroscopes and, increasingly, miniature accelerometers.
These accelerometers generally produce an analog output signal that must be passed through an A/D converter and changed to a digital format for use by the computerized navigation system. All A/D converters introduce error-sometimes on the order of a few hundred æGs-which reduces the accuracy of the measured acceleration. For some INS applications, such seemingly trivial error can be significant. "If you eat up ¼ of your sensitivity with A/D error, that's a big deal," says Bert Egley, a design engineer at Systron Donner's Inertial Division.
As a solution to this problem, Egley and a team of engineers developed, over a period of ten months, a miniature accelerometer that produces an inherently digital output. Called the Vibrating Quartz Accelerometer (VQA), it eliminates A/D-conversion errors and makes integration easier for Systron Donner Inertial Division's customers.
At the heart of the VQA is an assembly of two crystalline-quartz force-sensing crystals and a disk-shaped pendulous mass. A pair of flexure hinges support the mass along one edge and allow themass to move along the accelerometer's sensing axis. Attached to opposite sides of the mass across the flexure hinge, the two crystals restrain this motion.
Formed into each force-sensing crystal is a pair of tuning-fork-like tines. Crystalline quartz is highly piezoelectric, and a set of electrical oscillator circuits bonded to the crystal exploit this property to drive the tines at 16 kHz, their fundamental oscillatory mode.
Applying an acceleration load along the sensing axis causes the pendulous mass to move. As it moves it loads the two crystals, tensing one set of tines and driving them to a higher frequency, and compressing the other set and driving their frequency lower. "What we have here are guitar strings, continuously plucked by the oscillator," says Egley. "Load them and their frequency changes."
By finding the difference between the two frequencies, the sensor determines the magnitude of the acceleration event. For every G of acceleration, each crystal's frequency changes by about 8 Hz. The sensor's operating range is ±100G with a resolution of 50 æG. Surprisingly rugged, it will survive 1,000G shocks and 10G random vibration from 20 Hz to 2,000 Hz. To reduce the VQA's temperature sensitivity, engineers mounted copper plates to each side of the quartz assembly. The plates also help improve the sensor's vibration performance. Positioned within a few thousandths of an inch of the pendulous mass, they work in conjunction with the inert gas inside the case to provide squeeze-film damping of the mass.
Early prototypes exhibited an unexpected problem in which the VQA's output would make brief unexpected jumps from 16 kHz to 87 kHz. Using a detailed FEA model created with ANSYS, Egley studied the relationship between the electrode positions and the third harmonic mode. By subtly redesigning the electrodes he found a way to avoid energizing the third mode while only slightly affecting the efficiency of the primary mode.
"This device competes with analog units costing from $2,000 on up," says Egley. "In high volume, we can get this price down to a few hundred dollars apiece."
Additional details...Contact Scott Orlosky, Systron Donner, Inertial Div., 2700 Systron Dr., Concord, CA 94518, (510) 671-6619.