A device that measures both translational acceleration and rotation exploits the dynamics of a vibratory structure that resembles a clover leaf. This device is related to other vibratory accelerometers and vibratory gyroscopes in which vibrations in suitably shaped structures are excited electrostatically and measured either capacitively or piezoresistively. The basic principle of this device could also be applied to a different structure, provided that, like the clover-leaf structure, it vibrates in substantially degenerate modes and provided further that it has a small mass imbalance.

The figure depicts the clover-leaf structure in the present device. This structure vibrates in two degenerate modes. A small mass imbalance defines the shape of the modes in that it causes the node lines of the vibrational pattern to lie along (1) a line that runs through the geometric center of the structure and the location of the mass imbalance and (2) a line perpendicular to the aforementioned line. In the presence of a nominal constant (e.g., zero) rotation or constant translational acceleration, it is possible to rotate the node lines to make them coincide with the Cartesian axes of symmetry of the structure; this is accomplished by applying an electrostatic force of such a magnitude as to contribute a negative spring-stiffness component that compensates for the mass imbalance.

A Planar Resonator that resembles a four-leaf clover is symmetrical except for a small mass imbalance at the location of the circle. The node lines of the degenerate vibrational modes lie along one or the other set of dashed lines, depending on whether or not the mass unbalance is compensated electrostatically.

The vibrational dynamics are such that a change in the translational acceleration perturbs the compensation, causing the node lines to rotate back toward alignment with the mass unbalance. In operation in an open-loop mode, the rotation of the node lines can be deduced from the amplitude and phase relationships among the outputs of the capacitive vibration sensors. Alternatively, the device can be operated in a closed-loop mode in which the signals are processed into feedback control signals that adjust the electrostatic force to keep the node lines from rotating; in this case, the feedback control signal serves as an indication of the angular velocity or translational acceleration. It is possible to measure rotation and translational acceleration simultaneously and separately because the translation- and rotation-related capacitive-sensor outputs come out in quadrature with each other.

This work was done by Roman Gutierrez and Tony K. Tang of Caltech for NASA's Jet Propulsion Laboratory.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
NASA Resident Office - JPL; (818) 354-5179

Refer to NPO-20620

Motion Control Tech Briefs Magazine

This article first appeared in the October, 1999 issue of Motion Control Tech Briefs Magazine.

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