Acceleration Strain Transducer Containing Cantilever Flaps

Naval Undersea Warfare Center

Wednesday, April 01 2009

Sensitivity is maximized along one direction.

A recently invented acceleration strain transducer is based on the principle of a conventional spring-and-mass acceleration transducer combined with a linear strain sensor that measures the acceleration-induced deflection of the spring. The invention is compatible with any of a variety of linear strain sensors, including conventional foil resistance strain gauges, fiber-optic and fiber-laser strain sensors, and electrically-conductive-liquid strain sensors.

The invention encompasses numerous versions; the figure depicts a basic version to illustrate the principle of operation. The transducer includes a linear strain sensor stretched and bent over the tops of an array of angled polymeric cantilever flaps extending from a base. The strain sensor is bonded to the outermost two flaps (but not bonded to any of the other flaps) by an adhesive. The flaps closer to the middle are longer than the flaps closer to the ends; this configuration, in combination with the tension in the stretched strain sensor, keeps the strain sensor in contact with the tips of the flaps.

The flaps on opposite sides are set at opposite angles in a quasi-truss arrangement designed to maximize the strain response to acceleration along the y axis and to suppress or at least minimize the strain response to acceleration along the x axis. The flaps and the strain sensor act as both inertial masses and springs such that in acceleration along the -y or +y direction, the tension (and thus the longitudinal strain) in the linear strain sensor increases or decreases, respectively. The change in measured strain is taken as the indication of y-axis acceleration. On the other hand, because of the opposite tilts of the flaps on opposite sides, acceleration along the x axis causes the flaps on one side to move upward and outward and the flaps on the opposite side to move downward and inward. The strain contributions of these opposing motions tend to cancel, so that the net strain applied to the strain sensor (and, hence, the x-axis acceleration strain response) is close to zero. In addition, the flaps are made sufficiently wide along the z axis to impart insensitivity to acceleration along that axis.

An important consideration affecting the design of such an accelerometer is the need to prevent or at least minimize vibration of the strain sensor in a string mode, which vibration would yield spurious oscillating strain readings and could be excited by acceleration along the x or z axis. Such vibration can be minimized by mounting the strain sensor in enough tension to push the vibrational resonance to a frequency well above the maximum frequency of acceleration that one seeks to measure.

This work was done by Jason M. Maguire of the Naval Under sea Warfare Center.
NUWC-0004