Force measurement using vibrational spectroscopy

The stress dependence of ultrasonic velocities is used as the physical basis for a device that measures large forces. A solid aluminum cylinder serves as a load-bearing element, and noncontacting electromagnetic-acoustic transducers are used to excite and detect resonant vibrational modes; The problem of vibrational damping at the ends of the cylinder is solved by making the diameter slightly larger in a central section, so that certain resonant modes are ''trapped,'' with negligible amplitude al the ends. Nearly complete temperature compensation is achieved by taking the ratio of two resonant frequencies with approximately the same temperature dependence but with much different stress dependencies. The measurement of force with vibrational spectroscopy provides several advantages over conventional resistive strain gages, including greater resolution, insensitivity to transducer and bond degradation, durability in harsh environments, and insensitivity to bending moments. The resolution in strain is approximately 4 x 10(-8). The upper limit of measurable stresses approaches the yield strength of the aluminum cylinder; this is approximately 325 MPa, which corresponds to a strain of 4 x 10(-3).