Controlling stiffness of a frame spring by changing the boundary condition with an SMA actuator

A semi-active device was developed to reduce structural vibrations by adjusting the stiffness of the device that connects the structure to the base. The idea is to change the boundary condition of the device to adjust its stiffness. The selected construction was a circular frame (a ring). While the frame is pushed vertically against the base it behaves as a spring. During the compression the ring expands horizontally. Restricting the deformation in horizontal direction makes the frame stiffer. The controllable stiffness ratio of the device from soft to stiff mode was determined analytically to go up to 6.4. The influence is caused purely by changing the boundary condition. It does not depend on dimensions or material parameters as long linear elastic material behaviour is valid. A string made of shape memory alloy (SMA) was used as an actuator, which was attached to the frame on the horizontal diagonal. The change of boundary condition was controlled by the gap between the frame and horizontal constraint. The operation mode, stiff or soft, was selected by controlling the temperature of the SMA string, which in turn controls the phase composition of the SMA. Experimental tests were carried out to verify the functionality of the device. Both static and dynamic loading tests indicated that the control of boundary condition can be utilized to change the stiffness. The average stiffness ratio of each test set was typically from 3.4 to 4.1. The maximum stiffness ratio was 5.0.