Bidirectional torsional negative stiffness mechanism for energy balancing systems

A new concept of an integrated bidirectional torsional negative stiffness mechanism is introduced which allows for passive energy balancing of mechanical systems by reducing actuation requirements and improving energy efficiency. This novel design is a modular device, is bidirectional and is easily integrated and customised for different applications. The energy balance concept is achieved by employing a negative stiffness system to couple with a positive stiffness system of the mechanical system to create a zero stiffness system which can be driven with lower energy requirements. The bidirectional torsional negative stiffness mechanism proposed here uses a series of pre-compressed springs around a bidirectional torque shaft to convert decreasing force in the springs into increasing torque output through geometric reconfiguration to generate the negative stiffness characteristic. The kinematics of the negative stiffness mechanism were derived and a physical model was assembled from LEGO^® components for validation. An analytical model was developed for prediction and the numerical results showed that a satisfactory performance can be generated to match the torque requirements. An experimental demonstrator was then built and tested to verify the predictions from the analysis. To show the impact of the energy balancing concept on actuator efficiency, a representative case study is made of a tendon-driven morphing active camber mechanism. The performance of the optimised bidirectional negative stiffness device is investigated to shows an improvement by the kinematics tailoring.