A force–torque sensor based on a Stewart Platform in a near-singular configuration

It is well known that a parallel mechanism at a singular configuration can gain one or more degrees of freedom instantaneously, and at such a configuration it cannot resist externally applied force/torque along certain directions. At near-singular configurations, small applied force/torque in a certain specific direction can give rise to large forces in the links, thereby resulting in mechanical magnification in link forces. This key idea is used, with a Stewart Platform, in a near-singular configuration, to design a directionally sensitive force–torque sensor. The concept of near-singular configuration and magnification is developed analytically and numerically with the help of a simple planar truss with rotary and flexure joints. A finite element analysis shows that a properly designed flexure joint approximates a rotary joint reasonably well, thus avoiding friction and non-linearities associated with rotary joints. The concept of force magnification and flexural joints is next extended to a Stewart Platform at a near-singular configuration. It is verified, using finite element analysis, that the Stewart Platform at a near-singular configuration with flexural hinges shows large forces in the legs for small external forces and torques applied in certain directions, and thus can be a good design for a highly sensitive force–torque sensor for certain components of applied force/torque. It is also shown, from a singularity analysis of the Stewart Platform, that sensitivity to other components of external force/torque can be obtained by using different near-singular configurations. The theoretical concepts are demonstrated with a prototype sensor which is sensitive to two components of the externally applied force and one component of the externally applied moment.