Estimating grip forces with a tactilely transparent finger exoskeleton for pinch grip force assistance

A grip force estimation method is presented for an assistive finger exoskeleton which preserves the tactile sensation by allowing direct contact of the wearer´s fingerpad and object. The wearer´s grip force is estimated using two side-mounted custom load cells that are contacting to the lateral sides of the wearer´s fingertip whereas the palmar surface of the finger is exposed through the opening between the load cells. The outputs of the load cells reflect the amount of grip force and are also inevitably susceptible to the lateral force given by frictional traction of the fingerpad skin during object manipulation. A calibration equation for the grip force estimation was built as a combination of two linear functions of the load cell outputs. The grip force estimation error in presence of the lateral force was evaluated for 10 participants, and exhibited 0.653±0.287 N (mean±SD) in RMSE value.