  This paper develops an accurate force control algorithm for series elastic
actuators (SEAs) based on a novel force estimation scheme, called transmission
force observer (TFOB). The proposed method is designed to improve an inferior
force measurement of the SEA caused by nonlinearities of the elastic
transmission and measurement noise and error of its deformation sensor. This
paper first analyzes the limitation of the conventional methods for the SEA
transmission force sensing and then investigates its stochastic
characteristics, which indeed provide the base to render the accurate force
control performance incorporated with the TFOB. In particular, a tuning
parameter is introduced from holistic closed-loop system analyses in the
frequency domain. This gives a guideline to attain optimum performance of the
force-controlled SEA system. The proposed algorithm is experimentally verified
in an actual SEA hardware setup.
