Mechanical design and impedance compensation of SUBAR (Sogang University’s Biomedical Assist Robot)

Abstract

SUBAR (Sogang University's Biomedical Assistive Robot), an advanced version of the EXPOS [1-2], is a wearable robot developed for assisting physically impaired people. It provides a human with assistive forces controlled by the human intention. Since it applies geared DC motors, however, the control efforts are used mainly to overcome the resistive forces caused by the friction, the damping and the inertia in the actuator. In this paper, such undesired properties are rejected by applying a flexible transmission. With the proposed method, it is intended that an actuator exhibits zero impedance without friction and generates the desired torques precisely. Since the actuation system of SUBAR has a large model variation due to human-robot interaction, a control algorithm for the flexible transmission is designed based on robust control theory. In this paper, the mechanical design of SUBAR including the flexible transmission and its associated control algorithm are discussed. They are also verified by experiments.

Joint force control of parallel robot for ankle rehabilitation

“…Combination of (7), (10), (11) and (14) therefore gives the new current command (15). It should be noted that D can be rewritten as (16), where U D' is a matrix consisting of non-null space vectors from the singular value decomposition of D and D' is a diagonal matrix consisting of the singular values corresponding to the column vectors of U D'.…”

“…In order to ensure that the actuator force is being followed as closely as possible when significant interaction is present between the robot and the environment (i.e. M ext ), an additional modification is applied to the force command before it is passed through the control law (15). The modified force command, F c ' is shown in (18) and it basically aims to obtain an identity transmission matrix between F c and F act along the non-null space directions by pre-scaling the non null space components with the inverse of their corresponding singular values in (17).…”

“…Based on (11) and (13)(14)(15)(16), the approximate relationship between the actuator forces, the commanded forces and the disturbance forces when the acceleration term is negligible can be given as (17). Where = [1 (1 )K f ] -1 is the singular value corresponding to the null space vector in the commanded force to actuator force transmission matrix.…”

“…By considering the actuator current as the input to the system shown in (4) and treating the rest of the dynamic terms and external torque as disturbances, a disturbance observer based approach [15] can be used to control the actuator forces. In this approach, the nominal plant would be the direct feed through term K t K a D, which does not have any dynamics, thus simplifying the design of the disturbance observer based controller.…”

“…Frictional forces are normally accounted for by applying friction compensators [14], which relies on accurate modeling of frictional forces and availability of velocity information. Another possible solution for reducing effects of friction is the use of disturbance observer based control schemes [13,15]. In parallel robots, significant coupling can exist between the different degrees of freedom.…”