Active Loading Control Design for a Wearable Exoskeleton with a Bowden Cable for Transmission

Abstract

Exoskeletons with a Bowden cable for power transmission have the advantages of a concentrated mass and flexible movement. However, their integrated motor is disturbed by the Bowden cable’s friction, which limits the performance of the force loading response. In this paper, we solve this problem by designing an outer-loop feedforward-feedback proportion-differentiation controller based on an inner loop disturbance observer. Firstly, the inner loop’s dynamic performance is equivalent to the designed nominal model using the proposed disturbance observer, which effectively compensates for the parameter perturbation and friction disturbance. Secondly, based on an analysis of the stability of the inner loop controller, we obtain the stability condition and discuss the influence of modeling errors on the inner loop’s dynamic performance. Thirdly, to avoid excessive noise from the force sensors being introduced into the designed disturbance observer, we propose the feedforward-feedback proportion-differentiation controller based on the nominal model and pole configuration, which improves the outer loop’s force loading performance. Experiments are conducted, which verify the effectiveness of the proposed methods.

Study on the Control Method of Knee Joint Human–Exoskeleton Interactive System

“…Through experiments in previous studies, we have verified that the parameter perturbation of the inner loop and the poor anti-interference ability can deteriorate the dynamic performance of the outer loop [ 31 ]. Furthermore, it is concluded that the inner loop based on the DOB can improve the dynamic performance potential of the outer loop.…”

“…Therefore, the power supply, controller, and servo motor are integrated near the center of body weight. The fixation mechanism of the knee joint [ 31 ] is shown in Figure 1 , which is driven by a servo motor through a Bowden cable. Two lightweight carbon fiber boards (LCFB) and hinge mechanisms can realize rotation in the sagittal and coronal planes.…”

“…The dynamic equation of the lower limb can be simplified as where is the muscle torque of the human, is the interaction torque, and is the gravity torque. Combined with Equations (1)–(5), the block diagram of the inner loop [ 31 ] can be obtained as follows.…”

“…However, the bandwidth of the filter is limited by the stability conditions of the DOB. The stability conditions are described as [ 31 ] where is the modeling error of the speed loop. It should be noted that Equation (12) is essential for designing a DOB based on a servo motor instead of knee joints.…”

EXOSMOOTH: Test of Innovative EXOskeleton Control for SMOOTH Assistance, With and Without Ankle Actuation

“…[1], [2]), in the latest years there is a tendency towards a hardware simplification and a task-specific design of exoskeletons, e.g. [3]- [6], Ekso Vest (Ekso Bionics, Richmond, CA, USA), LegX and suitX (US Bionics, Inc., Emeryville, CA, USA). In parallel, several control paradigms have been studied to improve the usability of these machines and cope with the limited sensing and actuation capabilities imposed by the hardware simplification.…”