Design and Control of a Variable Stiffness Actuator Based on Adjustable Moment Arm

Abstract

For tasks that require robot-environment interaction, stiffness control is important to ensure stable contact motion and collision safety. The variable stiffness approach has been used to address this type of control. We propose a hybrid variable stiffness actuator (HVSA), which is a variable stiffness unit design. The proposed HVSA is composed of a hybrid control module based on an adjustable moment-arm mechanism, and a drive module with two motors. By controlling the relative motion of gears in the hybrid control module, position and stiffness of a joint can be simultaneously controlled. The HVSA provides a wide range of joint stiffness due to the nonlinearity provided by the adjustable moment arm. Furthermore, the rigid mode, which behaves as a conventional stiff joint, can be implemented to improve positioning accuracy when a robot handles a heavy object. In this paper, the mechanical design features and related analysis are explained. We show that the HVSA can provide a wide range of stiffness and rapid responses according to changes in the stiffness of a joint under varying loads by experiments. The effectiveness of the rigid mode is verified by some experiments on position tracking under high-load conditions. Index Terms-Adjustable moment arm, compliant joint, lever mechanism, variable stiffness actuator (VSA).

A New Soft RCC Device with Pneumatic Regulation

“…The advantage of an actuator having two rolling diaphragm seals, instead of classical bearings, is its ability to perform a movement with reduced friction. Unlike a large portion of the variable stiffness systems presented in the literature, where the impedance has the same direction as the actuation [20][21][22], in this design it is possible also to adjust the compliance in the not-actuated directions. This is achieved by modifying the pressure p in the chamber A, while the actuation is modifying the pressure p B in the chamber B.…”

“…In this paper, the authors attempted to apply the RCC concept to a new linear variable stiffness pneumatic actuator, which was presented for the first time in [19], and named for convenience using the acronym PATuCCo (Pneumatic Actuator with Tunable-Compliance Constraint). To date, a large number of variable stiffness actuators have been developed but, in general, stiffness is controllable only in the same direction of the actuation [20][21][22][23][24][25]. In [19], the authors proposed a novel pneumatic actuator with tunable compliance that is able to modify the general stiffness of its internal constraints due to the use of rolling diaphragm seals for separating the actuator's chambers.…”

Design of a Variable Stiffness Joint Module to Quickly Change the Stiffness and to Reduce the Power Consumption

“…Particularly in some cases, the system should instantaneously change the stiffness to absorb the energy during the contact with the environment, or to inject energy into the system in other cases. Further, decoupling the control of stiffness and the position can be achieved by coupling the endpoints of the springs either through lever arm mechanism [7]- [9], [30], [33], [34], or cam mechanism [15], [34], [35]. Also, implementing a lever arm mechanism for varying the stiffness by changing the transmission ratio between the internal elastic element and the motor would affect the compactness.…”

Robust Trajectory Tracking Control for Variable Stiffness Actuators With Model Perturbations

“…Variable stiffness actuators (VSAs), a kind of compliant actuators, have been introduced to develop new-generation robots because of its abilities to increase safety in human-robot interaction, to satisfy dynamic requirements, and to provide adaptability in unknown environments (Vanderborght et al, 2013; Grioli et al, 2015; Guo et al, 2015; Wolf et al, 2016; Pan et al, 2017). VSAs are usually multi-input multi-output (MIMO) non-linear systems, where the stiffness and position of the VSAs can be adjusted simultaneously by decoupling control methods (Kim and Song, 2012). However, in these actuators, the stiffness variation brings physical modifications, which requires the controllers to transit among different working conditions quickly.…”