Design and experiment of a SMA-based continuous-stiffness-adjustment torsional elastic component for variable stiffness actuators

Abstract

The variable stiffness actuator (VSA) is widely applied in robotics and human-robot interactions, generating controllable torque with sufficient compliance to ensure safety and robustness. At present, typical VSA design usually adopted pure mechanical structures that integrated an additional motor for stiffness adjustment, making it hard to meet the demand of lightweight joint actuation due to large actuator size and weight. As a new functional material, shape memory alloy (SMA) shows the prospect of lightweight actuation due to its high power-to-weight ratio. In this paper, a compact SMA-based torsional elastic component (SMA-TE) for the VSA is proposed. The SMA-TE with torsional stiffness is implemented by the confrontational arrangement of paired spiral SMA springs. The ceramic heating plate and semiconductor refrigeration plate are installed in the elastic component shell, and the thermally conductive medium is filled between the internal gap. The torsional stiffness of the SMA-TE can be continuously adjusted via a feed-forward cascade controller. Repeated experimental results show that the SMA-TE can precisely maintain constant stiffness and continuously adjust its stiffness between 2.6 Nm rad −1 and 6.8 Nm rad −1 when performing precise square wave stiffness tracking under variable frequencies.

Design and development of a shape memory alloy-powered rotary variable stiffness actuator embedded with an agonist-antagonist mechanism

“…One category of such actuators is an SMA-basedcontinuous-stiffness-adjustment torsional elastic component for variable stiffness actuators. 12 Different designs have different advantages depending on their mechanisms. Some include beam-based Compliant Transmission Elements (CTEs), 13 allowing for larger deflections and thus extending the VSA stiffness variability range.…”

Design and Analysis of a Novel Variable Stiffness Joint for Robot

“…The ball obtains the largest kinetic energy, and it will be hit farther. (2) The energy utilization rate of the system = the kinetic energy obtained by the ball/the total input energy of the system. When the joint stiffness is in the AC segment, K = 14.74~364 Nm/rad.…”

“…Recently, with the increasing demand for physical human-robot interaction (pHRI), compliant joints that can improve the safety and adaptability of the interaction process have attracted more and more attention [1]. In general, compliant joints are used to improve the safety of pHRI or the dynamical adaptability to the environment, as well as enhancing the energy efficiency [2]. Rigid joints have a strong positioning and bearing capacity, but they lack compliance and shock absorption due to the rigid structure of the joint [3].…”

“…When maintaining a constant stiffness, the requirement for the stability of the motor is high, which reduces the energy utilization of the joint. (2) The range of the joint variable stiffness is limited. (3) The variable stiffness structure of the joint has a large force variation, a single force position, and a complex control.…”

Mechanical design and analysis of a reconfigurable variable stiffness mechanism based on permanent magnetic springs and swing guide mechanism

“…4 Robots with variable stiffness joint actuators can be flexible when interacting with humans to ensure safety, and rigid when working in fine detail to improve motion accuracy. 5,6 At present, variable stiffness joint actuators mainly adopt the following schemes: (1) Using intelligent materials such as electro polymer, artificial muscle or shape memory alloy (SMA), so that the joint shows high toughness, high transmission strain, and inherent shock absorption capacity, but there are limitations such as expensive price, small output force, and slow response speed 7,8 ; (2) The impedance control based on force sensor has been widely used, however, it is limited by the accuracy of the force sensor and the bandwidth of the control system, and requires continuous energy consumption 9 ; (3) The structure of the mechanical variable stiffness adjustment device is flexible and diverse. 10 Some mechanical variable stiffness mechanisms can achieve stiffness adjustment within a certain range, but the mechanical structures are complicated and not easy to manufacture.…”