Design and Evaluation of a Variable Stiffness Module for an Open-End Tendon Antagonistic Actuator
L. Laohaphand and E. Pengwang
AI summary
Problem
Conventional lever-based variable stiffness actuators rely on bulky, independently motorized pivot regulators that increase distal joint inertia, limiting safe physical human-robot interaction.
Approach
The design replaces independent motors with a Coupled Dual Slider Crank Mechanism driven by antagonistic open-end twisted string actuators to remotely regulate lever pivots, minimizing weight and inertia while preserving a wide stiffness range.
Key results
- Novel CDSCM pivot-regulation mechanism design and kinematic modeling
- Fabricated prototype achieves 0.5 kg weight and 84 mm diameter
- Pivot regulation from 0 to 73% of active range under 1.0 Nm load in 0.618 seconds
- Maximum stiffness of 99.26 Nm/rad with empirical torque-deflection model correction
Why it matters
Enables safer, more responsive distal joint actuation for wearable exoskeletons and robotic manipulators by drastically reducing reflected inertia without sacrificing stiffness range.
Abstract
This paper presents a lever mechanism-based Variable Stiffness Module (VSM) for distal joints in physical Human–Robot Interaction (pHRI) robotics applications. Conventional lever mechanism-based VSA designs rely on bulky pivot-regulation mechanisms in an independent motor setup, resulting in high inertia that limits safe pHRI operation. We introduce a novel pivot-regulation mechanism, the Coupled Dual Slider Crank Mechanism (CDSCM), which integrates an antagonistic Twisted String Actuator (aTSA) with the lever mechanism. This method allows smaller actuators to handle a higher load and regulate the pivot from the distal position while preserving the lever mechanism’s wide stiffness range (0 −∞Nm/rad, theoretically) and reducing the weight of the VSM to 0.5 kg. A stiffness model is established to study the torque–deflection relation. The internal force relations of the CDSCM are formulated to guide motor selection. Experiments on pivot regulation demonstrate that the CDSCM can regulate pivots from 0 to 73% of the active range under 1.0 Nm external torque within 0.618 seconds. Actual torque and estimated torque profile are compared in the torque- deflection identification experiment. The maximum achievable stiffness of the VSM is 99.26 Nm/rad, which indicates the design rigidity issues. We address this issue by introducing empirical gain in the torque-deflection model to increase the model accuracy and analyze the VSM design via the SolidWorks static analysis (FEA) to identify the parts that over-deflected under the external load.