The Rhombic TSA-based Haptic Device (RTHD) is a novel parallel structure haptic device that combines a twisted string actuator (TSA) with a linearization mechanism. The RTHD consists of a simple mechanism that can render high linearized forces in the Y-direction throughout the workspace. The kinematic analysis demonstrates the advantages of the RTHD; simulation results compare the RTHD’s renderable end-effector force and velocity profiles to a capstan-pulley-based pantograph-type haptic device.
This paper introduces a novel 2-DoF haptic display, named the Rhombic TSA-based Haptic Device (RTHD), which integrates Twisted String Actuators (TSA) with a rhombus-type parallel linkage to achieve linearized high-force feedback in the Y-direction. Traditional capstan-pulley-based haptic devices (CPHDs) suffer from nonlinear transmission, limited workspace force uniformity, and increased inertia due to gearing constraints. To overcome these limitations, the authors design a parallel 2-DoF mechanism with symmetric dual TSA-driven linearization modules connected via a rhombus geometry. The nonlinear contraction behavior of TSAs—where output force diminishes as twisting increases—is addressed by embedding a kinematic transformation that maintains a constant transmission ratio between motor torque and output force. The proposed mechanism improves upon prior TSA-based solutions by reducing mechanical complexity and inertia while preserving high output force.
The kinematic modeling of the system confirms that the transmission from motor torque to Y-axis force remains linear, enabling consistent haptic feedback regardless of end-effector position. Simulation analysis demonstrates that the RTHD exhibits superior workspace manipulability, especially in the Y-direction, and achieves significantly higher renderable forces (up to 94 N) compared to the CPHD (12–15 N) under identical input torque and angular velocity conditions. Additionally, normalized force distribution maps confirm that RTHD maintains uniform force output across its entire reachable workspace, while CPHD performance degrades near the X-axis. The manipulability contour plots (Fig. 5) and renderable force–velocity comparisons (Fig. 6–7) support the claim that RTHD offers isotropic and linearized force capabilities over conventional mechanisms.
In conclusion, the integration of TSA and rhombus kinematics provides a low-cost, compact, and mechanically efficient solution for haptic rendering. While the current implementation addresses linearization in the Y-direction, future work will explore expansion to full 2D or 3D linearization and validation through experimental prototypes. The research highlights the potential of TSA–structure synergy not only in haptics but also in other high-force, lightweight actuation scenarios.