Transtiff: A Stylus-shaped Interface for Rendering Perceived Stiffness of Virtual Objects via Stylus Stiffness Control

TL;DR

Transtiff investigates how stylus stiffness interacts with visual cues to shape perceived stiffness of virtual objects in VR. By designing a stylus-shaped interface driven by McKibben artificial muscles, the authors demonstrate on-demand stiffness control that creates the illusion of a hard object feeling soft. A psychophysical study shows cross-modal influences between stylus stiffness and visual deformation cues, and subsequent evaluations confirm tunable stiffness rendering across materials like sponge, plastic, and tennis balls. The work advances VR haptics by providing a compact, grip-style interface capable of realistic tool-mediated stiffness rendering and highlights design considerations for future stiffness-control haptic devices.

Abstract

The replication of object stiffness is essential for enhancing haptic feedback in virtual environments. However, existing research has overlooked how stylus stiffness influences the perception of virtual object stiffness during tool-mediated interactions. To address this, we conducted a psychophysical experiment demonstrating that changing stylus stiffness combined with visual stimuli altered users' perception of virtual object stiffness. Based on these insights, we developed Transtiff, a stylus-shaped interface capable of on-demand stiffness control using a McKibben artificial muscle mechanism. Unlike previous approaches, our method manipulates the perceived stiffness of virtual objects via the stylus by controlling the stiffness of the stylus without altering the properties of the real object being touched, creating the illusion of a hard object feeing soft. Our user study confirmed that Transtiff effectively simulates a range of material properties, such as sponge, plastic, and tennis balls, providing haptic rendering that is closely aligned with the perceived material characteristics. By addressing the challenge of delivering realistic haptic feedback through tool-based interactions, Transtiff represents a significant advancement in the haptic interface design for VR applications.

Figures (15)

• Figure 1: Experimental overview. The participants use the HMD to receive visual stimuli showing the deformation of the stylus or object and manipulate a stylus with a rubber joint to explore objects of different stiffness (a spring-loaded cylindrical mechanism).
• Figure 2: Stylus/Cylinder deformation. The visual stimuli consisted of a stylus appearing soft on the left and a cylinder appearing soft on the right, both designed to visually convey stiffness.
• Figure 3: Variations of the stylus used in the experiment, showing different combinations of hardness levels (50 or 70) and joint lengths (10 mm, 15 mm, or 20 mm), along with a condition without a rubber joint.
• Figure 4: Cylinder spring conditions and mechanism. The left side illustrates the five predefined stiffness levels of the cylinder's spring mechanism (spring constant: 0.5N/mm), while the right side depicts the cylinder size and stiffness control mechanism.
• Figure 5: Experimental environment. Participants used the stylus to interact with a cylinder extending through a hole in the table, representing an object with variable stiffness.