A Fabric-based Approach for Softness Rendering

In this chapter we describe a softness display based on the contact area spread rate (CASR) paradigm. This device uses a stretchable fabric as a substrate that can be touched by users, while contact area is directly measured via an optical system. By varying the stretching state of the fabric, different stiffness values can be conveyed to users. We describe a first technological implementation of the display and compare its performance in rendering various levels of stiffness with the one exhibited by a pneumatic CASR-based device. Psychophysical experiments are reported and discussed. Afterwards, we present a new technological implementation for the fabric-based display, with reduced dimensions and faster actuation, which enables rapid changes in the fabric stretching state. These changes are mandatory to properly track typical force/area curves of real materials. System performance in mimicking force-area curves obtained from real objects exhibits a high degree of reliability, also in eliciting overall discriminable levels of softness

Design and characterization of a fabric-based softness display

To enable a realistic tactile interaction with remote or virtual objects, softness information represents a fundamental property to be rendered via haptic devices. What is challenging is to reduce the complexity of such an information as it arises from contact mechanics and to find suitable simplifications that can lead an effective development of softness displays. A possible approach is to surrogate detailed tactile cues with information on the rate of spread of the contact area between the object and the finger as the contact force increases, i.e. force/area relation. This paradigm is called contact area spread rate. In this paper we discuss how such a paradigm has inspired the design of a tactile device (hereinafter referred to as Fabric Yielding Display, FYD-2), which exploits the elasticity of a fabric to mimic different levels of stiffness, while the contact area on the finger indenting the fabric is measured. In this manner, the FYD-2 can be controlled to reproduce force-area characteristics. In this work, we describe the FYD-2 architecture and report a psychophysical characterization. FYD-2 is shown to be able to accurately reproduce force-area curves of typical objects and to enable a reliable softness discrimination in human users

Rendering Softness: Integration of Kinesthetic and Cutaneous Information in a Haptic Device

While it is known that softness discrimination relies on both kinesthetic and cutaneous information, relatively little work has been done on the realization of haptic devices replicating the two cues in an integrated and effective way. In this paper, we first discuss the ambiguities that arise in unimodal touch, and provide a simple intuitive explanation in terms of basic contact mechanics. With this as a motivation, we discuss the implementation and control of an integrated device, where a conventional kinesthetic haptic display is combined with a cutaneous softness display. We investigate the effectiveness of the integrated display via a number of psychophysical tests and compare the subjective perception of softness with that obtained by direct touch on physical objects. Results show that the subjects interacting with the integrated haptic display are able to discriminate softness better than with either a purely kinesthetic or a purely cutaneous display

A Device for Mimicking the Contact Force/Contact Area Relationship of Different Materials with Applications to Softness Rendering

In this paper a fabric yielding softness display (FYD-2) is proposed, where the stretching state is controlled using two motors, while the contact area is measured in real-time. In previous works, authors proposed a fabric-based device, with embedded contact area measurement system, which was proved to provide subjects with a compelling and naturalistic softness perception. Compared to it, FYD-2 exhibits reduced dimensions, a more accurate sensorization scheme and an increased actuation velocity, which allows to implement fast changes in the stretching state levels. These changes are mandatory, for example, to properly track typical quadratic force/area curves of real materials. Furthermore, FYD-2 is endowed with an additional degree of freedom that can be used to convey supplementary haptic cues, such as directional cues, which can be exploited to produce more immersive haptic interactions. In this work we describe the mechanical design and the mathematical model of the device. The reliability in real-time tracking of stiffness and force-area curves of real objects is also demonstrated

A Finite element model of tactile flow for softness perception

Touch is an extremely dynamic sense. To take into account this aspect, it has been hypothesized that there are mechanisms in the brain that specialize in processing dynamic tactile stimuli, in a way not too dissimilar from what happens for optical flow in dynamic vision. The concept of tactile flow, related to the rate of expansion of isostrain volumes in the human fingerpad, was used to explain some perceptual illusions as well as mechanisms of human softness perception. In this paper we describe a computational model of tactile flow, and apply it to a finite element model of interaction between deformable bodies. The shape and material properties of the bodies are modeled from those of a human fingertip interacting with specimens with different softness properties. Results show that the rate of expansion of isostrain volumes can be used to discriminate different materials in terms of their softness characteristics

Modulating the Perceived Softness of Real Objects Through Wearable Feel-Through Haptics

In vision, Augmented Reality (AR) allows the superposition of digital content on real-world visual information, relying on the well-established See-through paradigm. In the haptic domain, a putative Feel-through wearable device should allow to modify the tactile sensation without masking the actual cutaneous perception of the physical objects. To the best of our knowledge, a similar technology is still far to be effectively implemented. In this work, we present an approach that allows, for the first time, to modulate the perceived softness of real objects using a Feel-through wearable that uses a thin fabric as interaction surface. During the interaction with real objects, the device can modulate the growth of the contact area over the fingerpad without affecting the force experienced by the user, thus modulating the perceived softness. To this aim, the lifting mechanism of our system warps the fabric around the fingerpad in a way proportional to the force exerted on the specimen under exploration. At the same time, the stretching state of the fabric is controlled to keep a loose contact with the fingerpad. We demonstrated that different softness perceptions for the same specimens can be elicited, by suitably controlling the lifting mechanism of the system

On the Role of Affective Properties in Hedonic and Discriminant Haptic Systems

Common haptic devices are designed to effectively provide kinaesthetic and/or cutaneous discriminative inputs to the users by modulating some physical parameters. However, in addition to this behavior, haptic stimuli were proven to convey also affective inputs to the brain. Nevertheless, such affective properties of touch are often disregarded in the design (and consequent validation) of haptic displays. In this paper we present some preliminary experimental evidences about how emotional feelings, intrinsically present while interacting with tactile displays, can be assessed. We propose a methodology based on a bidimensional model of elicited emotions evaluated by means of simple psychometric tests and statistical inference. Specifically, affective dimensions are expressed in terms of arousal and valence, which are quantified through two simple one-question psychometric tests, whereas statistical inference is based on rank-based non-parametric tests. In this work we consider two types of haptic systems: (i) a softness display, FYD-2, which was designed to convey purely discriminative softness haptic stimuli and (ii) a system designed to convey affective caress-like stimuli (by regulating the velocity and the strength of the “caress”) on the user forearm. Gender differences were also considered. In both devices, the affective component clearly depends on the stimuli and it is gender-related. Finally, we discuss how such outcomes might be profitably used to guide the design and the usage of haptic devices, in order to take into account also the emotional component, thus improving system performance

A novel tactile display for softness and texture rendering in tele-operation tasks

Softness and texture high-frequency information represent fundamental haptic properties for every day life activities and environment tactual exploration. While several displays have been produced to convey either softness or high-frequency information, there is no or little evidence of systems that are able to reproduce both these properties in an integrated fashion. This aspect is especially crucial in medical tele-operated procedures, where roughness and stiffness of human tissues are both important to correctly identify given pathologies through palpation (e.g. in tele-dermatology). This work presents a fabric yielding display (FYD-pad), a fabric-based tactile display for softness and texture rendering. The system exploits the control of two motors to modify both the stretching state of the elastic fabric for softness rendering and to convey texture information on the basis of accelerometer-based data. At the same time, the measurement of the contact area can be used to control remote or virtual robots. In this paper, we discuss the architecture of FYD-pad and the techniques used for softness and texture reproduction as well as for synthesizing probe-surface interactions from real data. Tele-operation examples and preliminary experiments with humans are reported, which show the effectiveness of the device in delivering both softness and texture information

W-FYD: a Wearable Fabric-based Display for Haptic Multi-Cue Delivery and Tactile Augmented Reality

Despite the importance of softness, there is no evidence of wearable haptic systems able to deliver controllable softness cues. Here, we present the Wearable Fabric Yielding Display (W-FYD), a fabric-based display for multi-cue delivery that can be worn on user's finger and enables, for the first time, both active and passive softness exploration. It can also induce a sliding effect under the finger-pad. A given stiffness profile can be obtained by modulating the stretching state of the fabric through two motors. Furthermore, a lifting mechanism allows to put the fabric in contact with the user's finger-pad, to enable passive softness rendering. In this paper, we describe the architecture of W-FYD, and a thorough characterization of its stiffness workspace, frequency response and softness rendering capabilities. We also computed device Just Noticeable Difference in both active and passive exploratory conditions, for linear and non-linear stiffness rendering as well as for sliding direction perception. The effect of device weight was also considered. Furthermore, performance of participants and their subjective quantitative evaluation in detecting sliding direction and softness discrimination tasks are reported. Finally, applications of W-FYD in tactile augmented reality for open palpation are discussed, opening interesting perspectives in many fields of human-machine interaction

The effect of indentation force and displacement on visual perception of compliance

This paper investigates the effect of maximum indentation force and depth on people's ability to accurately discriminate compliance using indirect visual information only. Participants took part in two psychophysical experiments in which they were asked to choose the 'softest' sample out of a series of presented sample pairs. In the experiments, participants observed a computer-actuated tip indent the sample pairs to one of two conditions; maximum depth (10mm) or maximum force (4N). This indentation process simulates tool operated palpation in laparoscopic surgery. Results were used to plot psychometric functions as a measure of accuracy of compliance discriminability. A comparison indicated that participants performed best in the task where they judged samples being indented to a pre-set maximum force relying solely on visual cues, which demonstrates the effect of visual information on compliance discrimination. Results also show that indentation cues such as force and deformation depth have different effects on our ability to visually discriminate compliance. These findings will inform future work on designing a haptic feedback system capable of augmenting visual and haptic information independently for optimal compliance discrimination performance