This paper describes a new tactile rendering technique to produce lump sensations in virtual soft surfaces. Most softness displays reported so far reproduced only simple and uniform soft objects; nevertheless many real objects around us provide complicated softness. To reproduce various different softness sensations, we have previously developed a softness display utilizing a flexible sheet that wraps around a fingertip to stimulate fingertip cutaneous sensations. The device controls the softness sensations by changing tension of the flexible sheet, as well as the contact area between the sheet and a fingertip. During our pilot experiments, we found that users feel lump sensations from the softness display in particular conditions. Based on the findings, this paper proposes a technique to reproduce sensations of soft objects that contain lumps beneath their surfaces. The proposed technique can vary the size of the rendered virtual lumps through tension control; larger tension produces smaller size of lump sensation, and vice versa. Results of experiments using human subjects confirmed the validity of the proposed method.

This paper presents a 2-DOF controlled softness display and its application to a tactile tele-presentation system. It has been reported that softness feelings can be displayed by reproducing contact area, or contact width, on a fingertip. Several softness displays based on the contact area control have been reported in past studies, but all of them had only one degree-of-freedom (DOF) for their contact area control. Resultantly, the reproduced contact area was symmetric, which could only produce uniform surface feelings. In some practical deformable surfaces, however, the contact conditions are not uniform over fingertip surfaces; for example, if there is a small lump beneath a soft deformable surface, resultant contact area can become asymmetric. To reproduce such asymmetric contact conditions, a softness display should have more controlled DOFs. This paper reports our trial to realize a multi-DOF controlled softness display. As a first step, this particular paper describes a 2-DOF controlled display. The display has two DC motors which independently control both sides of the contact area. Resultant asymmetric contact area can facilitate, e.g., discrimination of lump location in soft deformable surfaces. The display was integrated into a tactile tele-presentation system together with a contact width sensor, which measures asymmetric contact condition. Using the tele-presentation system, discriminations of lump location are demonstrated.

This paper presents a tele-presentation system for tactile softness. Tactile softness was estimated at a remote site, which was then reproduced at a master site, by focusing on change of contact width between a fingertip and an object. To realize the tele-presentation system, a new contact width sensor was developed based on an optical principle. The sensor was designed to have a soft structure to resemble human fingers. The prototype sensor was evaluated for five different samples. The result showed that the sensor could discriminate five samples from the contact width variation, although one pair of samples showed only a slight difference. The tele-presentation system that integrates the developed contact width sensor and a newly designed softness display was developed and evaluated. The evaluation result showed that a user can discriminate softness of a remote object using this system, although the perceived softness feelings did not perfectly match the real feelings of the remote objects, which would be due to insufficient optimization of the contact width sensor. This would be improved by improving the sensor to have a similar dimensions and stiffness as real human fingers, which will be addressed in our future work.