Until now, it was difficult for technical sensors to sense the softness of a material. Now researchers have developed a new haptic device that can mimic the human perception of softness.
Every day we use our sense of touch in numerous situations to sense the properties of objects. However, it is much more difficult to reproduce this tactile sensation technically. While the elasticity of materials can be determined relatively easily, sensing the softness of a surface is a complex task. Many sensory and cognitive processes play a role. Replicating these processes digitally represents a major challenge for robotics.
Models of haptic perception developed so far have not been able to distinguish between two main components of human touch: the signals that arise from the sensory feedback from the skin of the fingertip and the kinesthetic signals that reflect the force exerted on the finger joint. “If you press a marshmallow with your fingertip, you can easily feel that it is soft. But if you place a hard cookie on top of that marshmallow and press it again, you can still tell that the soft marshmallow is underneath, even if your fingertip touches a hard surface. We wanted to find out if we could develop a robotic platform that could do the same,” explains Mustafa Mete from the Reconfigurable Robotics Laboratory (RRL) at the Swiss Federal Institute of Technology in Lausanne (EPFL).
With the SORI model (Softness Rendering Interface), Mete’s team has now succeeded in presenting such a platform. By splitting into skin and kinesthetic signals, the SORI device is able to mimic the perception of the softness of materials that have both soft and solid properties – like a cookie on the marshmallow or a leather-bound book. “It should not be understood as a softness sensor for robots, but as a digital transmission of the feeling of ‘touch’, similar to sending photos or music,” explains Mete.
The perception of softness depends on the person
The picture shows a SORI device in use. It is equipped with motorized origami joints, which can be adjusted either more firmly or more flexibly. The joints are covered with a silicone membrane. A stream of air inflates the membrane to varying degrees and envelopes the fingertip, which is placed in the center of the membrane.
Studies in neuroscience and psychology show that skin signals depend on how much skin comes into contact with the surface of the material. When there is greater contact with the material surface, it is perceived as softer. The perception is therefore also person-dependent and depends on how large and firm the human finger is. “For our study, we first had to create parameters for the geometries of a fingertip and its contact surface in order to estimate the softness signals for this fingertip,” says Mete.
From space to microsurgery
The SORI device’s tactile capabilities could be useful in many areas, such as unmanned space or deep sea exploration. In medicine, it could potentially be useful as an important sensory feedback for surgeons during robot-assisted surgery. SORI is also likely to be helpful in agriculture: its sensitivity could allow robots to harvest soft fruits and vegetables without crushing them.