Published papers

The rendering of tactile sensations has recently been attracting attention in the fields of multimedia and communication. This study focuses on thermal sensation, which is one of the important elements for rendering a tactile sensation. Humans feel an object by hand movements such as tracing, grasping, and stroking. In response to such movements, it is necessary to take into account the change in the contacting point on the system. However, most conventional studies on thermal rendering did not take such situations into account in their system design. This study aims to fill this gap by proposing a control method for reproducing thermal conductance with the detection of a single contacting part. In this study, a copper plate attached to a Peltier device is regarded as a thermal system, and the system is modeled with three parts. Heat disturbance observers and heat inflow observers are then allocated to each part, and the thermal conductance at any single part is virtually reproduced. Moreover, a detection method of single contacting part is introduced in the proposed system. Combining the detection method with the controller enables different thermal sensations to be rendered depending on the contacting part.

This paper proposes a method for controlling the stiffness of a thermally driven actuator by leveraging the phase transition of a low-boiling-point liquid. Using a Peltier device for cooling, the reduction in actuator stiffness can be precisely adjusted. Experiments demonstrated the actuator's effectiveness, particularly in handling fragile objects with precision, ensuring delicate interactions with contact surfaces. Since stiffness is adjusted within a safe temperature range, this approach shows promise for applications involving direct human contact, such as the outer surfaces of social robots (robot skin).

Soft robot skin is one of the key factors for physical human-robot interaction (pHRI), and many studies have proposed various kinds of robot skin. In particular, two functionalities, the ability to convey warmth to humans and to detect contact, are significant among the roles of robot skins. The former is known to create comfort and a positive impression of robots and help build trust in them. The latter enables more personalized human support. However, temperature feedback and contact detection require sensors on the skin's surface; this can disturb the heat conduction from the skin surface and soft, smooth tactile feelings. This paper proposes a system design for fluid-based soft robot skin, simultaneously enabling contact detection and temperature control without attaching a sensor to the skin surface. We developed a skin prototype based on the proposed design and conducted experiments to verify its capabilities.

Thermal properties are significant for recognizing an object's material but cannot be determined via visual and stiffness (or tactile)-based recognition techniques. Most studies have used temperature as a complementary part of multimodal sensing; however, the thermal signal is an unexplored capability that can be beneficial for recognizing target objects. Since changes in thermal responses can result from both material properties and initial temperature, realizing robust and high-accuracy recognition in different environments is a challenging issue. To tackle the issue, this paper proposes a novel strategy for material identification that can actively measure heat flow by heating and cooling a robot gripper, enabling the extraction of the thermal properties of contact materials regardless of the object's initial temperature variation (referred to as 'active heat flow sensing'). We use a robotic task as an example of one possible application of the proposed strategy. For this, we developed a gripper pad embedded in a temperature control system and heat flow sensor to monitor the thermal exchange during contact with a target object. The paper conducted some experiments divided into two scenarios. The first experimental results show that active heat flow sensing is realized within 0.4 sec from first contact for 100% classification of four heated materials. The second experimental results show that the three materials, whose thermal properties are largely different, can be classified within 0.7 sec from first contact using different initial temperatures of the training and test data. These results suggest robustness against environmental change, which has been difficult using conventional temperature-based methods.

Safe and pleasant physical human-robot interaction (pHRI) is essential in the robotic skin and related control paradigms. Among approaches, integrating tactile sensing and thermal techniques enables the robot to provide pleasant and gentle touch perceptions, thus enhancing likability and trustworthiness during interactions with humans. However, achieving both accurate tactile sensing and desired thermal comfort perceptions poses significant challenges. In response to these challenges, this paper presents the development of a novel soft skin named Thermo Tac. This innovative skin not only possesses the ability to sense physical touches effectively but also provides thermal comfort sensations. The integration of these capabilities is realized through a carefully designed and strategic system. The developed skin is integrated with vision-based tactile sensing and water circulation systems to evaluate its performance and effectiveness in realistic interaction scenarios. The results of the experiments highlight the potential of ThermoTac as a promising solution for safe and stable human-robot interaction, paving the way for advanced tactile sensing and thermal comfort in robotics.

In this paper, we address robotic haptics with the following question: is it possible to design a heating/cooling system that can fulfill both the necessity of keeping the inner robot actuators at a desired optimal heat and, at the same time, use actuators energy loss in terms of heat conveyed, through a soft robotic cover, as pleasant thermal sensation to a human when the robot is touched? We propose a solution to address this question in terms of mechatronic design. Indeed, internal heat generated by actuator loss can be transferred to the robotic haptic cover and a cooling tank for thermal display purposes. A suitable switching control strategy is divided to fulfill the double objective of keeping the actuators’ temperature under a given threshold whilst displaying the desired temperature to the soft robot's cover. We assess our ideas with an experimental set-up using one actuator and our original soft cover technology with a preliminary user study.

We propose a new robotic cover prototype that achieves thermal display while also being soft. We focus on the thermal cue because previous human studies have identified it as part of the touch pleasantness. The robotic cover surface can be regulated to the desired temperature by circulating water through a thermally conductive pipe embedded in the cover, of which temperature is controlled. Besides, an observer for estimating heat from human contact is implemented; it can detect human interaction while displaying the desired temperature without temperature sensing on the surface directly. We assessed the validity of the prototype in experiments of temperature control and contact detection by human hand.

This study proposes a circulating water system with human blood circulation capacity to utilize the robot’s self-heating effectively. Water channels (robot’s blood vessels) are embedded in the skin covering the robot, controlling and managing the thermal conditions of the robot system. In the process of dissipating the heat generated by the actuator, the heat is transferred to the robot skin in contact with people and objects to enable recognition of the target materials and give a pleasant warmth for human interaction. The proposed method is an innovative way to solve two traditional problems: ”how to avoid overheating of the actuator” and ”how to improve robot skin’s perception ability.”