Influence of the Size Factor of a Mobile Robot Moving Toward a Human on Subjective Acceptable Distance

Service robots working around humans are expected to become widespread in the next decade. There have been numerous works for developing autonomous mobile robots, starting as early as the 1980s. For example, Crowley developed the Intelligent Mobile Platform (IMP) which moved around a known domain according to given commands (Crowley, 1985). The issue in the earlier works was how to navigate a robot in a room. HelpMate (Evans et al., 1989) was a mobile platform intended to be used in hospitals for carrying medical records, meal trays, medications, etc. In the 1990s, robots were developed which were equipped with manipulators and executed tasks such as moving objects. Bishoff (1997) developed a mobile robot called HERMES, which is an upper-body humanoid equipped with two arms with hands and an omni-directional vehicle. HERMES recognizes objects around it using stereo vision, and executes tasks such as moving an object from one place to another. Recently, service robots that can execute more complicated tasks using three-dimensional distance sensors and more powerful actuators have been actively developed (Borst et al., 2009; Graf et al., 2009; Droeschel et al., 2011). Along with the development of such service robots, service robot contests have been held such as RoboCup@Home League (RoboCup Federation, 2011), in which mobile service robots compete for accuracy, robustness and safety of task execution in home-like environments. We have also developed an experimental care service robot called IRIS (Hiroi et al., 2003). This robot understood a patient’s commands through spoken dialogue and face recognition, and performed several care tasks such as carrying bottles or opening/closing curtains in a real environment. The other feature of IRIS was its safety; IRIS was equipped with various devices for physical safety, such as arms with torque limiters (Jeong et al., 2004). Safety is the most important issue for this kind of robot, and there have been many studies on keeping a robot safe for humans. Here, we consider two kinds of “safety.” The first one is the physical safety of avoiding collisions between a robot and humans; physical safety is the most important requirement for a mobile robot working around humans. The other is mental safety, which means ensuring that the robot does not frighten people around it. Mental safety is as important as physical safety; if a robot’s appearance or behavior is frightening, it will not be accepted by people even if it is physically harmless.