  In robotics, many control and planning schemes have been developed to ensure
human physical safety in human-robot interaction. The human psychological state
and the expectation towards the robot, however, are typically neglected. Even
if the robot behaviour is regarded as biomechanically safe, humans may still
react with a rapid involuntary motion (IM) caused by a startle or surprise.
Such sudden, uncontrolled motions can jeopardize safety and should be prevented
by any means. In this letter, we propose the Expectable Motion Unit (EMU),
which ensures that a certain probability of IM occurrence is not exceeded in a
typical HRI setting. Based on a model of IM occurrence generated through an
experiment with 29 participants, we establish the mapping between robot
velocity, robot-human distance, and the relative frequency of IM occurrence.
This mapping is processed towards a real-time capable robot motion generator
that limits the robot velocity during task execution if necessary. The EMU is
combined in a holistic safety framework that integrates both the physical and
psychological safety knowledge. A validation experiment showed that the EMU
successfully avoids human IM in five out of six cases.
