21-06-2012, 01:42 PM
An atlas of physical human–robot interaction
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Introduction
The extension of application domains for robotics, from factories to human environments, is growing, due
to the elderly-dominated scenario of most industrialized countries, the desire of automatizing common daily
tasks, and the lack or high cost of local human expertise. Safety and dependability are the keys to a successful
introduction of robots into human environments. Robots for physical assistance to humans should reduce
fatigue and stress, increase human capabilities in terms of force, speed, and precision, and improve in general
the quality of life; on the other hand, the human can bring experience, global knowledge, and understanding
for a correct execution of tasks [1]. Only dependable robot architectures can be accepted for supporting
‘‘human-in-the-loop’’ conditions and human–robot teams. Application domains asking for human augmentation
and substitution by robots include everyday houses and offices, but also unmanned warfare operations,
mainly in USA [2], and robot companions as well as humanoids, the robots with ‘‘kokoro’’ (heart) diffused in
Japan [3]. Moreover, teleassistance and the use of computers and devices for remote medical care pave the way
to the future use of robots in domestic environments. Researchers worldwide are studying the social factors
related to the introduction of robots in human environments and often their attention is focused on the cognitive
interaction with machines.
Safety in pHRI
In the complexity of a HRI, the physical viewpoint is mainly focused on the risks of collisions occurring
between the robot and its user: too high energy/power may be transferred by the robot, resulting in serious
human damages. Severity indices of injuries may be used to evaluate the safety of robots in pHRI. These
should take into account the possible damages occurring when a manipulator collides with a human head,
neck, chest or arm. Several standard indices of injury severity exist in other, non-robotic, domains. The automotive
industry developed empirical/experimental formulas that correlate human body’s acceleration to injury
severity, while the suitability of such formulas is still an open issue in robotics.
Mechanics and actuation for pHRI
Intrinsic safety
The first important criterion to limit injuries due to collisions is to reduce the weight of the moving parts of
the robot. A prototypical example of this is the design of the DLR-III Lightweight Robot [20], which is capable
of operating a payload equal to its own weight (13.5 kg). Advanced light but stiff materials were used for
the moving links, while motor transmission/reduction is based on harmonic drives, which display high reduction
ratio and efficient power transmission capability. In addition, there is the possibility of relocating all the
relevant weights (mostly, the motors), at the robot base, like it was done for the Barrett Whole Arm Manipulator
(WAM) [27]. This is a very interesting cable-actuated robot, which is also backdrivable, i.e., by pushing
on the links, it is possible to force motion of all mechanical transmission components, including the motors’
rotors. In the case of a collision, the lighter links display lower inertia and thus lower energy is transferred
during the impact. On the other hand, compliant transmissions tend to decouple mechanically the larger inertias
of the motors from those of the links. The presence of compliant elements may thus be useful as a protection
against unexpected contacts during pHRI. More in general, a lightweight design and/or the use of
compliant transmissions introduce link [28] and, respectively, joint [29] elasticity.