Dana Kulic, Elisabeth A. Croft
International Conference of
Industrial Robotics, 2005
Summary
Typical robotic industrial applications see robots
segregated in order to ensure safety, this is not acceptable in more and more
flexible environments which require interaction with operators, while ensuring
safety conditions. In order to ensure safety a complete system must then
incorporate the following features: safe mechanical design, human friendly
interfaces, safe planning and control strategies; the authors focus on the
third aspect. There in the literature mainly three methods to mitigate risk:
redesign to eliminate hazard, control the hazard through electronic or physical
safeguards and warn the operator during operation or by training. While the
last has been proven to not be effective, specially in presence of untrained
users, the first one appears to be the case in which the aim is reducing the
results of a possible impact and not the impact itself.
The key issue then is to
understand when safety is threatened, this can be done using tactile and force
sensors to identify unplanned contact or by considering every object in the
environment as an obstacle and use real-time obstacle avoidance strategy. The
proposed algorithm introduces a danger
index, if the level of danger is low the plan can proceed, otherwise there
will be a corrective decision, this safety system performs then in
one-step-ahead condition, moving towards the location with the lowest danger.
The danger index appear to
be the multiplication of distance factor, velocity factor and inertia factor,
the multiplication ensures no active evasive action which would cause the
evasion of the robot also in safe conditions.
The Distance factor is
obtained considering a scaling factor kD and it is build so that
when the distance between the critical point and the object is large (greater
than Dmax) then the factor would go to 0. In a similar fashion the
velocity factory is obtained. The inertia factor appears to be the ratio
between the effective inertia at the critical point and the maximum safe value
of the robot inertia.
·
Model
A one dimensional example
is performed showing the robot’s behavior when there are not obstacle, one
obstacle and two obstacles on the two robot’s sides. It is shown that in the
case of two obstacles, the robot is capable of avoiding the obstacles and
reaches a stable point with no oscillation.
The full algorithm
(regarding all the robot arm) results in a local sequential planner,
backtracking cannot be considered since it’s the case of real-time motion. In
this case the damping force is obtained as: Fd=-Bq’ where q’ is the
measured joint velocity. To calculate distances a set of spheres representation
is used, being a ceservative method ensuring safety.
Parameter selection is
obtained through physical characteristics of the robotic arm, Dmin
can be estimated based on the maximum robot deceleration and velocity and Vmax
can be estimated on indices of injury such as Gadd Severity Index or Head
Injury Criterion.
Dmax is based
on the physical size of the robot and Vmin must be smaller than 0 in
order to ensure stability in the system.
Ranges for distances and
velocity boundaries should be large so that there is time to react before the
danger is imminent, for the velocities this ensures reduction of the effective
damping.
Key
Concepts
Safety, danger index
Key Results
Simulation has confirmed
expected behavior, showing no oscillation in case of two obstacle in the robots
proximity. The set of spheres ensures for the experiment a conservative system,
so that highly accurate position sensing is not required. The method can be
used for redundant and non-redundant manipulators, generating robot motion by
minimizing the danger index.
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