Manufactures from different
domains ranging from industrial to aerospace are preferring
Nondestructive testing for quality control technique.
With automated NDT,robots and scanners
are used to increase the speed and repeatability factor of NDT
techniques, creating much more efficiently produced precision
measurements.Automated NDT scanner requirements are needed to achieve
high quality control and precision measurements.
Recently many manufacturers of
industrial robots have produced robotic manipulators with excellent
positional accuracy and repeatability. An industrial robot is defined
as an automatically controlled, re-programmable, multipurpose
manipulator, programmable in three or more axes.In the spectrum of
robot manipulators, some modern robots have appropriate attributes to
develop automated NDT systems and subsist with the challenging
situations seen in the aerospace industry.
ARTICULATED ARM ROBOTS
Industrial robotic arms present precise
articulated mechanical links whose functions are alike to a human
arm.Their links are jointed to provide rotational motions and handle
objects within a certain volume. Off the shelf industrial robots are
recognized as a polyvalent and robust solution for many applications:
welding, material handling,palletzing , laser cutting,machine
tending, machining, etc.
Comparing to typical Cartesian gantry
systems, the concept of an articulated arm provides a system with
greater ability. Such robots can help advanced NDT methods if they
meet the basic standards of automated NDT testing: data acquisition,
repeatability, precision and accuracy.
MOTION CONTROL
The
major difficulty of using robotic arms for automated NDT is the
proprietary motion controller design that these robot use. With such
control, we are rigidly fixed into proprietary programs and limited
motion control capabilities. More notably, we have limited control
and not enough information of the robot positions when it moves from
one point to another. Automated NDT systems require acquiring data on
the fly while the robotic arm is moving. So, the motion control
system needs to handle extremely fast control changes (fast PID
controller loops). For example, in applications that require
the robot to move fast and perform contour following motions around a
complex surface, smooth and precise trajectories must be maintained
during the robot movement. When the PID loops close at a slow pace,
the robot will not move on required precise trajectories. This may
results in a jumpy motion of the robot and losses of the NDT signal
measurements. Current industrial robots offer slow PID control loops,
estimated around 10 times less than the required speed to perform
fast and accurate NDT scanning of complex parts.
DATA ACQUISITION
Required data acquisition speed is
another challenging factor of using industrial robots for NDT
application.Capturing NDT data in real time while using a robotic arm
is challenging feat because it requires real-time robot position
monitoring.That means a direct encoder feedback has to be made
available on the robot,which is generally not the case.After being
processed by the motion controller unit,all these standard robot
controllers will provide low rate of position feedback at around 200
Hz. The rate at which position feedback refreshes itself has a
proportional impact on the inspection speed, as the robots “true”
position must be attached to each measurement point and no
interpolation is allowed.
By working on the main challenges
identified above, industrial robots can be replaced eventually by the
conventional Cartesian scanners in selected NDT application if motion
control functions and encoder feedback monitoring are customized.But
at this moment without such refinement in terms of the support data
transfer, link and communication protocols, an efficient use of
articulated robotic arms for NDT is still a work in progress.
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