WAM Arm – User’s Manual
www.barrett.com
© 2008 Barrett Technology®, Inc.
Document: D1001, Version: AH.00
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Jacobian-Inverse - Jacobian-Inverse
is a computationally slow and burdensome operation in
robotics, generally calculated off-line on a powerful processor, and virtually barring true realtime
Cartesian control. It is a matrix inversion in the formal sense, which means that it must be well
conditioned (the robot must be far from singularities) and it must be square (so only non-
redundant robots can have a Jacobian-Inverse).
Jacobian-Transpose -
A
Jacobian-Transpose
is derived from the Jacobian with no computation,
allowing realtime Cartesian control of robots with redundant degrees-of-freedom. A mathematical
matrix transpose affects only the order in which the matrix elements are written into and
subsequently read from memory.
Kinematics -
The science of motion which treats motion without regard to the forces which cause
it, specifically all the geometrical and time-based properties of position.
Micro-Yielding -
Micro-yielding
occurs when instantaneous cable tension loads combined with
bending-induced (tension) stresses in the outmost cable fibers exceed the yield strength in the
material, causing some of the fibers to break.
Pretension -
The process of adding additional tension to a cable during the assembly process.
Puck™ -
An ultra-miniature brushless servo controller fit into an ultra-high-precision encoder.
Developed over several years by Barrett Technology, this puck weighs only 44gms and is only
35mm (dia) x 17 (high) with connectors. The Puck™ fulfills both definitions of a Controller (see
Controller).
Stiffness -
The
stiffness
of a spring or similar mechanism (with units of force/distance, or force x
distance in the case of trosional mechanisms) is defined as the slope of the curve of the restoring
force relative to the displacement from equilibrium. So, in a robot stiffness can be measured at the
endtip of the arm with the arm outstretched. In robotics high stiffness is always better.
Whole Arm Manipulation -
There are two meanings to
Whole-Arm Manipulation
, both
exemplified in Barrett’s WAM arm:
1.
The inherent capability of a robot to control contact forces with inherent safety and
robustness all along its link and joint surfaces, from the base of the arm through its end.
This capability is especially enabling when applied to a kinematically redundant arm.
2.
A holistic approach to robot design enhances safety and performance while reducing
intrinsic manufacturing, shipping, training, software, footprint, power, and servicing
costs. For example, conventional robot designs continue today to depend on legacy drive
technologies that introduce friction, backlash, torque fluctuations, or generate poor aspect
ratios. Then conventional designers go to great lengths to attempt to mask these effects at
the expense of safety or other performance metrics. The holistic approach leverages
simple, (high-speed cable) drives that do not introduce these problems in the first place
and yet exceed the highest standards of conventional performance.
