The goal of this work was to develop a new
mechanical architecture-for actively powered
haptic ("force-feel") hand-controllers and robotic
manipulators - that offers enhanced performance in
terms of combined work-space size, position and
force control accuracy, and payload. For hand-controllers,
these attributes have obvious impact on
the subjectively perceived "feel" of interactions with
virtual or telemanipulated environments.
The moving structural components of a hand-controller
or manipulator form the mechanical
linkage that couples force and motion between the
device's actuators (motors) and end-effector. Thus,
the linkage's architecture directly determines not only
the work space but also transmission characteristics
such as dynamic range (comparing maximum usable
force to friction) and bandwidth (frequency content of
deliverable force and motion) that define control
accuracy and contribute to payload capacity.
A novel three-degree-of-freedom (DOF) linkage
architecture was devised, offering significant
improvement over prior technology with respect to
the above listed performance criteria. The innovative
architecture (figure 1) is a mechanism composed of
10 rigid links that connect 12 single-DOF rotary
joints. The links and joints are arranged in three loops
that couple a handgrip or robot end-effector (D) to
three rotary actuators (A, B, and C). The moving links
do not carry actuator weight and inertia since A, B,
and C are all mounted on a common base (unlabeled
link 1). This frees more of the actuator force budget
for useful work, that is, payload. Because the base
link does not move, more powerful and typically
heavier actuators can be used without the need for
more massive moving support structures. This lower
inertia improves acceleration response and expands
structural bandwidth for better control at high
frequencies.
The architecture requires no belt, cable, pulley,
or screw transmission elements, all of which can be
backlash-, friction-, or compliance-prone and that
would therefore compromise end-effector force and
position control. The actuators can be embodied by
COTS (commercial, off-the-shelf) rotary electric
motors and the other joints by COTS backlash-free,
low-friction ball bearings. Enhancing usable human
and actuator force levels while minimizing losses to
friction increases dynamic force range and is critical
for back-drivability and the feel of hand-controllers.
The multiple-loop configuration of the linkage
forms a so-called "parallel" mechanism, universally
acknowledged as being structurally stiffer than
competing "serial" designs. Greater stiffness is key to
enhanced structural and control bandwidth, as well
as improved position control accuracy.
The linkage work-space volume is correspondingly
larger than that of prior three-DOF parallel
configurations, approaching the work space of serial
devices. This new architecture's work space is
bounded by the singularity at the sphere of maximum
reach typical of all arm-like devices and by
singularities in the sphere's equator-the plane
defined by the three actuator axes. In general, work-space
singularities correspond to locations at which
the linkage looses its ability either to move or to
apply force in one or more directions.
The linkage is composed of the minimum
number of links and rotary joints in which all three
actuators can be supported on a common base,
giving the architecture fewer component parts than
other three-DOF, spatial, all-rotary joint mechanisms.
Fabrication and assembly constraints for the linkage
are relatively simple: only the intersection of joint
axes within two of the mechanism loops and the
parallelism of joint axes in the third loop are mandatory.
A closed-form potential energy analysis
demonstrates that, in theory, the linkage can be
statically balanced for all base link orientations and
also provides a three-step link mass-distribution
procedure to achieve the balance. Perfect static
balance obviates the need for actuator or external
forces to support unbalanced linkage weight for any
pose, freeing up more force for useful work.
Finally, the three-DOF architecture is scalable
from large crane-sized construction and material-
handling equipment down to micro-machines for use
in applications such as minimally invasive surgery.
Arm- and finger-scale three-DOF force-feel joysticks
(figure 2) demonstrate a portion of this scalability.
Point of Contact: B. Adelstein
(650) 604-3922
dadelstein@mail.arc.nasa.gov
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