Introduction to the
Personal
Assistant tm
The Personal Assistant is
a culmination of engineering design and development over the past twelve
years. Conceived in 1993, this design was a departure from previous
mechanical Hands, including not only anatomically similar mechanical Hands,
but arms as well, packaged into a supporting chassis. This effort
began with a complete redesign of previous mechanical Hands, having fewer
parts and better actuators. Compact packaging of finger joint actuators
allowed for anatomically proportioned lower and upper arm sections; proportion
defined as link lengths, not currently diameters. Design and packaging
for a three-axis shoulder onto the arm was next, followed by integration
of the arm assembly into a support chassis measuring only 9 inches in diameter
by 19 inches long. The working load for each hand is 10 lbs, using
a design factor of safety of three. No-load joint speeds were designed
for 90 degrees /sec. All fabrication drawings (over 100) will be
completed by December, 1997, and prototype development could begin as funding
allows. A specifications list on the personal assistant may
be viewed in the figure #2.
Fundamental Design
Assumptions
Selection of body, arm, hand and head
dimensions to closely match the 50th percentile, Male Military recruit
would result in the closest match of link lengths to the general population
of potential teleoperators.
Typical range of motion for human joints
measure up to 90 degrees. Human hand to eye coordination response
is approximately 3/4 second. Therefore, robot joint speed (no load)
was chosen to be 90 degrees per second.
Robot manipulation includes derivatives
of power and precision grasping. Power grasping requirements were
derived from an automotive industry study concluding that most objects
manipulated by a person during daily activity weigh under 4 lbs.
Load capacities of each hand were increased to 10 lbs. to accommodate testing
with heavier objects.
Precision grasp requirements are derived
from angular joint resolution. Angular resolution at a given robot
joint was determined to be +/- 2 degrees.
Unlike industrial robot design requirements,
a high degree of accuracy (position of the robot arm in space to a programmed
point) and repeatability (return of robot arm to a point as a function
of linkage tolerances) are not tightly required in this design, since these
attributes are not emphasized in performing object manipulation by humans.
Design to incorporate ease of manufacturability,
ease of assembly and ease of repair.
Design considerations to include manufacturing
from plastic wherever possible.
