perm filename JPL.PAP[1,VDS] blob sn#047569
filedate 1975-02-26 generic text, type T, neo UTF8
COMMENT ⊗ VALID 00002 PAGES
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00002 00002 CORRECTIONS AND ADDITIONS TO AI PAPER. NUMBERS CORRESPOND TO THE
CORRECTIONS AND ADDITIONS TO AI PAPER. NUMBERS CORRESPOND TO THE
NOTED LINES ON THE PAPER.
1) Hydraulic power is characterized by high power levels in a
small volume and continuous high stall torque capability with little
actuator heating. However, the interface hardware, such as the
motor driven pump, servo valves, and hoses are bulky, difficult to
easily integrate into a compact configuration, and also expensive.
Electric power is characterized by a simple ,all electric, computer
and power source interface at the expense of lower power to weight
ratios and motor stall torque duty cycles limited by heating . The
wide variety and availability of electric motors make them attractive
for rotary joints. The size and loading of the JPL manipulator does
not require high power, suggesting the use of electric motors for
2) To properly employ this type of motor, it should be
matched to a low driven load inertia. In addition, specialized
electronics are required to obtain acceptable performance levels
3) but, as the arm moment of inertia and gravity torque vary
considerably with the configuration of the joints, it is necessary to
make an estimate of the average inertia and gravity torque load
handled by each joint when selecting the proper reduction ratios.
This next section is to be added as a description. You can
put it under Introduction, or maybe another section like background
or furthur work.Feel free to change things a little to make them fit
The JPL manipulator is an outgrowth of the manipulator
designed at Stanford University and used exclusively for their
Hand-Eye research work. The original manipulator has been operating
reliably for about 3 years with few maintainance or design problems.
Several new versions of the original design have been built. These
reflect new developments in the areas of improved electronics and
mechanical hardware. A brief description of some of the research
and development work being carried out at Stanford University follows
under 4 main categories: Drive trains, Structure, Feedback, Control.
The Stanford arm was one of the first small manipulators
designed to use the USM Corporation's Harmonic Drive. This compact
high reduction unit exhibits high torque capability, high efficiency,
low backlash and a simple in-line mounting arrangement. The original
arm featured a low inertia DC motor directly coupled to the harmonic
drive with a slip clutch mounted on the output shaft. This was done
because calculations indicated that should the arm run into a solid
object at high speed, the angular acceleration reflected through the
harmonic drive could cause damaging overloads. In actual practice,
this condition was rarely encountered. At the expense of increased
inertia, the slip clutches have been replaced with larger Harmonic
Drives giving an adeguete margin of safety. Besides backlash, which
controls the maximum gain and dead band amplitude on an unloaded
joint, the joint stiffness determines the natural frequency of the
arm. The lowest spring constant element in each link is generally
the Harmonic Drive. Thus, there is a trade-off between the inertia
contribution of a larger harmonic drive and reduced windup (increased
stiffness). Much thought has been given to motor selection. The
original design used conventiional, permanent magnet d.c. motors
characterized by high speeds and low continuous stall torques.
Presently, these motors are being replaced by high torque, lower
speed "torque motors" which give better performance but require a
controller with current limiting to prevent demagnetization of their
strong permanent magnet fields. These torque motors permit the use
of high efficiency, low reduction ratio (<100/1) transmissions which
means that the joint is easily back drivablle. This allows for
torque control of the joint enabling sensing of loads and forces.
Present studies center in the area of ultra high performance electric
motors and very high torque capability precision transmissions with
the goal of approaching the performance of hydraulic actuatorswhile
maintaining the simplicity of an all electric manipulator.
The arm design is characterized by the placement of
the drive elements at or physically in the joint that is being
operated. This places a gravity load and increased moment of inertia
on the shoulder joints in a manipulator of this configuration. Other
alternatives such as cable or band operated joints are valid
possibilities, but the increased complexity created by the cable
guides and tensioning elements seems to more than offset the
advantages of their potentially high performance
In the original design, care was taken to produce a
configuration which could be easily manufactured in prototype
quantities using the machine shop facilities at Stanford University.
Most of the components are lathe turned parts with a few simple
milling operations required. No castings, or complicated setups are
required by the design. Large diameter widely spaced bearings on all
rotary joints ease the machining tolerance requirements and provide a
stiff and rugged structure by allowing large shaft sections. All
components are machined from aluminium with the execption of a few
small motor related components which are made of stainless steel.
This speeds the machining process, produces a corrosion resistant
manipulator, and allows the use of a colored anodize finish. As all
the sections are holllow tubular members, little shrouding is
necessary to hide or protect components. The prismatic joint is an
unmachined drawn square section aluminium tube carefully selected for
straightness and freedom of flaws. This boom rolls on 16 phenolic
faced bearings which guide it while preventing rotation or play. As
mass in the shoulder results in relatively small increases in overall
arm inertia, shoulder sections are heavy and sturdy to provide the
necessary rigidity to eliminate the need for structural bending
calculations in the solution programs.
Traditionally, arms of this type have been designed without
much consideration to the feedback elements. While it may not be
much of an inconvenience to mount a pot on a shoulder joint, it is
not an easy matter to incorporate a pot into an outer joint without
it getting in the way, or else being vunerable to damage.
For this reason,the original arm was designed to use integral
pots mounted directly on each joint. There are no set screws,no
gears, no mounting clamps, or limit stops; just a resistive element
and a wiper cemented to the joint. The resolution is high, and the
repeatability good as the coupling between pot and joint is rigid.
It was thought that by having large diameter pots, high linearity
could easily be obtained. This is true, but experience shows that
it is a much simpler matter to plot a curve of of pot output vs.
displacement, store it in the computer and then use this correction
table when reading the pots to get the actual values. This procedure
has been successfully used with the Stanford Arm.
Recent experiments with optical encoder components indicate
that it is now practical to build reliable compact incremental
encoders into each joint as an integral unit. For a long time the
price of such devices, when coupled with suitable read and counting
electronics, was high. Now, with improved electronics and a more
competitive encoder industry the use of integrated encoders is a
reality. The latest Stanford arm will have such encoders shortly.
For a long time, velocity information was developed by
differentiating the position feedback signal. This resulted in jerky
motion, especially when very slow trajectories were commanded. Adding
analog tachometers to each joint has greatly improved the performance
of the arm. Not only does it produce greater smoothness, but also
allows an increased gain, with resulting improved accuracy and
shorter servo time. Here too, these analog tachometers can be
replaced by digital type optical tachometers.
The Stanford arm operates in a trajectory control mode. This
means that every motion is precalculated to produce a terminal device
motion which is controlled in position and velocity vs. time. This
desired trajectory produces acorresponding trajectory requirement for
each of the 6 arm joints. The performance properties of each joint
are known and predicted for each arm configuration, with individual
joint velocity or torque limits dictating the maximum overall
performance. Deviations from the predicted drive requirements are
interpreted as external force imputs on the arm. This information
can be used to perform tasks such as tightening screws to a given
torque, pushing objects along a table surface, turning a crank, or
stacking blocks where the stack height is not known. More precise
force sensors are being developed at the present time. This includes
a 6 component wrist mouunted force balance and individual force
sensors in the fingers. Tactile feedback in the form of a one bit
signal has been looked at, but present work is centered around using
discrete force sensors for both tactile and local force feedback. As
an example: a finger with 8 discrete analog force sensors has been
developed which allows a detailed description of the contact surfaces
of a grasped object and the forces exerted on it.