Mechanics for Robot Hands and Arms
compression and
extension) or a gas
spring to
compensate for the
weight of a robot’s
arm mass. Many
builders find that an
arm of a decent
length requires a
fairly hefty high
torque motor to
even lift the arm,
not including any
sort of payload. For
example, an
arm/claw
configuration that is
a foot long,
extended, and is
weighed at the end
at one pound, a servo or gearmotor of 192 ( 16 x 12) oz in
of torque will be required to just lift the arm with no
payload. That’s a pretty hefty servo. Add a 16 oz payload
and you’re asking for 384 oz in of torque required; that’s
for a pretty short robot arm.
FIGURE 4. Revenge of
the Nerds robot.
Gas Springs to the Rescue
while back. The arms were about two feet long and the
weight at the end of the arm was about three pounds, or
( 24” x 48 oz) or 1,152 oz in of torque — again, with no
payload. I placed a three inch lever inside the robot body
facing down in line with the axis of the robot’s arm. These
levers were 1/8 the length of the arm, so I needed eight
times the force on a three inch lever arm to compensate for
the arm’s weight, or ( 8 x 3 lbs) 24 pounds of force in a
spring. It turned out that the gas springs that I bought had
a force of about 40 pounds — more than enough to
compensate for the arm’s weight.
I thought a minute, “Why not over-compensate for the
weight of the arm by adding a bit more upward force to
give me even more payload capacity?” My shoulder
gearmotor had sufficient torque to pull the arm downward
with no payload attached, and the extra force allowed an
even greater payload. It made the customer happy as the
robot could pick up a fairly heavy six to seven pound
payload. I had to make sure that the three inch levers on
the arms were not allowed to poke through the shoulders,
so the arm’s upward motion was limited to about 15º past
the horizontal, using a DPDT polarity-reversing limit switch.
Even a simple large rubber band around the shoulder
shaft of a small robot’s arm can be wound so that the band
pulls the back surface of the shaft downward so that the
arm’s mass in the front will be lifted upward. I used this
method in a robot that I designed for a Boy’s Life Magazine
robot construction article that I wrote in 1987. There are
many mechanical ways a robot builder can jury rig a way to
help a robot move its arms with a payload.
Coil springs can work for mass compensation of robot
arms but they are not linear in their force. As you compress
or stretch a coil spring, more force is required the greater
the distance. Gas springs (shown in Figure 5) are fairly
linear in their force throughout their working stroke length.
They are quite often used in cars to compensate for hood
or trunk lid weight, as well as in applications such as photo
copier and printer lids, and for medical instrument uses.
I used a set of surplus gas springs to compensate for
arm weight in a large promotional robot that I made a
Arms that Compensate for Gravity
I was in the Portland, OR airport recently, waiting for a
flight when I ran across a must-have and uniquely-titled
book in a bookstore — Packing for Mars, by Mary Roach.
She discusses many space topics that are quite interesting
and few people really understand. One chapter dealt with
“The Alarming Prospect of Life Without Gravity.” In the
early days before spaceflight, scientists and doctors were
truly concerned that astronauts
might suffer all sorts of
maladies in space, including
stoppage of the heart or loss
of blood circulation. Most
astronauts — after a period of
adjustment — enjoy zero G,
but a few have found the loss
of weight on the arms a bit
frustrating as they float out of
control, especially after many
months in space. Fortunately
for these astronauts, moving
objects of a large mass are
fairly easy to accomplish,
though acceleration and
deceleration of that mass is
FIGURE 5. Gas springs
from Ace Controls.
78 SERVO 09.2011
