The battery-operated chair,
intended as a work of art, looks like an ordinary
kitchen chair until it collapses into six pieces —
four legs, a chair back, and a seat which houses the
robotics. For a moment, the chair seems lifeless.
Then, it begins to move, rolling first to one leg,
then another, gathering itself, so to speak. If a leg
lands in an inopportune position, the seat nudges
it into the proper orientation. After attaching its
limbs, the chair seat still rests on the ground.
Then, the chair slowly rises in a process designer
Max Dean compares to a fawn gaining
its footing (See it in action at roboticchair.com/documentation.php). Dean first thought of the
chair in 1985. To build it, he teamed with artist and
industrial designer Matt Donovan and Raffaello
D’Andrea, a professor at ETH Zurich, Switzerland.
The biggest challenge, says Donovan, was developing
a way to lift the 28-lb chair off the ground.
He started with a 26-mm servomotor fitted with a
gearhead that provided a 1,526:1 reduction ratio
for a peak torque of 4.5 Nm. He added his own 9:1
gearbox for a total reduction ratio of 13,734:1.
“We solved the problem with sheer torque, flat
out power,” says Donovan. The trade-off is a corresponding
drop in speed. “Motors with enough
torque to stand the chair up quickly would be too
big and increase the weight so much we couldn’t
get enough battery power to run them.”
The lifting problem solved, Donovan turned
to the challenge of attaching the legs. He fitted
each leg with a projecting rod pierced crosswise
by a hole. The rods fit into corresponding brackets
in each corner of the chair that link to the lifting
mechanism.
The docking process involves two steps. First,
the chassis (seat) maneuvers to slip the rod into
the bracket. Then, a servomotor drives a pinion,
the drive pin, through the holes to anchor the leg
in place. The rods have a second hole drilled at 90°
to the first so the legs can dock no matter which
side they fall on. The chair collapses when the
drive pins retract.
Retracting and installing the drive pins requires
more speed than torque, so Donovan chose
a 26-mm gearmotor with a 43:1 reduction ratio for up to 1.2 Nm of peak torque, placing one in each
corner. The same size motor propels the chassis to
find the legs and chair back. The chassis can even
rise up on off-center wheels to drive off of any
obstacle on which it has landed. That power comes
courtesy of a servomotor with a 592:1 reduction
ratio for up to 4.5 Nm of peak torque.
The chassis also attaches to the chair back and
raises it to a vertical position. After maneuvering
next to the chair back, a grabbing mechanism links
to a bracket on the bottom of the chair back. For the
sake of symmetry, Donovan chose a lower-torque
gearmotor to run the drive pin that attaches the
chair back. After the back is attached, a different
mechanism raises it.
The robotic chair runs via a wireless link from
a PC-based centralized controller. Because of the
problem’s indeterminate nature, the chair operates
open loop with input from an overhead vision
system. The idea was to reduce hardware as much
as possible, shifting complexity to the software.
Make Contact
MicroMo,
Clearwater, Fla.,
micromo.com