arm is shown in blue, and is
connected by two half inch bolts to
the end of the crank arm and the
middle of the rocker arm.
The rocker arm is shown in
green, and it connects to the frame
(considered the fourth bar in a
four-bar linkage) by another half
inch bolt that acts as a hinge. The
rocker arm extends 9-1/2 inches
forward of the middle hinge point.
The crank arm is turned by an
AmpFlow motor with both
planetary gear and roller chain
reduction, but the specific
arrangement of Polar Vortex’s four-bar linkage provides additional
Figure 1 shows the retracted
arms with the crank arm at a
theoretical “zero” angle. Figures 3
and 4 show that as the crank arm
starts moving at a constant rate,
the rocker arm moves very slowly
at first, but with great leverage.
That means the robot can lift a lot
of weight a short distance; the
rated weight is about 60 degrees.
A torque limiter in the roller
chain reduction stage limits the
current draw to about 60 amps —
well within the limits of our Vantec
RSFR-48e speed controller. The
clutch slips if the arms encounter
an immovable object. That
happened in one fight in Miami
Polar Vortex had Shrederator
stopped and against the bumpers
in a corner, and was under him
lifting. Shrederator’s shell was
wedged into the bumpers, so we
were basically trying to lift the
entire BattleBox. Without the
clutch, we could have easily stalled
the motor and burned out a speed
Figure 5 shows a photo of the
lifter motor, planetary gearbox, and
roller chain reduction assembly.
The unloaded current draw is
about 12 amps at 24 volts.
The design also makes it easy
to install mechanical stops instead
36 SERVO 07.2014
Rocker Angle versus Crank Angle
0 30 60 90 120
Crank Angle (deg)
150 180 210 240 270 300 330 360
FIGURE 3. Rocker angle vs. crank angle.
i o 40
ees of Crank Rotation
150 180 210
FIGURE 4. Four-bar leverage.