It takes three of these EDF- 50 ducted fans to make the
HoverBot run: one for lift and two for propulsion.
propelled. It’s the opposite situation for small wheeled
robots; they stop on a dime after cutting power to the drive
motors. Low drag conditions mean that driving the
HoverBot requires a consideration of Newton’s Laws of
Motion. The controlling program doesn’t plan in terms of
distance and heading. Instead, it plans in terms of time and
acceleration.
The HoverBot’s lift and propulsion comes from three
ducted fans (these are GWS EDF- 50 ducted fans). The first
fan directs air down to lift the HoverBot on its skirt. The
other two fans generate a horizontal force that drives and
Parts List
QTY
4
4
1
2
2
3
DESCRIPTION
#6-32 bolts, 2 inches long
#6-32 nylocks
Roll of two inch wide Scotch mailing and storage tape
#4-40 bolts, 3/4 inch long
#4-40 nylocks
GWS EDF- 50 ducted fans
• Correplast (corrugated plastic) 5/32 inches thick, available from
either a plastics distributor or sign company.
steers the HoverBot. Relays arranged in an H-bridge circuit
control the direction the ducted fans spin, and therefore the
thrust direction of each fan — except the lift fan. A single
relay controls this fan so the HoverBot can take off and
land. Perhaps that’s a slight misnomer; lift-off is only two
inches high.
Even though the force the two horizontal fans generate
is small, the HoverBot’s low friction lets the fans build up
high speeds (in fact, the HoverBot will travel faster than my
wheeled CheapBots). The drive fans attach to the HoverBot
off-center. This allows the two fans to drive and steer the
HoverBot like little rocket engines on a satellite. When fired
in the same direction, the HoverBot accelerates either
forward or backwards.
Recall your high school physics class that accelerating
backwards doesn’t necessarily mean the HoverBot moves
backwards. If the HoverBot is initially moving forward, the
backwards acceleration will first slow it down and bring it
to a halt before it even begins to move (displace) it
backwards. How long the fans have to drive the HoverBot
backwards to reverse its travel depends on how long the
HoverBot was accelerating forward.
Firing one fan forward and the other fan backward
spins the HoverBot around its center. The longer the fans
spin an initially stationary HoverBot, the faster it spins and
the longer the fans must work to stop the rotation. I
suspect it’s possible to spin the HoverBot long enough for
centripetal force to tear it apart. Therefore, it’s important
not to overdrive the HoverBot in a turn. It’s best to use a
small burst to begin its rotation and then wait awhile for it
to slowly approach its desired final heading before reversing
thrust.
Building a HoverBot
In writing this article, I realized how involved the
HoverBot has been, so my article will be broken into a few
parts. This month, we’ll cover the construction of the
HoverBot body sans its skirt. Next time, we’ll build a skirt
and add the electronics. So to begin with, collect the
materials listed in the Parts List to build the HoverBot.
The plan is to build two decks: one for the top and one
for the bottom. The bottom deck houses the lift fan,
nozzle, and skirt. The top deck houses the batteries, robot
controller, fan electronics, and drive fans. Separating the
two decks are bolts and spacers. We’ll need to align all the
holes in both decks and prepare the bottom deck for the
fans and skirt.
60 SERVO 09.2010
