rods to the front segment.
• Glue gun to mount the servos.
• Epoxy to secure the mounted
magnets to the servo horns.
■ FIGURE 4. Magnet and servo sequencing
for straight movement forward (top) and a
clockwise turn (bottom).
miniature servo in the front segment
and the three miniature servos in the
rear segment. I used four channels of
a Futaba T9CAP transmitter and
matching receiver to sequentially
activate the servos according to the
sequences shown in Figure 4.
The top of the figure shows
the robot moving forward in a
straight line using basic inchworm
locomotion. Note the timing of
magnet actuation, depicted in red. By
advancing one of the two pushrod
servos relative to the other, the robot
can be directed to turn clockwise or
you’ll require the following materials:
• One 2. 5 x 5. 5 x 1 inch plastic
project box, cut in half about 1-3/4
inches from one end.
• Four GWS Servo Nano Pro/Std
or equivalent servos. The Nano Pro
is a 13g device that measures 27 x
12 x 22 mm. About $11 each from
• About one foot of bare solid wire
to mount the magnets to the
The following instructions assume
a one evening project. Feel free to
improve upon the basic design as
your time and needs dictate. In
addition to the usual shop tools,
• A four channel R/C transmitter/
• Two 4-40 six-inch threaded stainless
steel push rods with clevis, trimmed
• Two closed solder lugs, one
soldered to the unthreaded
end of each push rod.
• Two 0.25” x 0.2”L
Neodymium magnets; $1
for the pair, less shipping, from
• Two 1” 4-40 bolts and
four nuts to mount the push
■ FIGURE 5. Front segment of
the inchworm showing servo
with magnet rotated away from
the surface cutout.
First, securely mount each
magnet to a servo horn using the bare
wire. I used a circular horn and drilled
a pair of extra holes in the horn to
accommodate two loops of wire to
mount the magnet. Cover the
magnets and the bare wire with
epoxy, as shown in Figures 5 and 6.
Each magnet face should be flush with
the outer edge of its servo horn.
Next, take the plastic project box
and (using a hacksaw) cut the box to
create the front and rear segments.
The dimensions of each segment
depend on the servos you have
available for the project. Mount the
servo horns with magnets on two
servos and fit a servo in each
segment, noting where the magnet
and horn must clear the case. Mark
the case and then (using a drill)
create an oval for the magnet and
servo horns. The face of the magnet
should just touch the surface below
the segment, as in Figure 6. Note that
the magnets rotate along the long
axis of the robot.
Next, mount each of the
pushrods on a 4-40 bolt; attach the
bolts to either side of the front
segment, using a drill to create holes
for the bolts. Use a glue gun to mount
the servos carrying the magnets. With
a pair of diagonal pliers, cut notches
in the case for each of the two
remaining servos, as in Figure 6.
Secure the servos with hot glue and
attach the horns and pushrods. Place
a 1” cube of foam over each servo
and attach the case cover on the front
and rear segments.
Plug the servos into the appropriate channels on your receiver and
connect power to the receiver. I found
a three foot, 22 gauge power cord
provided ample flexibility. Finally,
calibrate your transmitter and
move the servo horns so that you can
easily control the magnet servos
and the position of the rear segment