48 SERVO 01.2012
to cut it off shorter. That way, the balloon could hang
freely but have support when it was pressed against the
object to be picked up. The funnel helped confine the
balloon and force the gripper to flow around the object
I decided to put a fabric pressure sensor in the funnel.
That way, the Arduino controlling the servo could sense
when enough pressure was applied to the balloon. (Plusea
also has a great instructable at instructables.com
describing how to create pressure and bend sensors.)
These sensors can be very sensitive and also remarkably
accurate in reading pressure or bending.
I created mine out of craft foam from a local craft
store. It is flexible, but has enough structure that it
doesn’t collapse. It is also much cheaper and easier to
acquire than the neoprene that Plusea uses. You need a
material that can be compressed, but won’t stay that way
after the pressure is removed [Figure 12].
Instead of sewing the edges together, I used Bond
527 multi-purpose cement. It is flexible and also adheres
well to the foam. I have found it difficult to stitch
together the foam and get just the right amount of
pressure. The conductive thread was sewn in a crisscross
pattern. The Velostat middle layer was cut smaller than
the outer pieces so that the glue did not adhere to it and
cause compression. Velostat is available from the EMF
safety site at from www.lessemf.com [Figure 13].
Conductive thread was left sticking out about an inch
on both sides to allow connection to a wire running to
the Arduino [Figure 14].
I tested the syringe and found that I only needed to
pull the plunger back to 30 ml in order to get a good
vacuum. So, I used a hacksaw to cut off the barrel of the
syringe. If it was eventually going to fit inside of a robot, I
would need it as small as possible. I also cut the plunger
off about 3 cm longer than the barrel [Figure 15].
I drilled a hole in the servo arm and also through the
plunger. A #6 bolt with washers and nuts holds the
plunger pretty stable as the servo pulls it back and pushes
I had some aluminum U channel and cut a piece
about 20 cm long. I had a plastic servo-mounting bracket
to make a stable platform for the servo. I attached the
servo to the plastic mount and then bolted the mount to
the U channel aluminum with #6 bolts and nuts.
A piece of flat bar aluminum made the syringe
support. The support has to rotate when the plunger is
moved. In my mechanism, the syringe must be able to
move up and down as the arm pushes or pulls in and out.
So, I put one #6 bolt to hold the barrel, and another bolt
into the U channel. The U channel bolt was not tightened
so that the whole assembly can rotate [Figure 16].
Wires from the pressure sensor go to the Arduino. I
used the same setup as for a photoresistor on the Arduino
site at www.arduino.cc/playground/