be a couple of force sensitive resistors as mentioned
previously. Force sensitive resistors are made up of a
conducting polymer that changes resistance as pressure is
applied. The resistors are nestled inside of a cutout section
in the pentagonal body, and they flank the center mounting
hole for the servo drive shaft. The body section containing
the center mounting hole looks a bit like a peninsula jutting
out into the center cutout, and two somewhat cloudy
looking blocks extend from the edges of the center piece to
the walls of the cutout. The resistors are positioned partially
underneath the cloudy blocks. It’s a very clever design.
As the main body of the device presses against
something, the cutout causes the
body to flex about the servo drive
shaft. The cloudy blocks transfer
the force to the resistors, and the
device sends out a changing
voltage corresponding to the
force exerted on the resistors. It’s
an elegantly compact device —
like everybody’s new favorite
spherical droid, BB8.
I Find Your Lack
Implementing the Force
servo arm is not quite as simple
as slapping it onto a 25-tooth servo spline. Well, it can be
that simple if you have the right servos — ones that don’t
have a potentiometer and already have an extra PWM cable
waiting for whatever device will replace the missing pot.
Such servos include the MKS DS6630 and other MKS servos
normally used for camera gimbals.
If you don’t have these specialized servos, it just takes a
few modifications to enable a servo to work with the Force
arm. In fact, modifying a servo to interface with the device
is an operation very similar to a fundamental hack that is as
essential for any roboticist to master as self-control for a
Padawan training to become a Jedi knight: modifying a
servo for continuous rotation.
Generally, servos rotate over a limited range —
potentiometers themselves have only a limited range. To
enforce the limited range, the gearbox on a servo includes
a pin on one of the gears. Confined to a slot, the pin stops
the gearbox from turning beyond the determined range.
In many instances, it is useful to modify a servo for
continuous rotation. With their compact size and high ratio
gearbox, servos can make a great motor for small robots
for use either in a drive train or in
mechanisms like arms. For the
Force servo arm — since you’re
replacing the potentiometer —
you’re already halfway to
Taking out the stop is also
essential because the Force servo
arm would not sense any force
from the stop when the gear
reached the limit of its range —
the force would be acting on the
gearbox and not the arm.
Without any force acting on the
arm, it would keep going until it
encountered the force it was
looking for. It would never hit its
mark, like a Stormtrooper firing
his blaster and never ever hitting
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SERVO 06.2015 71
Twin brothers hack whatever’s put in front of them, then tell you about it.
EXPOSING THE PCB.