70 SERVO 06.2015
he namesake of this magazine is the humble
workhorse of the robotics world. Servos may not
normally be flashy or glamorous, but they are
essential building blocks for a panoply of
mechanisms ranging from drive trains to end-effectors. The reason for the servo’s ubiquity is
the elegance with which it solves an essential problem:
moving something to a certain position. Sometimes, you
want to do more than just move something to a certain
position. Sometimes just setting a servo-powered claw to a
closed position will end up putting something like a Darth
Vader force choke on whatever hapless object you want to
pick up. If that object is fragile, then whatever task you
were hoping to accomplish is as doomed as the Death Star
after the rebels discovered its fatal defect.
The Force servo arm — available from ServoCity —
hopes to solve that problem. The compact device sits atop
a servo in place of a regular horn, and allows you to
position the servo according to the force it exerts. Not only
is the force strong with this device, it can be weak too — if
that’s what’s needed to avoid crushing fragile cargo.
These are the Sensors
You're Looking For
The Force servo arm (designed by Aleksey Zaitsevsky) is
a compact device made up of an ABS body roughly
pentagonal in shape, with what appears to be some force
sensitive resistors and a small PCB (printed circuit board). A
PWM cable extends from the PCB and promises easy
integration into your servo-based projects. The underside of
the device features a counterbored spline cut hole for
fastening to a 25-tooth spline — like you find on a standard
Futaba servo. It comes with a screw for attachment to a
servo drive shaft. The pointed end of the ABS body contains
four 2 mm diameter mounting holes, spaced 5 mm apart.
The Force servo arm is designed to take the place of a
potentiometer in a standard servo. Servos work based on
the principle of negative feedback — kind of like when Luke
was parrying with the orb-shaped training remote and
getting a little blast every time he failed to trust the Force.
The controller will send a (control) signal to the servo
based on what position the user of the program selects. As
the motor (and gearbox) turns, the position of the
potentiometer (which is connected to the gearbox) is
checked against the control position. If it doesn’t match the
control position, the motor keeps turning. When the signal
from the potentiometer matches the control signal, the
position has been reached and the servo stops.
At the core of this feedback loop is the potentiometer.
A potentiometer is a small device that looks like a knob,
and the knob can turn over a limited range. The
potentiometer works like a variable voltage divider, and
when the knob turns it is effectively changing the resistance
of the second resistor in the voltage divider. Thus, the
change in the output voltage from the potentiometer
corresponds to the rotational position of the servo.
The Force servo arm takes the place of the
potentiometer. The specs on the Force arm are a bit
mysterious like the crossguard lightsaber-wielding Kylo Ren.
The product manual on the ServoCity website provides
plenty of detail on the physical design of this, with
thoroughly annotated dimensions and other structural
details. There is not much detail, however, on the circuit
used for sensing force. The secrecy may be due to its status
as a patent pending invention, with the application not
having been published.
Visual inspection of the device reveals what appear to
by Bryce Woolley and Evan Woolley
The Force Servo
Disclaimer: If you are not a Star Wars fan,
please forgive the plethora of puns.
THE FORCE SERVO ARM