the setup is relatively simple. We open the radio menu and
go to the settings menu. From there, it’s a long scroll (with
beeps at every button push) to the “Aux Ch” menu. From
there, you scroll to find the desired channel, and press the
left/right arrow buttons to select its function. In this case,
we assign channel 7 to the “PIT TRIM” knob (Figure 11).
Once that’s done and you are back to the home menu,
power-up your quad and twist the assigned knob. Your
claw should open and close. If it doesn’t, recheck the
polarity of the servo plug, the radio settings, and that the
claw isn’t assembled too tightly for the servo to move.
Now that the gripper was on the quad and working, I
was ready for a flight test. This is where some of the
First off, I couldn’t get the gripper to hold onto
anything tightly enough to survive a bench test, much less
takeoff. Here’s where not having any M4 hardware came
back to haunt me. The linkages had enough slop that the
gripper could open about 1/8” or more when in the closed
The servo also just doesn’t have enough torque to
move the whole assembly when the 6-32 hardware is
tightened down enough to reduce the slop.
This was partially remedied by resetting the servo horn
so that the fully “open” position of the servo was only
about half open on the actual gripper. This means that the
servo had more throw than it needed when closing, and
could help compensate for some of the loose connections.
The main limitation now was the low torque of the servo
and the little mechanical advantage provided by the gripper
Nevertheless, I was finally able to get it to hold a pack
of stickers from my consulting company. I tried hanging the
stickers from a small rope with a piece of plastic hose over
it to give the gripper something rather squishy to hold onto.
It didn’t really have enough leverage to crush the tubing, so
the plain rope turned out to work the best.
I also experimented with the position of the rope in the
gripper, and found that in the hole of the end effector
worked the best (Figure 12). I tried putting it in the gullet
18 SERVO 12.2017
Figure 12: Holding a rope in the gripper’s hole seemed to work
best in my tests.
Figure 11: Set the channel 7 input to the appropriate knob/switch in
your radio’s menu.