What Type
of Gripper
Should
You Use?
I am a coach for a
middle school robotics
club, and we have been
using VEX components
including the VEX Claw
kit shown in Figure 3. In
the beginning, the
students wanted to have
a Sumo-type competition,
placing LEGO robots built
by the school’s robotics
class against VEX robots of the
robotics club. One of the first things
the kids came up with as a ‘weapon’
was to use the VEX robot claw to
grasp an opponent. I explained to
them that the basic premise of the
competition was to shove your
opponent out of a ring — rather than
grabbing it and tossing it out. I spent
a few minutes asking them, “Just how
are you going to accomplish this with
the claw and robot’s arm?”
The VEX Clawbot robot shown in
Figure 4 is one that I had built earlier
for my November column; we are
using it as a classroom test bed. The
school had purchased a large amount
of VEX products in previous years (as
seen in Figure 5) and their teacher,
Tylor Hankins and I felt that we should
be able to use the large amount of
VEX structural materials on hand in
conjunction with the VEX ARM Cortex
microcontroller as a base for the
club’s competitive robots.
We have since decided to
construct a smaller version of the
NASA Mars rover, Curiosity using
mostly ServoCity and Actobotics
structural members, motors, and
wheel systems. Brian Petty and his
team at ServoCity have been most
cooperative in helping us in design
issues and the selection of
components. I will have a write-up on
that when it is completed.
Positioning the
Gripper or
End-Ef fec tor
One of the reasons that industrial
robots are so much more expensive
than experimental robot arms and
grippers is the speed, accuracy of
movement, and the ability of the
robot’s arm and wrist to position the
gripper rapidly and precisely, and in
the correct orientation for the
required task. Looking at the VEX
Clawbot in Figure 4, there are only
two axes of motion. The arm is raised
and lowered in a single arc, and the
gripper opens and closes.
The motors used on this robot do
not include shaft encoders on the arm
motor nor the gripper, but that is not
necessarily a bad thing as a remote
operator can actually add a third axis
of motion by differentially steering the
base with the two sets of wheels. This
will allow the robot to ‘zero in’ on a
target object, grasp it, and move it to
some other location.
The students and I sat down and
talked about the usefulness of the
claw in a robot vs. robot competition.
Why even use a claw or gripper? Why
not just rely on the sloped ramp on
the front to lift the opponent and
shove him out of the arena?
Before suggesting that approach,
I tried to instill upon them the
difficulty of using some sort of gripper
as a weapon against another robot.
We talked about the fact that the
claw only operated in a single
horizontal plane. In addition, it was
only ‘horizontal’ when the claw was
lowered all the way to the
floor/tabletop. The axis of the arm’s
motion was a single vertical arc.
Basic Arm and
Gripper Control
Using a Sensor
They sat in groups and discussed
ways that the claw could be
positioned in order to disable an
opponent. The group wanted to use
the VEX ultrasonic distance sensor and
couple it with control of the arm and
claw. The ultrasonic sensor can be
seen in Figure 4 as the red object on
the front of the wheeled base.
The group realized that the
sensor would detect many different
objects, but the only object of interest
would be the opponent’s robot, and
that should be the only object in the
area. Since it had a broad detection
beam, using a claw to grasp
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Advances in robots and robotics over the years.
SERVO 05.2015 77
Figure 4. VEX ClawBot with nice gripper.
Figure 5. VEX kits at Ridgefield
Middle School.
