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something on the robot would be
useless, as it could not ‘see’ any sort
of detail. Therefore, they decided
that using the ramp and
programming the robot in RobotC
to head in the general direction of
what the sensor ‘saw’ was the best
approach.
Designing a Robot
to Use an Arm with
a Gripper or Claw
Many experimenters like to
begin with a basic
mobile robot
platform and later
decide to add a
manipulator with a
gripper. This
approach can work
well most of the
time, but starting
with a robot design
to incorporate an arm
(or arms) is better.
Realizing that a
useful arm or pair of
arms requires a large
work envelope
should always be
taken into consideration.
The CrustCrawler rover in Figure
6 (first offered by Alex Dirks in 2010)
is a good example of a design with a
clear work area around the articulated
arm with a base top area of 18” x 14”
and a 4” clearance in the body for
electronics. The CrustCrawler Nomad
rover uses the Parallax pneumatic
wheels and gearmotors for good
traction, and the motor’s weight
offsets any of the arm’s payload mass
in the front of the robot. Always make
sure the robot’s structure, sensors,
and other appendages do not
interfere with the arm’s operation.
The fact that the arms move in a
wide area and have a certain mass —
including a prospective payload —
should be taken into account as a
constantly moving center of gravity
can topple a robot. Parallax
considered this when it designed the
Gripper kit for their popular Boe-Bot
shown in Figure 7.
Note that the heaviest component
— the servo — is mounted on the end
opposite the actual grippers. This
allows for a heavier payload without
tipping the robot, and is just good
engineering practice.
I have used this gripper on both
the Boe-Bot and the ActivityBot and
have found that the parallel jaw
gripper has a better grasping ability
due to the rubber inserts in the jaws.
This small tabletop robot is a great
platform for gripper tests, sensor
applications, and use in contests.
Positioning a
Gripper with an
Articulated Arm
As I mentioned in the beginning,
our hands are useful because we can
position them so our thumb and
fingers can grasp or manipulate
something. The Clawbot shown back
in Figure 4 is a great educational tool,
but the actual claw or gripper can
only grasp an object that has some
sort of vertical surface, or an
appendage close to the floor, or on a
table top where the two pincers can
surround and close in on an object.
The VEX robot was designed to allow
students to understand motions and
the programming required to perform
a specific task.
As robot experimenters, we do
not need the speed or accuracy of an
industrial robot, but positioning a
gripper can still be accomplished with
a series of servos arranged in a way to
create three or more degrees of
freedom at the end of the arm.
As humans, we are limited in our
hand’s motions, since we cannot
rotate joints in a continuous motion
like a motor or servo. Joint rotation at
our wrist is limited to 180° as we twist
our radius and ulna bones; our
shoulder, elbow, metacarpals, and
phalanges can move even less.
One of CrustCrawler’s robot arms
is shown in Figure 8 with two Robotis
Figure 8. CrustCrawler pro series
robotic arm.
Figure 7. Parallax gripper added
to Boe-Bot.
Figure 6. CrustCrawler Nomad rover
AX12 arm.