Dynamixel rotary actuators moving the gripper’s
pincers — one actuator for each pincer. The wrist
motion uses another Dynamixel, and the arm joints
use one each, with two working together at the base
as the required torque is the greatest at this point.
Another in the base rotates the arm. Eight actuators
drive seven axes of motion to allow a good bit of
‘payload’ manipulation with an affordable robot arm.
The mobile robot shown in Figure 9 utilizes a
different approach to driving a robot’s arm joints: the
linear actuator. A linear actuator uses a motor-driven
‘screw’ to move a ‘nut’ up and down on the screw,
depending on the rotation of the screw. The nut is
attached to an inner sleeve that moves in and out
similar to a hydraulic cylinder.
The particular linear actuators that I used on this
robot are from ServoCity, and produce 500 Newtons
or 112 pounds of force. The shorter one at the
bottom moves 50 mm, and the other 100 mm. The
complete robot structure is built with Actobotics/
ServoCity structural components, wheels, and motors.
Our arms, legs, and all other motion-producing limbs
are moved in this way by linear muscles. This type of
actuator is ideal for many types of robot arms, but not
all. Looking at the robot’s arm, the long actuator is
placed six holes up out of 25 holes, or a 1: 4. 16 ratio.
So, dividing 112 pounds by 4. 16 is about 27 pounds
of lifting force at the end. The short actuator moves
the arm and support tube fore and aft.
Two geared RobotZone servos were used to
maneuver the gripper, which used a miniature Firgelli
linear actuator (shown in Figure 10) to move the
gripper’s pincers. These little actuators are no bigger
around than your finger, yet can have a force of up to
40 pounds and can be configured to operate just like
Types of Grippers for Robots
There are many types of grippers used in robots.
Some use magnetic force and others utilize vacuum
suckers to pick up payloads. Some even use
deformable sand or coffee grounds in a bladder to
encase an object when the air is removed from the
bladder. Various types use hydraulic, vacuum, or
pneumatic air systems to move pistons — much like
our human muscles. I’m going to limit the rest of this
article to electro-mechanical servos or motor-driven
grippers and arms for robots.
One of the most popular types of
experimental/hobbyist grippers is the parallel jaw gripper.
The Parallax ActivityBot and the VEX Clawbot shown earlier
utilize parallel jaw grippers in that the two pincers move in
and out — all while remaining parallel to each other. Notice
that the Parallax gripper has two flat gripper fingers that
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Figure 10. Firgelli L12-S- 2
Figure 9. ServoCity/Actobotics mobile robot with arm.
Figure 11. Robotiq finger model 2085.