Servo - May 2015

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 a servo.

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.

Common Robot Gripper Configurations

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