Servo - May 2015

78 SERVO 05.2015

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.