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26.2.1 REVOLUTE COORDINATE
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Revolute coordinate arms, such as the one depicted in Fig. 26-1, are modeled after the human arm, so they have many of the same capabilities. The typical robotic design is somewhat different, however, because of the complexity of the human shoulder joint. The shoulder joint of the robotic arm is really two different mechanisms. Shoulder rotation is accomplished by spinning the arm at its base, almost as if the arm were mounted on a record player turntable. Shoulder flexion is accomplished by tilting the upper arm member backward and forward. Elbow flexion works just as it does in the human arm. It moves the forearm up and down. Revolute coordinate arms are a favorite design choice for hobby robots. They provide a great deal of flexibility, and they actually look like arms.
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26.2 ARM TYPES
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FIGURE 26-1 A revolute coordinate arm.
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26.2.2 POLAR COORDINATE
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The work envelope of the polar coordinate arm is the shape of a half sphere. Next to the revolute coordinate design, polar coordinate arms are the most flexible in terms of the ability to grasp a variety of objects scattered about the robot. Fig. 26-2 shows a polar coordinate arm and its various degrees of freedom. A turntable rotates the entire arm, just as it does in a revolute coordinate arm. This function is akin to shoulder rotation. The polar coordinate arm lacks a means for flexing or bending its shoulder, however. The second degree of freedom is the elbow joint, which moves the forearm up and down. The third degree of freedom is accomplished by varying the reach of the forearm. An inner forearm extends or retracts to bring the gripper closer to or farther away from the robot. Without the inner forearm, the arm would only be able to grasp objects laid out in a finite two-dimensional circle in front of it. The polar coordinate
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FIGURE 26-2 A polar coordinate arm.
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REACHING OUT WITH ROBOT ARMS
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FIGURE 26-3 A cylindrical coordinate arm.
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arm is often used in factory robots and finds its greatest application as a stationary device. It can, however, be mounted to a mobile robot for increased flexibility.
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26.2.3 CYLINDRICAL COORDINATE
The cylindrical coordinate arm looks a little like a robotic forklift. Its work envelope resembles a thick cylinder, hence its name. Shoulder rotation is accomplished by a revolving base, as in revolute and polar coordinate arms. The forearm is attached to an elevatorlike lift mechanism, as depicted in Fig. 26-3. The forearm moves up and down this column to grasp objects at various heights. To allow the arm to reach objects in three-dimensional space, the forearm is outfitted with an extension mechanism, similar to the one found in a polar coordinate arm.
26.2.4 CARTESIAN COORDINATE
The work envelope of a Cartesian coordinate arm (Fig. 26-4) resembles a box. It is the arm most unlike the human arm and least resembles the other three arm types. It has no rotat-
FIGURE 26-4 A Cartesian coordinate arm.
26.3 ACTIVATION TECHNIQUES
ing parts. The base consists of a conveyer belt-like track. The track moves the elevator column (like the one in a cylindrical coordinate arm) back and forth. The forearm moves up and down the column and has an inner arm that extends the reach closer to or farther away from the robot.
26.3 Activation Techniques
There are three general ways to move the joints in a robot arm:
Electrical Hydraulic Pneumatic
Electrical actuation is done with motors, solenoids, and other electromechanical devices. It is the most common and easiest to implement. The motors for elbow flexion, as well as the motors for the gripper mechanism, can be placed in or near the base. Cables, chains, or belts connect the motors to the joints they serve. Electrical activation doesn t always have to be via an electromechanical device such as a motor or solenoid. Other types of electrically induced activation are possible using a variety of techniques. One of particular interest to hobby robot builders is shape-memory alloy, or SMA, as discussed previously. Hydraulic actuation uses oil-reservoir pressure cylinders, similar to the kind used in earthmoving equipment and automobile brake systems. The fluid is noncorrosive and inhibits rust: both are the immediate ruin of any hydraulic system. Though water can be used in a hydraulic system, if the parts are made of metal they will no doubt eventually suffer from rust, corrosion, or damage by water deposits. For a simple home-brew robot, however, a water-based hydraulic system using plastic parts is a viable alternative. Pneumatic actuation is similar to hydraulic, except that pressurized air is used instead of oil or fluid (the air often has a small amount of oil mixed in it for lubrication purposes). Both hydraulic and pneumatic systems provide greater power than electrical actuation, but they are more difficult to use. In addition to the actuation cylinders themselves, such as the one shown in Fig. 26-5, a pump is required to pressurize the air or oil, and values are used to control the retraction or extension of the cylinders. For the best results, you need a holding tank to stabilize the pressurization. For small robot arms, the Lego Technics pneumatic cylinders, controls, and tanks can make the task of actuating an arm quite easy. An interesting variation on pneumatic actuation is the Air Muscle, an ingenious combination of a small rubber tube and black plastic mesh. The rubber tube acts as an expandable bladder, and the plastic mesh forces the tube to inflate in a controllable manner. Air Muscle is available premade in various sizes; it is activated by pumping air into the tube. When filled with air, the tube expands its width but contracts its length (by 25 percent). The result is that the tube and mesh act as a kind of mechanical muscle. The Air Muscle is said to be more efficient than the standard pneumatic cylinder, and according to its makers it has about a 400:1 power-to-weight ratio.
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