Issue link: https://wardsworld.wardsci.com/i/1504662
2 Manipulator geometries An important design for robotic manipulators is the articulated arm, a configuration with rotary joints that resembles a hu- man arm. An extension of this idea is the selective compliance assembly robot arm (SCARA) configuration (Fig. 3), which has been extensively applied to assembly in manufacturing. SCARA manipulators have high stiffness during vertical motions, but they may be fitted with mechanical and sensory control devices that allow freedom in lateral motions during assembly opera- tions. Cylindrical configurations having a rotary joint at the base and a prismatic joint at the shoulder are useful for simple material transfer and assembly. One of the earliest manipulators, mar- keted in the late 1950s, was based on a configuration that had two rotary axes at the shoulder; this configuration has disap- peared from commercial application. Overhead gantry (Carte- sian geometry) robots are used for high-payload applications and those requiring linear positioning over large work spaces. Degrees of freedom Some manipulators have simple mechanical configurations involving only two or three degrees of freedom of movement. Most robotic manipulators have six degrees of freedom so that they can position a part or approach a part with any desired position and orientation. The wrist is positioned at a desired x, y, and z position in the work space. Then, the end effector is rotated to a desired orientation (roll, pitch, yaw). In effect, the wrist represents the origin of a three-axis coordinate system fixed to the gripper. Moving the first three joints of the arm translates this origin to any point in a three-axis coordinate system fixed to the work space; motion of the final three joints (in the wrist) orients the gripper coordinate system in rota- tion about an origin at the wrist point. While not all robots are constructed this way, the mathematics of mechanical motion (kinematics) is considerably simplified when these axes of the wrist do not intersect. Types of joints Robotic mechanisms can have joints that are prismatic (linear motion), articulated (rotary motion), or a combination of both types. While many robots use only articulated joints to imitate the human arm, limited actions can be produced by using prismatic joints alone. Robot locomotion can also be obtained by the use of wheels and treads. Walking robots have been developed that use articulated legs. Joint actuators Actuators for moving joints of a robotic mechanism are usually electric motors. In the past, larger robots were built with hy- draulic actuation due to their high payload capacity and ability to work in explosion-prone environments. In the 1990s there was a trend by the major manufacturers to use electric actua- tors in most of their industrial robots. Actuators can be placed directly at the joints; however, the weight and bulk of these motors and associated gear transmis- sions limit the performance of the robot, particularly at the wrist of industrial robots. Another design involves placement of the actuators in the base of the robot and transmission Robotics (continued) Fig. 2: Metal inert-gas welding robot. (Credit: Aaron Roeth Photography) Fig. 3: A model for the selective compliance assembly robot arm (SCARA). The arm has three links. Horizontal links move sideways and the third (vertical) link moves up and down. A typical SCARA is shown here with a drill, a chuck, and drill bit. (Credit: Richard Pasala) + ward ' s science