The design of a robot manipulator is modeled on the human arm, but with some differences. For example, a robotic arm may extend telescopically, ie sliding cylindrical sections within another to lengthen the arm. Robotic arms also can be constructed so that they bend like an elephant trunk. The clamps are designed to mimic the function and structure of the human hand. Many robots are equipped with specialized grippers for gripping specific devices such as a rack of test tubes or an arc welder. The joints of a robotic arm are usually driven by electric motors. In most robots, the gripper is moved from one position to another, changing its orientation. A computer calculates the joint angles needed to move the gripper to the

desired position, a process known as inverse kinematics. Some multijointed arms are equipped with servo controllers, or feedback, controllers that receive data from a computer. Each joint of the arm has a device to measure its angle and send that data to the controller. If the actual angle of the arm is not equal to the angle calculated for the desired position, the servo controller moves the joint until the arm's angle matches the computed angle. The drivers and associated computers must also process the data collected from cameras that locate objects to be grasped, or information from sensors located on the clamps that regulate the grasping force. Any robot designed to move in an unstructured or unknown environment needs multiple sensors and controls (eg, ultrasonic or infrared sensors) to avoid obstacles. Robots such as NASA planetary vehicles require a large number of sensors and computers on board some very powerful to process the complex information that allows them to move. That is particularly true for robots designed to work in close proximity to humans, such as robots that assist persons with disabilities or serving meals at a hospital. Safety must be integral to the design of robots for human service.