havior of the human shoulder, wrist, and elbow. Other robots move in straight lines similar to a crane. Robots rarely have a pair of arms, and are usually stationary. If a robot moves that is all it does. Examples are delivery robots rolling down halls delivering mail or supplies. The hands and arms of early robots were pneumatically powered (air pressure) or hydraulically powered (fluid pressure). Flexible tubes carried the pressurized substances to the joints. Now, electric motors located at the joint give the robot greater precision and control, but slow down its movements. All robot manufacturers dream of joints with human-like tendons. Most robots are blind and are insensitive to their surroundings. Some have sensors triggered by light, pressure, or heat that can create a crude picture of what is happening. ROBOTIC ARMS There are four types of robot arms that are used today. Degrees of freedom are the axes around the arm in which it is free to move. The area a robot arm can reach is its work envelope. Rectangular arms are sometimes called "Cartesian" because the arm's axes can be described by using the X, Y, and Z coordinate system developed by Descartes. Descartes is a famous French philosopher, scientist, and mathematician. If a pen were attached to the arm, it would draw a rectangle which would be its work envelope. Imagine a graph where X would be side to side, and Y would be in and out on the graph. Up and down would be Z which runs through the graph and describes depth. Z also adds the third dimension. A cylindrical arm also has three degrees of freedom, but it moves linearly only along the Y and Z axes. Its third degree of freedom is the rotation at its base around the two axes. The work envelope is in the shape of a cylinder. The spherical arm replaces up and down movements along the two axes with a rocking motion of the arm. The spherical arm's work envelope is a partial sphere which has various length radii. The last and ...