Degrees of Freedom Robotic motion is described in terms of degrees of freedom. A degree of freedom is one movement, either back and forth (linear) or around in a circle (rotational). Your own arm has seven degrees of freedom: three in the shoulder, one to bend the elbow, one to rotate the wrist, and two to move the wrist up and down and side to side. To access any point in space (within reach) from any angle, a robot needs six degrees of freedom: three arm movements and three wrist movements. Some robots have more than six degrees of freedom, some less, depending on the kind of work they do. The basic six degrees of freedom–called waist, shoulder, elbow, roll, pitch and yaw. Learn more about the basic six degrees of freedom and how they function.

Nitinol wire is a unique metal (known as a "shape memory alloy") that can be used to activate robotic parts. When heated, usually by passing electricity through it, nitinol wire contracts up to 10% of length. (Ordinarily, metal expands when you heat it–that’s part of why nitinol wire is so unusual.) The contractile force is surprisingly strong, approaching 22,000 pounds per square inch. Even a hair-thin nitinol wire six thousandths of an inch thick can lift 11 ounces.

Not all robotic actuators are electric. Hydraulic and pneumatic actuators rely on oil and gas, respectively, to move things in and out. In both, plungers called pistons move in or out when they are pushed or pulled. Hydraulic systems are used instead of electric when there is a danger of sparks igniting fumes, for example, in the painting area of an automobile factory. Pneumatic systems are often used in grippers, the part of a robot that picks something up. The compressibility of gas makes the gripper more responsive, and less likely to crush what it’s picking up.

Air muscles are simple pneumatic devices that contract by thickening, a lot like our own muscles do. When pumped up with air, an air muscle shortens by as much as 40% of its length. With a power-to-weight ratio of 400 to one, air muscles can provide substantial pulling force. Since they are mostly made of soft plastic and rubber, air muscles can be bent around curves and can work when wet or even underwater.

To do work, a robot needs to be fitted with what’s called an "end effector": a gripper or a tool such as a welder, saw, or spray gun. Vacuum grippers use suction to pick up fragile things like sheets of paper or glass. Magnets activated by electricity, called electromagnets, are used for picking up and releasing metal objects.