Spherical Robots, Spherical Robot, Spherical
Spherical Coordinate Robots
Spherical Robot: A spherical robot is a robot with two rotary joints and one prismatic joint; in other words, two rotary axes and one linear axis. Spherical robots have an arm which forms a spherical coordinate system.
The Structure of Spherical Robot
The spherical robots' structure is the one you can see at the left side of this picture. As you can see, it has two rotary joints and one linear. Thus a spherical work envelope is formed. Of course, it is not a sphere, but reachable places can still be calculated in a polar coordinate system. As you can understand, there can be more than three joints. However, these three are the basic ones that form the work envelope. Further joints would add more flexibility, but wouldn't radically change the reachable area. For example, if the gripper could rotate, this would be a 4-axis robot while the work envelope wouldn't change at all. A robot with an R-R-R structure with three rotary joints can be seen at the right side of the above picture. As you understand, robots of this kind have a near-spherical work envelope. Most modern industrial robots are derivatives of this type.
Spherical Coordinate System
In mathematics, a spherical coordinate system is a coordinate system for three-dimensional space where the position of a point is specified by three numbers: the radial distance of that point from a fixed origin, its polar angle measured from a fixed zenith direction, and the azimuth angle of its orthogonal projection on a reference plane that passes through the origin and is orthogonal to the zenith, measured from a fixed reference direction on that plane. The radial distance is also called the radius or radial coordinate. The polar angle may be called colatitude, zenith angle, normal angle, or inclination angle.
The Stanford arm
The Stanford arm was designed in 1969 by Victor Scheinman who at the time was a student working at the Stanford Artificial Intelligence Lab - SAIL. The arm has five rotary joints and one linear joint. The mechanical structure is the same I discussed above plus three rotary joints. The arm is all-electric, it had DC motors with potentiometers for position feedback and analog tachometers for velocity feedback. Also, it can be regarded as one of the first robots designed specifically for computer control. It seems needless to say as you probably understand it already - the Stanford arm is significant as it helped to form the knowledge base about robot control we use today.
Unimate is another historical spherical manipulator. It is widely regarded as the first industrial robot. In 1961 first Unimate began its work at General Motors. At the beginning, it was used for die casting and spot welding as these applications proved to be financially beneficial. This success spawned the industrial robot industry as we know it today. It featured hydraulic drive and could execute commands stored on a magnetic drum. Unimate, cherished by George Devol and Joe Engelberger, was manufactured by the first industrial robot company - Unimation. Another interesting twist of fate took place in 1977. In that year Unimation acquired Victor Scheinman's company - Vicarm. Using this new knowledge that originated from the Stanford arm project they designed Unimate PUMA. In 1989 Unimation was acquired by Stäubli. As you see this type of robot arms is quite important because of its place in history.However, nowadays you won't see many of them actually being used, as articulated (anthropomorphic) robot arms are more flexible, and the software and hardware development allows us to use them with remarkable results.