Robot Structures

When selecting a robot for a particular task a number of decisions have to be made. The first of these is the structure of the robot required. There are a number of different structures commonly available and each has its own set of limitations and benefits.

Jointed Arm

The jointed arm robot closely resembles the human arm. This structure is very flexible and has the ability to reach over obstructions. It can generally achieve any position and orientation within the working envelope in eight different ways. This can cause control problems. When driving these robots in their natural co-ordinate system (joint space) the motion of the robot from one point to another can be difficult to visualise as the robot will move each joint through the minimum angle required. This means that the motion of the tool will not be a straight line. This structure of robots is used for a wide range of applications including paint spraying, arc and spot welding, machine tending, fettling, etc.

SCARA (Selective Compliance Assembly Robot Arms)

SCARA robots are specifically designed for peg board type assembly and are heavily used in the electronics industry. They are very stiff in the vertical direction but have a degree of compliance in the horizontal plane that enables minor errors in placement of components to be accounted for. These robots tend to be fairly small and capable of operating very accurately and at high speed. They are used for assembly, palletisation and machine loading.

Tricept and Hexapod Robots

Tricept and hexapod robots use linear motors to control the position of the tool. The tricept uses three of these legs in conjunction with a central pillar to hold the head rigidly in position and then has a standard wrist mounted on it to achieve the orientation. A hexapod uses six legs and achieves both position and orientation using them. Both of these structures give very rigid robots but both have the disadvantage of small working envelopes and limited orientation ability. These structures tend to be used for machining operations where machine tool level tolerances are not required but greater flexibility is.

Cartesian Co-ordinate Robots

This structure is most often seen in machine tools and co-ordinate measuring machines due to its high rigidity. It produces a robot that is very accurate and repeatable but which lacks flexibility as it cannot reach around objects. These robots are very easy to program and visualise but require a large volume to operate in. They are mainly used for pick and place operations and operations requiring great accuracy. Their linear joints are difficult to seal and this makes them unsuitable for working in damp and dusty environments.

Cylindrical Co-ordinate Robots

Very similar to the Cartesian co-ordinate robots these robots have good rigidity and are good for jobs requiring straight line moves. Programming them is simple as their motion is easy to visualise and they are good for reaching into cavities which makes them ideal for machine tending. The disadvantages of these robots is their inability to reach around objects and the amount of clearance required behind the robot. The linear joint makes them unsuitable for working in dusty or damp environments as it is difficult to seal.

Polar Co-ordinate Robots

The polar co-ordinate robot structure was the first one to be used in industry. The main reason for this was that it was ideal for hydraulic drives. Since the advent of all electric robots this structure has been all but replaced by the jointed arm robot but many of the old hydraulic robots are still working well in industry doing spot welding and many other tasks. A notable exception to this is the pendulum robot. This structure is effectively a polar robot hung from a gantry as a pendulum. It produces a very fast accurate robot as the centre of mass is at the centre of rotation of the major joints giving it a small moment of inertia. Pendulum robots are currently used for assembly, welding, gluing etc.