Robot Structures and Their Characteristics
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 or Articulated Arm
Figure 1 Left: Jointed Arm Robot Structure, Right: with parallelogram between the shoulder and elbow joints. 
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.
By adding a parallelogram structure, shown in figure 1 right, provides additional benefits of a “closed kinematic chain” as opposed to the “open kinematic chain” shown in figure 1 left. The parallel geometry of the parrelogram, where there are more than one chain of joints and links connecting the base to the end-effector, increases the manipulators stiffness to maintain positioning accuracy particularly at the limits of the robot reach and when larger payloads are being carried. However the addition of this extra structure does reduce the workspace of the robot .
SCARA (Selective Compliance Assembly Robot Arms)
Fig 2: SCARA Robot Structure
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
Fig 3: Tricept Robot
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. They are a type of “parallel” robot due to their “closed kinematic chain”. 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
Fig 4: Cartesian Co-ordinate Robot Structure
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. They come in a vast range of sizes from small ones for pick and place of small parts to ones that span automated production lines, a gantry, carrying large assemblies or even robots dangling downwards to move parts and assemblies from one automated cell to the next.
Fig 5: Delta Robot “FlexPicker” from ABB Ltd. 
The delta is another parallel robot. A very fast robot (arguably the fastest) but with only 4 axes, so 4 degrees of freedom, so very limited movement. Largely used in food industry for fast pick and place applications for lightweight products (e.g. chocolates, biscuits, croissants etc.) as they can only take small payloads of up to 8kg . Therefore they have to be easy and safe to clean with water so their electrics are kept out of the way at the top and covered additional protection during clean down. Can also be used in other end of line applications i.e. picking and packing.
Consists of one or two jointed arms. A two arm collaborative robot will typically to resemble a human and be a similar size to a large adult. Main design features that distinguish them from a normal industry jointed arm is that they are designed for safety with soft parts and they stop quickly with any collision detected. Designed with the intention of working next to humans on a production line without the usual safety precautions particularly physical guarding or light curtains. Some have screens as faces.
Fig 6: A collaborative robot with two 7 degree of freedom arms plus cameras made by ABB and called “YuMi”. 
Fig 7: Single arm collaborative robot by Fanuc.
Other Robots from History
There are designs that are now no longer manufactured:
Cylindrical robot which consist to a vertical linear joint which had a rotary joint angling a radial linear joint. These had good rigidity and were good for jobs requiring straight-line moves, reaching into cavities, so ideal for machine tending and simple to program as their motion is easy to visualise. However they had an inability to reach around objects and required a large amount of clearance behind. Their linear joints make them unsuitable for dusty or damp environments.
Polar Co-ordinate Robots were the first ones to be used in industry as it was ideal for hydraulic drives. It did produce 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.