Introduction to Robotics

Industrial robots are particularly designed to manipulate a workpiece or tool in free space. In order to freely position and orientate such objects there is need for six degrees of freedom, three for positioning (e.g. X Y Z) and three for orientation (e.g. euler angles A B C).

Thus, the typical serial industrial robot consists of 6 joints (of linear- and/or rotary type), assembled in such a configuration that the “necessary” 6 degree of freedom are achieved. Figure 1 shows a typical industrial robot, made of rotary joints alone in a configuration known as “anthopomorh” or say human- like.

Figure 1. ABB® robot, a serial layout

The serial industrial robot is built up by joints typically including the following components:

A motor

A gear

A bearing

A measurement system

Today most robots use electrical motors (reliable, well-known, globally available technology). However, in heavy-duty operations, fluid power (hydraulic) motor drive systems are well suited. As a rule of thump; a hydraulic motor would be 10 times stronger than a similar sized electrical motor.

A gearing system is very important in the serial structured arm, mainly for two reasons:

1) to reduce the speed in order to be able to drive the robot end-effector (tool- center- point (TCP)) slow enough,

2) a gearing mechanism increases the lifting capacity of the joint. Bearings are found between rotary and static parts of the construction.

Measurement systems are of type position, speed and force/torque and are used in the control loop of a robot joint.

Joints are connected with beam elements in a serial manner (Figure 2.),; the three inner joints are often referred to as the robot arm while the three outer joints are named as the wrist.

A tool (often changeable) is connected at the very end of the arm (TCP). Coordinate systems (right handed Cartesian) are assigned to the tool and the base (fundament) of the robot.

Figure 2. Classical joint/beam structure and its components

Figure 3. ABB® control system

The robot control system (often in a box next to the robot), see Figure 3, has its main task to control the movement of the individual joints so that the TCP follows a pre-defined path with a given speed profile.

Figure 4. ABB® teach pendant

In order to program the robot there are normally two approaches;

1) Teach programming where the operator uses a pendant (joystick, see Figure 4) to drive the robot manually to the desired locations and store them in the control system of the robot. The operator guides the robot through its path and by this teaches the robot where to go and what to do.
After teaching, the robot can endlessly repeat such motion with a very high repeatability. Teach programming is a very easy and intuitive way of machine programming and new operators does only need a few hours to grasp the main concepts. However, teach programming can be very time-consuming, and the robot cell will be un-productive during programming and
testing of new programs.

2) Offline programming is the second approach to robot programming and requires some environment (Figure 5.) where you can program and test programs disconnected from the robot itself. Today, this is normally achieved by PC based software solutions where you have a graphical representation of the robot which can be programmed and tested without disturbing the real machine. Finally, the ready-to- run program can be downloaded to the real robot with a minimum down-time in production. The major challenge in offline programming is accuracy between the virtual simulations and the real robot cell; all deviations between these two environments will affect the path accuracy. So, calibration of the virtual world with respect to the real world will require some special attention.

Figure 5. IGRIP® offline robot programming environment

A modern robot control system offers extra functionality through built- in- functions and possible add-on solutions/equipment to be purchased. A few essentials of the latter are “I/O cards” allowing the robot to speak and listen to its surrounds, and “extra servo axis” where the robot can be attached to a moving rail system, extending the working area of the machine. Built- in- functions are related to the more basic and classical operations of the robot, aiming to help the programmer in those typical situations. E.g. the “palletizing function” is an advanced programming function where the robot control system automatically increment the location of the robot, according to a pre-defined pattern, each time the robot is sent towards the pallet. Thus, the robot will always reach the next free position of the pattern.

In modern industry, industrial robots have found various application areas. Material handling operations is probably still the largest. Because of its high repeatability the industrial robot is well suited to cover repetitive task such as moving objects between machines in a manufacturing cell. By utilizing the I/O capability of the robot it can receive and send messages between machines at the shop-floor, acting as a cell-controller.

Another larger application area is welding operations. Especially spot-welding in car manufacturing, but also advanced beam -welding systems are available from many robot manufactures.

Washing, spraying and various painting operations are carried out by lightweight robot arms, especially targeted for such operations. Instead of using a joystick to teach the robot an experienced operator can grab, guide and teach the lightweight arm manually by his own hand movements. This is a variant of the “teach” programming principle, especially targeted to catch the knowledge of experienced human operators.

Typical machining operations (e.g. polishing, grinding, milling, turning etc.) has been challenging for the industrial robot. The accuracy requirements of such operation are often too strict for the robot system to follow. Machining operations often require a complex path planning, often carried out in an offline programming environment.

As the prize of technology drops compared with the ever increasing cost of labor the industrial robot is gaining market shares. Not only the larger sized companies but also the small and medium sized industry can nowadays afford to invest in these modern machines. Ongoing research and development around the world indicates that the robot systems of the future is moving towards higher flexibility, allowing for human friendly rapid programming methodologies, opens for high accuracy machining operations etc. I general, modern robotics is developing towards the versatile needs of the typical smaller sized enterprise of our region.

Suggested literature for further reading and in-depth studies:

Introduction to robotics, mechanics and control, Craig, J.J, Pearson Education, ISBN 0-13-123629-6

Introduction to robotics in CIM systems, J.A. Rehg 5 ed., Prentice Hall, ISBN 0-13-060243-4, 2003
Handbook of Industrial Robotics, Editor Shimon Y. Nof.. John Wiley and Sons, ISBN 0-471-17783-0,1999
Introduction to Robotics, P.J McKerrow, Addison-Wesley, ISBN 0-201-18240-8, 1991.
T.K.Lien: Industrirobotteknikk, Tapir Forlag, Trondheim 1993. (Norwegian)
Matere og Orienteringsinnrettninger. Estensen et al. SINTEF rapport STF17 A80043 (Norwegian)