AUTOMATION AND ROBOTICS:INDUSTRIAL ROBOTS

INDUSTRIAL ROBOTS

Definitions

Industrial robots have become an important and indispensable component in today’s world of flexible automation. After the initial technical problems and high financial risk that impeded the willingness to invest, they have become an important means of automation in recent years.

The large number of handling devices in use can be divided into parts handlers and industrial robots (programmable handling devices).

Parts handlers are used in a variety of industrial sectors. They are mainly equipped with handling devices with grippers that execute set motion processes in specified cycles. The motion processes are controlled by means of limit stops or control cams. These components present the biggest disadvan- tage of the parts handler because of the amount of effort required to retool the device if the process is changed.

Industrial robots are defined by ISO 8373 as automatically controlled, reprogrammable, multi- purpose manipulators that are programmable in three or more axes and may either be fixed in place or mobile for use in industrial automation applications.

The number of installed industrial robots in all sectors of the most important industrial nations is shown in Figure 18.

The automotive, electrical, and electronic industries are the largest users of robots. The predom- inant applications are welding, assembling, painting, and general handling tasks. Flexibility, versa- tility, and the cost of robot technology have been driven strongly by the needs generated by these industries, which still account for more than 75% of the world’s stock of robots. In their main application areas, robots have become a mature product exposed to enormous competition by inter- national robot manufacturers, resulting in rapidly falling unit costs. However, the robot unit price accounts for less than 30% of the average work cell investment. A considerable share of total in- vestment cost is attributed to engineering and programming the robot and putting it into operation.

Classification and Types of Robots

Each of the industrial robot elements is linked to one another by linear guides and revolute joints in a kinematic chain, and the actuation of these elements forms the axes of the robot. Kinematics is the spatial allocation of the movement axes in order of sequence and construction.

Figure 19 illustrates the mechanical configurations of different types of robots.

The main axes help to position the final effector (tool or workpiece) spatially. The hand or adjacent axes are primarily responsible for the orientation of the tool and are therefore usually made of a series of revolute joints.

The following information may be useful for the selection of the industrial robot best suited to the respective application:

• The load-bearing capacity is the largest weight that has to be handled at the specified motion speed in relation to the robot flange. If the work speed or the reach is decreased, a larger weight can be moved.

• The mechanical structure refers to the kinematic chain as an ordered sequence and the type of motion axis on the robot.

• The number of axes is the sum of all the possible movements by the system. The higher the degree of freedom, the lower the system accuracy and the higher the costs. Therefore, it is advisable to limit the number of axes to the amount required.

• The work space is calculated from the structure of the kinematics and its dimensions. Its shape is also dependent on the movement areas of the individual axes and actuators.

• The positional accuracy determines the deviation during the run up to freely selected positions and orientations. The repetitive accuracy is the difference when the run-up to a spatial point is repeated.

Within a wide range of contructional types and variants, four versions have proved to be particularly suitable in practical operation:

1. Cartesian robots

2. Articulated robots

3. SCARA robots

4. Parallel robots

An extreme rigid structure is the first characteristic of Cartesian robots, whose single axes move only in the direction of the Cartesian space coordinates. For this reason, Cartesian robots are partic- ularly suitable for operating in large-sized working areas. Major applications include workpiece pal- letizing and commissioning.

Standard articulated robots consist of six axes. They are available on the market in a large variety of types and variants. Characterized by a cylindrical working area occupying a relatively small vol- ume, articulated robots easily allow failures to be repaired directly. Articulated robots are used pri- marily for spot welding, material handling, and painting as well as machining.

SCARA robots have a maximum of four degrees of freedom but can also be equipped with just three for very simple tasks. These robots are used for assembly tasks of all types, for simple loading and unloading tasks, and for fitting of electronic components to printed circuit boards.

The six linear driving axes of parallel robots are aligned between the base plate and the robot gripper so as to be parallel. Therefore, positioning accuracy and a high degree of rigidity are among the characteristics of parallel robots. The working area is relatively small. Parallel robots are partic- ularly suitable for tasks requiring high accuracy and high-range forces, such as workpiece machining.

Major Robot Components

8.3.1. Power Supply

Three main power sources are used for industrial robot systems: pneumatic, hydraulic, and electric. Some robots are powered by a combination of electric and one other power source. Pneumatic power is inexpensive but is used mostly for simpler robots because of its inherent problems, such as noise, leakage, and compressibility. Hydraulic power is also noisy and subject to leakage but is relatively common in industry because of its high torque and power and its excellent ability to respond swiftly to motion commands. It is particularly suited for large and heavy part or tool handling, such as in welding and material handling, and for smooth, complex trajectories, such as in painting and finishing. Electric power provides the cleanest and most quiet actuation and is preferred because it is self- contained. On the other hand, it may present electric hazards in highly flammable or explosive environments.

The typical series of sequences to be carried out by the electrical drive of a robot axis is shown in Figure 20.

The reference variables, calculated in interpolation cycles by the motion-controlling device, are transmitted to the axle controller unit. The servodrive converts the electric command values into couple of forces to be transmitted to the robot axle through the linkage. The internal control circuit is closed by a speed indicator, the external one by a path-measuring system.

Measuring Equipment

Measuring devices able to provide the axis controller unit with the suitable input quantity are required for robot position and speed control. Speed measurement is carried out directly over tachometer generators or indirectly over differential signals of angular position measuring systems. Optoelec- tronical incremental transducers, optoelectronical absolute transducers, as well as resolvers are among the most common devices for robotic angular measuring. For the path measurement of linear motions, the measuring techniques used by incremental and absolute transducers are suitable as well.

Control System

The major task of the robot control system consists of piloting one or more handling devices ac- cording to the technologically conditioned handling or machining task. Motion sequences and op- erations are fixed by a user program and are carried out by the control system unit. The necessary process data are provided by sensors, which therefore make it possible for the unit to adapt to a certain extent the preset sequences, motions, and operations to changing or unknown environmental conditions.

Figure 21 shows the components of a robot controller system.

Data exchange with superset control system units is actuated through a communication module, such as for loading the user program in the robot control unit or exchanging status data. The process control device organizes the operational sequence of the user program which provides instructions for the motion, gripper, sensors, and program flow. The robot axes as well as the auxiliary axes involved in the task to perform are driven by the motion-controlling device.

There are three kinds of motion control:

1. Point-to-point positioning control

2. Multipoint control

3. Continuous path control

The axis controller carries out the task of driving the robot axes according to the reference variables. The sequential progression from one axis position to the next is monitored and readjusted comparatively to the actual positions. An appliance (teach pendent) allows the robot to be operated and programmed by hand. The operator can easily access all control functions in order to run the robot in manual mode or determine the program flow.

The control system also determines two major performance measures of the robot: its accuracy and repeatability. The first indicates the precision with which the robot can reach a programmed

position or orientation. The second indicates the tolerance range over which a programmed motion can be repeated several or many times. While the accuracy of robots is difficult to establish in a standard way, repeatability has been standardized. Typically, industrial robots will have a repeatability better than ±0.01 in. and more precise robots will be within ±0.003 in.

Gripper

The gripper is the subsystem of an industrial robot able to transfer the power transmission from the workpiece to the industrial robot to make sure the robot has recorded the workpiece’s exact position. A gripping system has to carry out the following tasks:

• Temporarily maintaining the allocation of the workpiece and the gripping device, defined on the basis of the gripping center line

• Recording the static forces and moments produced by the workpiece

• Recording the kinetic forces and coupling of forces produced by the acceleration during the operating sequence

• Recording process-bound forces (e.g., joining forces)

According to the complexity of the gripping task, the robot gripper can be equipped with several subsystems. A driving mechanism (electric, pneumatic, or hydraulic) is required if the gripping force is produced kinematically, which is often the case with mechanical grippers. A holding system allows the workpiece position to be fixed opposite the gripping kinematic device. The gripping area is among the major criteria for evaluating the flexibility of a gripping device.

The object (workpiece) to be manufactured, the space available in the building, and the required cycle time of handling and assembly systems play a major role in the decision which robotic gripper to use. If, for example, the standard gripper version proves to be inadequate for specific operations, it can be equipped with interchangeable gripping systems by the creation of standardized interfaces.

Programming and Robot Simulation

Programming

The robot control system contains a programming method by teaching, by using a teach pendent, by showing and actually leading the robot manipulator through the desired motions, or by programming.

A programming method is the planned procedure that is carried out in order to create programs. According to IRDATA, a program is defined as a sequence of instructions aimed at fulfilling a set of manufacturing tasks. Programming systems allow programs to be compiled and also provide the respective programming assistance (see Figure 22).

If direct methods (online programming methods) are used, programs are created by using the robot. The most common method is teach-in programming, in which the motion information is set by moving towards and accepting the required spatial points, assisted by the teach pendent. However, the robot cannot be used for production during programming.

One of the main features of indirect methods (offline programming methods) is that the programs are created on external computers independent of the robot control systems. The programs are gen- erated in an offline programming system and then transferred into the robot’s control system. The key advantage of this method is that the stoppage times for the robot systems can be reduced to a minimum by means of the configuration of the programs.

Hybrid methods are a combination of direct and indirect programming methods. The program sequence is stipulated by indirect methods. The motion part of the program can be defined by teach- in or play-back methods or by sensor guidance.

Robot Simulation

Computer-assisted simulation for the construction and planning of robot systems is becoming more and more popular. The simulation allows, without risk, examination and testing on the model of new experimental robot concepts, alternative installation layouts or modified process times within a robot system. ‘‘Simulation is the imitation of a system with all its dynamic processes in an experimental model which is used to establish new insights and to ascertain whether these insights can be trans- ferred to real situations’’ (VDI Guideline 3633).

The starting point for the development of graphic 3D simulation systems was the problem of planning the use of robots and offline programming. Independent modules or simulation modules integrated into CAD system were created. To enable a simulation to be conducted, the planned or real robot system must first be generated and depicted as a model in the computer. The abstraction level of the simulation model created must be adjusted to the required imitation: ‘‘as detailed as necessary, as abstract as possible.’’ Once the model has been completed, an infinite number of sim- ulations can be carried out and modifications made. The objective is to improve the processes and eliminate the possibility of planning mistakes.

Figure 23 shows an example of the visualization of a robot welding station.

Simulation is now considered one of the key technologies. Modern simulation systems such as virtual reality are real-time oriented and allow interaction with the operator. The operator can change the visual point of view in the graphical, 3D picture by means of input devices (e.g., data gloves)

and thus is able to take a look around in the ‘‘virtual world.’’ The virtual objects can be interactively manipulated so that modifications can be executed more quickly, safely, and comfortably.

The main advantages and disadvantages of simulation can be summed up as follows:

• Dynamic analysis of modeled processes and verification of executed modifications

• Increased planning security due to the evaluation of the project planning before realization

• Reduction in planning time

• Reduction in development costs for the realization of robot systems

• Expenditure savings due to reduction of run-through times and stock levels and optimization of resource utilization

New Applications

In the 1980s, as industrial robots became a recognized indicator of modern production facilities, thought was also given to the use of robots outside the factory. The term service robots was coined for such new robot systems, which, however, still have no valid international definition today. The International Federation of Robotics (IFR) November 1997, Stockholm, Sweden suggested the fol- lowing definition in 1997: ‘‘A service robot is a robot which operates semi or fully autonomously to perform services useful to the well-being of humans and equipment, excluding manufacturing oper- ations.’’

Because of the multitude of forms and structures as well as application areas, service robots are not easy to define. IFR has adopted a preliminary system for classifying service robots by application areas:

• Servicing humans (personal, safeguarding, entertainment, etc.)

• Servicing equipment (maintenance, repair, cleaning, etc.)

• Performing autonomous functions (surveillance, transport, data acquisition, etc.)

Following are examples of application fields for service robots.

Courier and Transportation Robots

The HelpMate, introduced in 1993, is a mobile robot for courier services in hospitals. It transports meals, pharmaceuticals, documents, and so on, along normal corridors on demand (see Figure 24). Clear and simple user interfaces, robust robot navigation, and the ability to open doors or operate

elevators by remote control make this a pioneering system in terms of both technology and user benefit.

Cleaning Robots

A service robot climbs surfaces on suction cups for cleaning (see Figure 25), inspection, painting and assembly tasks. Tools can be mounted on the upper transversal axis. Navigation facilities allow accurate and controlled movement.

Refueling by Robot

Refueling a car by robot is as convenient and simple as entering a parking lot. Upon pulling up to the refueling station, the customer inserts a card and enters the PIN code and the refueling order. The robot locates the car, opens the tank flap, and docks onto the tank cap. Once the cap is open, the robot pumps the right grade and amount of fuel into the tank (see Figure 26).

Medical Robot

This mechatronic assistant system was developed by Siemens AG and Fraunhofer IPA. It is used for different operation tasks. The system consists of an operation robot (hexapod) whose kinematics concept is based on the Stewart platform. These kinematics allow extreme accuracy in micrometer ranges and a high level of stiffness despite the robot’s relatively small size (see Figure 27).

The operator controls the operation robot from an ergonomic operation cockpit similar to a flight simulator. Tactile back-coupling of the motions helps the operator operate the system. The operation cockpit is also assembled on a hydraulic hexapod.

Assistance and Entertainment

This mobile robot has been created to communicate with and entertain visitors in a museum (see Figure 28). It approaches the visitors and welcomes them to the museum. Speech output is accom- panied by movement of the robot head. The robot gives guided tours in the museum. Moving its head up and down symbolizes the robot looking at the object it is currently talking about. Expla- nations are further accompanied by pictures or video sequences shown on the screen of the robot.