Abstract

The external appearance of robots disguises the careful thought that has been used in their design. An overview of the requirement analysis shows a similarity with machine tools. Differences in the physical appearances arise from the range of programmable motions. These vary in type and arrangement giving a choice of working envelopes. This has huge a bearing on the nature of the control system. Encompassing actuators, sensors and a microprocessor core, operation relies on synchronised internal events. The pattern varies according to the sequence of programmed motions. These often lead to complex velocities and accelerations for each of the robot joints. Considered as a system of energy stores and energy dissipating elements, these motions are further complicated by natural transient responses. Mathematical modelling enables predictions to give reliable timing of robot events. This is explored in manufacturing and hazardous environment applications where the robustness is utilised.

The common image of a robot is coloured by its history which is associated with science fiction writers. The word robot was first introduced in 1922 by the Czechoslovakian playwright Karel Capek in his work Rossum's Universial Robots. Practical robots did not appear till 1960's. Some of the early applications were in the die casting operations for automobile component manufacture. Over the proceeding ten years developments took place to advance the robot control systems. This trend has continued in more recent years with the increased availability and features of VLSI (very large scale integration) electronic devices.

The function of a robot might be considered to assist man in doing useful work. This at first sight appears to be true of a plethora of machines and so a definition is critical in establishing subject bounds. The International Standards Organisation (ISO) has a formal definition:

'An automatically controlled, reprogrammable, multipurpose, manipulative machine with several degrees of freedom, which may be either fixed in place or mobile for use in industrial automation applications.'

In terms of this definition robot arms have degrees of freedom similar to that of a human operator.

Movement

Movement is the primary methods by which the robot performs useful work. This may apply in two ways:

a) The robot could move the workpiece past a stationary tool b) The robot could move the tool over the stationary workpiece

In both cases the motion is described by translation and rotation about three axis to give six degrees of freedom (see figure 1).

Comparison with machine tools

Comparing the definition of a robot with the milling machine, which shares similar motions, enables a more reliable representation of a robot to emerge:

a) Motion comparison

i) Same six degrees of freedom
ii) The motions are only achievable with additional fixtures such as turntable attachments
iii) Some of the motions may only be manually activated
iv) Motion is imparted to both the workpiece and tool
v) Relatively small working envelope for bulk size of machine

b) Forces

i) Milling machine has a large stiff structure which contrasts with the flexibility of many robot frames.

ii) The forces in a milling machine are substantially greater than those of a robot of similar bulk size.

iii) Power supplies and drives are substantially larger in milling machines than those in most robots.

c) Programming

i) There is no common programming language between robots and milling machines. Both may use a variety of languages which are particular to manufacturers and controllers.

ii) Both robots and milling machine share an ability to accept point to point and continuous path programmed control.

iii) Robot programming may be carried out interactively with the operator leading the motions. Milling machine programming requires preparation of tool paths using data from a drawing of the component to be machined.

Robot characteristics

Robots may be classified into two broad categories (see figure 2) [1]:

a) Stationary robots - the robot arm is attached to a pedestal and positioned within reach of its workpieces.

b) Mobile robots - the robot arm is attached to a platform that is equipped with a traction system enabling it to move large distances to the workpiece.

Under both categories the robot arms may be further subdivided according to the arrangement of linear (prismatic) joints and rotary (revolute) joints used to sweep the working envelope. The major types of robot arms are (see figure 3):

a) Cylindrical coordinate robot
b) Spherical (polar) coordinate robot
c) Cartesian (rectangular) coordinate robot
d) Revolute coordinate (jointed arm) robot
e) Selective compliance assembly robot arm (SCARA)

Robot construction

The major components of a robot are:

a) Actuators
b) Sensors
c) Microprocessors and electronics

The actuators used by the robot vary according the nature of the power supply. Three common power sources and so types of actuator classifications are:

a) Hydraulic - high speed and acceleration, leakages cause problems
b) Pneumatic - similar to hydraulic but producing less power, used for manipulators
c) Electrical - very common, numerous sizes and technologies

Sensor technologies used in robots depend of the structure of the robot and level of performance in terms of range of motion, speed and accuracy. Two classes of sensors are used to sense motions in revolute joints and prismatic joints .

The microprocessor forms the hub of the robot controller with connections to both the actuators and the sensor systems. Power amplification forms one aspect of the interface electronics required for the actuators. Additional uses provide a user interface allowing programming operations to be performed.

Robot applications

Industrial applications of robots are varied, but some of the more common are summarised in the table below [2]:

a) Dismantling of unexploded bombs and missiles
b) Decontamination of chemical leakage sites
c) Remote inspection of suspect packages

The fourth dimension of time

The fourth dimension of time influences all aspects of robots.

a) Manufacture - current trends in manufacturing rely on minimising work in progress and inventory levels both of which rely on precise knowledge of process cycle times. The use of robot work cells in a manufacturing environment brings a predictable level of performance with respect to time that can be changed under programmed control.

b) Control - Inside the robot the microprocessor used in the control system is massively governed by a clock source which drives a sequence of events following the steps of the control program. The timing also extends to the interaction of the microprocessor with the electronic interfaces and the sensor systems which may be synchronous or asynchronous [3].

c) Movement - External evidence of the fourth dimension is apparent in the robot motions which vary in velocity and accelerations both of which may be non-linear functions of time when following complex profiles. Internal to the robot is a software kernel which controls the behaviour of the joints in response to commands. The control signal is applied via the actuator in accordance with the position realised though the sensor system. This cycle of events where the kernel generates an actuator signal to ensure the actual position is the same as the requested position takes a finite time period to settle. This gives rise to a transient response of the robot which is a function of time and the various energy stores (kinetic energy in masses) and losses (friction) . [3].

Conclusion

The external appearance of robots reveals only part of the design effort in making it an invaluable tool in many applications. Extracting the maximum benefit comes from matching the requirements to the capabilities of the robot not just in its working envelope which is a physical attribute but the attention to detail in its control system. This extends from the power supply and actuators through to the sensors and control system. The latter is key to ensuring the timely issuing of signals, for up to six degrees of freedom, according to the programmed sequence of instructions. Compensation in this process, through mathematical modelling of the natural flow of energy in the system with respect to time, is required to ensure reliable operation in all environments.

References

[1] Fu K.S., Gonzales R.C., Lee C.S.G.
Robotics, control and sensing, vision and intelligence
McGraw Hill Book Company 1988
ISBN 0 07 100421 1

[2] Groover M.P.
Industrial robotics: Technology, programming and applications 1986
McGraw Hill Book Company 1988
ISBN 007 100442 4

[3] Medvedev V.S., Lescov A.G., Juschenko A.S.
Control systems of manipulation robots (In Russian)
Moscow Nauka 1978

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