Robotics softwareand CADCAM

by Mike DoonerThe Open University

Robotics software integrated into a CADCAM system is a major stepforward towards the planning and designing of manufacturing cellson a systems basis. Such is the appeal and commercial potential ofthis new software tool that already two major CADCAM vendorshave marketable products, while others are announcingdevelopment plans. The real rewards will come when this softwarecan be used to effectively program robots off-line; meanwhilesimulation brings its own rewards, and certainly there is no denyingthat robot animation does no harm to the vendor's high-technologyand progressive image. This paper looks collectively at severalavailable systems, summarises their features and comments on theirusefulness.

Introduction

Few can doubt that one of the mostimpressive applications for computer-aided design and manufacturing(CADCAM) systems is the ability to si-mulate, animate and program the in-dustrial robot. Conceptually it is a fineexample of CADCAM: robot/workcellgeometry can be evaluated and, usinginformation created during simulationand design, robot programs can bedeveloped and verified off-line. Visuallyit can be spectacular to preview a robotoperating within its working en-vironment, particularly if the represent-ation is both solid and colour. It couldeven be argued that its promotionalvalue is the software's greatest asset atthe moment. However, robotics soft-ware is much more fundamental andimportant than mere visualisation(although this is useful in itself). It isanother step towards producing an in-tegrated . computer-aided engineering

system that will accommodate, in thiscase, an essential and central com-ponent in advanced manufacturing.

As a simulator, the technique has dis-tinct technical and economic attrac-tions, particularly for feasibility study,where it is necessary to evaluate certainrobot attributes and, further, to makean assessment of the potential of theworkplace scenario. All this is achievedthrough computer graphics and prior toinstallation.

Companies that are likely to makeuse (and cost-effective use since thesystems are expensive) of this tool arethe major users, consultants and imple-menters of industrial robotic systems.The automotive industry, a good exam-ple of the former, has almost a monop-oly of robot applications and has tore-implement, from time to time, itsflexible production automation. Conse-quently, any technique that will helpreduce downtime and highlight andeliminate potential problems must be

regarded as hugely cost-effective.Another obvious home for this tech-nique is the robot system suppliers ormanufacturers, whose job should beeased by graphically demonstratingrobot application to their clients.

Developments

Interactive graphics simulation of robotsystems (as distinguished from dynamicsimulation of manipulators) based onthree-dimensional modelling softwarebegan its life as university research pro-jects and notably Nottingham Uni-versity was probably first in producingprototype software. Nottingham's workwas of a practical nature and severaltesting industrial case studies demon-strated the potential of the technique.The Nottingham research environmentwas perhaps suitable for this work sincethere was the essential blend of com-puter science and robotics engineeringamong the staff at the time. (It is worthnoting that other groups that havedeveloped similar work also have, orhave acquired, this mix of expertise.)The Nottingham group has continuedto develop the system (with UK Scienceand Engineering Research Councilfunding) along practical lines, and re-cently a company was formed to offerconsultancy and to market the soft-ware. Two other rival systems that com-pete commercially with GRASP(Nottingham's software) are McAuto'sPLACE and Computervision's Robogra-phix. McAuto's software (parent com-pany McDonnell Douglas) wasdeveloped as a result of its involvement

Computer-Aided Engineering Journal December 1984 217"

with the Integrated Computer-AidedManufacturing (ICAM) project (a US AirForce funded research programme intoadvanced manufacturing methods forthe aircraft industry). Computervision,never slow to miss a commercial op-portunity, used robotics know-howfrom the company Automatix todevelop its product. These three sys-tems are now available commercially,and many others are being developedby CADCAM vendors — althoughthese are mainly US companies.

Generic features

From the point of view of the user, ro-botics software exhibits certain features,through these features the user can:

• display geometrical models ofmanipulators• define postures and movement• generate and play back sequences.

All these features are necessary in pro-viding a tool that will allow practicalrobot workcells to be designed and re-alistically simulated.

In functional terms, and to realise theabove features, the software can be re-garded in modular form with modulesfor:

• geometrical and kinematic model-ling• control and programming• analysis.

A fourth module is one of output, i.e.the generation of program control datafor the purposes of off-line program-ming.

The geometric modeller, an essentialcomponent in any CAD system, pro-vides the facilities for three-dimensionalrepresentation. The type of modeller is,however, an important issue in roboticssoftware. Wire-frame modellers, for in-stance, although adequate for visual-isation purposes, are not true modellers(i.e. they do not represent solid or evensurface properties) and as such do notallow geometrical interference to beautomatically detected.

In addition to geometry, the kine-matics of the manipulator needs to be

represented. Manipulators are mechani-cal linkages or structures with joints(translating or revolute) that allowmovement and the end-effector (thelowest part in the kinematic chain) tobe positioned and oriented in space. Itis therefore necessary to reproduce thecharacteristics of manipulation and torepresent types of movement as well asjoint constraints.

Control is required at two levels.Control software first allows manipula-tor information to be accepted in end-effector space, as opposed to jointspace. This procedure is realised byhaving co-ordinate transformationswhich derive joint data from user-specified workcell co-ordinate data. Thisconversion is by no means easy, con-sidering that multiple solutions (or per-haps no solutions) may exist for aparticular manipulator configuration,and in the context of a general-purposetool the software should be capable oftransforming many industrial robotscontained within the system's file ofrobots.

At a higher level of control, processes

Fig. 1 Seam welding workcell simulation using GRASP

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[Courtesy of BYG Systems, Nottingham]

Computer-Aided Engineering Journal December 1984

which can occur independently andwhich reflect the operation of the work-cell need to be specified and co-ordinated (multi-robot installations,machine operations etc.). Both thesecontrol functions can be embedded inprogramming software to allow the op-erator easy use.

Supporting these functions is theability of analysis. Analysis in this con-text means the detection of joint con-straints, to ensure the manipulator isoperating within its kinematic limi-tations. A further element of analysis isthe ability to check for interference orclashes between geometrical entities ofthe model. Practically interpreted, thismeans that fouling can be checked, say,between a robot manipulator arm andsome part of its environment. Ofcourse, geometrical interference can beobserved visually, but for complex situ-ations this is not always satisfactory.

Design and simulation

A robot workcell could typically com-prise the various physical equipmentthat represents the manufacturingsystem, and in addition to the robot(and gripper or tool) there arecomputer-controlled machines, materialhandling equipment, storage areas andthe component part or workpiece. TheCADCAM system, with its general-purpose modelling facilities, is able toproduce geometrical representations ofthese objects.

The procedure for designing work-cells using robotics software followsgeneral lines:

• Retrieve a robot from file and createthe cell geometry.• Develop the task sequence andprogram the robot.• Display and simulate the operation.

All this is usually done interactively anditeratively. Computer analysis (jointconstraint and interference checking) iscarried out during the design until a sa-tisfactory solution is obtained.

It is highly probable that the systembeing used will have a library of most ofthe commercially available industrialrobots. Thus the user is presented withthe ability to quickly assess and com-pare some characteristics of severalrobots of his choice. It is further pos-sible, at least in principle, for the user tocreate alternative robotic designs, orperhaps modify existing industrial ones(although it is amusing to imagine thereaction of some robot manufacturerswhen faced with a request from a clientto perhaps shorten or lengthen a sec-tion of a robot arm).

Robot construction or design is gen-erally difficult because specialist knowl-

bFig. 2 Three-dimensional graphics displays from the PLACE workcell simulator and evalu-ator system

[Photographs courtesy of McAuto]

a Robot motion/joint limit information is graphically displayed in the top right-hand cornerof the screenb The display in the bottom left-hand corner is an actual program sequence generated byPLACE

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Fig. 3 Three-dimensional workcell environment created on a Computervision CDS 4000 system using Robographix application software[Photograph courtesy of Computervision]

edge of kinematics is required.However, provided that certain rules arefollowed, the nonexpert can be provid-ed with the tools to construct newrobots that appear on the industrialscene.

Once a workcell design has beenconceptualised, and, of course, therobot's environment has been model-led, the next phase is to evaluate therobot's capability to perform its in-tended task. During this phase the userrelies on computation to provide cer-tain facilities and analysis. For example,software is provided that allows manip-ulator data to be specified in terms ofend-effector co-ordinates, rather thanjoint co-ordinates. This makes the roboteasier to program. It is further possibleto specify a program in a robot-independent way; i.e. the task is speci-fied in terms of object co-ordinates.Thus it is possible to compare severalrobots performing the same task.

Simulation can now be used to verifythe design in certain ways. First, theprogram can be checked for oper-ational logic; secondly, sequence timescan be displayed for particular sectionsof the path, or the total work cycle timecan be calculated.

Test sites

Prototype versions of the software are,in some cases, currently being tested ata number of customer test sites —called beta sites. These sites are spe-cially chosen so that they may assist insystem evaluation and are normallymanned by experienced users. As aresult, production versions may incor-porate new hardware and software fea-tures which have been suggested bybeta site users. Generally, most of thesesites are concerned with the technicalproblems in producing robot controlprograms from CADCAM model data.

One practical problem faced by usersis that differences exist betweencomputer-modelled workcells and realrobot installations. There are methodsof dealing with such errors, and onecompensation technique is to include acalibration probe mounted on the faceplate, of the robot which records theactual spatial arrangement of the work-cell at various points. The result is acomputer model which accounts forchanges during workcell installation, as

well as physical variations in the parti-cular robot.

A more fundamental problem con-cerns the need to develop standardisedprogramming interfaces. Many differentrobot languages and operation func-tions exist and so far attempts to stan-dardise on a specific robot languagehave not been successful.

Conclusions

CAD systems, which are becoming in-creasingly popular in manufacturing in-dustry, are being recognised asadvanced software tools which can beused for performing the subtasks in-volved in robot programming. If existingusers Of off-line programming systemsare provided with facilities for off-linetest and program evaluation — all pos-sible with computer graphics simulation— then robotics software will becomerecognised as an efficient tool, capableof reducing the workcell planning andprogramming phase, and of making off-line programming a more economicalfeature in manufacturing systems.

M. Dooner is Lecturer in CADCAM with the Faculty of Technology, The Open University,Walton Hall, Milton Keynes, Beds. MK7 6AA, England

220 Computer-Aided Engineering Journal December 1984