Robot Kinematics Development Environment:Application to a Configurable Redundant Manipulator

for Heavy Robotics ∗∗∗∗

DAVID PUIG1, MARTIN MELLADO1, JUAN V. CATRET1 & EMILIO RUIZ2

1 Department of Control Engineering and Systems (DISA)Polytechnic University of Valencia (UPV)

Camino de Vera s/n, E-46022, Valencia, SPAIN

2 Institute for Systems, Informatics and Safety (ISIS)EC-Joint Research Centre (JRC)

T.P. 210, I-21020 Ispra (VA), ITALY

Abstract: This paper presents a development environment for robot kinematics designed to support thedevelopment of a robotics software controller. The development environment called Virtual Robot Simulatorconsists mainly of a kinematics simulator with open software architecture designed to allow the implementationand validation of direct and inverse kinematics algorithms for any kind of mechanical structure. By means ofsoftware templates providing a fixed application interface, the user can implement new kinematics modules andverify their behavior on a 3D graphical computer simulation of the real workcell. Some examples are shown inthe paper for a configurable redundant robot designed as a multi-purpose heavy payload prototype manipulatorfor the maintenance of thermonuclear fusion reactors, laser welding and remote inspection of storage areas.

Key-Words: - Robotics, Robot Kinematics, Robot Control System Design, Robot Simulation, ComputerGraphics, Heavy Robotics, Redundant Robots

∗ This work has been partially funded by projects from JRC (13787-1998-03 F1EP ISP ES), Generalitat Valenciana (AE98-01), UPV

(PII-19990576) and FEDER-CICYT (Ref. 1FD97-2158-C04-0x TAP).

1 IntroductionCurrent industrial robotics controllers are closed inthe sense that they apply only for a given type ofmechanical system and for some industrialapplications, but rather limited for advancedapplications development and for research purposes.Most Robot manufacturers protect their owncontrollers preventing internal access or, in a fewcases, limiting it to the integration of sensorinformation in the motion generation process. Againstthis approach motivated for commercial reasons,research institutions are largely working with the goalof developing open robot control systems for easyexternal access. Some works, discussions andrecommendations on this line are [1], [2], [3], [4] , [5]and [6].

A significant example and one of the mostimportant improvements, is the case of GENERIS.During last seven years, the EC Joint Research Centre(JRC) of Ispra–Italy has been working on developingGENERIS, a Generalised Software Control System

for Industrial Robots within the frame of a EuropeanCommission project supported by the EC DGXIIICompetitive Support Activity program. The DGXIIIhas decided to finance the GENERIS projects inorder to provide the European market with a low costinnovative software product issued from the JRCIspra research experience on advanced roboticscontrol system architectures.

1.1 GENERISThe GENERIS software package is a real-time

system with a truly scalable, modular and openarchitecture to control virtually any type of industrialmanipulator [7], [8]. It possesses advanced control,calibration and tuning features being highlyconfigurable and completely independent from thehardware platform (CPU, bus system andinput/output boards). The control system, based onthe VxWorks real-time multitasking operating system(from Wind River Systems Inc.), can be easilyenriched with user-made components and library

functions using the GENERIS OEM DevelopmentToolkit. In addition, GENERIS has a multi-machinearchitecture that facilitates the manufacturing processcontrol of a real workshop containing several robots,auxiliary devices (as conveyors, rotating tables, etc.)and process sensor systems (seam tracking systems,force sensor systems, vision systems). Furthermore,GENERIS can also be easily integrated in an existingdistributed system managed by a factory supervisorthat remotely controls and monitors the roboticsdevices. JRC is continuously upgrading the productby integrating new technologies as field busses, auto-tuning systems, and new application processes.

GENERIS has been tested and implemented onseveral industrial robots dedicated to materialhandling and to welding processes, but also on tele-operated manipulators used for the maintenance ofthermonuclear fusion reactors. This case has been asignificant test for GENERIS validation, because ofthe special characteristics of the robot considered, ascommented on a later section.

In order to optimize the performance of thesystem, GENERIS is divided into the HumanMachine Interface (HMI) and the Numerical Control(NC) executing the time-critical processes on a targetsystem:• The HMI runs on a remote Host, typically a

Microsoft Windows 95/98/NT/2000 or UNIXstation using TCP/IP link with NC.

• The NC (Fig. 1) runs on the Target processorusing VxWorks and can be interfaced to one orseveral remote HMI Host stations forprogramming, debugging, controlling andmonitoring. The HMI applications can bedistributed among different host platforms as theuser wishes.

Actuators Servo-Control &

Management

Motion Planning and

Generation

I/O Interface I/O boards

Driver

Actuators Management

Control Loops

Micro- interpolation

Homing Procedure

Robot Kinematics

Orientation Control

Trajectory Library

Path Planning Library

Driver for field buses

Driver for intelligent axes control

boards

Driver for hand-box

Local monitoring

Custom Filtering Functions

Custom Processes

Figure 1. GENERIS NC Functional Model

The development of robot kinematics moduleshaded on Fig. 1 was subcontracted as a researchcollaboration project established between JRC andthe Tele-Robotics Research Line of DISA – UPV, inorder to provide:• A generic inverse kinematics resolution function

(in ANSI C code) for GENERIS in order to solveall the configurations of the redundant RIALTOmanipulator but also most of industrial robotsavailable on the market.

• A valuable testing platform for kinematics wherekinematics algorithms and methods could bedeveloped and validated.The result of this project went further obtaining a

3D opened graphical simulation system based onthese two goals, but also including features as off-line robot programming and simulation, and on-lineprogramming and real-time monitoring. The resultingsystem, called Virtual Robot Simulator (VRS), is atruly opened graphics system useful for SMEintegrators and manufacturers of robotics andautomated machinery, as well as for research andeducational purposes.

In this paper, the design of the VRS application isexplained starting from the general features of thesystem to the method used for the user’s kinematicsdevelopment and integration, with special attentionto the architectural design and giving some examplesof applications.

2 Virtual Robot Simulator (VRS)VRS is a C++ application developed on MS VisualC++ environment by the DISA-UPV Tele-RoboticsGroup, with a representation based on the OpenGLgraphic library under Windows operating systems(Fig. 2). VRS can be applied on multi-robot systemsfor their off-line programming and simulation as wellas for on-line programming and monitoring.

The easy and friendly user interface of VRSreduces notably learning time, even for new users forthis kind of applications, being adequate foreducational, research and industrial purposes. Forexample, DISA-UPV research on new virtual realitytrends, such as haptic systems, is based on VRS [9].Another field of research based on VRS is the 3D-mapping for robot-arms [10]. VRS has been alsosuccessfully used for robot off-line programming forrapid prototyping in industrial applications, thanks toits flexible architecture that allows data integrationfrom and to other applications. The VRS opensoftware architecture design is explained in the nextsection.

Figure 2. Graphic Interface of Virtual Robot Simulator

2.1 VRS Open ArchitectureThe VRS software architecture is based on four parts(Figure 3), the VRS Kernel, the user interface, a set ofinternal modules required as external access interfaceand a set of external components that the user canreplace with his/her own components according tosimple coding and compiling conventions.

The VRS Kernel has been designed through ahierarchical structure of classes (Figure 4). Thishierarchy has been implemented following theprinciple of inclusion, where an entity is modeled as aset of smaller parts. In this way, the simulator class isa compound of geometric models of the environmentand the robots. The environment represents a set ofobjects and parts while a robot is defined by anordered set of links. Finally, a link, object or part isdefined by sets of geometric primitives. Thesimulator class offers an interface to the view class,which implements the user interface and handles I/Odevices (disk, monitor, keyboard and mouse) in suchway that when the user’s action generates an event, amethod (or more) of the simulator class is called.

RobotKinematics

VR Kernel

VREALInterface

KinematicsInterface

GenerisInterface

View

VREALGraphicsLibrary

GenerisRemoteInterface

UserApplication

Mouse

Screen

Disk

Keyboard

Figure 3. VRS Open Software Architecture

Simulator

Robot

AdapterLink Tool

Environment

Object Part

Tool Status

Figure

Element

LinePoint VDA_Curve VDA_Surf…

Windows

Figure 4. VRS Kernel Classes

2.2 VRS External Access Library (VREAL)The most powerful tool to adapt VRS to the user’srequirements is the Virtual Robot External AccessLibrary (VREAL). VREAL is implemented as aDynamic Link Library in order to make possible theinteraction between user’s application and VRS.

VREAL provides an interface formed by a set ofalmost 50 functions (in addition to constant and typedefinitions) whose objective is to allow the user’sapplication to manage elements defined on VRS(robots and environment) and even create newelements on VRS, such as objects generated fromsensor data. The functions are organized in 5independent sets:• Functions to initialize and to close VREAL• Functions to manage files for robots and the

environment loading on VRS• Functions to manage loaded robots in VRS, such

as moving the robot in both Cartesian and Jointspaces, asking for robot status, selecting tool andtool center point (TCP), operating the tool andmuch more.

• Functions to handle an auxiliary list of figuresthat the user’s application can create and modifyon VRS.

• A function to handle some functionalities of theuser interface of VRS, mainly to configure theview parameters (zoom, point of view, referenceview point, …).With the help of VREAL, the graphical

representation is transparent to the user, who onlymust concentrate the effort on its own application.Hence, the user leaves the usually arduous graphicalpart to VRS which acts as a “true” virtual cell withrobots and environment, reflecting the actions ofuser’s application.

2.3 Robot Kinematics ModuleRobot Kinematics Module is a software modulecoded in C and composed of:• Constant and type definitions• A set of functions

The module can be integrated into three differentapplications, as shown in Fig. 3:• GENERIS NC• Virtual Robot Simulator• User’s application

Note that the first one runs on VxWorks while theother two under Windows O.S. This requirement hasforced that the module is implemented on two levels:• ikLib defines the export interface for Windows

environment.• ik is the ANSI C computation module

Actually robot kinematics module is the secondone, while the former is like a shell used to adapt theinterface to the development environment as forexample MS Visual C++. Usually the interfaceconversion is trivial.

An important requirement to the kinematicsmodule is that its code can be modified and compiledwithout modifying VRS or user’s application. Thisrequirement implies that its external interface, that isthe possible functions to be called and theirparameters, must be fixed. Eight functions wereconsidered enough to solve this problem, accordingto GENERIS NC constraints:• ikInitKinematics: this function can be used

to initialize the kinematics module for a specificrobot. The initialization parameters of this robotare defined on an ASCII file whose name ispassed as a parameter of the call

• ikSetConfig: this function sets the armconfiguration (framework) to use for inversekinematics resolution. Some parameters are rangecheck, solution selection or fixed orientation

• ikReadConfig: this function gives back theprevious framework

• ikSetTcpFrame: this function can be used todefine a Tool Center Point (TCP)

• ikKinematicsProblemDH: this function mustbe implemented to solve the kinematics problemaccording to Denavit-Hartenberg method

• ikKinematicsProblem: this function is used tosolve kinematics problem by any other method

• ikInverseKinematics: this function mustsolve the robot inverse kinematics according tothe current framework

• ikInverseKinematicsMixed: this functionmust solve the robot inverse kinematicsaccording to the current framework but only forthe robot-arm, keeping the wrist invariable

ik

ikLib

rcflikp

WindowsApplication

Figure 5. Robot Kinematics Module Structure

This module is multi-robot in the sense that thefirst function can be called as many times as wishedaccording to the number of robots required in theenvironment. As a consequence, the functions arecalled with a robot identifier that specifies therequired robot.

Two auxiliary internal modules are also includedon the Robot Kinematics Module, one for auxiliaryfunctions (likp) as transformation handling,orientation conversions (for example between Eulerangles, quaternions and rotation matrices) and othermathematical functions and another for data structuredefinition and file management (rcf). The overallstructure of the Robot Kinematics Design is shownon Fig. 5.

The specific resolution method implemented forthe kinematics of GENERIS is out of the scope ofthis paper, but the modules have been developed tobe re-entrant in the GENERIS real-time multi-robotenvironment.

3 Application ExamplesThe assessment platform called RIALTO is aCartesian gauntry heavy-payload robot, with 4degrees of freedom (XYZW) able to carry a payloadup to 6.5 tons over the whole workspace (2.2m x 3mx 6.5m) with an accuracy of 0.1 mm (Fig. 6). Theaxes velocity can range from 0.1 to 10 mm/sec. TheX and Y axes are driven by two ball circling screwsbut the vertical Z axis by four worm screws.Brushless motors/gearboxes are used to drive screws.Additional degrees of freedom are available throughspecific robots connected to the flange support, as the5 axes angular Ansaldo Power Manipulator (PM)robot-arm, or the 5 axes spherical robot-arm fromGAER (in project), giving in each case a redundantmechanical structure that can be defined in aconfigurable way.

Figure 6. RIALTO platform

To explain the possible configurations consideredfor RIALTO, the following notation has beendefined:• The prismatic axes of the Cartesian robot are

called X, Y, and the vertical axis Z.• The roll axis of the Cartesian robot is called W.• The Ansaldo PM axes are W (base), R (shoulder),

A (elbow), B (pitch), C (roll).• The Gaer Manipulator axes are W (waist), R

(shoulder), T (arm extension), A (twist), B (yaw)Note that when Ansaldo PM o GAER

Manipulator are attached to Rialto, only one Wworks, giving 8 proper axes.

Obviously, inverse kinematics for such redundantrobot presents lots of known problems, mainly forrobustness resolution, solution selection andconfiguration continuity during Cartesian trajectoryexecution. To overcome these problems, two mainapplications were analyzed and simpler robotconfigurations were established, making a distinctionbetween axis to be consider as external or auxiliaryaxes (not relevant for inverse kinematics) and axes tobe considered for inverse kinematics. In this way, atmost 6 axes are considered at a time by the inversekinematics whilst the remaining axes are maintainedat a nominal position or moved with or without run-time modification of the geometrical model used bythe inverse kinematics.

3.1 Manipulation ApplicationAn application for material handling and remoteinspection of storage areas of nuclear material wasdefined using the Ansaldo PM robot attached toRIALTO.

Figure 7. RIALTO platform with Ansaldo PM attached

Two special configurations were taken into account:• RA1 considers XYZ of Rialto as external axis and

considers the 5 axes of Ansaldo PM for IK• RA2 considers RA axes of Ansaldo PM as

external axes and XYZ of Rialto and BC ofAnsaldo PM for IKOn Fig. 7 the robot model for this application is

shown as modeled on VRS for inverse kinematicsverification. Please note the similarities with Fig. 6.

3.2 Laser Welding ApplicationA laser welding application was analyzed using theGAER Manipulator together with RIALTO.

Four special configurations were taken intoaccount for Rialto:• RG1 considers XYZ of Rialto as external axis and

considers the 5 axis of GAER (WRTAB) for IK• RG2 considers XZ of Rialto as external axis and

considers Y and the 5 axis of GAER (WRTAB)for IK

• RG3 considers YZ of Rialto as external axis andconsiders X and the 5 axis of GAER (WRTAB)for IK

• RG4 considers Z of Rialto and RT of GAER asexternal axis and considers XY of Rialto and ABof GAER for IKOn Fig. 8 the robot model for this application is

shown as modeled on VRS for inverse kinematicsverification.

3.3 Industrial RobotsThe most extended types of uncoupled industrialrobots were also considered in the generic inversekinematics module, as any combination of:• Robot-arm types: Anthropomorphic, Spherical,

Cartesian, Cylindrical, Scara, Pendular and anyone with three or less axes.

Figure 8. RIALTO platform with GAER attached

• Wrist types: in-line, roll-pitch-yaw, triple rollwrist, triple displaced turn, bend-swing-twist,offset wrist, pitch-roll and roll.Industrial robots for all the commercially

available combinations were modeled on VRS andthe inverse kinematics verified.

4 ConclusionThe needs to develop and evaluate the robotkinematics software module for GENERIS NC robotcontrol system has led to the development of amultipurpose application called Virtual RobotSimulator. In this paper its use as a generaldevelopment and testing system for robot kinematicsmethods has been presented, detailing the softwarearchitecture design and user’s possibilities to take thebest of such a design. The application to the complexcase of a configurable redundant robot manipulatordesigned as a prototype system for the maintenanceof thermonuclear fusion reactors, laser welding oflarge components and remote inspection of nuclearstorage areas has served to demonstrate the featuresof the system.

References:[1] Open System Architecture for Controls within

Automation Systems, EP 6379 & EP 9115,OSACA I & II Final Report, 1996

[2] W. Sperling & P. Lutz, Designing Applicationsfor an OSACA Control, International MechanicalEngineering Congress and Exposition, (TheASME Winter Annual Meeting), 1997

[3] R. Alami, S. Fleury, M. Herrb, F. Ingrand, F.Robert, Multi Robot Cooperation in the MarthaProject, IEEE Robotics and Automation MagazineSpecial Issue on `Robotics & Automation in theEuropean Union', Vol. 5, N° 1, March 1998.

[4] B. Kneifel, Open Architecture: Where should wespend our effort?, Proceedings of IRA-NIST Near-Term Objectives Workshop on Open ArchitectureControl for Robotics, 2000

[5] G. Rutledge, Open Architecture Interfaces toRobot Controllers, Proceedings of IRA-NISTNear-Term Objectives Workshop on OpenArchitecture Control for Robotics, 2000

[6] G. Rutledge, Open Architecture Interfaces toRobot Controllers, Proceedings of IRA-NISTNear-Term Objectives Workshop on OpenArchitecture Control for Robotics, 2000

[7] E. Ruiz, M. Ricci, A. Dosio, A GeneralisedSoftware Control System for Industrial Robots, 5th

IEEE International Workshop on AdvancedMotion Control, Coimbra University, June, 1998,pp.411-16.

[8] E. Ruiz. GENERIS: The EC- JRC GeneralisedSoftware Control System for Industrial Robots.International Journal of Industrial Robot, Vol.26,No.1, 1999.

[9] R. Zotovic, M. Mellado, V. Jornet, J.V. Catret &D. Puig. Diseño, Implementación y Control de unSistema Háptico con Realimentación Sensorial enTele-Robótica (in Spanish). 2º CongresoInternacional Interacción Persona-Ordenador(Interacción’2001), to be published, Salamanca(2001)

[10] J.V. Catret, M. Mellado & D. Puig, OpenGraphics System for 3D-Mapping Validation onRobotics, 8th IEEE International Conference onElectronics, Circuits and Systems(ICECS’2001)¸to be published, 2001.