ROBOSIM, a simulator for robotic systems, was developed by NASA to aid in the rapid prototyping of automation. ROBOSIM has allowed the development of improved robotic systems concepts for both earth-based and proposed on-orbit applications while significantly reducing development costs. In a cooperative effort with an area university, ROBOSIM was further developed for use in the classroom as a safe and cost-effective way of allowing students to study robotic systems. Students have used ROBOSIM to study existing robotic systems and systems which they have designed in the classroom. Since an advanced simulator/trainer of this type is beneficial not only to NASA projects and programs but industry and academia as well, NASA is in the process of developing this technology for wider public use. An update on the simulators's new application areas, the improvements made to the simulator's design, and current efforts to ensure the timely transfer of this technology are presented

Cook, George E.

Fernandez, Ken

Hinman, Elaine M.

English

NASA Technical Reports Server

N91-23059
ROBOSIM, A SIMUlaTOR FOR ROBOTIC SYSTEMS
Elaine M. Hlnman and Ken Fernandez, Ph.D.
Marshall Space Flight Center
George E. Cook, Ph.D.
Vanderbflt University
Nashville, Tennessee
ROBOSIM, a simulator for robotic systems, was developed by NASA to aid in the rapid prototyping of
automation. ROBOSIM has allowed the development of improved robotic systems concepts for both earth-
based and proposed on-orbit applications while significantly reducing development costs. In a cooperative
effort with an area university, ROBOSIM has been farther developed for use in the classroom as a safe and
cost-effective way of allowing students to study robotic systems. Students have used ROBOSIM to study
existing robotic systems and systems which they have de_4gned in the classroom.
Since an advanced simulator/trainer of this type is beneficial to not only NASA projects and programs but
industry and academia as well, NASA in the process of developing this technology for wider public use. In
this paper we give an update on the simulator's new application areas, the improvements made to the
simulator's design and current efforts to ensure the timely transfer of this technology.
INTRODUCTION
ROBOSIM is a computer graphic robotic simulator developed by NASA-MSFC to aid in rapid
prototyping of automation concepts. Cooperative development of a commercial and classroom version of
ROBOSIM has been carried out by MSFC and Vanderbilt. In addition, ROBOSIM has boon used in
graduate courses at Vanderbilt as a trainer in robotic design.
Several applications of ROBOSIM will be described in this paper. While these are not the only current
uses, they do show the usefulness of ROBOSIM in diverse fields -- from spaceflight to manufacturing to
surgical aid development.
In the course of making ROBOSIM accessible to the larger robotics simulation and development
community, several ports have been made to other workstations. A couple of these are described in more
detail below.
DEVELOPMENT
ROBOSIM was originally developed in a VAX environment and provided graphical output to
TEKTRONIX 4014 terminals and Evans & Sutherland graphics terminals [3]. Ports to the GTI Poly 2000,
HP350SRX workstation, InterPro 360, and Silicon Graphics IRIS machine also exist [1, 2]. The TEKTRONIX
terminal provides a low-cost and quick way to see the robot and workcell during the creation process, while
the other systems allow real-time or near real-time viewing of robot motions.
To use the original ROBOSIM, the user created shapes and interconnecting joints using the ROBOSIM
programming language, once a robot was modelled, motions could be generated either by user supplied
routines or by using the default control algorithms [4]. The ROBOSIM program structure is shown in Figure
1.
309
https://ntrs.nasa.gov/search.jsp?R=19910013746 2020-03-19T17:39:38+00:00Z
fIncludes ROBOSIM language, solid
models, joint specifications, link
definitions, path control, and
view control
Includes graph generation,
screen control, mass properties,
Jacobian components, and a
numerical method
operation, matrix
operation, low-level
input/output,
ROBOSIM hardware
emulation, error
diagnostics
FIGURE 1. Tri-level structure of ROBOSIM
310
PORTS
The advent of high speed graphics workstations provided the impetus to port ROBOSIM to such a
workstation [6]. However, to remain compatible with the VAX version of ROBOSIM, the VAX version has
been kept as the kernel, with various features added to the front and back ends.
The TEKTRONIX 4014 terminal and Evans & Sutherland PS330 graphics system only allowed wireframe
representations of the robot and workcell. The second implementation of ROBOSIM was on an HP350SRX
graphics workstation. This allowed solid graphics, faster screen access, and facilitated the development of the
R2 graphical interface discussed in the next section. The Starbase graphics library and X-Windows
capabilities available on this workstation were used in the implementation of pull down menus and shaded
graphics in the new ROBOSIM user interface.
A parallel development has been to integrate ROBOSIM onto the Intergraph InterPro 360 graphics
workstation [2]. The Intergraph windowing environment, Environ V, was used in interfacing ROBOSIM with
the InterPro workstation. Again, the foundation of ROBOSIM remained the same, however, the
ROBOSIM-created graphical libraries were generated using Intergraph's graphic capabilities, and displayed in
an Environ V window.
GRAPHICAL EDITOR
Once ROBOSIM was ported to the HP workstation, the 3D graphical editor, R2, was developed to
provide an easier and more flexible interface to ROBOSIM [3]. The ROBOSIM kernel handles the
rudimentary simulation tasks including the generation of primitive solids and rotation and translation
operations performed on those objects. When modelling using the ROBOSIM programming language, the
user was required to keep track of each object's dimensions, position and orientation with respect to other
objects it may be connected to. In R2, menus and a mouse are used to generate the link primitives and to
define the interconnecting joints. Since R2 is a graphical editor, the user may quickly and easily see and
modify the robot workcell as it is being constructed. However, complete compatibility with ROBOSIM is
maintained by having R2 generate ROBOSIM code. This way the capabilities of the original ROBOSIM are
maintained while the enhancements of R2 are provided. Now, the user can design robots in the ROBOSIM
code, by using R2, or through the use of custom programs.
SIMULATION LIBRARY
The following description of the ROBOSIM simulation library is from the paper by Springfield, Cook,
Anderson, and Fernandez [3]. The library was first implemented on the HP-9000 workstation equipped with
an SRX graphics accelerator.
The simulation library and environment provides methods to access the data structures created by
ROBOSIM. The robots and other objects are specified and loaded into memory. These structures remain
resident in memory while the simulation is running. The library provides an interface to these structures so
that the user does not have to understand what is happening at that level. The library provides higher level
facilities much like an actual robot programming language.
The simulation package allows one to use the robots that have been designed. The package consists of a
library of C functions that operate on the f'des created by ROBOSIM. Although this package is far from
complete, it allows simple simulations to be run. It also provides a framework in which to test the major
components for the simulator: collision detection and dynamics. Having the simulator as a library of C
routines allows more flexible methods for running simulations. Very specific and efficient simulations can be
written in C which call the simulation functions directly. However, even at this level, much of the internal
data structure is hidden from the user. This level of programming roughly corresponds to programming a
robot in its programming language. For instance, one can tell a robot to move along a straight line or move a
particular joint. Using these same routines, a very flexible, user-friendly interface can be built up, allowing an
311
interactive way to do simulations that arc not too complicated, or that do not require great speed.
ROBOSIM provides most of the information required by the simulator through the files it creates.
However, some information is not directly provided, but it can be determined from what is there. This
involves the information required by the collision detection algorithm. ROBOSIM provides the
Denavit-I-Iartenberg parameters , the A-matrix, the pseudo-inertia matrix, and a list of points which describe
the physical structure of the robot. The internal data structure also includes areas that are not currently used,
but will be at a later time. These include minimum and maximum joint angles, velocities, and aeceleratious.
The structure also includes information related to the graphics display. The simulation package acts as
intermediary between the user and the internal representation.
The simulation program that the user writes can turn on collision detection, request solutions to inverse
kinematics problems, and display results graphically. The user can use the general numerical Jacobian method
for inverse kinematics or provide an exact solution for the robot. The user simply passes the address of the
function to the simulator, and the simulator will then use that function when solving inverse kinematics for that
robot. A proposed extension to the simulator will allow the recognition of the possible robot configurations
for which exact solutions exist. The exact solutions to these configurations would then be used instead of a
numerical method, freeing the user from having to solve and code it himself. Several uses of the simulations
system can be found in later sections.
Collision detection is very important in simulation of robots, one usually wants to know if the robot has
collided with its environment or with itself. The collision detection algorithm implemented here is very similar
to the POCODA (Polygon Collision Detection Algorithm) algorithm given in Scott E. Waiter's dissertation
from Cornell [7]. The collision detection algorithm has only two weak points. It does not handle concave
polygons, and it will not signal a collision if one object is completely inside of another. The stipulation
concerning concave polygons is not serious. ROBOSIM does not generate concave polygons unless they are
the result of a custom object. Although R2 does not check for concave polygons, this feature could be
implemented. In fact, algorithms exist to split concave polygons into convex polygons. Either of these
features could be implemented fairly simply. The problem of not detecting a collision if one object is
completely inside of another derives from the fact that the algorithm used is a polygonal collision detection
algorithm and not a solid object one. However, assuming two objects start off outside of each other and
movements are sufficiently small, then this should not prove to be a problem. This condition also prevents the
ability of one object to pass through another (i.e. a movement is large enough that two objects do not overlap
at any point). This algorithm does not detect collisions in the volume swept by an object moving between
positions with another object, but rather only overlap of the objects at the starting and ending positions. But,
if the distance between the positions is smaller than the smallest object, then there should be no problems.
APPLICATIONS
ROBOSIM has been used in several different applications. As a rapid prototyping robotic simulation
system it is especially applicable when designing custom robotics for specific tasks. However, standard
industry robots can also be modelled. Control algorithm development, positioning and task viewing studies,
and workcell configuration are tasks for which ROBOSIM has been used.
CLASSROOM
ROBOSIM is used in an introductory graduate-level robotics classat Vanderbilt University. It is useful
for aiding the students in visualizing transformations and perspective operations applied to items they have
designed themselves. They can build objects and link them into kinematic chains. Once a robot is designed,
the inverse kinematics can be solved and various control algorithms tested in graphics. Using graphic
simulation, students have access to robotics development tools without the expense of providing robotics
hardware for a classroom/laboratory environment.
312
SURGICAL POSITIONER DEVELOPMENT
ROBOSIM has been used by Dr. Bob Galloway and Allison Balogh of Vanderbilt to aid in development
of a surgical positioner for use in brain surgery [3]. The positioner would be used to determine points of
entry into the brain. Currently this is done by precomputing the location of the points with respect to an
external reference. Greater flexibility would be provided by having a way to immediately see these positions.
The current research is to see if a robot of sufficient accuracy can be built. ROBOSIM has been used to
design the positioning arm and study its work envelope. The robot needs to be able to reach all positions on
the head, while avoiding collisions with it. Currently, different positioner arm configurations are being
studied.
WELDING
ROBOSIM has been used in a couple of welding simulation applications, one was in the development of a
control algorithm for robotic welding of a Space Station airlock [2]. This application was developed on the
InterPro workstation. A wireframe representation from this application is shown in Figure 2.
FIGURE 2. Airlock and Robot during weld.
313
Another welding application was in the testing and simulation of a downhand welding algorithm [5,8]. In
this case a six degree-of-freedom robot performs the welding while a two degree-of-freedom positioner holds
the workpiece. One implementation of this algorithm used a 2 degree-of-freedom table as the positioner,
while another implementation used a PUMA 560 with 4 locked degrees of freedom as the positioner. In both
cases, the object is to keep the weld feed in the downhand position to avoid pooling of the field. Based on
the status of objects in the workceU, path information is generated and simulated by ROBOSIM in order to
perform the weld. Joint constraints and collision detection are included in the path generation. Once a path
is found, it may be sent to the actual robots and executed.
CONCLUSIONS
The previous examples point out a few of the areas in which the use of ROBOSIM is beneficial. Although
ROBOSIM was originally developed at NASA for use in space robotics, it is useful to the larger robotics
community. The commercial version currently under development will include the enhancements made in the
second and later implementations of ROBOSIM. In this version, X-Windows may be used by multiple users
to develop their applications. Although the processing speed of a workstation will be required for real-time
and near-real-time simulation runs, workcell development can be carried out at a terminal through the
common X-Windows interface.
In closing, ROBOSIM is a useful tool for robotics simulation in academia, government and industry.
ACKNOWLEDGEMENTS
The authors would like to express their appreciation to the Technology Utilization Office at MSFC, and in
particular to Mr. Ismail Akbay for his support of this work.
REFERENCES
[1] Fernandez, K., R., and E. M. Hinman, "The Use of Computer Graphic Simulation in the
Development of On-Orbit Tele-Robotic Systems," Proceedings of SPIE--The International Society for
Optical Engineering, Advances in Intelligent Robotics Systems, 1986.
[2] Wilson, S., L., Interfacing of a Robot Simulation Program with Graphic Utilities of an Intergraph
Interpro 360 System, Master's Thesis, Vanderbilt University, 1990.
[31 Springfield, J., G. E. Cook, K. Andersen, and K. R. Fernandez, "ROBOSIM: A Simulation Package
for Robots," University Programs in Computer-Aided Engineering, Design, and Manufacturing. ASCE
Seventh Annual Conference, July 23-26, 1989.
[4] Fernandez, K., R., "ROBOSIM User's Guide," Internal Publication of the Electrical Engineering
Department, Vanderbilt University, Nashville, TN, July 1986.
[5] Fernandez, K., R., Robotic Simulation and a Method for Jacobian Control of a Redundant
Mechanism with Imbedded Constraints, Ph.D. Dissertation, Vanderbilt university, 1988.
[6] Springfield, J., ROBOSIM Workstation Extensions, Master's Thesis, Vanderbilt University, 1988.
[7] Walter, Scott Edward, Polygonal Collision Algorithm, Ph.D. Dissertation, Cornell University, 1985.
I8l Fernandez, K., R., and G. E. Cook, "Computer Graphic Simulation of an Algorithm for Controlling
Downhand Position in Robotic Welding," SME paper MS86-209, Presented at the SME Conference
on Robotic Solutions in Aerospace Manufacturing, March 3-5, 1986.
314


ROBOSIM, a simulator for robotic systems

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server