Application of bond graph methodology to concurrent ... - IEEE Xplore

Application of Bond Graph Methodology to Concurrent Conceptual Design of Interdisciplinary Systems J E E Sharpe and R H Bracewell Engineering Design Centre Lancaster University Lancaster LA14YR UNITED KINGDOM

Abstract Exploitation of the bond graph methodology's unified

object and rule based computer aided conceptual design tools.

modelling approach over many energy domains has enabled a comprehensive, generic, computer-aided conceptual design tool

These methods, rules and object structures may work on many different knowledge planes within a design in addition to the normal direct functional plane. Bond Graphs have shown themselves capable of handling many different situations including aspects of economic analysis, tribology and manufacturing systems, all of which naturally impinge on an optimal design. In an earlier paper we have established the notion of the 'Causal Lattice' which provides coupling between these different functional planes [l].

(Schemebuilder) to be developed. This paper describes how bond graph fragments representing individual components are assembled into complete system models from which steady state performance and dynamic simulations may be computed. It is concluded that the bond graph methodology provides an excellent basis for the concurrent conceptual design of intcrdisciplinary products through to the generation of a "rapid prototype" I. INTRODUCTION.

In our work developing computer aided conceptual design tools, we are very mindful of the need to embrace the nature of the 'Causal Lattice' so as not only to structure the design of the primary functions but also include the many other aspects of design on a consistent basis.

The increasing occurrence of multidisciplinary product development has removed many of the traditional constraints to design and has given the designer a bewildering freedom of choice as to the best solution to any particular problem. Whilst methods of analysis have largely kept pace with this development, very little has been done to support the designer in the creation or synthesis of these systems. As products become more complex and highly integrated, design teams will find it increasingly necessary

This paper uses the development of a commercial mechatronic product as an example to demonstrate how the inherent causal structure of Bond Graphs may be applied to the concurrent conceptual design of interdisciplinary systems.

to have a common language, independent of traditional engineering disciplines, in which to communicate. This should allow resolution of conflicts and ensure common standards of design quality assurance.

11. COMPUTER AIDED TOOLS FOR CONCEPTUAL DESIGN

Schemebuilder has been conceived as a major design aid covering the design process from initial concept to the complete detail design. One of its essential features is the ability to automatically generate mathematical models of the many proposed schemes for the solution of the design

For energetic systems the use of Bond Graph methodology with its common structure and clear causal rules across cngincering domains provides a very natural approach to design and allows the development of an integrated suite of

7

I SpecificationI Function Model I System Design I System Parameter Design I Simulation I 3D Modelling I Prototype I

- 1

Process of Conceptual Design

( *

Fig. 1. The Conceptual Design Procedure

problem. Thus the alternative means of achieving particular functions may be assessed before any final choice is made.

best alternative design and must be included in a full simulation.

This process of conceptual design is shown diagramatically in Fig 1, whilst the overall structure of Schemebuilder is shown in the diagram in Fig 2. This includes not only the direct support to the Conceptual Design process but also the supporting design data and expert advice that is integrated into the system. Many of the advice and data structures are built around notions of the importance of the energy covariables and their product or integrals.

The development of a Function Means Tree for a portable intravenous drug infuser is shown in Fig 3.

The designer using Schemebuilder need not be aware of the underlying bond graphs, operating instead at the word graph level in pursuit of a satisfactory conceptual design. However, the appropriate model representation may be accessed at any time, either as a complete bond graph or as an appropriate component fragment. 111. DEVELOPMENT OF ALTERNATIVE FUNCTIONAL SCHEMES

Once the top-level working principles of the system to be designed have been established together with a minimum set of functional specifications relating to input and output variables, it is possible to establish the alternative energy domains in which the desired functions can be achieved. Applying stored, rule-based conceptual design principles, it is possible to develop a tree structure of required functions and alternative solutions, known as a Function Means Tree [ 2 ] . The development of the tree is initially at the level of the required abstract functions but subsequently embraces the physical embodiment of those desired functions. The embodiment naturally introduces often undesired secondary functions, for example unwanted inertia, stiffness or energy dissipation. It may also introduce problems of manufacture and availability, which must be considered in choosing the

As the Function Means Tree is developed, the bond graph fragments relating to pure functions and physical embodiments are automatically a5sembled into alternative bond graph models which may be used to generate automatically the appropriate sets of differential and algebraic equations for use in simulation or subsequent parameter optimisation. IV. BOND GRAPH FRAGMENTS

The bond graph structures of the working set of schemes for the current design are stored in an associative database along with all other relevant attributes of the chosen devices. For a typical class of component, several alternative bond graph fragments of successively increasing complexity may be held in the model library. The port-based component representation allows this fragment library to be stored offline and the appropriate one retrieved whenever the designer wishes to change the level of detail of a particular device model. Fig 4 shows typical fragments held for a simple permanent magnet DC servo motor. If required the model could be developed to the levcl of inclusion of brush behaviour and noise paths. It is of utmost importance that the basic overall causal structure of different levels of model should remain the same. The actual behaviour of the physical system does not change just because we have chosen whether or not to model a particular aspect. Level 0 models simply introduce a default word graph element with the appropriate energy and signal ports. The 1st level provides the minimum functionality to provide the correct causal structure and is used for steady state

-

Man Machine Interface

1

‘sche”

MetaCard

Design Process. Contiit G a X l MWIlS.

KnowledgeEngineering Core

~

Principlesof Layout

Component. and Function. Database. FunctionData. FunctionCosts. Spatial Data. Advice New Compn’t Data Bond Graph Fragm’t Yourdon Fragm’t

3 D Sdid Modeller

StructurdHousing.

Detail Design

New Component Details

Manufactwe

Simulation Engine. Yourdon Dag Bond Graph Model. Causality Assignment 1st Ord Diff. Equations. Steady State Operation. EigenAnalysis. “e Response Sim. system Performance. Life Cyde Calculations

I

I I

Physical Prototype Computer Simulation.

of Product Behaviour Prototype Behaviour Comparative Study

I

Fig. 2. An overview of the Conipulzr Aided Conceptual Design Tools

calculations and the initial sizing of the fundamental

generation and wear rates in a safety critical gearbox,

parameters of a system design. Full concurrent understanding of complex interactions, for ex‘ample the effects of manufacturing errors on gear vihration, noise

requires the development of the interdisciplinary causal lattice requiring complex models of perhaps level 3 or 4[1].

9

Fig. 3. Function Means Tree for Ineavenous Infuser

Figure 5 shows the developed Bond Graph of one of the alternative systems created by the process of the Function Means Tree.

the early stages of prototype development. It is very straightforward using the Component database to rapidly try many alternatives.

Assistance with the detailed choice of components and the required level of model is provided in the form of contextsensitive hypertext design data and intelligent knowledgebased prompts and checks.

VI. KNOWLEDGE ENGINEERING ANI) A t ITOMATIC TRIITI1

V. SIMULATION OF CHOSEN DESIGN

The state differential equations are automatically generated by the application of rules which follow the essential causal structure of the chosen system that is to be simulated. The results of the simulation derived from the bond graph of the preferred embodiment of the infuser, showed that the design as embodied with appropriate functional parameters, was viable and was capable of meeting the desired performance specification. Results of the simulation are shown in Fig 6. This provides the mechanism for replacing

MAIKIENANCE

The Schemebuilder integrated set of Computer Aided Conceptual Design Tools is written in KEE, a Lispbased software development environment combining objectoriented programming with rule and frame based expert system support and automatic assumption-based truth maintenance (A'I'MS). Bond graph entities are represented by KEE units (frames), with connectivity and causality information stored in appropriately named slots. This allows incremental, rulebased causality propagation during wembly of the bond graph, whilst the ATMS allows infcrrcd information to be retracted if the user decides to break a connection, or change a component.

components of any scheme. In Schemebuilder therefore, not only are the functional attributes, parameters and bond graph fragments held for each component but also spatial information. This enables the components of any chosen scheme to be assembled in a 3D solid modeller, as shown in Fig 7. This figure shows the physical form of the intravenous infuser, with the operational Components aligned and assembled. The use of functional axes based on the defined energy covariables of the bond graph fragments of each component greatly assists in this whilst the forces and torques from the simulation also automatically relate to the solid model.

Level 1 Level 2

Level 3 Fig. 4. Bond Graph Fragments for Permanent Magnet DC Motor

Further rule chaining consolidates a completed bond graph into its simplest possible form and provides qualitative reasoning about the expected system behaviour. A particular advantage of the rule based approach is the ease of providing the inexperienced user with explanations as to what operations are being performed and why

VIII. CONCLUSION

The development of the integrated set of conceptual design tools has demonstrated that the essential causal structure of bond graphs have an importance well beyond that of simulation. They contain the essential understanding of physical systems and may therefore be used not only as the basis of analysis but also for the synthesis of complex interdisciplinary systems in a truly concurrent manner.

VII. PHYSICAL STRUCTURAL.LAYOUT

An important part of any design process is the ability to lay out and correctly arrange the operational functional

-. .........I.

.:......... .:."...-^..,...... ............-... +. .

'"'y""

Q

Epicyclic

h d Screw 1

I

i

SE Fig. 5 . Block Diagram of Intravenous Infuser and Generated Bond Graph

11

components of any scheme. In Schemebuilder therefore, not only are the functional attributes, parameters and bond graph fragments held for each component but also spatial information. This enables the components of any chosen scheme to be assembled in a 3D solid modeller, as shown in Fig 7. This figure shows the physical form of the intravenous infuser, with the operational components aligned and assembled. The use of functional axes based on the defi9ed energy covariables of the bond graph fragments of each component greatly assists in this whilst the forces and torques from the simulation also automatically relate to the solid model.

Fig. 4. Bond Graph Fragments for Permanent Magnet I X Motor

Further rule chaining consolidates a completed bond graph into its simplest possible form and provides qualitative reasoning about the expected system behaviour. A particular advantage of the rule based approach is the ease of providing the inexperienced user with explanations as to what operations are being performed and why.

VIII. CONCLUSION

The development of the integrated set of conceptual design tools has demonstrated that the essential causal structure of bond graphs have an importance well beyond that of simulation. They contain the essential understanding of physical systems and may therefore be used not only as the basis of analysis but also for the synthesis of complex interdisciplinary systems in a truly concurrentmanner.

VII. PHYSICAL STRUCTURALLAYOUT

An important part of any design process is the ability to lay out and correctly arrange the operational functional

..-....... -,..-...-,....,.*......................-.......: i,,,,. : . 2

'".Y"''

. i j2:

Battery 1

Lead Screw 1

Epicyclic

"-Dd

ri------.--

.....,_............ 8

L

1""

..-

4 Syringe 1

A

R

i'

I

SE Fig S Block Diagram of Intravenous Infuser and Generated Bond Graph

12

ACKNOWLEDGEMENTS Response of Flow Rate into Patient to Blood Clot 1

We would like to acknowledge the help of our colleagues in the EDC and the support given by the Science and Engineering Research Council in preparing this paper. REFERENCES

[l] J.E.E. Sharpe and E. M. Goodwin, "The Application of Bond Graphs in Complex Concurrent Multi-Disciplinary Engineering Design," Proc. Int. Con5 on Bond Graph Modelling ICBGM'93, 1993. La Jolla, California, pp. 2328.

Blood Clot Formation

-0.2

-0.4 0

10

20

30

40

50

60

70

80

Time from 0 to 1Ml Seconds

Fig. 6. Simulated Behaviour of Intravenous Infuser

90

100

[2] J. Buur, A Theoretical Approach to Mechatronics Design, PhD Thesis, 1990, Institute for Engineering Design, Technical University of Denmark.

Fig. 7. 3D Solid Model of Intlavenous Infuser