Computer Simulation And Modeling

Transcription

1 Computer Simulation And Modeling The key to increased productivity in Scientific and Engineering analysis Professor Ralph C. Huntsinger California State University, Chico USA Bialystok Technical University POLAND

2 Computer Modeling and Simulation The use of simulation is as old as the practice of engineering itself, what has been very useful is the use of the computer to assist one in the analysis of large complex engineering and scientific problems. Lately the advent of special high level computer software packages has made the use of the computer even easier and has increased the productivity of the scientists and engineers that use these tools. Many of these software tools were very specialized and applied to a narrow subject field. They are quite useful, however, it is very difficult to apply them to another similar area.

3 Computer Modeling and Simulation The latest development is the appearance of several general purpose high level software packages that are able to be applied to any problem of mathematical modeling that contains non-linear, non-homogeneous, simultaneous, high order, ordinary and parabolic partial, differential equations with associated algebraic equations and discontinuities. This is a real productivity booster as these packages come with graphical user interfaces (GUI s) and statistical and engineering analysis tools to form an intelligent simulation environment.

4 Computer Simulation and Modeling of Dynamic Systems Computer Simulation can be used to represent in great detail the performance of real world systems which are often very complex (e.g. space craft, control systems, mechanical and electrical systems, etc.) Simulation may be used as an aid to design, to diagnose the cause of system malfunction, to evaluate performance under fault conditions, for operator training, as embedded systems for predictive control, or to gain a deeper understanding of the dynamics of an existing or proposed dynamic system.

5 Computer Modeling and Simulation Intelligent Simulation Environment This special environment includes specialized computer languages that make modeling and simulation of complex dynamic systems a straight forward and easy task. The time to analyze a complex system is reduced many times by a factor of ten or more. This leaves the scientist and engineer more time to think about the real problem instead of spending most of the time on the details of the solution.

6 Computer Simulation and Modeling High-level continuous systems simulation languages are application-oriented software systems designed to assist Engineers and Scientist to mathematically model and analyze the behavior of piecewise-continuous systems described by differential equations. For the system analyst, these languages provide a straight forward, easy to use analytical tool for the simulation of dynamic systems. Powerful systems-oriented functions coupled with extensive man-machine inter-active capabilities allow the user to concentrate on his/her simulation studies with minimal burden from the computer system.

7 ESL The European Space Agency Simulation Language. One of the great features of ESL is its ability to handle discontinuities accurately. This was one of the European Space Agencies original specifications as when a thruster on a satellite is fired it must happen at the specified time and not at the nearest integration interval. Examples of the use of ESL are the simulation of a six legged robot combined with 3D animated graphics with a human controller in the loop and the entire system running on multiple platforms in real time, as a training simulator for a process control filtering system in a large water treatment plant, and of course for satellite orbit analysis.

8 ESL program sample study model rocket( real: max_ht := real: FUELo); -- Output max height, input initial fuel. real: height,drag,thrust,velocity,fuel,mass; constant real: G/9.81/,Mrk/300.0/,burn/20.0/; logical: power, done/false/; initial --Initial conditions. height:= 0.0; height':= 0.0; FUEL:= FUELo; max_ht:= 0.0;

9 ESL program sample dynamic --Rocket dynamics. velocity:= height'; -- Drag is proportional to velocity squared. drag:= 0.5 * velocity * abs(velocity); -- Mass of rocket and its current fuel. Mass:= Mrk + FUEL; power:= FUEL > 0.0; -- Thrust is constant until fuel exhausted. thrust:= if power then else 0.0;

10 ESL program sample -- Flight equation. height'':= (thrust - drag)/mass - G; -- Fuel Mass equation, fuel burn constant. FUEL':= if power then -burn else 0.0; -- Detect maximum height. when height' < 0.0 then done:= true; max_ht:= height; -- record the max. end_when;

11 ESL program sample -- Save results for later analysis by DISP. prepare "rocket",t,height,velocity,thrust, FUEL,drag,Mass; terminate done; --when rocket starts decent. communication Tabulate t,height,velocity; end rocket;

12 ESL program sample -- Experiment real: FUELo, max_ht; -- Set integration parameters algo:= rk5; cint:= 15.0; tfin:= 120.0; -- Do simulation for varying initial fuel. for FUELo:= step loop -- Call the model to do a simulation run. rocket(max_ht := FUELo); print "With fuel ",FUELo:6.1," kg", " height achieved was ",max_ht:-10.1," m."; end_loop; end_study

13 The European Space Agency Simulation language (ESL) The European Space Agency Simulation language (ESL) was originally written to meet the simulation requirements of the European Space Agency. It started out as a general purpose Continuous Systems Simulation Language (CSSL) and can still be used for that purpose. It has a comprehensive supporting software environment and it can be used in any field where dynamic systems are studied and their simulations need to be used.

14 Continuous Systems Simulation Language Version Four CSSL-IV is a user-oriented software system designed to support analytical simulation. Availability, ease of use, and powerful support tools allow CSSL-IV to be applied effectively to a broad range of technical problems. Application areas range from aircraft flight dynamics, nuclear reactors, chemical processes, and physiological dynamics to a wide variety of control systems. CSSL-IV provides access to many powerful programming tools which speed simulation development and verification. Powerful systems-oriented functions coupled with extensive man-machine inter-active capabilities allow the user to concentrate on his/her simulation studies with minimal burden from the computer system. I

15 ACSL ACSL (pronounced "axle") is an Advanced Continuous Simulation Language. It was introduced 25 years ago, as a commercially available, modeling and simulation language designed for simulating continuous systems. Based on the CSSL (Continuous System Simulation Language) standard, established by the Technical Committee of the Society for Modeling and Computer Simulation International, [SCS].

16 Advanced Continuous Simulation Language ACSL ACSL Sim was designed to help the engineer or scientist, mathematically model and analyze the behavior of a continuous system described by time-dependent, nonlinear differential equations and/or transfer functions. Although continuous systems by their nature are time-dependent, ACSL Sim lets you designate the independent variable as something other than time, such as distance or angle. This gives the user the flexibility to model a multitude of dynamic systems.

17 ACSL screen picture

18 Modelica Dymola Modelica is a bit of a paradigm shift from the previous generation of Modeling languages. The language standard returns to the original concept of the "equation" where arbitrary expressions can appear on both sides of the equals sign. (e.g. r*i = v) The Modelica syntax was designed to support symbolic manipulation of model equations in order to generate the set of assignment statements used during the simulation.

19 Modelica Dymola When speaking of Dymola model definitions, one uses the terms component (not block), connectors (not ports) and connections (not wires). This is to differentiate Dymola from the previous generation of graphical modeling systems which are block diagrams. Blocks are characterized by directed flow (wires with arrows) while components are characterized by connections (wires without arrows) defining the coupling of connectors of components Components contain either graphical models in a hierarchical model structure or Modelica code.

20 Modelica (Simulink) Diagram

21 Modelica Dymola Connectors declare the set of component variables which can be connected to other components. It is here that the concept of efforts and flows becomes important. The "flow" keyword prefix to a declaration means that all connected flows must sum to zero. (e.g. Kirchoff's current law). Examples of flow variables include forces, torques, electric current and hydraulic fluid. Without the "flow" prefix, a connector variable is presumed to be an effort. Connected efforts must be set equal to each other. Examples of efforts include angular rates, voltage and pressure.

22 Dymola 5

23 Modelica Dymola Symbolic manipulation can also solve or simplify differential algebraic equations (DAE) of any order at translation time, rather than solving such problems numerically when running the simulation. The performance savings can be significant for systems modeled in terms of DAE's (multibodymechanical systems, mechatronic systems, hydraulics, electronics)

24 Computer Simulation and Modeling CSSL s are a useful productivity increasing mechanism for the busy engineer and scientist. They allow one to work smarter not harder in the solution of complex dynamic systems. Instead of writing lots of computer code and debugging long involved programs, one can describe the system with one of these software packages in a short time and the detailed code is generated automatically. More time can be spent thinking about the real problem instead of working on the details of the numerical analysis and the computer coding.

25 Computer Simulation and Modeling of Dynamic Systems Solving scientific and engineering problems should be fun and not a tedious chore. Modern high level computer simulation environments make this a reality. However, always remember, Computer Simulation is a hopeful fake when the real thing is just too much Quote: John McLeod, P.E. Founder of the Society for Modeling and Simulation International