Roy E. Crosbie

Multi-Rate Simulation Techniques for Electric Ship Design

The power and propulsion system for an electric ship is a complex combination of electrical, mechanical, thermal and fluid dynamic components. Effective analysis of its behavior requires detailed computer simulations. Simulations of the complete system can be computationally very demanding, and execution times can be particularly problematical when multi-run optimization studies or real-time simulations are required. One approach to reducing the computational load in these simulations is to partition the system into a number of subsystems with different dynamic ranges. This allows different parts of the system to be solved with different time steps, thus avoiding the use of a small time step that is dictated by the fastest components, to simulate slow components that could be solved satisfactorily with longer time steps. The result is a simulation with many fewer individual calculations and therefore faster execution. This approach is referred to as multi-rate simulation. Although it offers more efficient execution, it also raises additional questions regarding the accuracy and stability of the simulation. Multi-rate simulation has been investigated using the example of an unmanned underwater vehicle (UUV) in a joint study by California State University, Chico, the University of South Carolina, and the University of Glasgow. The simulation language ESL promises to be a valuable aid in evaluating multi-rate techniques.

A model curriculum in modeling and simulation: do we need it? Can we do it?

An international debate on the need for a model curriculum for graduate programs in modeling and simulation (M&S) continues to grow. As the use of M&S continues to expand to new application areas, and its importance as a key enabling technology in the 21/sup st/ century continues to be recognized, many questions are being asked by both universities and corporations concerning the proper basis and content for advanced studies in M&S. Corporations and government bodies are experiencing rising demands for new recruits with broad exposure to the concepts and methodologies of M&S and who are capable of contributing to the increasingly important M&S activities within the organization. Many recruiters are, however, frustrated in their efforts to define productive sources in US universities that meet these needs.

Low-cost, high-speed, real-time simulation for electric ship power systems

Real-time simulation of power electronic systems for electric ships is an essential technique for testing hardware and software in the loop. An important parameter of these simulations is the frame-time necessary to capture the dynamics of the system being simulated. Modern power electronic systems, using higher frequency pulse-width modulation (PWM) converter control, demand frame times that are significantly shorter than those found in most real-time simulators. Simulations have been developed for 6-pulse and 12-pulse, 3-phase voltage-sourced converters connected back to back and to ac systems at either end. The hardware-in-the-loop capabilities of the simulator have been tested by interfacing the simulation platform, installed on one computer, to simulated hardware on separate computers.

ESL - A new Continuous System Simulation Language

ESL is a new Continuous System Simulation Language (CSSL) which is being developed under contract from the European Space Agency. It is based on the concepts outlined in Refer ence 6. The main features of the new language are: (1) Models can be built from submodels (2) Separation of model and experiment (3) Advanced discontinuity-handling (4) A parallel segment feature. The implementation of ESL requires both an interpreter and a translator version of the language. The interpreter translates the users program into an intermediate code (H-code) which is then interpreted at run-time. The translator may be used to convert the H-code to FORTRAN-77 to produce a more efficient ex ecutable program of production runs. The entire system (with the exception of a few low-level routines) is being written in FORTRAN-77. A prototype version of the language was installed in August 1983 and is currently undergoing evaluations. Details of this version are given with examples of its use.

Developments and Applications of Multi-rate Simulation

Multi-rate simulation techniques offer advantages to the computer simulation of large scale dynamic systems. Each part of the system is solved using the most appropriate time step and numerical integration method. The approach can be particularly advantageous in real-time applications, where it is essential to complete each set of calculations in the allotted real-time interval. The ESL Simulation language has parallel segment features which makes it particularly suited to the realization of multi-rate simulations. Experiences of using ESL and the Virtual Test Bed (VTB) to realize a multi-rate simulation of an underwater unmanned vehicle (UUV) are described. A non-real-time version of the simulation with 5 different frame rates has achieved speed increases of the order of 500 times with no significant loss of accuracy, making a real-time implementation feasible.This research has included analysis of the stability of multi-rate methods and these are summarized in the paper.

More on a model curriculum for modeling and simulation

At WSC 00, one of the authors (Crosbie) suggested that the development and publication of a model curriculum for MS programs in modeling and simulation would facilitate the development of such programs. This paper presents a first draft of a model curriculum developed by a small group at the McLeod Institute of Simulation Sciences at California State University, Chico. The aim of the draft is to stimulate further discussion in the M and S community with the goal of arriving at a generally acceptable outline that can serve as a guideline for new programs.

Advances in high-speed real-time multi-rate simulation techniques for ship power systems

Real-time simulations of modern power electronic systems require very short frame times of the order of a few microseconds or less. Recent research has developed techniques using digital signal processors or field-programmable gate arrays to meet this need. Integrating these high-speed simulations in a complete power system simulation often requires multi-rate simulation techniques and integration of the high-speed platforms with conventional platforms based on a real-time operating system such as a real-time version of Linux.

FPGA-accelerated Simulink simulations of electrical machines

This paper presents work that uses field-programmable gate arrays (FPGAs) to accelerate Simulink simulations of electrical machines on a desktop computer by more than 100 times. These simulations operate and can be modified transparently in the Simulink environment with no FPGA expertise required. This offers the prospect of low-cost, high performance simulation workstations supporting large simulations that do not require excessive execution times. Additionally, the work includes multi-rate and limited non-linear capabilities.

Using Attached Processors to Achieve High-Speed Real-Time Simulation

Commercially available real-time simulation systems, usually based on a real-time version of Linux, can support real-time simulations with frame times as low as 10 to 20 microseconds. There is, however, a growing class of problems for which shorter frame times are essential. This is true for example of hardware-in-the-loop simulations of many power electronic and automobile engine simulations. The problem can be addressed by the use of an attached processor that executes the time-critical parts of the simulation without the danger of operating system interrupts during a real-time frame. Both digital signal processors and field-programmable gate arrays have been used for this purpose. Examples based on power electronic systems are presented. The design of a low-cost system capable of achieving frame times as low as 500 nanoseconds is described.

Advances in High-Speed Real-Time Simulation

The demands of modern applications for ever increasing performance for real-time simulations have posed considerable challenges to simulation developers which are still not fully met. An effective response to these challenges has involved an interesting combination of a return to old-style programming techniques such as multi-rate simulation and low-level program design with a focus on individual machine cycles, with new technologies (DSP, FPGA, cell architecture) and algorithms (state transition, QSS). Some of these possibilities remain to be explored and the prospects for future research are very encouraging.