Dale Word

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.

High-Speed Real-Time Simulation for Power Electronic Systems

Real-time simulators have been used as an aid to power system design for many years. Over time, scaled-down physical models and analog computer-based simulators have given way to real-time simulators based on digital technology. Another trend has been the need for shorter and shorter frame times for these digital real-time simulators. Modern power electronic systems use highfrequency Pulse-Width Modulated (PWM) controllers. Indications are that frame times of 2 μS are needed for PWM switching frequencies of 10 KHz. As frequencies increase further it is possible that frame times of less than 1 μS may be required. In order to achieve these very short frame times, the implementation of this type of simulator requires careful selection of the methods and technologies used. The first involves thorough analysis of the integration method chosen, to ensure that it provides the performance, stability and accuracy required. Additionally, the choice of computing platform is crucial to provide the computational support to meet these very aggressive timing requirements.

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.

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.

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.