John G Pearce

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.

Design and simulation of a non-linear, discontinuous, flight control system using rate actuated inverse dynamics

This article presents the novel non-linear controller design method of rate actuated inverse dynamics. The rate actuated inverse dynamics controller design uses a novel variable structure control based anti-windup method to ensure that the actuator does not become overdriven when rate or deflection limits are reached. This allows the actuator to remain on both rate and deflection limits without the system becoming unstable. This is demonstrated in a non-linear simulation of a missile body rate autopilot using a multivariable controller designed using rate actuated inverse dynamics methods and, for comparison, a controller designed using robust inverse dynamics estimation. The simulation is performed with an advanced solver which uses a discontinuity detection mechanism to ensure that errors do not occur during the simulation due to the presence of multiple discontinuities. The results show that using a smaller actuator, with reduced rate limits, is not possible with the robust inverse dynamics estimation design. Conversely, the rate actuated inverse dynamics design demonstrates excellent performance, despite the actuator limiting in both deflection and rate of deflection. This illustrates the possibility of using smaller, less powerful actuators without sacrificing system stability.

Interactive simulation on minicomputers Part 2 - Implementation of the ISIS language

This paper describes the mechanisation of the ISIS interactive continuous-system simulation language, with particular emphasis on the compilation process. The language has extensive facilities, including line-by-line syntax checking and instant diagnostics. The special problems and constraints of minicomputer operation are discussed. Compilation takes place in two stages, the first occurring line by line as the program is entered into the host computer. The second step consists of a single pass that produces low-level interpretive code that can be executed immediately. The compiler itself is written in FORTRAN with a minimum of machine-dependent subrou tines for handling input/output., terminal graphics, keyboard interaction, and the like. As a result, ISIS is highly portable ; it has already been instal led on six different types of computers.

Scattering of energetic ions by solids — a simulation

This paper describes a digital computer simulation of an experiment which measures the energy-intensity distribution of noble gas ions scattered by crystal line solids. The discussion emphasises the simula tion techniques employed (in particular, the choice of integration method), the methods of relating com puter input data to the experimental variables, and the transformation of computer results into a form directly comparable to experimental data.

A hybrid cell-sharing technique for solving ion-bombardment problems

This paper describes a hybrid cell-sharing technique which allows a limited amount of analog equipment to be used to solve the large number of equations associated with ion-bombardment problems. The tech nique was implemented on a HS7/PDP8L hybrid computer for the case of an argon ion interacting with two atoms of a gold crystal and the results agreed well with those obtained by digital simulation.