"Click Here to return to main index".......A Simple Pendulum

ArcSimp can also simulate digital gates and digital systems and it can do this at the same time as it is simulating analogue systems. Because of this mixed systems such as analogue to digital convertors and digital to analogue convertors can be modeled.

This power is of great value in may key stages of education. Imaging building logic and gate test circuits with no batteries and boards to give confusing bad connections. Or simulating a 10 metre pendulum or a long inclined plane without moving from your computer keyboard. All this can be done with any Acorn Archimedes computer or RISC PC.

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Simulation programs have been used ever since the early (electromagnetic relay!) computers where a �bug� really meant an insect stuck between contacts!. ArcSimp follows this long tradition and takes full advantage of the power of the Arm and the Risc Os Wimp desktop. It is written in ANSI C under the ACORN Desktop Development Environment which should help to take care of the insectevorae.

Building a simulations require some knowledge of mechanics and dynamics but the simulations, once built, can provide fascinating material for further experiment and study by a wide range of users.

In the simulation building stage, the mouse is used to drag �function blocks� into a window and to make �connections� between them. Each �function block� can be given �parameters� which configure it to the required task.

When the model is complete, it can be �run� to show both graphical and numeric outputs. ArcSimp also allows a two-dimensional representation of the physical object to be drawn on a further graphics window in which the x and y positions of the parts of this object can be linked to the numerical output of the simulation.

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Using ArcSimp the movement of the bob of a pendulum can be shown as:-
a) A series of numbers giving its position in space against time -
and
b) A continuous plot of those numbers against time. -
and/or
c) A moving pictorial representation.

Very large models can be built on the Archimedes limited only by the memory available and the users patience. Each function block takes about 640 bytes of memory and an average of .4 milliseconds to run on an ARM 3 with no floating-point accelerator. On a StrongArm the performance is improved by a factor of 10. The basic program is 740K bytes. An average of 5000 blocks (StrongArm 50,000) can be �run� in one second hence a typical 100 function block model would thus need about 800K of memory and when set to recalculate every 20 milliseconds (2 milliseconds StrongArm) of simulated time would run just about in �Real Time�. Some functions such as the sine function and integrator take longer to run than average and if a large number are used may slow the simulation down. There is also a limit to the speed at which the plots and graphics can be drawn on the screen.

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Simulation programs running on very large computers can be used to model, for example, an aircraft and at the same time model the automatic pilot. The combined simulation can then be tested to find out whether the aircraft with it�s automatic pilot works to specification. In the early stages of test this can be much cheaper than building the aircraft and autopilot and conducting flight trials, and it is a great deal safer if there is something seriously wrong with the design! Later during flight trials the results of the simulation runs can be compared with the real aircraft. This allows adjustments to be made and tested as confidence builds up in both the real and simulated system.

ArcSimp on the Archimedes is powerful enough to do smaller, but yet significant, studies of this kind, from showing and studying problems in mechanics and dynamics to designing and testing control systems. However with the advent of the StrongArm Processor the RISC PC will be able to undertake simulation tasks of very considerable significance.

All of us are familiar with control systems - We are reading a book as dusk falls, so we switch on a light. We read all night! and, when daylight comes, we switch off the light. In doing this we make ourselves part of a simple on/off or �bang-bang� control system designed to keep the light on the book sufficient for comfortable reading.

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Such a control system does not result in a constant illumination, if we need that we will have to make the control system more complicated. We might add some way to reduce the bright daylight at midday - perhaps window blinds, and/or something like a dimmer switch to control the amount of artificial light. Oh and don�t forget the possibility of a trivial solution - brick up the window and turn on the light! A design engineer will need to know which works best? Which is cheapest? Which will the customer like best? He needs to try out the ideas and show them to his customer without spending too much money, and this is where simulation comes into it�s own.

There are many pitfalls in control engineering, imagine the window blinds and dimmer switch solution to the lighting problem, . Now to test it out:- We are reading with the blinds half drawn at midday when a cloud covers the sun. We have to decide whether to turn up the artificial light or to get up and draw back the blinds. We decide to draw the blinds, but as we operate the control the cloud passes - but by now the blinds are opening. We have discovered one of the major problems of control systems - response time. If the response to change is too slow, the desired result is not obtained. We may well start moving the blinds so late that we are still opening them after the cloud has passed, causing a change of brightness greater than if we had done nothing at all!

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Any control system can have faults, and it can be very expensive to build it and find them when it is installed. Can you imagine the cost of keeping a new office block unused for six months because the heating system is frying the penthouse and freezing the lobby!

Over the years engineers have found that the use of simulation can help them avoid such problems. Many simulation programs have been written to help in such tasks and ArcSimp is a complete re-write for the Archimedes Desktop of one program which has been used for many years in industry. We hope that this manual will help you to enjoy using it.

Dynamic Spread Sheets

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It does this by providing functional elements such as addition, multiplication, integration and differentiation which can be easily entered onto a pictorial `worksheet' and connected together.

The advantage of this approach is that these functional elements can represent physical processes in the real world, enabling many simulations to be set up without first generating their mathematical equations of motion - an approach which is often easier for those trained as engineers rather than mathematicians.
For a reference to these techniques see the chapters on `State Variable Diagram' and `Digital Computers in `Modern Control System Theory and Design' by S.M.Shinners ISBN 0-471-55008-6

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Another advantage is that the elements of digital gate logic can easily be expressed in the same way - even allowing representations of rise-time and electrical noise to be added. Because of this ArcSimp can simulate mixed analogue digital models.

ArcSimp runs as a task in the RISC OS multitasking environment and multiple copies can be run and interconnected. This allows studies of multiplex control systems and systems with other forms of redundancy.

ArcSimp is fully compliant with the new StrongArm Processor family and is thus potentially very fast indeed giving a highly cost-effective means of demonstration, design and test of a wide range of simulation subjects. There is no limit to the size of simulations which can be built using the professional version of ArcSimp other than available machine memory.

The heart of a simulator is the integration algorith - the one used gives vitually all of the advantages of the predictor-corrector approach without the time penalty of a double computation at each `step' - the corrections being applied `on the fly'. This approach was originally developed to enable real time floating point operation on a much earlier (and slower!)) generation of computers. It has stood the test of time and has been used extensively for the design of real-time industrial control systems.

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The program design of ArcSimp is modular and can easily be adapted to meet the special requirements of users. Some features, such as the pictorial representations of Digital Gates, can be changed by the user. If the Professional version is purchased and the user has access to ACORN C/C++ the user can add special purpose functions with only a modest knowledge of 'C' and minimal involvement with the ACORN DESKTOP

The output of simulations is presented in numeric format and also in data v time on a specially designed `moving map' display and also in x y format where the displayed points may be linked to moving sprites giving the ability to build up a dynamic picture of the simulated real world.

A novel use of ArcSimp is to use it as a `dynamic spread sheet' the individual functions acting as `cells' in the sheet - design variables can then the scanned automatically to find the optimum values of parameters in the design process.

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