Software For Fluid Power Technology

From Editor

VisSim - Visual Simulation - a program for modelling and simulation of complex dynamic systems

Why is that so? I think the users quickly come to realize that to use modelling software one must have a "deep" knowl-edge of modelling and simulation as well as a very sound educational background and thus the perceived advantage of specialized software as not requiring specialist users is quickly dispelled. After all when carrying out a dynamic simulation one must know the answers beforehand (it sounds like a paradox but unless you know what results to expect you will not know if results are wrong!). Although some universities use commercial specialized software in their post-graduate work, most young engineers become familiar with block diagram modelling in Dynamics and Control courses and are experienced with packages like Matlab/Simulink and VisSim, thus the advantage of prepackaged models etc. is not a great incentive to purchase a dedicated package. Some people are comfortable working with specialized programs, some, like me, prefer to have full control over their work and thus use the general purpose software. Finally new devel-opments in control and mechatronics need software which provide, seamlessly interfaced, tools for linear analysis, for modelling of hybrid analogue-digital systems, optimization, real-time (hardware-in-the-loop), analogue/digital control and intelligent control (neural nets, fuzzy logic) applications. These requirements are easily handled by advanced gen-eral software packages.

The subject of this review is VisSim (Visual Simulation) from Visual Solutions Inc. a Windows-based program for modelling and simulation of complex dynamic systems (VisSim won Control Magazine's 1999 Readers Choice Award in the Software Simulation category). Over the years I worked with ACSL, CSMP, Simulink, etc. however, over the last few years, I used exclusively VisSim as my modelling/simulation tool. Why did I choose VisSim over the other soft-ware? VisSim provides a fully integrated environment for engineers and scientists to analyze, design and test both gen-eral engineering and control systems. It could be argued that software like ACSL or Simulink provide similar capabili-ties. They do, however, they have some drawbacks. ACSL is Fortan based software and thus require a Fortan compiler and Simulink requires Matlab and Matlab toolboxes (often requiring programming code). Some advantages of VisSim over other similar software programs are, in my opinion, low demand on computing resource (the software takes ap-proximately 10-15 MB of disk space), very fast solvers, integration with other software tools like linear analysis, digital signal processing, neural nets, real-time hardware-in-the-loop simulation. All these tools are accessed from within the main program without the necessity to call up any other software, and as modelling tasks are performed using graphic interface we have a fully integrated control system design environment where all design and simulation tasks can be completed without writing a single line of code. Other useful tools of the VisSim are C-code generation which allows to convert a verified simulation model into an executable C-language program and VisSim Viewer, a run-time, view only version of VisSim. An important difference for academic users of the software is that VisSim Professional software (a version proceeding the current version of the software, currently version 3.0) is provided free of charge to students and university teachers. Finally, VisSim is very easy to use and has, in my opinion, a much shorter learning curve than competitive systems (in my Control class at the University students learn to operate VisSim after initial 3 hours hands-on tutorial followed by a couple of hours of tutorials on building and solving complex models). Less time training - more time studying or working!

Most block oriented software packages have an intuitive "drag and drop" block diagram interface. All three packages use graphic representation of modelling blocks, however more elaborate graphics provided by Simulink or ASCL mean that fewer number of modelling blocks can be displayed on the modelling canvas at the same time. VisSim blocks are very simple - small, rectangular blocks with a symbol of operation, and thus it is very easy to build complex block dia-grams on a single canvas and still be able to have a good overview of the model.

The visual block diagram interface offers a simple method for constructing, modifying and maintaining complex system models. Complex models will usually have a hierarchical structure. VisSim allows grouping of symbols in so called compound blocks (a bitmap can be placed on top of a compound block) and thus generate a model in layers. A maxi-mum number of blocks in a model is limited to 64000 blocks (for example, a turbine control model which includes tur-bine control, valves, valve actuators and the turbine itself required approximately 5000 blocks).

The VisSim family of modelling software is shown in Fig. 1. The core of the modelling system is VisSim Pro currently in version 4.0 which is an incremental improvement on previous versions and models. Models developed with previous versions of VisSim are fully compatible with the new version, however, there is no backward compatibility which sometimes can be a nuisance. VisSim can be used to model and simulate linear, non-linear, continuous and discrete systems. The models can have multiple inputs/outputs. Thus a user can develop a hybrid model which uses both contin-uos and discrete blocks (these can have different sampling rates). Parameters of blocks can be dynamically changed during simulation, and values of all parameters and variables can be monitored during simulation. To model a system we use modelling blocks from pull-down menus (or using icons on toolbars). As blocks which belong to the same class are grouped together (e.g. arithmetic blocks, non-linear blocks, signal producing blocks) it is very easy to quickly access the relevant blocks. Clicking on the block with a right mouse button brings a panel for changing block parameters and gets help on the particular block. If a user previously saved a model into a file (e.g. model of a relief valve) he can insert this model into a current model as a sub-component. This way, after a while, a user has a library of models which are fully reusable. To navigate large, complex models with multiple hierarchical levels (compound blocks) VisSim uses Explorer-like interface which provides a hierarchical map of a VisSim diagram. VisSim provides seven integration routines, including Backward Euler for simulation of stiff systems and implicit Newton Raphson equation or a custom solver. All these routines are fully configurable.

Fig. 1: VisSim Software

VisSim/Analyze allows the user to perform a frequency domain analysis of the model or subsystem. V/Analyze line-arizes the model of the system about a specified operating point. A linearized system can be represented in state space or transfer function form and can be analysed using Nyquist, Bode and root locus plots. Vector and Matrix Operation handle vector and matrix algebra (buffer, dot product, invert, multiply, transpose, etc.). These operations simplify model design and are useful for analysis of linear systems. VisSim allows interfacing with other software and transfer of data using DLL and DDE capabilities. For example, C Expression Block allows a C command or expression to be part of a VisSim diagram, and auto DLL generation tool automatically creates a DLL from a compound block (requires VisSim C-Code option). VisSim comes with a toolbox of various compound blocks, e.g. PID block, various signal producing blocks etc. Numerous examples of models come as part of the software package.

The VisSim Viewer allows users to distribute VisSim models and simulations to others who are not licensed to use VisSim. The recipient of the model cannot change the model structure but he/she is free to change simulation and blocks parameters, and perform interactive "what -if" scenarios on the model. Full output options (histogram, plot and stripchart blocks) are available. It is a useful tool for exchange of models and collaboration with colleagues and custom-ers.

Capabilities of VisSim can be integrated with Mathcad as Mathcad worksheets can be embedded as components in Vis-Sim. The 'function blocks' which represent components of a system can be Mathcad worksheets (thus we can use, for example, Mathcad capability to solve differential equations). This feature is useful for both Mathcad and VisSim users. Simulink modelling blocks can be now used in VisSim, and Matlab interface allows Matlab commands, functions and .m files to be accessed from a VisSim session and expressed as a block in the VisSim model. There is also ActiveX compatibility, which could be of interest to users developing real-time models. For details of other features of VisSim and associated tools the reader is referred to the VisSim website. Typical screen shots are shown in Fig. 2 and 3.

Fig. 2: Screen shot of Door System (top level)

Fig. 3: Door system - gearbox model (lower level)

VisSim is used now by many major companies spanning industry spectrum including: ABB, IBM, Parker Hannifin, Caterpillar, Chrysler, Eaton, Ford, General Motors, Boeing, British Aerospace, Lockheed-Martin, NASA, Northrop-Grumman, Pratt and Whitney.

In summary, VisSim in an excellent simulation software, fully scalable and expandable to handle very complex model-ling and simulation tasks. The big advantages of VisSim are integrated simulation environment, relatively low cost (and minimal or no cost to universities and students), low computational resources required to run the software, integration and interface with other design software and user friendliness. It cannot be stressed enough though that to obtain satis-factory results the user must have good background in dynamics, control theory and modelling/simulation.