Hydraulic system modelling is a powerful tool in design work. For a complex system with many parameters it's difficult to realise the effect of a component change. Also the time to calculate this effect will be reduced with a simulation model and there will be more steps in the iterative process to optimise the system solution. In this work is presented the design task for a hydraulic lift - lowering system on a forklift truck. The system includes a gas accumulator for energy saving. The accumulator is complex to design in this application. But with a proper simulation model of the accumulator the design work is considerably simplified.

It can be stated that the value of a simulation model depends on its accordance to the real system in the aspect of simulation purpose. The understanding of the system and the system working conditions is very important. The forklift truck example also shows that some component can be "ruff'' modelled and anyway the simulation result is good enough for its purpose. However, to establish that insight the theoretical modelling must be validated by experimental tests.

The work also include a troubleshot for oscillation problem in a jet engine device. Here it's clear that the component models must be detailed to show the real system dynamical behaviour. A credit from the model building is that it compels to look deeper into the system components design and that will increase the knowledge and understanding of the system.

Sometime there is no need of physical component models. Instead the system looks at as a "black box". For example can an analysis of input and output signals give enough information for a control algorithm. This work includes a study of this form of "System Identification".

Today there are many simulation program and simulation techniques available on the market. One of them is Matlab with included toolboxes. Advantages with Matlab are that it's spread and well known by most students. Math Works who has developed Matlab has a big staff of engineers that develop and adjust their products to new hardand software technologies.

dSPACE is another powerful tool in the product developing process. dSPACE is a measurement and control card for real-time processing. The system can be included in the toolbox Simulink in Matlab. Matlab and dSPACE have been successfully used in this work.