Whenever a computer simulation of a model of any

physical process or a system is done in parallel to its physical counterpart

can be referred to as the real time simulation. The

virtual representation of physical system runs simultaneously and for the same

duration as the physical system. They may share common input variables and come

out with comparable output. One good example for a RTS can be operation

of the Fuel Injection System of a modern day computer controlled car engine,

the onboard computer (Engine Control Unit) calculates the duration of operation

and the interval between each operation based upon the throttle input, camshaft

position, inputs from Oxygen Sensors, inputs from NOx sensors etc. all of which

are measured in real time. Another suitable example are the computer games where

outcomes are generated based on user inputs in real time.

A computer does all the computations using an

operating system which eventually does all of its calculations in the form of

0’s and 1’s. All the differential equations, state equations or any mathematical

functions representing a physical system will converted to a discrete system of

0’s and 1’s, these will be solved by the computer simulation software using

their own solvers. The solvers use different numerical methods to do the

computations and each may take different amount of time and produce results

with different accuracy.

To work with RTS the simulation associated would

be for discrete time with a constant step size. Variable time-steps simulation

is not suitable for RTS and hence the time is incremented in equal step sizes

called Simulation Time Steps and the simulation itself is often called Fixed

Time Simulation.

As mentioned earlier the differential equations

and the mathematical functions representing the model are solved to perform the

i/o operations and to obtain the output of the model. However during a

‘discrete non real time simulation’ the actual time required to solve the

aforementioned equations and functions may be more or less than the simulation

time step. But in case of real time simulation it is necessary that (apart from

the precise modeling of the physical system) all the computations are done

within the simulation time step so that the model under test can accurately

represent the functioning and perform all the I/O operations of its equivalent

real or physical system.

If the computations are not complete within the

simulation time step the real time simulation results are not accurate which is

also referred to as ‘overrun’. Moreover if the computations are done before the

simulation time step is complete then the remaining time, called the

‘idle-time’ is simply lost, which is in contrast to the accelerated

simulations where the remaining time would be used to perform the computations

of the subsequent time step.