A method of closed loop simulation of a computer model of a physical system and an actual real-time hardware component of the physical system, the method comprising: generating at a simulation processor an input waveform for the component from a simulated model of the physical system using an initial waveform as a response waveform of the component; sending from the simulation processor through a network to a real-time playback and record device (RTPR) connected to the component the input waveform, the RTPR resetting the component to an initial state and playing back the input waveform in real-time to the component, the RTPR receiving a real-time response waveform generated from the component; receiving at the simulation processor from the RTPR through the network the generated response waveform of the component; using one or more waveform relaxation, WR, converging methods at the simulation processor to enable convergence of the generated waveforms affected by hardware induced distortions; and performing a WR method (404, 406, 408, 401, 412) of the one or more WR converging methods with the generated response waveform to provide a closed loop simulation response; characterised in that: said WR method is at least a modified time windowing method being applied to the generated response waveform fixed within a time window [0,T] growing from zero, to the simulation end time, T w =T, where a converged part of the waveform of an iteration, T k, is repeated in subsequent iterations, T k+1, to avoid manual re-initialization of the real-time hardware component at every iteration, and wherein the increasing time window T w is a continuous time interval from zero to a time that a waveform difference from a previous iteration, T k-1 W, to a current iteration, T k W is smaller than a pre-defined threshold constant. generating at a simulation processor an input waveform for the component from a simulated model of the physical system using an initial waveform as a response waveform of the component; sending from the simulation processor through a network to a real-time playback and record device (RTPR) connected to the component the input waveform, the RTPR resetting the component to an initial state and playing back the input waveform in real-time to the component, the RTPR receiving a real-time response waveform generated from the component; receiving at the simulation processor from the RTPR through the network the generated response waveform of the component; using one or more waveform relaxation, WR, converging methods at the simulation processor to enable convergence of the generated waveforms affected by hardware induced distortions; and performing a WR method (404, 406, 408, 401, 412) of the one or more WR converging methods with the generated response waveform to provide a closed loop simulation response; characterised in that: said WR method is at least a modified time windowing method being applied to the generated response waveform fixed within a time window [0,T] growing from zero, to the simulation end time, T w =T, where a converged part of the waveform of an iteration, T k, is repeated in subsequent iterations, T k+1, to avoid manual re-initialization of the real-time hardware component at every iteration, and wherein the increasing time window T w is a continuous time interval from zero to a time that a waveform difference from a previous iteration, T k-1 W, to a current iteration, T k W is smaller than a pre-defined threshold constant.