The Integrator is, so to speak, the core component of ModelicaDEVS. Its purpose is to integrate a (piecewise constant) function originating from another DEVS block that is connected to the Integrator's input port.
The picture illustrating the behaviour of the Integrator shows a sine signal (blue), the integrated sine function (red), its first derivative (green) and its second derivative devided by 2 (pink).
