In the case of the integral-action element (I element) the output variable x and the input variable y are combined using the expression
The output variable x(t) arises through the integration of the input variable y(t). The parameter KI is referred to as the integral action coefficient, its inverse value TI = 1/KI is called the time constant of the I element.
The following diagram shows the step response and the block symbol of the I element. From the constant input variable for t > 0 a linear characteristic is generated at the output of the I element, the slope of which is given by KI.