This block defines a discrete-time PI controller by the formula:
// State space form:
x(ti) = previous(x(ti)) + u(ti)/Td;
y(ti) = kd*(x(ti) + u(ti));
// Transfer function form:
y(z) = kd*(c*z-1)/(z-1)*u(z);
c = 1 + 1/Td
where kd is the gain, Td is the time constant, ti is the time instant of the i-th clock tick and z is the inverse shift operator.
This discrete-time form has been derived from the continuous-time form of a PI controller by using the implicit Euler discretization formula.