?? y ( t ) = ??? i = 1 n b i ?? u ( t - d - i ) - ??? i = 1 n a i ?? y ( t - i ) where : ( 7 ) ?? y ( t ) = y ( t ) - Y ss ( 8 ) ?? u ( t ) = u ( t ) - u ss ( 9 ) and t is time, ai and bi are real numbers, d is a time delay, u(t) is an input and y(t) an output.