The paper investigates robust stability of an embedded robust controller for optimal reference tracking of electrohydraulic steering systems. The regulator is synthesized on the base of multivariable system identification and H-infinity cost function. In order to describe the system in robust control framework a small uncertainty is introduced into the model. Then the system is represented as a $\mathrm{M}-\mathrm{\Delta}$ interconnection which allows to calculate the structured singular value ( ${\mu}$ ) of the closed loop system with the H-infinity controller. This singular value is a measure of the loop stability in presence of bounded variations in the model characteristics in frequency domain or in its parameters. The present paper proves that the closed loop system keeps its stability in presence of input multiplicative uncertainty caused by the inherent nonlinearities in the hydraulic steering units. The performance of the H-infinity controller is experimentally verified on laboratory test bench.
