Ruan, J., P. R. Ukrainetz, and R. Burton. “FREQUENCY DOMAIN MODELLING AND IDENTIFICATION OF 2D DIGITAL SERVO VALVE”. International Journal of Fluid Power, vol. 1, no. 2, Oct. 2000, pp. 49-58, https://journals.riverpublishers.com/index.php/IJFP/article/view/651.