Wang, T., M. Cai, K. Kawashima, and T. Kagawa. “MODELLING OF A NOZZLE-FLAPPER TYPE PNEUMATIC SERVO VALVE INCLUDING THE INFLUENCE OF FLOW FORCE”. International Journal of Fluid Power, vol. 6, no. 3, Nov. 2005, pp. 33-43, https://journals.riverpublishers.com/index.php/IJFP/article/view/566.