This paper aims to enhance the speed stiffness of the electro-hydro-mechanical system (EHMS). Focusing on the variable speed pump-controlled hydraulic motor system (VSPCMS), a typical EHMS, a mathematical model was established to discuss the coupling mechanism between multiple system parameters under the load condition, the physical meaning of system speed stiffness was explained, and the law of speed stiffness was disclosed under the action of multi-source variables. Moreover, several experiments were carried out to explore how the system speed stiffness is affected by three leading influencing factors in the VSPCMS, hydraulic motor displacement, leakage coefficient (system temperature and pressure) and motor shaft inertia. The experimental data agree well with the theoretical results: the system speed stiffness is positively correlated with hydraulic motor displacement and hydraulic motor shaft inertia and negatively with leakage coefficient. The research findings lay a theoretical basis for enhancing system speed stability, reducing speed fluctuations and optimizing system design.
electro-hydro-mechanical system (EHMS), Variable speed pump-controlled hydraulic motor system (VSPCMS), speed stiffness, multiparameter coupling
 Shimoaki M. (1992). VVVF-controlled hydraulic elevators. Mitsubishi Electric Advance 61(12): 13-15.
 Helbig A. (2002). Injection molding machine with electric-hydrostatic drives. The 3rd International Fluid Power Conference, Aachen: Shaker Verlag, 67-82.
 Olugbenga MA, Carl DC. (2013). A new active variable stiffness suspension system using a nonlinear energy sink-based controller, vehicle system dynamics. International Journal of Vehicle Mechanics and Mobility, 51(10): 1588-1602.
 Jiang WL, Zhu Y, Zheng Z, Zhang S. (2015). Nonlinear vibration mechanism of electro-hydraulic servo system and its experimental verification. Journal of Mechanical Engineering, 51(4): 175-184. https://doi.org/10.3901/JME.2015.04.175.
 Chen L, Zhou KK, Li ZX. (2010). Dynamic characteristics fitting of air springs and numerical analysis of air suspensions with variant stiffness. Journal of Mechanical Engineering, 46(4): 93-98. https://doi.org/10.3901/JME.2010.04.093.
 Meuser M, Vollmer F. (2003). Using piezo actuators to improve the load stiffness of servo hydraulic drives. Hydraulic and Pneumatic, 47(10): 1-6.
 Winnicki A, Olszewski M. (2009). Sliding mode control of electro-hydraulic servo system. Wydawnictwo PAK, 55(3): 174-177.
 Wang Y. (2010). Variable structure control for variable pump controlling variable motor. Journal of Beijing University of Aeronautics and Astronautics 36(12):1453-1456. https://doi.org/10.13700/j .bh.1001-5965.2010.12.008
 Sun L. (2012). Fuzzy self-tuning PID controller in SEHS. Applied Mechanics and Materials 214: 765-770. DOI: 10.4028/www.scientific.net/AMM.214.765.
 Jia YF, Gu LC. (2014). Model and Conditional PID Compensation Control on Flow of Hydraulic Source Driven by Permanent Magnet Servo Motor. Journal of Mechanical Engineering 50(08): 197-204. https://doi.org/10.3901/JME.2014.08.197
 Gu HR, Jiao SJ. (2010). Investigation into speed stiffness for milling system of totally-hydraulic milling machine. Chinese Journal of construction machinery 8(1): 14-16, 34. https://doi.org/10.15999/j.cnki.311926.2010.01.008.
 Tong W, Liu SD. (2000).The effect of oil contamination on the velocity-load characteristic of the throttle velocity-modulating circuits. Journal of South China University of Technology (Natural Science Edition) 28(06): 95-99.
 Gu LC., Liu PJ., Chen JC. (2011). State recognition technique of hydraulic system based on electrical parameters information fusion. Journal of Mechanical Engineering 47(24): 141-150. https://doi.org/10.3901/JME.2011.24.141
 Yang B, Gu LC. (2016). Analysis and on-line measurement method of kinetic energy stiffness for mechanical electro-hydraulic system. 2016 13th International Conference on Ubiquitous Robots and Ambient Intelligence (URAl), China: Xi’an, 604-609.
 Manring ND., Luecke GR. (1998). Modeling and designing a hydrostatic transmission with a fixed-displacement motor. Journal of Dynamic Systems, Measurement, and Control 120(1): 45-49.
 Kugi A, Schlacher K, Aitzetmuller H, Hirmann G. (2000). Modeling and simulation of a hydrostatic transmission with variable displacement pump. Mathematics and Computers in Simulation 53(4): 409-414.
 Gold PW, Schmidt A, Dicke H, Loos J. (2001). Viscosity-pressure-temperature behaviour of mineral and synthetic oils. Journal of Synthetic Lubrication 18(1): 51-79.
 Yang B, Gu LC, Liu Y. (2017). A graphical technique of kinetic energy stiffness identification for mechanical electro-hydraulic system. Journal of Vibration and Shock 36(04): 119-126. https://doi.org/10.13465 /j.cnki.jvs.2017.04.019.
 Sun YG, Qiang HY, Yang KR, Chen QL, Dai GW, Dong M. (2014). Experimental design and development of heave compensation system for marine crane. Mathematical Modelling of Engineering Problems, 1(2): 15-22. https://doi.org/10.18280/mmep.010204
 Piancastelli L, Burnelli A, Cassani S. (2017). Validation of a simplified method for the evaluation of pressure and temperature on a RR Merlin XX head. International Journal of Heat and Technology, 35(1): 549-558. https://doi.org/10.18280/ijht.350311
 Li J, Yan S, Zhou YH, Yang KX. (2015). Simulations and comparisons of D-section cylinder in the different Re flow. Mathematical Modelling of Engineering Problems, 2(4): 23-28. https://doi.org/10.18280/mmep.020405
 Wang TC, Xie YZ, Yan H. (2016). Research of multi sensor information fusion technology based on extension neural network. Mathematical Modelling of Engineering Problems, 3(3): 129-134. https://doi.org/10.18280/mmep.030303