## 1 Introduction  
Traditional hydraulic control systems are of two basic types: pump control and valve control [1]. Valve control systems respond quickly to valve and load inputs but are less efficient due to throttling and overflow losses [2; 3]. Pump control systems work efficiently since both system pressure and flow are closely matched with load requirements [4; 5] but have a critical drawback: slow response. The variable speed pump control (VSPC) system is a new class of pump control system, using a variable speed motor to drive a fixed displacement pump, which is applicable to hydraulic elevators [6], shield tunneling machine [7] and water distribution systems [8; 9]. Compared with traditional pump control systems, the VSPC system has advantages of lower cost, higher reliability and higher energy-efficiency. However, response time is poor, due to the high inertia of the electric motor and the low overload capacity of the inverter driving it, preventing their use in applications requiring rapid response.  
Two classes of combined valve-pump control may be identified: serial control and parallel control, balancing the respective advantages of each, with respect to dynamic response and energy efficiency [10]. To accelerate the response of VSPC systems, [PERSON] [11] proposes a valve-pump series control system, in which a flow control valve is connected in series to the outlet of the pump. [PERSON] et al. propose a variable speed hydraulic control system based on an energy regulation strategy [12; 13], in which an energy regulation device releases energy to improve dynamic response serial valve-pump control systems are not suitable for high power systems, due to the control valve installed in the main circuit limiting maximum flow of the system. However, valve-pump parallel control systems can avoid this disadvantage, as the parallel control valve in the main circuit does not limit the system flow. Recent research on parallel control systems mainly focus on electrohydraulic actuators in a hybrid primary flight control system [14; 15]. The current parallel control system is of a replenishing type, requiring an additional oil supply device and current research does not explain the role of pump control and valve control in the combined system and its corresponding influences on dynamic response.  
To improve the response of VSpace systems, this paper develops a leaking valve-pump parallel control (LVPC) system, in which a leaking control valve is placed in parallel to a variable speed pump to collectively regulate system flow. In the paper, the LVPC system is designed and its working principles explained. Mathematical models are constructed, an experimental system set up and the influence of different valve-pump weights on system response are discussed, demonstrating the role of pump and valve control in the combined system. Thus, the LVPC provides a new method for improving dynamic response of pump control systems and will enrich the control schemes of hydraulic systems. LVPC might have potential value in application requiring of fast response. Beyond the particular application on a hydraulic test rig for volumetric machines, the same method could be maybe applied on a turbomachinery test rig to regulate for example the head of the testing model, which are classically also equipped either with a variable speed pump.