Research On Valve-pump Parallel Control Systems For Speed Regulation

Traditional hydraulic motor speed servos have two basic types: valve control andpump control. The valve control system responds fast to valve and load inputs, but isless efficient, so this arrangement is only suitable to applications required rapidresponse with low power. The pump control system is more efficient since both flowand pressure are closely matched with load requirement, but responds slowly，so thisarrangement is only suitable to the applications required high power with slowresponse. Recently, the valve-pump combination control is rapidly developing, whichallows full play to advantages of valve control and pump control. This paper proposeda totally new valve-pump parallel control system, that is valve-pump parallel variablestructure control systems for motor speed servo with large power. Control mode willvary with control requirement during speed regulation process, and the proposedsystem will achieve comprehensive performance for hydraulic motor speed systemwith large power, such as low-speed performance, rapid response to load variation andhigh efficiency.Main work and achievements are as following:(1) Overview the principle and feature of valve-pump combination systems, andpoint out those existing valve-pump combination systems cannot achieve abovecomprehensive performance.(2) Design the valve-pump parallel variable structure control systems, and there aretwo key points, one is that the control valve can work at leaking status andreplenishing status, and then the system can be under leaking valve-pump parallelcontrol mode and replenishing valve-pump parallel control mode; the other is toregulate effect of valve and pump control by varying Kvp; establish the mathematicalmodels of the proposed system under different control modes, analyze systemparameters, which indicate that the system natural frequency doesn’t change, whenintroducing valve control into traditional pump control system, but total leakingcoefficients and damping ratios become greater and vary widely with operating points(valve opening and load pressure), and which will decrease velocity stiffness andmake system more susceptible to disturbances.(3) Design the structure of open circle system and closed circle system invalve-pump parallel variable structure control, establish balance flow equations ofclosed circle system under the replenishing valve-pump parallel control (RVPC) mode and leaking valve-pump parallel control (LVPC) mode, then analyze the relationshipof different flow, and point out that in the closed circle system of the valve-pumpparallel variable structure control, both the replenishing circuit and flushing circuit arenecessary, and replenishing circuit should not be canceled because of the exiting ofthe replenishing controlled by the valve, and the flushing circuit has the function ofbalancing system flow besides heat exchanging. Establish the foundation of parametermatching between hydraulic simulation systems and practical systems，and point outthat to simulate the practical system, should configure the simulation systemaccording to pump displacement, hydraulic motor displacement and inertia,respectively. The test system is established according to a hydraulic hoist unsed incoal, which is a kind of hydraulic system with high power and high inertia.(4) Propose the concept of valve weight（Kv） and pump weight（Kp）, establishthe relationship of traditional control method (valve control and pump control) andvalve-pump parallel control; Propose the concept of valve-pump weight ratio, that isKvp=Kv:Kp, to judge which mode plays a leading role in control process and whichcontrol mode is in.(5) Establish RVPC and LVPC modes, and study the influence of different Kvponsystem dynamic performance, and obtain some laws of valve-pump parallel control:the response is pyramidal when Kvp=1:0～0:1, and is slow at two sides (valve controland pump control) and rapid at middle (valve-pump parallel control), so thevalve-pump parallel control responds faster than single valve control and pumpcontrol both for input singal and load disturbance; the system mainly controlled valveresponds faster than that mainly controlled pump for the same sum of Kvand Kp,because open loop gain of valve control is greater than that of pump control; the ratioof valve and pump flow is proportional to Kvp; compared with the leaking valve-pumpparallel, the replenishing valve-pump parallel control has more advantages, becausethe latter can make damping ratio more stable, which contributes system predictionand control, in addition it can further improve response and velocity stiffness byraising Ps, and is more suitable to achieve rapid response to load disturbance.Simulation and experiment show that Kvp=1:0.2is preferable.(6) Study low velocity performance under different modes. Leaking valve controland leaking parallel valve control obtain good low velocity performance by increasingcontrollable leakages to increase damping ratios. Moreover, the increase of dampingratios caused by leakage controlled by the valve just compensates the reduction of damping ratios due to the friction negative slop (Stribeck effect), which furtherenhances low velocity performance. The valve initial voltage is essential to lowvelocity performance, and then is proposed. Study the influence of different initialvoltages and temperatures on low velocity performance. The experiment results showa sound low velocity performance could be obtained, if the initial voltage is set aroundthe theoretical value, which makes leaking parallel valve control more practical ataspect of improving low velocity performance.(7) Establish a simulation mode on AMEsim platform and set up a test system onLabView platform by virtual instrument technology, and study dynamic performanceof valve-pump variable structure control in a duty circle. Both simulation andexperiment results show that comprehensive performance is achieved by changingcontrol modes in control process, the leaking parallel valve control mode is applied tostart and stop stages and improves low velocity performance, the replenishing parallelvalve control mode is applied to the uniform stage with high speed and achieves fastresponse to load disturbance, the parallel pump control mode is applied to accelerationand deceleration stages and makes full use of its high efficiency; control modes switchsmoothly and both the variations of valve and pump are continuous and matchprevious supposition.In all, the valve-pump parallel variable mode control establishes a flexible controlmechanism by using two channels of valve control and pump control, where controlmodes vary with control requirement, which make the valve coordinate the pump, andmake full use of advantages of valve control and pump control, and enriches thecontrol modes of hydraulic speed regulation systems, and makes electro-hydraulicsystems more flexible and suitable, and could achieve comprehensive performance ofhydraulic speed regulation systems with large power, such as low-speed performance,rapid response to load variation and high efficiency.