1852_4缸發(fā)動(dòng)機(jī)油底殼組合鉆床Ⅱ主軸箱,_4,發(fā)動(dòng),機(jī)油,組合,鉆床,主軸 畢業(yè)設(shè)計(jì)文獻(xiàn)翻譯院 （ 系 ） 名 稱 工 學(xué) 院 機(jī) 械 系專 業(yè) 名 稱 機(jī) 械 設(shè) 計(jì) 制 造 及 其 自 動(dòng) 化學(xué) 生 姓 名 黎 振 廷指 導(dǎo) 教 師 賈 百 合2012 年 03 月 10 日黃 河 科 技 學(xué) 院 畢 業(yè) 設(shè) 計(jì) (文 獻(xiàn) 翻 譯 ) 第 1 頁INVESTIGATION ON DYNAMIC PERFORMANCE OF SLIDE UNIT IN MODULAR MACHINE TOOL （對(duì)組合機(jī)床滑臺(tái)動(dòng)態(tài)性能的調(diào)查報(bào)告）出處： Peter Dransfield, Hydraulic Control System-Design and Analysis of TheirDynamics, Springer-Verlag, 1981文獻(xiàn)作者：Peter Dransfield,翻譯頁數(shù)：p139—144對(duì)組合機(jī)床滑臺(tái)動(dòng)態(tài)性能的調(diào)查報(bào)告本報(bào)告處理調(diào)查利用有束縛力的曲線圖和狀態(tài)空間分析法對(duì)組合機(jī)床滑臺(tái)的滑動(dòng)影響和運(yùn)動(dòng)平穩(wěn)性問題進(jìn)行分析與研究，從而建立了滑臺(tái)的液壓驅(qū)動(dòng)系統(tǒng)一自調(diào)背壓調(diào)速系統(tǒng)的動(dòng)態(tài)數(shù)學(xué)模型。通過計(jì)算機(jī)數(shù)字仿真系統(tǒng)，分析了滑臺(tái)產(chǎn)生滑動(dòng)影響和運(yùn)動(dòng)不平穩(wěn)的原因及主要影響因素。從那些中可以得出那樣的結(jié)論，如果能合理地設(shè)計(jì)液壓缸和自調(diào)背壓調(diào)壓閥的結(jié)構(gòu)尺寸。本文中所使用的符號(hào)如下：s1-流源，即調(diào)速閥出口流量；Sel—滑臺(tái)滑動(dòng)摩擦力R 一滑臺(tái)等效粘性摩擦系數(shù)：I1—滑臺(tái)與油缸的質(zhì)量12—自調(diào)背壓閥閥心質(zhì)量C1、c 2—油缸無桿腔及有桿腔的液容；C2—自調(diào)背壓閥彈簧柔度；R1, R2 自調(diào)背壓閥阻尼孔液阻，R9—自調(diào)背壓閥閥口液阻Se2—自調(diào)背壓閥彈簧的初始預(yù)緊力；I4, I5—管路的等效液感C5、C 6—管路的等效液容：R5, R7-管路的等效液阻；V3, V4—油缸無桿腔及有桿腔內(nèi)容積；P3, P4—油缸無桿腔及有桿腔的壓力F—滑臺(tái)承受負(fù)載，V—滑臺(tái)運(yùn)動(dòng)速度。本文采用功率鍵合圖和狀態(tài)空間分折法建立系統(tǒng)的運(yùn)動(dòng)數(shù)學(xué)模型，滑臺(tái)的動(dòng)態(tài)特性可以能得到顯著改善。引言在組合機(jī)床正常工作中，滑臺(tái)運(yùn)動(dòng)速度的大小和它的方向以及所承受負(fù)載的變化都將以程度不同地影響其工作性能。特別是在工進(jìn)過程中?；_(tái)上負(fù)載的突然消失引起的前進(jìn)以及負(fù)載的周期性變化而引起的運(yùn)動(dòng)不平穩(wěn)性，都將影響被加工件的黃 河 科 技 學(xué) 院 畢 業(yè) 設(shè) 計(jì) (文 獻(xiàn) 翻 譯 ) 第 2 頁表面質(zhì)量，在嚴(yán)重的情況下會(huì)使刀具折斷掉。根據(jù)大連機(jī)床廠要求，作者采用有束縛力的曲線圖和狀態(tài)空間分析法建立組合機(jī)床滑臺(tái)的新型液壓驅(qū)動(dòng)系統(tǒng)一自調(diào)背壓調(diào)速系統(tǒng)的動(dòng)態(tài)數(shù)學(xué)模型。為了改善滑臺(tái)的動(dòng)態(tài)特性，有必要去分析找出滑臺(tái)產(chǎn)生前沖和運(yùn)動(dòng)不平穩(wěn)的原因以及主要的影響因素，但那必須通過計(jì)算機(jī)數(shù)字仿真和研究得出最后的結(jié)果。動(dòng)態(tài)數(shù)學(xué)模型組合機(jī)床滑臺(tái)的液壓驅(qū)動(dòng)系統(tǒng)一自調(diào)背壓調(diào)速系統(tǒng)的工作原理圖如圖 I 所示。這個(gè)系統(tǒng)是用來完成"工進(jìn)一停止一快退 ”的工作循環(huán)。當(dāng)滑臺(tái)在工進(jìn)時(shí)，三位四通換向閥處于圖示右位，油泵的供油壓力在濫流閥的有效作用下近似地幾乎保持恒定，該油液流經(jīng)過換向閥和調(diào)速閥后進(jìn)入油缸的無桿腔，以推動(dòng)滑臺(tái)向前移動(dòng)；與此同時(shí)，從油缸有桿腔排出的壓力油經(jīng)自調(diào)背壓閥和換向閥流回油箱了。在這個(gè)過程中，兩個(gè)單向閥和溢流閥的工作狀態(tài)始終都沒有任何變化。對(duì)與象組合機(jī)床滑臺(tái)的液壓驅(qū)動(dòng)系統(tǒng)一自調(diào)背壓調(diào)速系統(tǒng)這樣的復(fù)雜非線性的系統(tǒng)，為了便于研究它的動(dòng)態(tài)特性，建立一個(gè)僅著重考慮主要影響因素的合理簡單的動(dòng)態(tài)數(shù)學(xué)模型是尤其重要的[1][2]。從理論分析和試驗(yàn)研究的列舉中可以得知：該系統(tǒng)的過程時(shí)間是遠(yuǎn)大于調(diào)速閥的過程時(shí)間的，當(dāng)油缸無桿腔有效承壓面積很大時(shí)，調(diào)速閥出口流量的瞬時(shí)的超調(diào)反映為滑臺(tái)運(yùn)動(dòng)速度的變化是很小的[2]。為了更加拓寬和深入研究系統(tǒng)的動(dòng)態(tài)特性，使研究工作能在微型計(jì)算機(jī)上有效地進(jìn)行，本文章對(duì)原模型[2]做進(jìn)一步簡化處理，假定調(diào)速閥在系統(tǒng)的整個(gè)通過過程中輸出時(shí)候恒定的流量，這被看作其為流源。這樣，系統(tǒng)的動(dòng)態(tài)模型的結(jié)構(gòu)簡圖如圖 2 所示，它是由油缸、滑臺(tái)，自凋背壓閥和聯(lián)接管路等組成。 。功率鍵合圖是一功效流圖，它是按著系統(tǒng)的能量傳遞方式，以實(shí)際結(jié)構(gòu)為基礎(chǔ)，用集中參數(shù)把子系統(tǒng)之間的作用關(guān)系抽象地表示為阻性元 R、容性元 C 和感性元 I的三種作用元。采用這種方法建模物理概念清晰，結(jié)合狀態(tài)空間分析法可準(zhǔn)確地描述和分析線性系統(tǒng)，該法在時(shí)域中研究復(fù)雜非線性系統(tǒng)動(dòng)態(tài)特性的一種有效的方法。根據(jù)自調(diào)背壓調(diào)速系統(tǒng)各元件的主要特性和建模規(guī)則[1]，得出了圖 3 所示的系統(tǒng)的功率鍵合圖。圖中每根鍵上的半箭頭表示功率流向，構(gòu)成功率的兩個(gè)變量是力變量（油壓 P 或作用力 F）和流變量（流量 q 或速度 v） 。O 結(jié)點(diǎn)表示在系統(tǒng)中屬于并聯(lián)連接，各鍵上的力變量相等而流變量之和為零；1 結(jié)點(diǎn)表示在系統(tǒng)中屬于串聯(lián)連接，各鍵上流變量相等而力變量之和為零。TF 表示不同能量形式間的變換器，TF下標(biāo)注的字母表示力變量或流變量的轉(zhuǎn)換比值。鍵上的短橫杠表示該鍵上兩變量間的因果關(guān)系。全箭頭表示控制關(guān)系。在三種作用元中容性元和感性元的力變量與流變量之間具有積分或微分關(guān)系，因此，根據(jù)圖 3 可推導(dǎo)出具有九個(gè)狀態(tài)變量的復(fù)雜非線性狀態(tài)方程。本文對(duì)滑臺(tái)動(dòng)態(tài)特性的研究是從滑臺(tái)的前沖和運(yùn)動(dòng)平穩(wěn)性兩方面入手，用四階定步長 Runge-Kutta 法在 IBM-PC 微型計(jì)算機(jī)上進(jìn)行數(shù)字仿真 ,仿真結(jié)果分別如圖 4 和圖 5 所示。黃 河 科 技 學(xué) 院 畢 業(yè) 設(shè) 計(jì) (文 獻(xiàn) 翻 譯 ) 第 3 頁滑臺(tái)前沖滑臺(tái)前沖現(xiàn)象是作用在滑臺(tái)上的負(fù)載突然消失（如鉆削工作的情況)引起的。在此過程中，滑臺(tái)的負(fù)載 F、運(yùn)動(dòng)速度 V、油缸兩腔壓力 P3 和 P4 的變化可從圖 4 仿真結(jié)果看出。當(dāng)滑臺(tái)在負(fù)載的作用下勻速運(yùn)動(dòng)時(shí)，油缸無桿腔油液壓力較高．油液中聚集了大量的能量。當(dāng)負(fù)載突然消失時(shí)，該腔油壓隨之迅速降低，油液從高壓態(tài)轉(zhuǎn)入低壓態(tài)的過程中向系統(tǒng)釋放很多能量，致使滑臺(tái)高速向前沖擊。然而，滑臺(tái)的前沖使油缸有桿腔油液受壓引起背壓升高，從而消耗掉系統(tǒng)中的一部分能量，對(duì)滑臺(tái)的前沖起到一定的抑制作用。應(yīng)當(dāng)看到，在所研究的系統(tǒng)中，自調(diào)背壓閥的入口壓力要受到油缸兩腔油壓的綜合性作用。在負(fù)載消失的瞬間，自調(diào)背壓閥的壓力將會(huì)迅速地上升，并穩(wěn)定地處在高于初始背壓的數(shù)值以上。從圖中可見，自調(diào)背壓調(diào)速系統(tǒng)在負(fù)載消失瞬間油缸背壓力升高的幅度大于傳統(tǒng)的調(diào)速系統(tǒng)，所以，其油缸有桿腔中油液吸收的能量就多；結(jié)果，滑臺(tái)的前沖量比傳統(tǒng)調(diào)速系統(tǒng)要小大約20%?？梢姴捎米哉{(diào)背莊調(diào)速系統(tǒng)作為驅(qū)動(dòng)系統(tǒng)的滑臺(tái)在抑制前沖方面具有良好的特性，其中自調(diào)背壓閥起了很大作用?；_(tái)的運(yùn)動(dòng)平穩(wěn)性當(dāng)作用于滑臺(tái)上的負(fù)載作周期變化時(shí)（比如說銑削加工的情況） ，滑臺(tái)的運(yùn)動(dòng)速度將要產(chǎn)生一定的波動(dòng)。為于保證加工質(zhì)量的要求，必須盡可能地減小其速度波動(dòng)的范圍。而從討論問題的方便性出發(fā)來說，假設(shè)負(fù)載按正弦波的規(guī)律變化，從而得到的數(shù)字仿真結(jié)果如圖 5 所示。由此可見這個(gè)系統(tǒng)與傳統(tǒng)的調(diào)速系統(tǒng)有著相同的變化規(guī)律以及非常接近的數(shù)值數(shù)字。其中的原因是負(fù)載的變化幅度不大，油缸兩腔的壓力也就沒有較大變化，從而最終導(dǎo)致自調(diào)背壓閥的作用不夠明顯顯示。改善措施通過研究的結(jié)果表明，以自調(diào)背壓調(diào)速系統(tǒng)作為驅(qū)動(dòng)系統(tǒng)的滑臺(tái)，其動(dòng)態(tài)特性要比傳統(tǒng)的調(diào)速系統(tǒng)好的。要減少滑臺(tái)的前沖量，就必需在負(fù)載消失的瞬間時(shí)候迅速提高油缸有桿腔的背壓力；要提高滑臺(tái)的運(yùn)動(dòng)平穩(wěn)性就需增加系統(tǒng)的剛性，主要措施在于減小油液的體積。從系統(tǒng)的結(jié)構(gòu)得知，油缸有桿腔與排油管之間有一很大的容積，如圖 6a 所示。它的存在方面延遲和衰減了自調(diào)背壓閥的作用，另一方面也降低了系統(tǒng)的剛性，它會(huì)限制了前沖特性和運(yùn)動(dòng)平穩(wěn)性的進(jìn)一步改善。因此，改善滑臺(tái)動(dòng)態(tài)特性可從兩個(gè)方法進(jìn)行處理：即改變油缸容積和改變自調(diào)背壓閥結(jié)構(gòu)尺寸。通過一系統(tǒng)結(jié)構(gòu)性參數(shù)的仿真計(jì)算以及結(jié)果的比較可以得出這樣的結(jié)果：當(dāng)把油缸有桿腔與排油管間容積 V4 同無桿腔與進(jìn)油管間容積 V3 之比由原來的 5.5 改為 1 時(shí)，如圖 6b 所示，同時(shí)，把自調(diào)背壓閥閥芯底端直徑由原來的 10mm 增加為 13mm,阻尼三角槽邊長從原來的 lmm 減小到 0.7mm 時(shí)，可使滑臺(tái)的前沖量減小 30%,過渡過程時(shí)間明顯縮短了，滑臺(tái)的運(yùn)動(dòng)平穩(wěn)性也將會(huì)得到很大的改善。結(jié)論通過理論上的分析和計(jì)算機(jī)仿真研究實(shí)驗(yàn),很明顯的是自調(diào)背壓調(diào)速系統(tǒng)作為組合機(jī)床滑臺(tái)的驅(qū)動(dòng)系統(tǒng)是很有推廣使用價(jià)值的。影響滑臺(tái)動(dòng)態(tài)特性的主要因素是油黃 河 科 技 學(xué) 院 畢 業(yè) 設(shè) 計(jì) (文 獻(xiàn) 翻 譯 ) 第 4 頁缸內(nèi)部結(jié)構(gòu)性和自調(diào)節(jié)背壓閥的尺寸。假如能對(duì)其進(jìn)行合理設(shè)計(jì)，可使滑臺(tái)的動(dòng)態(tài)特性得到顯著地改善。同時(shí)，也說明了采用有束縛力的曲線圖和狀態(tài)空間分析法研究復(fù)雜非線性液壓系統(tǒng)的動(dòng)態(tài)特性是既方便又有效的方法。附錄 英文原文翻譯文獻(xiàn)：INVESTIGATION ON DYNAMIC PERFORMANCE OF SLIDE UNIT IN MODULAR MACHINE TOOL （對(duì)組合機(jī)床滑臺(tái)動(dòng)態(tài)性能的調(diào)查報(bào)告 ）文獻(xiàn)作者：Peter Dransfield,出處： Peter Dransfield, Hydraulic Control System-Design and Analysis of TheirDynamics, Springer-Verlag, 1981翻譯頁數(shù)：p139—144INVESTIGATION ON DYNAMIC PERFORMANCE OF SLIDE UNIT IN MODULAR MACHINE TOOLAuthor： Peter DransfieldThis paper deals with the investigation for slide unit's impact and motion stability in modular machine tool fay means of the method of power bond graph and state space analysis. The dynamic mathematical model of self-adjusting back pressure speed control system used to drive slide unit is established. Main reasons and affecting factors for slide unit impact and motion unstability are analysed through computer digital simulation, It is concluded from those that, if the structural dimensions of hydraulic cylinder and back pressure valve are designed rationally, the slide unit's dynamics will markedly be improved.NOMENCLATURESf flow sourceSei sliding friction force in slide unitR equivalent viscous friction coefficient in slide unitIi mass of slide unit and cylinderh mass of SABP valve spoolCi,C2 hydraulic capacitances of rod chamber and non-rod chamber in cylinder re-spec-tivelyC3 spring compliance of SABP valveR]rR2 hydraulic resistances of damping holesR9 hydraulic resistance of orifice of SABP valveSe2 presetting force of spring in SABP valveI4J5 equivalent liquid inertia in pipe linesC^Cg equivalent hydraulic capacitances in pipe lines黃 河 科 技 學(xué) 院 畢 業(yè) 設(shè) 計(jì) (文 獻(xiàn) 翻 譯 ) 第 5 頁equivalent hydraulic resistances in pipe linesV-j V^ oil-containing volumes in non-rod chamber and rod chamber respectivelyP,r:, P-i oil pressures in non-rod chamber and rod chamber respectivelyF load acted on slide unitV slide unit velocity* Department of Mechanical Engineering, Dalian Ur.iversity of Technology, Dalian. China.INTRODUCTIONDuring operation of modular machine tool, the changes of slid unit's speed and load acted on it in both magnitude and direction will affect working performar.ee to a different extent Particularly the impact caused by sudden vanishing of load and the motion unstability due to periodical change of load in operation will affect the surface quality of the workpiece machined, and the tool would be broken off under serious conditions, By using the method of power bond graph and state space analysis, the dynamic mathematical model of the system used to drive slide unit is established, that is called as self-adjusting back pressure speed control system and abbreviated to SABP system. In order to improve slide unit's dynamics, it is necessary to find out the main reasons and affecting factors, that must be based on computer digital simulation and study on the results.DYNAMIC MATHEMATICAL MODELThe schematic diagram of SABP system is shown in Fig.l, the system is used to perform the cycle of feeding, stopping and returning. Four way control valve works in the right position during slide unit's feeding. The supply pressure of the pump is approximately constant under the action of pressure relief valve, the oil through the control valve and pressure compensated flow control valve enters the non-rod chamber to put slide unit forward. At the same time, the oil from the rod chamber is discharged through SABP valve and directional control valve to tank. In this process, the state of two check valves and pressure relief valve is not changed, To establish the mathematical model as reasonably and simply as possible, consideration must be focused on main affecting factors for a complex non-linear system such as the SABP system. It is illustrated by theoretical analysis and test result ' , that the transient time of the system is much longer than that of the flow control valve, and the flowrate overshoot of the valve in transients affects very small to slide unit speed because of the ;large effective sectional area of non-rod chamber in cylinder. For investigating the system's dynamics widely and deeply, the initial modeltn is further simplified in this paper, and so the study can be efficiently made with microcomputer. It is assumed that the flowrate through the flow control valve isconstant in the whole transient process, and is denoted to a flow source.Fig.2 shows the structure diagram of the dynamic model of the system, it is composed of cylinder, slide unit, 黃 河 科 技 學(xué) 院 畢 業(yè) 設(shè) 計(jì) (文 獻(xiàn) 翻 譯 ) 第 6 頁SABP valve and pipe line; etc.By using the method of power bond graph and state space analysis in this paper, the dynamic mathematical model of the system is to be established- The power bond graph is a power flow diagram, which expresses abstractly the actions among sub-systems as three effects, i.e. resistance effect, capacitive effect and inertia effect, according to the way of energy transform, on the basis of practical structure and by means of method of lumped parameters. The model is characterized by a clear con-ception in physics, and non-linear system can be accurately analysed in combination with method of state space analysis, thus it is a effective method used in the dyna-mic investigation of complex non-linear system in the'timedomain.From main performances of components in SAEP system, the power bond graph of the system has been formed by means of the rule of model establishing ' and is shown in Fig.3. Half arrow in each bond indicates a direction of power How, two variahles of power are effort variable and flow variable. O-junction illustrates algebraic summation of flow variables at common effort, i.e. parallel connection, 1-junction does algebraic summation of effort variables at common flow, i.e. series connection. The symbol TF represents power transformer between two types of energy, and transforming modulus between efforts or flows is noted below the sym-bol TF. Short transverse bar across one end of each bond shows causality between two variables. A full arrow expresses a control action. Among three actions, there is an integration or differential form in capacitive effect and inertia effect between two variables. So state equation may be derived from Fig.3, there are nine state vari-ables in this complex nonlinear equation. Studying on the slide unit's dynamics is started with impact and motion stability. The equation is simulated by using the method of 4th order Runge-Kutta integration procedure on IBM-PC computer. Fig.4 and Fig.5 illustrate the results respectively.SLIDE UNIT IMPACTSlide unit's impact phenominon results from load's vanishing in the transients, ■ for example, the situation of drilling through workpiece, Fig.4 expounds the variations of the load and speed of slide unit, the pressures of chambers in cylinder. When slide unit motions evenly under the action of load, the oil pressure in non-rod chamber is very high, and there is a lot of hydraulic energy accumulated in side. The pressure decreases at once with load's discharging rapidly. During the process of oil pressure converting from high to low, the system absorbs some of the energy, so slide unit impacts forward with high speed. And then the oil in rod chamber is141黃 河 科 技 學(xué) 院 畢 業(yè) 設(shè) 計(jì) (文 獻(xiàn) 翻 譯 ) 第 7 頁compressed to increase back pressure, some of the energy is consumed, which plays a part of restraining the impact of the slide unit. It must be noted that inlet pressure of SABP valve telys on the interaction of pressures of two chambers, and increases rapidly at the instant of load's vanishing, and then stabilizes at some value greater than initial one. This pressure is also greater than one of traditional speed control system, therefore the energy can be absorbed much more in the rod chamber. In re-sult, the impact of slide unit in SABP system is 20% lawer than in traditional's. It is thus clear that slide unit with SABP system for driving has a good performance in restraining the impact and SABP valve plays an important part in that,MOTION STABILITYWhen load acting on slide unit varies periodically, such as the situation of mill-ing, slide unit's speed will bring about some pulse. In order to meet the requirements of manufacturing quality, the magnitude of the speed pulse must be reduced as small as possible. The variation of the load is assumed to be of sine wave, in order to sim-plify discussion of the problem, The result of digital simulation is shown in Fig.5 It can be seen that, the response of the system is the sameas traditionl's and the differ-ences between them are very small. The reason for this is that the variation of the load is not targe, there the pressures in chambers vary very little that is, the effect of the SABP valve is not obvious.IMPROVEMENTIt is shown by studying, that dynamics of slide unit which used SABP speed control system as driving system is better than that of traditional system. To reduce the slide unit's impact, the back pressure of rod chamber has to be increased rapidly in the transients of load's vanishing; on the other hand, to enhance the slide unit's motion stability, it is necessary to raise the system rigidity. However, main recom-mendation lies in decreasing the volume of oil. It is known from system structure that, there is a lot of oil-containing volume between the rod chamber and drain pipe as shown in Fig.6a. Because the volume exists, not only the effect of SABP valve is delayed and reduced, hut also the rigidity of the system is decreased. Therefore, it is hindered to further improve the impact and motion stability. To make the slide unit dynamics better, the structural dimensions of cylinder's chamber and the SABP valve must be designed suitably. Based on simulations under the various structural dimensions and comparison among the results, the following two measures can be taken for improvements:the ratio between volumeV4 and V3 is changed from 5.5 to 1 approximately, as shown in Fig,6b;142黃 河 科 技 學(xué) 院 畢 業(yè) 設(shè) 計(jì) (論 文 )文 獻(xiàn) 翻 譯 第 8 頁the bottom diameter of spool of the SABP valve is increased from 10 mm to 13 mm and.the length of side of triangular damping slot is decreased from 1 mm to 0.7mm the slide unites impact quantity can be reduced by 30%, and the time of dynamic response is shortened. In addition, slide unit's motion stability may be improved obviously. It is thus evident that improvements are very effective.CONCLUSIONSBy the way of theoretical analysis and computer simulating investigation, it is obvious that the SABP speed control system used as slide units driving system in modular machine tool is valuable for popularization and utilization, Main factors affecting the slide unit dynamics are the structural dimensions of the cylinder and the SABP valve. In the case of rationally designing, the slide unit dynamics may be obviously improved. Meanwhile, it is shown that the method of power bond graph and state space analysis is a very convenient and effective method in studying dynamics of complex non-linear hydraulic system.REFERENCES[1] Peter dransfield Hydraulic Control System-Design and Analysis of Their Dyna-mics Springer-Verlag, 1981[2] Dong Bengang, Zhang Zhixiang, Investigation on impact property of selfadjusting back pressure speed control system. Machine tool & Hydraulics No.2, 1987 GuangZhou, China, (in Chinese).