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指導教師：李實,數(shù)控銑床夾緊裝置液壓系統(tǒng)設計,班級：07機自二班學生：張宏學號：14071900503,,論文的結構和主要內容,第一部分：液壓系統(tǒng)的研究概述第二部分：液壓系統(tǒng)的設計第三部分：液壓原理圖的擬定和方案驗證第四部分：液壓元件的計算和選擇第五部分：估算液壓系統(tǒng)性能,背景：機電產品日趨精密復雜，對數(shù)控銑床的要求越來越高，液壓技術在國民經濟各領域得到了廣泛的應用。目的：通過對數(shù)控銑床的工況分析，設計出一個能夠提高加工精度，結構簡單，便于操縱的夾緊裝置液壓系統(tǒng)。方法：構思---查閱---篩選---組合---請教---修補,研究概述,液壓系統(tǒng)總體結構,工作原理：本設計采用缸筒固定液壓缸驅動夾緊裝置，夾緊裝置由液壓與電氣配合實現(xiàn)自動循環(huán)：Ⅰ工位夾緊缸夾緊→Ⅰ工位夾緊缸松開→Ⅱ工位夾緊缸夾緊→Ⅱ工位夾緊缸松開。以完成工件裝夾運動,液壓系統(tǒng)原理圖的方案論證,調速方案的分析和選擇：設計系統(tǒng)本身屬于小功率輕載，低速中壓系統(tǒng)且工程機械非經常性調速，故采用節(jié)流調速。油路循環(huán)方式的分析和選擇：液壓系統(tǒng)油路循環(huán)方式分為開式和閉式兩種，節(jié)流調速一般采用開式循環(huán)方式，其結構簡單散熱性好。,液壓系統(tǒng)原理圖的方案論證,液壓動力源的分析與選擇：節(jié)流調速系統(tǒng)一般用定量泵供油在無其他輔助油源的情況下，液壓泵的供油量要大于系統(tǒng)的需油量，多余的油經溢流閥流回油箱，溢流閥同時起到控制并穩(wěn)定油源壓力的作用。液壓回路的分析選擇與合成：根據(jù)系統(tǒng)的負載特性和特殊要求選擇基本回路，在本系統(tǒng)中考慮到安全的要求，設置了背壓回路，同時由于是兩個執(zhí)行元件先后動作，且沒有順序聯(lián)動關系，所以設置了互不干擾回路配以輔助性回路，如控制油路，潤滑油路、測壓油路等，可以組成一個完整的液壓系統(tǒng),液壓原理圖的擬定,,液壓元件的選擇依據(jù),液壓泵：>=＋≥4.38+0.5＝4.88MPa==840WY90S—4B5型封閉式三相異步電動機滿足上述要求，其轉速為1400r/min，額定功率為1.5kW。閥類：一般選擇控制閥的額定流量應比系統(tǒng)管路實際通過的流量大一些，必要時，允許通過閥的最大流量超過其額定流量的20%,,,,,,,,,液壓元件的選擇依據(jù),管件：Ⅰ工位夾緊液壓缸有桿腔和無桿腔油管的實際最大流量分別為3.21L/min和4.67L/min，Ⅱ工位夾緊液壓缸有桿腔和無桿腔油管的實際最大流量分別為14L/min和23.02L/mind==4.9m=4.1m(Ⅰ工位夾緊液壓缸)=11.1m=8.6m(Ⅱ工位夾緊液壓缸)同時考慮到制作方便，Ⅰ工位夾緊液壓缸兩根油管同時選用10×1（外徑10mm，壁厚1mm）的10號冷拔無縫鋼管。Ⅱ工位夾緊液壓缸兩根油管同時選用14×1（外徑14mm，壁厚1mm）的10號冷拔無縫鋼管。油箱容積：V=V=6×14=84L,,,,,,,估算液壓系統(tǒng)性能,液壓系統(tǒng)壓力損失驗算:油缸夾緊時:==0.0009MP（Ⅰ）=0.017MP(Ⅱ)油缸松開時:==0.0097MP(Ⅰ)=0.2MP（Ⅱ）系統(tǒng)閥的壓力損失都小于原先的估計值，所以滿足系統(tǒng)的使用要求。系統(tǒng)的發(fā)熱和溫升=H/KA=41.8滿足了許用溫升要求。至此，系統(tǒng)校核完畢，從整個過程來看，此設計滿足使用需求。,,,,,,,,,,,結論,在本次關于數(shù)控銑床兩工位夾緊裝置液壓系統(tǒng)的畢業(yè)設計中，我通過查閱現(xiàn)有資料，應用液壓傳動的一般原理及液壓系統(tǒng)的設計原理，進行了液壓系統(tǒng)的功能原理設計、執(zhí)行元件的選擇、閥類元件的選擇等工作。通過對液壓系統(tǒng)的性能計算和校核使系統(tǒng)符合最初設計，在設計過程當中，由于實踐經驗和知識水平的不足，關于設計計算、結構設計以及校核等方面都會有不完善之處。在今后的工作學習中我會更加詳盡的學習，提高設計的科學性和效率。使之更加完善，符合實際工程機械的要求,大學本科的學習生活即將結束。在此，我要感謝所有曾經教導過我的老師和關心過我的同學，他們在我成長過程中給予了我很大的幫助。本文能夠順利完成，要特別感謝我的導師李實老師，感謝各位系的老師的關心和幫助。,致謝,
CONTROL OF MOBILE HYDRAULIC CRANES
Marc E. MüNZER
Aalborg University
Institute of Energy Technology
Pontoppidanstr?de 101
DK-9220 Aalborg. Denmark
Email: mmun@iet. auc. dk
The goal of the thesis described in this paper is to improve the control of mobile hydraulic cranes. The thesis is split into five parts: a requirements analysis, an analysis of the current systems and their problems, an analysis of different possibiilities for system topologies, development of a new control system for the near future based on electro-hydraulic separate meter in / separate meter out valves, and finally an analysis of more advanced and complex solutions which can be applied in the more distant future. The work of the thesis will be done in cooperation with industry so the thesis will have more of an industrial focus than a purely theoretical focus.
Key words: Mobile Hydraulic Cranes, Control strategies, Separate Meter-in/Separate Meter-out.
1 INTRODUCTION
The goal of the thesis described in this paper is to improve the control of mobile hydraulic cranes. A mobile hydraulic crane can be thought of as a large flexible mechanical structure which is moved by some sort of control system, The control system takes its input from a human operator and translates this command into the motion of actuators which move the mechanical structure.
The definition of this control system is purposely left vague in order not to impose any constraints on its design. The control system consists of actuators which move the mechanical structure, a means of controlling the actuators, a means of supplying power to the actuators, and a way of accepting inputs from the operator. It is this control system which is the target of this thesis. The goal is to analyze the requirments made on the control system and present guidelines for the gesign of new control systems.
The thesis will be split into five parts:
1. Analysis of the requirements of the control system, from the perspective of the operator, the mechanical system, efficiency, stability, and safety requirements.
2. Analysis of current control systems and what their problems are.
3. Analysis of the different options for the control system: different types of actuators different types of control strategies, and different ways of organizing components.
4. Presentation of a new type of control system, which is commercially implementable. A system that will meet the needs of industry in the near future.
5. Analysis of more optimized systems, with higher performance, better efficiency, more flexible control, etc. This will be less commercially applicable but will be a starting point for more research.
2 SECTIONS OF THE THESIS
2.1 Requirements Analysis of the Control System
Before starting detailed work on developing new control systems, it is important to analyze what the exact demands are on the control system. The control system is influenced by many factors.For example: the mechanical structure it is controlling, the human operator, efficiency, stability, and industry requlations.
Industry regulations are the first requirements that have to be addressed. Things like hose rupture protection and runaway load protection make a lot of demands on the control system. After regulations, stability is the next most important requirement; without stability the control system can’t be used. Once stability has been assured, the performance requirements of the control system have to be set. They are determined by the mechanical structure of the crane and the human operator. The mechanical structure of a mobile hydraulic crane is a very necessary to keep the speed of the control system below this natural frequency or to develop
a control system which can increase this frequency. The human operator also impossible limits on the control system. If the control system is too slow or too fast then it is impossible for a human operator to give it proper inputs. And finally, once the requlations have been met, stability is assured, and the performance is at the right level, the power efficiency of the control system has to be optimized.
2.2 Analysis of Current Control Systems
Before designing a new control system it is good to analyze the current control systems to find out what their problems are. Current control systems are mainly hydraulic and can suffer from three main problems:
1. Instability
2. High cost
3. Inefficiency
2.2.1 Instability
Instability is a serious problem as it can cause injury to human operators or damage to equipment. When a system becomes unstable it usually starts to oscillate violently. To avoid instability in current systems, the designers either sacrifice certain functions which are desirable, or add complexity and cost. For example, in the crane shown in Figure 1, it would be desirable to have control over the speed. But due to the safety system that cranes are required to have, standard speed control is not stable. To add speed control requires a more complex and more expensive mechanical system.
The parameters of a hydraulic system, such as temperature or load force, also affect stability. A system that is stable with one set of parameters might be unstable with another set. To ensure stability over the entire operating range of the system, performance must sometimes be sacrificed at one of the parameter range.
2.2.2 High cost
Current systems are purely hydraulic-mechanical, so if the user wants a certain function, the user buys a certain hydraulic-mechanical component. Because most user have different requirements, there are many different variations of the same basic component. This means that many specialized components must be manufactured rather than one standard product. This drives up the cost of components.
2.2.3 Inefficiency
One form of inefficiency in current systems is due to the link between the flows of the two ports of the cylinder. This is because most valves use a single spool to control the flow in both ports. Because of this link, it is impossible to set the pressure levels in the two sides of the cylinder independently. Therefore, the outlet side will develop a back pressure which acts in opposition to the direction of travel, which increases the pressure required on the inlet side to maintain motion. Since the force generated by the actuator is proportional to the pressure difference between the two sides, the actual pressures in the cylinder don’t affect the action of the cylinder. For example, the action of the cylinder for 0psi/600psi would be the same as 1000psi/1600psi. However, in the second case, the power supply would have to supply much more power. This extra power is wasted.
2.3 Different Options for Control Systems
Current control systems use hydraulic actuators with directional/proportional valves to control the movement. However there are many different options for controlling a cylinder. Options range from new high performance electro-hydraulic valves, to separate meter in / separate meter out (SMISMO) valves, to hydraulic bus systems, to intelligent actuators with built in power supplies, to pump based control strategies. These systems all have advantages and disadvantages which need to be analyzed if the most optimum solution is to be chosen.
2.4 Near Future Solution
It is expected that even if it is proven that a completely new system topology is the optimum configuration, the crane manufacturers and component manufacturers will not accept the new technology overnight. This will most likely take time, so an interim solution will be developed.
This solution will be made up of micro computer controlled Separate Meter In / Separate Meter Out (SMISMO) valves (Elfving, Palmberg 1997; Jansson, Palmberg, 1990; Mattila, Virvalo 1997). SMISMO valves will make it possible to implement new control strategies which are more efficient and stable. The micro computer will make it possible to introduce flexibility to valves. Variants can be programmed in software. This eliminates the need to manufacture hundreds of different variants. The crane manufacturer will be able to choose the exact functions he wants in his valve, while the component manufacturer will have to manufacture only one valve. This will lower the cost, even though the performance will have increased.
2.5 Analysis of Higher Performance Solutions
This analysis will depend on the results of the analysis of different topologies. If it is shown that pump based control is to be the way of the future for example, then analysis will be performed in this area. Another area which will also be explored, is tool position control.
3 LABORATORY FACILITIES
As the focus of this thesis is on developing control strategies that can be implemented on commercial machinery, much emphasis will be placed on experimental results. Experimental results will be obtained from two systems. The first, a simple one degree of freedom crane, was designed as an experimental platform. The second is a real crane which was donated to the University by Hojbjerg Maskinfabrik (HMF) a Danish crane manufacturer. Refer to Figure 1.
Figure 1 Experimental Systems in Laboratory. Left: One DOF crane model. Right: Real
Mobile Hydraulic Crane
As there are currently no commercially available separate meter-in/separate meter-out valves, two separate valves will be used instead. A sample circuit of one cylinder is shown in Figure 2. The control algorithms which control the valves, will be programmed on a Digital Signal Processor (DSP)/Pentium dual processor system. The DSP will run the control code and the Pentium will do diagnostics and provide a graphical user interface.
Figure 2 Separate Meter In / Separate Meter Out Setup
4 CURRENT WORK
4.1 Flow Control by Direct Actuation of the Spool
Most flow control valves on the market today work with a pressure compensator (Andersen; Ayers 1997). The pressure compensator keeps a constant pressure drop across the main spool of the valve, which keeps the flow constant. However, the addition of a pressure compensator makes the valve more complicated than a simple single spool valve. Another way of doing flow control is to measure the pressure drop across the valve and adjust the spool position to account for this (Backé; Feigel 1990). This is not a new idea but has not been implemented commercially because of the high cost of pressure transducers and micro controllers. However, with the current drop in cost of micro controllers and pressure transducers this idea is now commercially feasible.
The concept is very simple, spool position is calculated from the Bernoulli equation using the pressure drop across the spool and reference flow.
Even though this is a simple equation, it is not easy to implement. The accuracy of the flow control is dependent on the precision of the position sensors and of the pressure transducers. Noise on the pressure or the position signals can cause stability problems. Filtering the noise, introduces delays in the control which can also affect stability. In addition the Bernoulli equation is not followed exactly over the entire operating range of the valve, so it may be necessary to store the valve characteristics as a data table or develop a more complex equation.
4.2 Cylinder Control Strategy
To control a hydraulic cylinder, the strategy has to be able to handle four different situations depending on the directions of the load and the velocity of the cylinder. Refer to Figure 3.
Figure 3 Different Situations in Crane Operation
The control strategies that have appeared in the literature are usually quite complex and depend on measurements of the cylinder position and velocity (Elfving, Palmberg 1997; Mattila; Virvalo 1997). They are also based on rather complex control algorithms. It is the goal of this thesis to start with a control strategy which is based on simple PI controllers and makes no demands for position and velocity of the cylinder. The performance of this system will be lower than a complex control strategy, but it may be easier to implement commercially because it has no need for special sensors and is easier to understand for the average engineer.
Another feature which needs to be acknowledged when designing a control strategy, is the type of valve used. Mobile hydraulic valves demand low leakage and since most mobile valves are spool valves, they usually have large overlaps. In addition, to make the cost of the valve acceptable to industry, the actuation stage on the spool is usually quite slow. This combination of large overlap and slow actuation makes it hard to implement many of the strategies that have been presented. Pressure control especially becomes difficult when there is an overlap and a slow actuator.
One example of a new strategy which is simple and robust is described as follows. Flow control is implemented on the inlet side and pressure control is implemented on the outlet side. The flow control is based on the Bernoulli equation. Pressure control is done by PI controller which maintains a low constant pressure to increase the efficiency and prevent cavitation. To work around large overlaps and slow actuation stage, the pressure controller only does meter out control. This means that if the controller wishes to raise the pressure, it can’t add flow to the cylinder, it can only decrease the opening of the meter out port. The benefit of this is that the only time that the spool has to cross the zero position is when the operator wishes to change the direction of motion of the cylinder. For the case where the load force and the velocity are in the same direction, this strategy has to be modified. In this case, the pressure reference of the pressure controller at the outlet is increased to a value which opposes the load force. The pressure reference is increased when it is noticed that the pressure of the inlet side is dropping. The pressure reference is also controlled by a PI controller. A schematic model of the controller system for the load lowering case is shown in Figure 4.
At the time of writing this paper the initial experimental tests had performed on the real crane shown in Figure 1. Stability was not achieved because the crane is equipped with a load holding valve. However, the load holding valve will be replaced with a pilot operated check valve, which should make it possible to stabilize the system. In current systems, the load holding valve serves two functions, load holding and runaway load protection. Due to the use of a SMISMO valve setup, the runaway load protection is built into the control strategy, therefore the only function which is necessary for the load holding valve to perform is load holding. A pilot operated check valve will be able to do this, without adding complex dynamics which upset the stability of the system.
Figure 4 Controller Strategy for Lowering of Load
5 CONCLUSION
Even though not much experimental work has been finished, a good start has been made and initial tests have been promising. The outline of the thesis has been developed and organized in a logical manner. The work is split into five parts, requirements analysis, analysis of current systems, analysis of different topologies, development of a near future solution, and development of a more optimum solution. At the end of the thesis, the control of mobile hydraulic cranes will have been improved.
6 ACKNOWLEDGEMENTS
This project is being funded in part by Danfoss Fluid Power A/S. The author would also like to thank Hojbjerg Maskinfabrik (HMF) A/S for the donation of the test crane.
隨車液壓起重機的軌跡控制
問題描述
這項方案是根據(jù)如圖1所示的多自由度隨車液壓起重機控制問題提出來的?？刂齐S車起重機要求操作人員技術相當高，它的操作機動范圍很小。如果可以讓現(xiàn)代的起重機實現(xiàn)遙控控制的話，操作人員只需要控制他手中的遙控器就可以控制起重機把重物放在他要求的任何地方。一個按鈕控制一個自由度方向上的轉動。因此只需要讓操作人員得到熟練的訓練他就可以每次控制更多的按鈕來實現(xiàn)多個自由度的轉動。
吊具總成
圖1所示為一臺隨車液壓裝載起重機部分液壓系統(tǒng)控制圖實例
這項工程的目標是設計一臺非熟練操作人員都能夠控制的移動式液壓起重機。操作人員根據(jù)吊具總成的合成軌跡控制一根操縱桿。這樣不同的自由度就可以同時被控制。
圖2測試起重機圖片
多數(shù)隨車液壓起重機的結構就像圖1所示的那樣，大多數(shù)都是非常柔性化的，因此當受載時它們就會彎曲。這樣做可以使起重機吊重比最低。事實上吊重頂端位置也是制約控制系統(tǒng)結構偏差的因素。這種問題可以通過一個好的位置偏差補償控制系統(tǒng)解決，這個系統(tǒng)還可以消除操作初期結構上發(fā)生的擺動。
繼續(xù)使結構軌跡偏差補償控制系統(tǒng)在起重機上進一步發(fā)展，起重機的裝載能力將可以大大得到提高。當這種在起重機里的擺動可以被控制系統(tǒng)抑制的方法能夠得到充分證明，在一個長的期限里可能有一個降低動力學安全系數(shù)的機會。這將使起重機生產商和用戶節(jié)省一大筆費用。
方案內容
現(xiàn)以一臺如圖2所示的HMF 680-4型隨車液壓起重機來分析這些問題。在這臺起重機的不同位置安裝了傳感器來監(jiān)視系統(tǒng)上的不同參數(shù)值，它們都是一些起重機上很重要的不同連接位置的壓力、流量、應變參數(shù)值。實驗測試可以證實起重機性能，所以可以通過精確的模型來測試起重機的性能。為了使所含蓋的幾個問題能夠描述得更清楚，這些問題被簡略的表述如下：
1. 分析系統(tǒng)要求說明書
系統(tǒng)的執(zhí)行標準分析已被完成。基于系統(tǒng)的這種要求連同確保系統(tǒng)的執(zhí)行的檢驗程序將被列入清單。
2. 機械子系統(tǒng)模型
許多技術模型已經存在，因此這些部件包括研究明確的模型局部動力學的表達方法。機械子系統(tǒng)的分析與局部模型偏差的詳細分析相同。這樣做是為了使計算的有效性能夠明確表達出來，同時使系統(tǒng)的動作在控制過程中能夠十分精確?；谶@種非常有前景的用公式表示一個數(shù)學子系統(tǒng)模型的方法已經完成，它將從起重機試驗臺的實驗結果中得到校驗。
3. 液壓子系統(tǒng)模型
跟機械子系統(tǒng)建模一樣，液壓子系統(tǒng)模型由液壓泵、不同的液壓閥、激勵源和液壓導管組成。然而，并不是這些都要建模，只是那些對系統(tǒng)動力學部件影響比較大的成分才建模。液壓子系統(tǒng)模型也需要用實驗的方法來證明。除此之外是否在對偏差進行補償時，系統(tǒng)中用了比重比較大的電液比例控制閥都必須被分析，即對機械結構的擺動進行分析?；谏鲜鲂拚?，對液壓系統(tǒng)如果有必要都要做。
4.分析和標準的解決反轉運動結構
起重機相對于底部有一個可以操作的特定空間，即吊具總成能達到的范圍。這是公認的起重機工作范圍。有的部位要通過不同的路線才可以達到。因此有必要在這些區(qū)域確定最佳的運動結構。有不同的參數(shù)標準，習慣上用起重機上總負荷的最小值，也就是在臨界狀態(tài)點的最小壓力值。為了做這個重要的結構壓力分析，基于實現(xiàn)這個運算法則的控制系統(tǒng)將進一步得到發(fā)展。
5.載荷判斷方案的發(fā)展
為了實現(xiàn)起重機結構偏轉補償，需要知道起重機承受的有效載荷。因此，有必要進行不同的載荷在線可能情況分析，這樣就可以判斷哪一個傳感器需要進行載荷復合鑒定?；谶@種鑒定方案分析，可以實現(xiàn)最終的運算法則。
6. 控制運算法則的發(fā)展
基于這種機械液壓子系統(tǒng)模型，一種吊具總成位置軌跡控制的控制規(guī)律將會得到發(fā)展。這種控制規(guī)律可以保證系統(tǒng)按照吊臂頂?shù)倪\動軌跡運行，并且系統(tǒng)在工作情況下保持穩(wěn)定。這包含在載荷判斷和運動學最佳參數(shù)方案的分析中。
7. 控制系統(tǒng)的執(zhí)行
最后系統(tǒng)的控制規(guī)律已經通過仿真試驗得出，應該實現(xiàn)通過處理器或者數(shù)據(jù)信號處理檢驗系統(tǒng)實物了，即測試起重機。用這種測試方法將可以實現(xiàn)對系統(tǒng)制定測試，到測試結束的整個過程。這種測試技術還可以對一些典型系統(tǒng)進行控制。