數控銑床夾緊裝置液壓系統設計畢業論文-資料下載頁

　　

【正文】 fter 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. Analysis of Current Control SystemsBefore 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. Instability2. High cost3. Inefficiency InstabilityInstability is a serious problem as it can cause injury to human operators or damage to equipment. When a system bees unstable it usually starts to oscillate violently. To avoid instability in current systems, the designers either sacrifice certain functions which are desirable, or add plexity 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 plex 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. High cost湖南理工學院畢業設計（論文）33 Current systems are purely hydraulicmechanical, so if the user wants a certain function, the user buys a certain hydraulicmechanical ponent. Because most user have different requirements, there are many different variations of the same basic ponent. This means that many specialized ponents must be manufactured rather than one standard product. This drives up the cost of ponents. InefficiencyOne 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. Different Options for Control SystemsCurrent 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 electrohydraulic 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. Near Future Solution It is expected that even if it is proven that a pletely new system topology is the optimum configuration, the crane manufacturers and ponent 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 puter 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 puter 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 ponent manufacturer will have to manufacture only one valve. This will lower the cost, even though the performance will have increased. 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.湖南理工學院畢業設計（論文）343 LABORATORY FACILITIESAs the focus of this thesis is on developing control strategies that can be implemented on mercial 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 S