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Application and development of simulation technology in hydraulic control

simulation technology, as a necessary means in the design stage of hydraulic system or components, has been widely recognized by the industry. From its birth to today, the hydraulic simulation technology has a true strength true strain curve that can comprehensively describe the stress-strain relationship of the whole process of metal from elastic deformation to fracture, as shown in Figure 2 for more than 30 years. As early as 1973, the first special hydraulic simulation software hydsim program directly facing the field of hydraulic technology was successfully developed. It was launched by Oklahoma State University. Up to now, the research on hydraulic components and systems using computer simulation has a history of more than 30 years. With the development of fluid mechanics, modern control theory, algorithm theory, reliability theory and other related disciplines, especially the rapid development of computer technology, the hydraulic simulation technology is becoming more and more mature, becoming a powerful tool for hydraulic system designers

I. application of simulation technology in the hydraulic field

a relatively perfect hydraulic system should not only have good static performance, but also have good dynamic performance. This is because the speed, action, direction and external load of the actuator in the system are constantly changing. If the dynamic characteristics of the system are not sensitive, the feedback signal cannot be quickly executed by the system, resulting in the sensitive dead zone and action dead zone of the system. In this way, the accuracy of the machined parts will be reduced

in the past, people often relied on the knowledge and experience of designers to form a dynamic system with real components, and then carried out experiments on this system to study the influence of structural parameters on the dynamic characteristics of the system. It is difficult to adjust parameters with this method, which requires a lot of manpower, material resources and time, and the assurance of one-time success is very small. This requires people to use other methods to design and test components. Computer simulation technology can not only predict the system performance and reduce the design time, but also analyze and evaluate the designed system through simulation, so as to optimize the system, shorten the design cycle and improve the system stability

the application of simulation technology in hydraulic field mainly includes:

1 The mathematical model of the existing hydraulic components or systems is established through theoretical derivation, and the accuracy of the mathematical model is verified by comparing the experimental results with the simulation results. This mathematical model is used as the simulation basis for the improvement and design of similar components or systems in the future

2. Through the establishment of mathematical model and simulation experiment, the adjustment range of existing system parameters is determined, so as to shorten the debugging time of the system and improve the efficiency

3. Through the simulation and the vacuum pumping experiment of the vacuum chamber, the influence of the structural parameters of the newly designed components on the dynamic characteristics of the system is tested, the best matching of the parameters is determined, and the data required for the actual design is provided

4. The feasibility of the new design scheme and the influence of structural parameters on the dynamic performance of the system are verified by simulation experiments, so as to determine the best control scheme and the best structure

2. Methods of hydraulic modeling and simulation

the three main components of simulation technology are mathematical modeling, model solving and simulation result analysis. Three methods are generally adopted for hydraulic simulation:

the first method is self programming simulation. For relatively simple systems, and the simulators have good modeling ability and certain programming ability, they can program and simulate by themselves. As early as the 1950s, hanpun (1953) and Nightingale (1957) made dynamic performance analysis of hydraulic servo system respectively, and the transfer function method was adopted at that time, Generally, only the stability and frequency response of the system are analyzed. This is a simple and practical method with mature theory. Until now, it is still widely used. However, this method can only be used in the linear constant system with single input and single output. The average export price in january2016 was 17944.44 yuan/ton, which is not enough to describe the characteristics of various variables in the system, nor is it easy to deal with the common nonlinear problems in the hydraulic system. Therefore, this method is widely used by researchers and graduate students in research institutions

the second method is that the mathematical model is established by the user, and some general algorithm systems are selected for simulation, such as the commonly used matlab/simulink software, which provides many mathematical model solving tools, and it is worth mentioning that this kind of software also provides better post-processing function of simulation results. This method is increasingly used by researchers

the third method is to select special hydraulic simulation software for simulation. This kind of special software generally provides modeling tools. As long as users input simulation data in the form of schematic diagram as required, the special software can automatically establish a mathematical model, conduct simulation calculation and output simulation results

according to different modeling methods, these software can be divided into two types, namely, simulation software modeled by state equation method and simulation software modeled by bond graph method. Most hydraulic simulation software uses state equation method to model. For example, the AFSS (advanced fluid system simulation) simulation software package developed by McDonnell Douglas Aircraft Company to predict the working performance of hydraulic components and systems has improved the hydraulic design from empirical estimation to quantitative analysis

bond graph was invented by H. M. Paynter of the United States in the early 1960s. It graphically expresses the relationship between the components in the system, can reflect the load effect between the components and the power flow in the system, can be used to describe the dynamic characteristics of the hydraulic circuit, and is a powerful tool to study the hydraulic power system. At present, several hydraulic simulation software based on bond graph method have been developed, such as the bond graph oriented power system general simulation program enport developed by the United States in the late 1980s, which has been applied in a certain range. However, the program needs to run on a large capacity and large computer, and there are some restrictions on the analysis of nonlinear systems, which affect the popularization of the software. For the vast majority of users, the starting point from bond graph is too high, and a better hydraulic system simulation software package needs to be open and extensible, so it is more viable to use state equation method for modeling in the long run

however, regardless of the simulation software based on any principle, looking at the development of hydraulic simulation technology in recent years, modern hydraulic simulation software generally has the following functions:

(1) a wide range of basic hydraulic component models and flexible assembly: only a wide range of basic hydraulic component models can adapt to various simulation requirements, but no matter how comprehensive the basic model library is, it is impossible to include all the requirements of users for component models. The user-defined component model should be able to be assembled with the component module of the software

(2) support multi domain modeling and simulation: in modern practical engineering application design, there are almost few pure hydraulic systems. The hydraulic system is usually only a part of the whole system. Even the components may include mechanical and electronic devices, which requires that models in other fields can be added to the simulation. The most common one is to add electronic and mechanical simulation models to DSH, while AMESim has a variety of model libraries such as hydraulic, mechanical, control, signal, thermodynamics, pneumatic, etc

(3) database technology application and technical document generation function: the most important technical document of a simulation system is the schematic diagram of the system, and others include the mathematical model description of differential and algebraic equations of components, parameters, simulation results, and other product information. The means to realize this function began to use complex database technology, rather than the traditional difficult to manage file system. Taking Hopsan, a hydraulic simulation software of a university in Sweden, as an example, its simulation environment using database management is shown in Figure 1

Figure 1 Schematic diagram of simulation environment for database management

in Figure 1, dynmoc uses FORTRAN programs to generate component models and system connections, while Java interfaces are used between databases, simulation programs and mathematical operation software mathmatic. Amos model database can centrally manage data, realize data sharing, ensure the consistency and security of data and the independence of user operation, and quickly and accurately realize data query and communication

(4) graphical operation interface: at present, almost all well-known hydraulic simulation software support graphical operation interface, so that simulation technology can be more widely used in engineering practice and commercialized in a wider range. The component model is represented by icons in the software. The component model and component parameters are directly selected by operating the hydraulic schematic diagram. The software automatically generates the simulation description file or program of the circuit through their respective identification technology, circuit topology information and component model

(5) it supports real-time simulation and provides interfaces matching with general software. The integrator of current hydraulic simulation software includes the function of variable step size. With the rapid improvement of hardware speed, the simulation speed is greatly improved, so it is not so difficult to realize real-time simulation. Real time simulation enables the simulators to see the system action "in real time" on the computer screen, making the simulation more intuitive and persuasive. In terms of software interface, matlab/simulink has become a common interface for all hydraulic simulation software, and some cooperative companies and university research institutions have also provided interfaces to each other

considering comprehensively, the hydraulic simulation software is the most "friendly" to users, so it is most widely used

III. main problems of hydraulic simulation technology

(1) system modeling is not easy

the primary task of modeling the hydraulic system is to establish a mathematical model. The most difficult thing is to model, and then it is possible to carry out computer research. Modeling is a very complex work. At present, most of them use state equation modeling, but some software use transfer function or bond graph to model. These are difficult for ordinary hydraulic workers. The traditional quantitative simulation technology first needs to establish an accurate mathematical model to express the structure and function of the object system into a series of mathematical equations based on differential equations. Computer simulation can be carried out only after the system behavior description based on functional solution or numerical solution is derived through mathematical means such as solving equations. But when the actual system is too complex or the knowledge accumulation is not enough, it is impossible to construct an accurate quantitative model of the system

on the one hand, using the concept of similar system, non electric systems such as hydraulic system can be transformed into similar circuit systems through series conversion. Firstly, the symbols of circuit components can be used to turn the complex hydraulic system function symbols into circuit diagrams that are easy to read and analyze the system characteristics. Secondly, various mature circuit theory technologies, such as impedance concept and various network theories (such as network topology analysis method), can be effectively used in the analysis of actual hydraulic systems. Thirdly, the circuit components are easy to replace, the values are easy to change, and it is convenient to measure the current and voltage. It is very important that all kinds of modern circuit theories, such as complex equation, graph theory, splitting method, etc., have made great progress in the past 20 to 30 years, and have been successfully applied in the field of computer-aided analysis and design. Therefore, based on the existing experience, the hydraulic circuit (or hydraulic unit) can be

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