Fig. 1: Basic structure of OHC-Sim
From the above-mentioned viewpoint, some exclusive simulation packages, such as Bath-fp (Richards et al. 1990), DSH-plus (Kett et al, 1995), HOPSAN (Linköping Univ., 1991), AME-Sim (Imagine, 1995) and Ideafp (Cho et al, 1996) have been developed. Using these simulation packages, an oil-hydraulic circuit can be constructed on the display with ease by connecting oil-hydraulic component icons similar to oil-hydraulic symbols. Then, based on the mathematical models for components registered in the database, the mathematical model for the whole circuit is automatically constructed and the dynamic behavior of the circuit is simulated. Thus, these simulation packages easily provide the environment where an oil-hydraulic circuit can be designed and improved with feedback from the simulated results.
OHC-Sim (Oil-Hydraulic Circuit Simulation) was developed in Japan (Nakada et al, 1996) with the support of JFPS (the Japan Fluid Power System Society, formally The Japan Hydraulics and Pneumatics Society), and has been improved and enhanced in the research committee of JFPS (Sakurai, 1999 and 2000). OHC-Sim has a user-friendly graphical user interface in Windows® environment, and provides easy design and improvement of an oil-hydraulic circuit on the basis of the simulated results by personal computer. Furthermore, the first version of the user-customized function, which is based on bond-graph, is newly developed (Sakurai, 2001). By using this user-customized function, users can register the models for their own new oil-hydraulic components to the database of OHC-Sim.
In this paper, OHC-Sim and the first version of the user-customized function are described synthetically. Firstly, the structure of oil-hydraulic component icon is discussed. Next, the main functions of OHC-Sim are shown through the simulation of oil-hydraulic positioning circuit. Finally, the simulation of the dynamic characteristics of a load sensing oil-hydraulic system and the user-customized function are described. This simulation becomes executable by the development of the user-customized function.
In OHC-Sim, the mathematical models for oil-hydraulic components registered in the database are represented by bond-graph, which represents a system model based on power train. In bond-graph, model structure can be made up easily by adding or removing bond-graph elements. This point is effective for con-struction and maintenance of the models for oil-hydraulic components in the database. Furthermore, in bond-graph, a system model can be easily constructed by connecting sub-system models. This point is very useful for deriving the mathematical model for whole oil-hydraulic circuit.
As a solver, BGSP for OHC-Sim is employed. BGSP (Kohda et al, 1991) is a simulation program based on bond-graph, had been developed in Japan, and has obtained a number of fruitful results. In developing OHC-Sim, BGSP was renewed and employed.
Furthermore, the first version of the user-customized function is newly added to OHC-Sim. By using this function, users can register the mathematical models for their own oil-hydraulic components to the database.
Fig. 2: Basic structures of oil-hydraulic component icons
Figure 2 illustrates two types of oil-hydraulic component icons. Both icons have three kinds of ports, that is, connecting, input and sensing ports. Connecting port is used for connecting oil-hydraulic component icons each other. Through this port, a physical quantity such as pressure or discharge is transmitted to or from another component. Input port is used for the input of an electric signal or the feedback of a sensed physical quantity. Sensing port is used for picking up physical quantities.
Fig. 3: Icons and bond-graph models for a pipeline
As seen from Fig. 2 (a), for the connecting port in type-1, only the property of the port is classified by color on the icon. Namely, the difference between input and output is specified. The kind of the physical quantity inputted to the port or outputted from the port is not specified. Hence, for some oil-hydraulic components, their models registered in the database become compli-cated. For example, in the case of the model for the pipeline whose dynamic characteristics are specified by the compressibility of the working fluid, the inertia of the working fluid and the pipe friction, its model in the database becomes complicated, because there are two types of the bond-graph models as shown in Fig. 3 (a). In these models, the physical quantities put in to the port and put out from the port are different. Namely, when adopting type-1 as the basic structure of oil-hydraulic component, some icons have plural models, and their data structures in the database are complicated. Therefore, it is time-consuming to construct, check and modify their data. Furthermore, in constructing the mathematical model for the whole circuit including such components, suitable models should be selected automatically. Therefore, the structure of the program for OHC-Sim becomes complicated.
To overcome the above-mentioned problems, type-2 is proposed. Its basic structure is shown in Fig. 2 (b). In type-2, the property of the connecting port, that is input or output of physical quantity, is specified on the icon by using arrow as shown in Fig. 2 (b). In addition to it, the physical quantity transmitted to or from the connecting port is specified on the icon, namely four kinds of physical quantities such as pressure, discharge, force and torque are classified by color respectively. This modification makes the data structure for a model registered in the database simpler, though the number of the oil-hydraulic component icon is increased. In the case of the pipeline whose dynamic characteristics are represented by the compressibility of the working fluid, the inertia of the working fluid and the pipe friction, the icon is equivalent to the bond-graph model as shown in Fig. 3 (b). As a result, their data structure in the database becomes simpler. Therefore, compared with the case where type-1 is adopted as the basic structure of the oil-hydraulic component icon, it becomes convenient to construct, check and modify the models in the database. In addition, when constructing the mathe-matical model for whole circuit, it is not necessary to select suitable model. Therefore, the structure of the program for OHC-Sim becomes simpler.
When adopting type-2 as the basic structure of oil-hydraulic component icon, oil-hydraulic component icons are connected each other based on the simple rule as follows:
(1) a port whose property is input has to be connected to another one whose property is output.
(2) the colors of these ports have to be same.
And then, the causality between oil-hydraulic compo-nents is determined uniquely, and becomes integral causality.
Figure 4 (b) shows the oil-hydraulic positioning circuit constructed by OHC-Sim. In this figure, SU1~SU4 are the components called sensor and used to sense physical quantities. Furthermore, by copy and paste of the sensor sensing a physical quantity, the physical quantity can be utilized easily. In Fig. 4 (b), by copy and paste of sensor SU1, the piston displacement sensed by sensor SU1 is used as the feedback signal of PI-controller PC1. Thus, in OHC-Sim, since the physical quantity sensed by a sensor can be utilized, a feedback loop can be represented easily. In addition, this function is useful when the internal or external pilot is represented (Sakurai, 1999a).
After constructing an oil-hydraulic circuit, parameters for each component have to be set. After the right button of mouse is clicked on an icon, the dialog box for setting parameter is opened. The dialog box for the fixed displacement pump used in Fig. 4 (b) is shown in Fig. 5. In the dialog box, parameters can be set by either SI or engineering system of units.
Fig. 4: Oil-hydraulic positioning circuit
After an oil-hydraulic circuit is constructed on the display and the parameters for all components are set, the simulation of the circuit can be carried out. Then, the parameters for simulation, that is simulation start time and time width for Runge-Kutta-Gill method, have to be set. After the simulation is carried out, the simulated results can be confirmed on the display. The simulated results for the oil-hydraulic positioning circuit are shown in Fig. 6. In this figure, Y-axis denotes the simulated results corresponding to the name of sensor such as SU1 or SU2 in Fig. 4 (b), and X-axis denotes time. As seen from the simulated results sensed by sensor SU1, the piston displacement is controlled sinusoidally by the PI-controller.
Figure 7 shows the experimental apparatus of a load sensing oil-hydraulic system (Sakurai, 1996). The load sensing oil-hydraulic system is composed of a variable displacement pump, a load-sensing valve (LS valve), a pressure-limiting valve, a proportional valve and a load. The proportional valve has a port, which senses the load pressure. The load pressure sensed at the sensing port of the proportional valve is transmitted to the LS valve through the Y-port of the proportional valve. The pressure at the pump discharge port is fed back to the LS valve. The LS valve operates in the flow-compensating mode. In this mode, the LS valve adjusts the angle of the swash plate at any spool position of the proportional valve in order to keep the differential pressure between the pump discharge and the load at a constant value, which is set by the spring of the LS valve. The pressure-limiting valve operates in the maximum pressure-limiting mode, and only determines the maximum value of the system pressure. In this ex-perimental apparatus, the load is composed of a power cylinder, an inertial mass and a metering valve which is used to adjust the damping of the whole system.
In a load sensing oil-hydraulic system, when the proportional valve is closed, the system is in the idling mode, and the angle of the swash plate is almost zero. When the system is in the idling mode, if the valve is suddenly opened, the swash plate is moved largely, and after that the system is in the flow-compensating mode. Then, the moment of inertia of the swash plate affects the dynamic characteristics of the system because of the sudden movement of the swash plate. Therefore, in the experimental apparatus shown in Fig. 7, a bypass valve was installed upstream of the power cylinder. In experiment, the opening area of the bypass valve was set, and an input signal was inputted to the amplifier for the proportional valve. Then, the system was in the flow-compensating mode. After that, the bypass valve was closed, and experiments were performed.
Since a load sensing oil-hydraulic system may become oscillatory and sometimes unstable in the flow compensating mode (Krus, 1988), the simulation of its dynamic characteristics is carried out in this mode. Therefore, the pressure-limiting valve is not taken into account. The load sensing oil-hydraulic system can be constructed in OHC-Sim as shown in Fig. 8. In this figure, the bypass valve installed upstream of the power cylinder is represented by metering orifice ST1. The proportional valve is regarded as an orifice, and modeled by orifice ST4. By using the user-customized function, a control cylinder and a swash plate, and a LS valve are registered and utilized. The pilot line which connects the Y-port of the proportional valve with the LS valve is regarded as a kind of a signal line which transmits the load pressure, because the spool dis-placement of the LS valve is small and the power through the pilot line becomes small. Therefore, in Fig. 8, the pilot line is represented by copy and paste of sensor SU2 sensing the pressure at the head end of the power cylinder.
Fig. 9: Control cylinder and swash plate registered by user-customized function
Figure 9 (a) shows the icon for the control cylinder and the swash plate registered to OHC-Sim by the user-customized function and the corresponding sub-window for setting parameters. The sub-window for setting parameters is composed of two areas, that is, the area for setting parameters and that for explanation of model and parameters.
The first version of the user-customized function is based on bond-graph. Therefore, when a new oil-hydraulic component is registered to OHC-Sim, firstly, the bond-graph model has to be derived. Figure 9 (b) shows the bond-graph model for the control cylinder and the swash plate. In this model, the volume and the piston mass of the control cylinder, and the moment of inertia of the swash plate are ignored. Only the spring of the control cylinder and the power transformation from fluid power to mechanical power at the piston of the control cylinder are taken into consideration (Sa-kurai, 1996).
Based on the derived bond-graph model, the file called Odb-file has to be made. Odb-file for the control cylinder and the swash plate is shown in Fig. 9 (c). In this file, the bond-graph structure for the component, the information about connecting, input and sensing ports, the constitutive equations for bond-graph elements, and output variables are defined. In addition, when a new oil-hydraulic component is registered to OHC-Sim, another three kinds of files, that is Icon-File, Par-file and Fig-file, are required. In Icon-File, the oil-hydraulic component icon, which is utilized to construct oil-hydraulic circuit, is stored. Par-file is necessary to construct the area for setting parameters in the sub-window for setting parameters. And, Fig-file is used to explain the model and parameters in this sub-window.
Figures 10 (a) and (b) are the simulated results on OHC-Sim and the comparison of the simulated results with the experimental ones (Sakurai, 1996). As seen from Fig. 10 (b), the simulated results agree with the experimental ones. And, it can be seen that the response of the system is oscillatory. And it is shown that the prediction of the dynamic characteristics of the load sensing oil-hydraulic system can be executable in OHC-Sim by the development of the user customized function.
(b) Simulated and experimental results
Fig. 10: Simulated and experimental results of the load sensing oil-hydraulic system
modify the models in the database. Next, the main functions of OHC-Sim were shown through the simula-tion of an oil-hydraulic positioning circuit. Finally, the simulation of the dynamic characteristics of a load sensing oil-hydraulic system and the features of the user-customized function were presented. This simulation becomes executable by the development of the user-customized function. However, in the first version of the user-customized function, it is necessary for users to know bond-graph. Therefore, in the next version of the user-customized function, to make OHC-Sim more useful, it is important that the user-customized function provides the environment where a new oil-hydraulic component can be registered without the knowledge about bond-graph.
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YS, KT, TN, TK
| Internet Site | http://boss.nakada.p.dendai.ac.jp/ohc-sim |
| Vendor | The Japan Fluid Power System Society (Formerly, The Japan Hydraulics and Pneumatics Society) |
| Location | 3 Kikaishinko Kaikan
3-5-8, Shibakoen, Minato-ku Tokyo 105-0011, Japan |
| Educational Version | Student Edition, Class Room Kit |
| Telephone number | +81 3-3433-8441 |
| Telefax number | +81 3-3433-8442 |
| nakada@cck.dendai.ac.jp info@jfps.jp |
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| Platforms | All Windows environments |