VIRTUAL PROTOTYPING OF A HYDRAULIC CUSHION FOR HIL

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1 VIRTUAL PROTOTYPING OF A HYDRAULIC CUSHION FOR HIL Joseba Landaluze Izaskun Portilla Ana Martínez Raúl Reyero IKERLAN Research Centre E Arrasate-Mondragon, Spain jlandaluze@ikerlan.es Miguel A. Otaduy University of North Carolina Department of Computer Science Chapel Hill, NC USA otaduy@cs.unc.edu KEYWORDS Virtual Prototyping, Real-Time simulation, simulators, hydraulic actuators, block diagrams. ABSTRACT This paper presents the virtual prototyping of a hydraulic cushion in a mechanical press, a very costly system which is not easily available. The final objective is HIL (Hardware In the Loop) simulation with a real industrial controller of the cushion. Bearing in mind that the controller closes all its loops every 1 ms, the requirements for Real-Time simulation of the Virtual Prototype are extremely strict. The modelling process on a DYMOLA-Simulink combinationbased platform is presented here, together with the results of its functioning compared with those of the real system. 1. INTRODUCTION The progressive integration of engineering applications, and the proliferation of electronics used in all types of products, have made way for a new design engineering based on Virtual Prototyping or Digital Prototyping. This is a group of techniques allowing interaction with digital models in a similar way to interaction with processes in the real world. It enables the design to be optimised and the functioning of parts of the system to be checked before they are finally integrated. From the Control Engineering point of view, an interesting aspect of Virtual Prototyping is the fact that it makes HIL (Hardware In the Loop) simulation possible. A problem frequently encountered by the engineer is that of having to set up and test the controllers that have been built, and this requires the construction of prototypes for carrying out integration and testing. However, in the case of complex or very costly mechatronic systems it can be a very long or expensive process, and there are often limitations to its setup owing to the urgency with which the final product must be presented. The Virtual Prototyping of certain parts of the system and Real-Time simulation with real components (HIL) are technologies increasingly in demand for improving the design, testing and integration cycle of the control systems. HIL-oriented Virtual Prototyping of systems means strict modelling requirements for Real-Time simulation. The modelling tools to be used, together with the platforms for Real-Time simulation, must therefore be selected with great care. The IKERLAN Technology Research Centre analysed the particular features of Real-Time simulation and the methods which could be used in order to obtain models executable in Real-Time, and chose one for the development of Virtual Prototyping and HIL simulations in Real-Time (Otaduy et al. 2000). This paper presents the Virtual Prototyping of the hydraulic cushion of a mechanical press, applying the development platform chosen, in order to be able to test the real electronic controller of the cushion in an HIL environment. A mechanical press and its controlled hydraulic cushion are a clear example of a very costly system, but without them the electronic controller of the hydraulic cushion - a basic and relatively inexpensive element - could not be set up and tested. IKERLAN, together with FAGOR ARRASATE (a major European supplier of mechanical presses), developed the controlled hydraulic cushion, which was then installed in a mechanical try-out press in FAGOR ARRASATE's installations in Mondragon. In contrast to the normal process to be followed for Virtual Prototyping, a Virtual Prototype of the press-cushion unit was later created and a real controller, analogous to the one installed in the press, was tested in Real Time. The real system made it easy to compare and validate the methods proposed. This paper firstly presents the development platform chosen for the Virtual Prototyping. There follows a description of the plant and it then goes on to describe the modelling of the hydraulic cushion on DYMOLA software. This is followed by the schema for the HIL simulation, and finally the results obtained with this platform are compared to results obtained on the real press. 2. VIRTUAL PROTOTYPING FRAMEWORK The Real-Time simulation model can be developed in several ways. Perhaps the most efficient way is manual coding of all the physical equations, but this process is long, difficult and prone to error. The first advantage derives from automatic code generators. These tools translate models developed with block diagrams

2 Dymola 20-sim ANSI C Compiler Machine Code Custom HW Topological Modeling Envirnoment Portable Models ANSI C MATLAB/ Simulink Code Handwriting EASY5 MATRIX X / SystemBuild Manual Equation Formulation Generic Modeling and Application Generation Environment Target Oriented C Code dspace Simulation Inplementation Environment ADI RTS Figure 1: Possible paths for Real-Time simulation or equations into a Real-Time executable program for certain target hardware, and the engineer only has to deal with formulating the system properly. Automatic code generators attempt to draft optimised code, but this may not always be achieved. An advantage which poses even more of a challenge comes with the generation of efficient formulations from topological models (Cellier and Elmqvist 1993). The more efficient these representations are, the more suitable they become for Real-Time simulation. However, as was the case with automatic code generation, the need for generality may lead to a lack of efficiency. Figure 1 shows schematically the possible paths for generating a Real-Time simulation. The choice between automatic processes and manual coding does not always depend on the efficiency of the model. If this is not the dominant factor, other aspects such as reusability and development time may prevail. At IKERLAN, studies were carried out for testing commercial software tools that could enable the creation of fully automatic applications. Focusing at all times on object-orientated modelling and automatic model manipulation, the characteristics of some packages were defined and a Virtual Prototyping framework was finally chosen (Otaduy et al. 2000). In accordance with the platform chosen, the first step consists of modelling the system on the object-oriented modelling package DYMOLA (this package was chosen mainly due to the quality of the code generated and its proficiency in solving algebraic loops). Classic off-line simulations are made in this package until adequate behaviour in all components of the system is achieved. Figure 2: Schema of the press and hydraulic cushion After removing the unnecessary elements, a sub-model is then created exclusively from the parts to be included in a Virtual Prototype. Subsequently, the code of the sub-model is obtained, a Simulink S-Function being created. The Real-Time application is obtained from Simulink and a dspace DS1103 board, with RTW (Real Time Workshop) software from The MathWorks, Inc. The model of what is to be included in the Virtual Prototype is thus developed in a Simulink environment, taking into special account the interface between the Virtual Prototype and the real elements, performed on the I/Os of the dspace DS1103 board. The time taken to carry out the Real-Time simulation must be taken into account, depending on the real elements included in the HIL simulation. Very restrictive requirements regarding sampling times and bandwidths mean that the models in the DYMOLA package must be simplified, until the Real-Time simulation can be performed. The platform chosen was conditioned by the state of the commercial tools analysed at the time ( ), and the fact that the modelling world is constantly evolving means that the platform has to be continually reviewed. Today similar results can be obtained with other analogous platforms based on commercial products (Otaduy et al. 2000). 3. MODELLING AND SIMULATION TEST OF THE HYDRAULIC CUSHION 3.1 Description of the system The controlled hydraulic cushion was installed in a try-out press at FAGOR ARRASATE. This is a single action

3 Figure 3: Model of hydraulic cushion in DYMOLA straightside mechanical press, with eccentric gears and four connection points, used for experimentation and testing of dies. It can exert a maximum force of kn with dies, and its maximum drawing speed is 14 spm (strokes per min). The hydraulic cushion installed in the press has the following specifications: 2 cylinders of 350 mm in length and 250 mm in diameter, each of which are controlled by a Rexroth 4WRDE32V600L servovalve. Figure 2 shows a schema of the mechanical press and hydraulic cushion. The upper chamber of each cylinder is at supply pressure and the servovalve controls the input and output of oil to the lower chamber. The weight of the cushion supported by the cylinders is kg. The friction forces of each cylinder are 1413 N for the static force and 8480 N for the Coulomb force. The force capacity of the cushion varies between 200 and 2000 kn. The typical drawing distance is 200 mm at 14 spm and 250 mm at 6 spm. The typical pre-acceleration distance is 30 mm. Each cylinder has a pressure sensor incorporated in each one of its chambers, plus a displacement sensor. An encoder located in the axis of the press slide allows its position to be known from its displacement function. The controller is built using Siemens SICOMP SMP-16 hardware, under the RMOS Real-Time Operative System. This controller receives the signals from the sensors of the cushion and the press and generates the set-point voltages for the servovalves. It implements position and force controls, which act in accordance with the cushion's functioning conditions. Communication is made with the press PLC by means of digital inputs/outputs. 3.2 Modelling of the hydraulic cushion in DYMOLA DYMOLA allows technological modelling of multidisciplinary systems. DYMOLA environment models fulfil all object-orientated modelling specifications. Automated symbolic procedures aimed at creating efficient model descriptions are also used for code generation. Most of the model is written in ODE format, whereas inline iterative methods are used to solve large or nonlinear algebraic loops. Thus, code is always created for DYMOLA models, even though this might not be efficient enough. Figure 3 shows the ideal continuous model of the mechanical/hydraulic part of the cushion produced in DYMOLA (v. 4.0c). Basically the model consists of two symmetrical parts, each corresponding to one of its cylinders, and rigidly joined. The position/force of the cushion is controlled by a servovalve which regulates the flow of oil in the lower chamber of the cylinder. Each cylinder supports a corresponding part of the weight of the cushion, in addition to the friction force. Interaction between the press slide and the cushion takes place by means of the Pos block. When the slide reaches the cushion an impact is produced and the slide drags the cushion with it during the drawing process. The inputs to the model are the position of the press slide (in blocks Pos and Pos1) and the set-point values for the servovalves (in blocks gain1 and gain2). The outputs from the model are the forces exerted by each of the cylinders (in sum1 and sum2) and the position of the cushion. Rod1 and Rod3 (Rod2 and Rod4) are displacements representing pressure pins, which change from the position of the cylinders to that of the cushion ring.

4 pos1 ptop pos = pos1 pos2 pos < pbot f = Kbot * (pos + pbot) pbot pos > ptop f = Ktop * (pos ptop) pbot = pos = ptop f = 0 pos2 Figure 4: Schema of the class Pos The design was basically made using DYMOLA's standard library elements, although some of these had to be adapted. Figure 5: Model of the cushion controlled in Simulink For the servovalve a standard library block was used, but with some specific parameters for representing the commercial Rexroth 4WRDE32V600L servovalve. The most significant values of the parameters are: qnom=0.01, dpnom=5e5, qleak=0, dpleak=250e10, k1=10, k2=2.78, omega0=1000, damp=0.87, vmax=1000. The value of the servovalve bandwidth (omega0) is very important as its dynamics greatly affect the results of the simulation, and also the time needed for it. For the interface between the press slide and the cushion the Pos class was made, represented by the equations shown in Figure 4. pos1 is the position of the slide and pos2 the position of the cushion. 3.3 Off-line simulation of the controlled cushion in Simulink Taking the hydraulic cushion model in DYMOLA as a basis, an S-Function was generated representing the model in Simulink. In order to study the cushion's behaviour a model of the complete controlled cushion was made in Simulink, completing the part taken from DYMOLA with the model of the controller. Figure 5 shows the block diagram of the model made. It basically consists of the S- Function SimStr, the control blocks Control1 and Control2 for each servovalve (Landaluze et al. 2000), and a block for generating set-point values. Figure 6 shows the type of graphs obtained. This graph corresponds to a complete cycle, performed at a drawing speed of 12 spm with pre-acceleration. The first second is used for correctly initialising the variables. The top graph shows the positions of the press slide and the cushion cylinders. In the bottom graph the force set-points are shown (800 and 400 kn) together with the force exerted by each cylinder. The force for each cylinder includes the force necessary to withstand the corresponding weight of the cushion, and the difference with respect to the set-point value is therefore approximately constant. The vibrations caused by the impact can be observed here. The parameters in the DYMOLA model must be tuned in order for the level of these vibrations to be correctly set up. Figure 6: Positions and forces obtained in off-line simulation 4. VIRTUAL PROTOTYPING OF THE CUSHION 4.1 Description of the virtual prototyping schema Following the platform chosen for the Virtual Prototyping, the complete model for the Real-Time simulation was made in Simulink, taking into consideration the variable adaptation, the interface to the outside world and all the components, and particularly events, which were not taken into account in the continuous model in DYMOLA, but which were fundamental for simulating the behaviour of the final product.

5 ControlDesk from dspace was used for the monitoring and configuration software, which was executed on the PC where the DS1103 board is installed. The data for the drawing conditions can be changed from this software. It also allows monitoring of the main simulation variables. Real-Time simulation was performed on a dspace DS1103 board, using the Runge-Kutta IV method, and with a fixed integration step of 1 ms. 5. RESULTS Figure 7: Schema of Virtual Prototype of the hydraulic cushion and real industrial controller Figure 7 shows the schema of the Virtual Prototype of the press and the controlled cushion. On the one hand the Real- Time simulation of the press and the cushion is shown, including the functions of the press PLC, and, on the other hand, the real cushion controller, made up of two parts: the controller itself and the monitoring and configuration software. On the left of the Figure the actual Real-Time simulation on a dspace DS1103 board can be seen, as well as hardware which emulates the cushion's control panel and performs the adaptation from the TTL level signals of the dspace board to the 0/24V level required by the industrial controller. It also makes the conversion to the SSI protocol required by the controller for the encoders. 4.2 Real-Time modelling of the hydraulic cushion In Figure 8 the complete model for the Real-Time simulation of the press and the hydraulic cushion can be observed. As with the model for the off-line simulation shown in Figure 5, the basic block is the S-Function SimStr created from DYMOLA. The diagram includes the logical part of the press PLC, plus the interface to the outside world via the DS1103 board. It must be pointed out that a controller identical to the one used on the real press was placed in the Virtual Prototyping mock-up and it was not even necessary to change the parameters of the position and force controllers. The values tuned for the real press were valid for the mock-up. This was due to the fact that the press/cushion model had already been validated beforehand with respect to the real press. Figure 9 shows the positions of the press and the cushion obtained by HIL simulation of a cycle, for 12 spm with preacceleration of 30 mm and 100 mm drawing. Figure 10 shows the force profile (drawing length in axis of abscissas) obtained during the drawing, the set-point values being 400 and 200 kn, different for the two cylinders. Some vibrations appear due to the moment of impact, but the exerted force follows the set-point value very well, with just a slight tracking error. Figure 11 shows the force profile produced by the cylinders when the force set-point for one of them is pulsed, with a frequency component of 10 Hz and 40 kn amplitude added to 400 kn (the apparent change in frequency is due to the variable used in the axis of abscissas). Figure 12 shows analogous graphs obtained on the real press. These also correspond to a speed of 12 spm and 100 mm drawing, with pre-acceleration. The top graph shows the force exerted by the two cylinders, the set-points being of 400 and 200 kn respectively. In the bottom graph, one of the cylinders produces a pulsed force of 10 Hz and 40 kn amplitude added to 400 kn. In general the responses are Figure 8: Model for Real-Time simulation of press and cushion

6 Figure 9: Slide and cushion positions in HIL simulation Figure 12: Drawing forces obtained in the real press Figure 10: Set-point and drawing forces in HIL simulation greatly reduced, and that multiple modelling alternatives can be rapidly evaluated. Also, HIL (Hardware In the Loop) technology for Real-Time simulation is advancing notably due to developments in microprocessors and the reduction in the price of DSPs. Amongst the alternatives on the market for Virtual Prototyping and HIL simulation, IKERLAN has recently chosen one based on the DYMOLA-Simulink combination. The Virtual Prototyping of a mechanical press and its hydraulic cushion has been presented here, together with the results obtained in the HIL simulation with the real controller. The results are analogous to those obtained on the real system, and they open up the possibility of applying these methods to complex systems that are costly to build. Figure 11: Drawing forces in HIL simulation very similar, particularly as regards quality, although on the real press less vibrations occur in the force profiles during the impact than with HIL simulation. 6. CONCLUSIONS An increasing number of object-orientated modelling tools with high quality code generating options are coming onto the market, which means that development time is being REFERENCES Cellier, F.E. and H. Elmqvist Automated Formula Manipulation Supports Object-Oriented Modeling. IEEE Control Systems, vol. 13. Landaluze, J., A. Milo, A. Auzmendi, A. Ormaetxea and J. Olazabalaga Modelado y Simulación de un Cojín Hidráulico Controlado. XIII Congreso de Máquinas Herramienta, October, Donostia-San Sebastian, Spain. Otaduy, M.A., A.I. Martínez, A. Vidarte, J. Landaluze and R. Reyero Object-Oriented Modeling for Real-Time Simulation. IMECE'2000. Symposium on Automated Modeling, November 5-10, Orlando, Florida.