190 adams中多体动力学与液压系统的联合仿真

1、Running Combined Multibody Hydraulic System Simulations within AdamsArne Jansson, Imagine, 5 rue Brison, 42300 Roanne, France Mohamed Yahiaoui, JLG Industries Inc., JLG Drive, McConnellsburg PA, 17233 9502 Claude Richards, Imagine, 5 rue Brison, 42300 Roanne, France

2、AbstractIn this paper experiences using an interface between Adams and the hydraulic software AMESim are described. The interface is presented using the example of simulation of an articulated boom lift. The assumptions and results of simulating the system in each of these software

3、 packages are discussed and the results are compared with the results using the combined simulation. These comparisons highlight that the multi-domain approach is essential in this example for realistic results.1 IntroductionIt is no longer necessary to appeal for multi-domaimulation. Simulation of

4、systems withmore than one domain are common. This is inevitable because systems are rarely singledomain. Thus, a pure hydraulic system is singularly useless. The hydraulic power produced must be used to do something. This normally involves a mechanical system. In addition, thesystem must be controll

5、ed. Already we have three doma (multibody) and control.: hydraulics, mechanicalVirtually all simulation software specializing in a specific domain include a basic capability in other domain. This gives a limited multi-domain capability. It is limited for two fundamental reasons:It is not feasible fo

6、r one software supplier specializing in one field to have strong expertise in a variety of other fields.Often the user interface is tuned to a specific domain and the other doma necessarily fit well into this user interface.do notThe alternative is to interface two or more packages to talk together

7、and join forces in a single simulation. This combined simulation can be performed in two ways:Combined multi-body hydraulic simulation1/1the subsystem from one domain is imported into the other domain and a single integrator performs the integration;the integrators of each software integrate there o

8、wn subsystem exchanging information at a pre-assigned interval - co-simulation.This presentation describes a multidomaimulation using Adams and the fluid powersimulation software AMESim. The hydraulic sub-system was imported into Adams using the general state equation (GSE) facility. The Adams integ

9、rator performed the integration.2 A two domain problemFigure 1The combined Adams-AMESim simulation will be presented using an articulated boom lift as an example. This mechanical device is designed to place workers and their tools and materials easily, quickly and safely in elevated work areas. They

10、 enhance work site productivity and safety by eliminating traditional scaffolding and ladders. The device modeled is shown in fig. 1 and is produced by JLG Industries Inc., a leading manufacturer and distributor of aerial work platforms.Articulated boom lifts have work platforms mounted at the end o

11、f an articulated boom, which in turn, is mounted on a self-propelled chassis. These machines are especially useful forCombined multi-body hydraulic simulation2/2reaching up and over machinery and equipment mounted on floors where access using scaffolding is impossible.Various models are available wi

12、th platform heights up to 150 feet. The articulated boom can be rotated through 360 degrees and raised or lowered from the vertical to below the horizontal. They may be maneuvered forwards and backwards and steered in any direction by the operator in the platform. This can be done, even when the boo

13、m is extended and fully elevated.3 The multibody simulationFigure 2Only the upper structure (i.e. above the chassis) is modeled in Adams as shown in fig. 2. All parts are modeled as rigid bodies and are connected by using different types of joint. Joint frictions are modeled as forces, which are dep

14、endent on joint reaction forces. Cylinder (jack) motions are achieved using motion statements and losses are modeled as forces acting aga t motion. Lower lift and mid lift jack synchronization is achieved by a coupler statement. Inpractice this is achieved by connecting the jackseries so that pressu

15、rized fluid is led fromthe lower lift jack to the mid lift jack. The goal is to maintain the platform level at all time.Note that this is really assuming that the hydraulic system behaves perfectly!4 The hydraulic simulationFigure 3 shows the hydraulic part of the articulated boom lift displayed in

16、AMESim. Attention is focused on two jacks: the lower lift jack and the mid-lift jack. These jacks areconnectederies so that pressurized hydraulic fluid is lead from the lower jack to the midCombined multi-body hydraulic simulation3/3jack. The object is to synchronize the displacement of the two jack

17、s so that the platform is always maintained level.Figure 3The multibody system is represented in a very simple way. At the end of each jack is a mass, optionally with friction, and attached to this is an external force duty cycle varying with time (but actually constant for the results presented). I

18、n contrast to the Adams model, the hydraulic system is much closer to reality but the multibody system is idealized. In particular the hydraulic coupling between the jacks is present but the mechanical coupling is not.Crucial in this system are the counterbalance valves. These would normally be atta

19、ched directly to the jacks. They perform two vital functions:they close when there is sudden depressurization - hence the platform will not come crashing down;they stabilize the system particularly when there is an overrunning load.From the perspective of the hydraulics expert, counterbalance valves

20、 are notoriously difficult to set up to give smooth stable behavior. From the perspective of the operator riding in work platform, such behavior is highly desirable. With AMESim, a competent hydraulics engineer can construct the system and tune it to behave smoothly in under a day. However, there is

21、 the question of how the hydraulics will behave when connected to a real multibody system.Combined multi-body hydraulic simulation4/4Figure 4Figure 5The plots in fig. 4 and 5 show the displacement and velocity of the lower and mid jacks as the boom lift is raised. This is after the system had been t

22、uned.Figure 6Figure 7Ideally, the ratio of the displacements of these jacks should be constant as they move perfectly synchronized. Figure 6 shows the ratio predicted by the simulation.Figure 7 shows the pressures in the two jacks as the system is raised compared with the system (pump) pressure. Thi

23、s information is important as it helps the designer assess the efficiency of the hydraulic system.5 The combined multibody hydraulic simulationThe hydraulic system was modified to include two special Adams interface blocks as shown in fig. 8. When these are included, tead of creating a normal execut

24、able, AMESim creates special code designed to interface with Adams. This uses the GSE facility as a door into Adams.Combined multi-body hydraulic simulation5/5Figure 8The interface is designed to work with Adams and AMESim running simultaneously. The multibody system is changed within Adams and the

25、multibody results can be examined within Adams. Similarly the hydraulics system can be changed or parameters adjusted within AMESim and the results plotted in AMESim. Thus, each domain has its natural graphicalinterface for constructing the system, changing diagrammatically below.dataanddisplaying r

26、esults.Thisisshowni-body hydraulic si6/6Adams results fileAMESim resultsmfileAMESim dataAdams executableAMESim hydraulic subsystemAMESim G U IAdams G U IFigure 9To run the combined system it is necessary to set simulation parameters very different from the default settings. The most consistent algor

27、ithm for integrating this and other combined simulations has proved to be GSTIFF. A typical value for the initial and minimum step sizes is 10 -12.The integrator proved very sensitive to some hydraulic parameters. Very small hydraulic volumes produce very small time constants. GSTIFF could cope with

28、 these only by using very small step sizes. The worst examples were in the hydraulic volumes when a jack was at the end of its stroke. If the dead volume was small, the simulation was very slow until the jack moved away from the extreme position.Another phenomenon, which led to slow simulation and o

29、ccasionally to simulation failure, was the sudden opening of valves. A step change in the valve opening could lead to complete failure. Ramping the valve open was a better solution. This is not too bad a restriction as in real life step changes to valve opening are not possible.The runs quickly esta

30、blished that the combined simulation produced results that were significantly different from the single domain results. In particular1. the hydraulic system was not perfect and the jacks were not well synchronized;2. the external forces used in hydraulic simulation were not representative of the for

31、ces predicted by the combined simulation;3. as a result of 2 the tuned hydraulics only system was not well tuned on the actual multibody system.The most interesting example of these effects is that, during part of the raising stage, the mid lift jack experiences an over running load. Once the simula

32、tion has pointed this out it is obvious. Unfortunately, it is not obvious before the simulation!Figures 10 to 13 correspond to figs. 4 to 7.Combined multi-body hydraulic simulation7/7Figure 10Figure 11Figure 12Figure 13It is clear that the assumptions of each of the single doma combined simulation h

33、ighlighted problems, which were domain runs.imulations are unacceptable. The missed completely by the singleLooking back on this exercise it was not performed in the best way. It would have been better to have delayed the combined simulation until after addition hydraulic simulations. The force valu

34、es from the Adams runs could have been down-loaded into a file. Equally well a sequence of values could have been read from the graphs. This data could have been used (with linear interpolation or cubic splines) as the force duty cycle. This would enabled the system to be tuned much better before th

35、e combined simulation was performed.6 The problemsCombined multi-body hydraulic simulation8/86.1 Numerical problemsIt is possible to perform the simulations described above because both the hydraulic and the multibody systems can be described in terms of ordinary differential equations or differenti

36、alalgebraic equations. Many other domapecific software also create models governed byequations of this type. An integrator is provided to solve these equations.It is interesting to note that the integrators employed by the Adams and AMESim are the same- Adams-Moulton, BDF (Gear type) and DASSL. Howe

37、ver, it is clear that algorithms have been extensively tuned to suit each domain.From a numerical point of view, the characteristics of the hydraulic governing equations have the following characteristics:They often have very small time constants, the equations can be pathologically non- linear and

38、it can be convenient to model some phenomena as discontinuities.Multibody equations can be stiff if certain types of rubber joints are included otherwise they are much less stiff than typical hydraulic system equations. The Lagrange multiplier approach prefers that step sizes are not too small - thi

39、s conflicts with the requirements of the hydraulic equations where very small step sizes are necessary during fast transient phases.It is worth discussing discontinuities briefly. We will describe a jump change in a state variable as a hard discontinuity. A jump change in the first or higher derivat

40、ive of a state variable will be described as soft discontinuity.It is possible to do relatively routine combined simulations provided the three following cautions are respected.1 The Adams integrator is not happy with hard discontinuities. This is not a serious problem as only a very few AMESim comp

41、onent models employ hard discontinuities and these are easily avoided. Soft discontinuities can also give problems but normally they can be eased by careful parameter setting. Thus, a step opening of a valve normally is passed to a state variable as a discontinuity in the first derivative. Changing

42、the step to a ramp makes this a discontinuity in the second derivative.2 The most pathological non-linear equations in hydraulic systems are those occurring in cavitation and air-release. These do occasionally cause failure in the combined simulations using the Adams integrator. Normally, when this

43、happens, the simulation can be persuaded to run by setting a very small maximum step size. The phenomenon is usually highly undesirable in the real system. Hence, having confirmed the problem exists, it is then necessary to do some redesign to eliminate it.3 To run the combined simulation using the

44、Adams integrator, it is necessary to select the stiff integrator and set initial and minimum steps sizes to values much smaller than is normal with multibody systems.The authors know of one company that has experimented with the interface in the opposite direction. They have used the callable Adams

45、facility to export the Adams multibody model and then import it into AMESim. It is not know how well this interface worked.Combined multi-body hydraulic simulation9/9The third possibility, running a co-simulation, has not been tried. It has the theoretical disadvantage that it de-couples a coupled system. In addition, it is not clear how the communication interval should be set. These problems become severe if three or more software are involved.6.2 Diversity of skillsGood