84.在ADAMS中的多体动力学和液压系统联合仿真a

Technical Bulletin n°111
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Latest update: Feb. 26th, 2002
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Running Combined Multibody Hydraulic System Simulations within ADAMS®3/14 Running Combined Multibody Hydraulic System Simulations
within ADAMS®
1. Introduction
It is no longer necessary to appeal for multi-domain simulation. Simulation of systems with more than one domain are common. This is inevitable because systems are rarely single domain. 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, the system must be controlled. Already we have three domains: hydraulics, mechanical (multibody) and control.
Virtually 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:
Ä It is not f easible for one software supplier specializing in one field to have strong expertise in a variety of other fields.
Ä Often the user interface is tuned to a specific domain and the other domains do not necessarily fit well into this user interface.
The alternative is to interface two or more packages to talk together and join forces in a single simulation. This combined simulation can be performed in two ways:
Ä the subsystem from one domain is imported into the other domain and a single integrator performs the integration;
Ä the integrators of each software integrate there own subsystem exchanging information at a pre-assigned interval --- co-simulation.
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Running Combined Multibody Hydraulic System Simulations within ADAMS®4/14 This presentation describes a multidomain simulation using Adams and the engineering systems simulation software AMESim. The hydraulic sub-system was imported into Adams using the general state equation (GSE) facility. The Adams integrator performed the integration.
2. A two-domain problem
Figure 1
The 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 enhance work site productivity and safety by eliminating traditional scaffolding and ladders. The device modeled is shown in Figure 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 of an articulated boom, which in turn, is mounted on a self-propelled chassis. These machines are especially useful for reaching up and over machinery and equipment mounted on floors where access using scaffolding is impossible.
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Running Combined Multibody Hydraulic System Simulations within ADAMS ® 5/14
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Various models are available with 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 boom is extended and fully elevated.
3. The multibody simulation
Figure 2
Only the upper structure (i.e. above the chassis) is modeled in Adams as shown in Figure 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 dependent on joint reaction forces. Cylinder (jack) motions are achiev ed using motion statements and losses are modeled as forces acting against motion. Lower lift and mid lift jack synchronization is achieved by a coupler statement. In practice this is achieved by connecting the jacks in series so that pressurized fluid is led from the lower lift jack to the mid lift jack. The goal is to maintain the platform level at all time.
Running Combined Multibody Hydraulic System Simulations within ADAMS®6/14
Note that this is really assuming that the hydraulic system behaves perfectly!
4. The hydraulic simulation
Figure 3 shows the hydraulic part of the articulated boom lift displayed in AMESim. Attention is focused on two jacks: the lower lift jack and the mid-lift jack. These jacks are connected in series so that pressurized hydraulic fluid is lead from the lower jack to the mid jack. The object is to synchronize the displacement of the two jacks so that the platform is always maintained level.
Figure 3
The 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). In 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.
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Running Combined Multibody Hydraulic System Simulations within ADAMS ® 7/14
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Crucial in this system are the counterbalance valves. These would normally be attached directly to the jacks. They perform two vital functions: Ä they close when there is sudden depressurization --- hence the platform will not come crashing down Ä they stabilize the system particularly when there is an overrunning load.
From the perspective of the hydraulics expert, counterbalance valves 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 the question of how the hydraulics will behave when connected to a real multibody system .
Figure 4
Figure 5
The plots in Figure 4 and Figure 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 tuned.
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Figure 6
Figure 7
Ideally, 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. This information is important as it helps the designer assess the efficiency of the hydraulic system.
5. The combined multibody hydraulic simulation
The hydraulic system was modified to include two special Adams interface blocks as shown in Figure 8. When these are included, instead of creating a normal executable, AMESim creates special code designed to interface with Adams. This uses the GSE facility as a door into Adams.
Running Combined Multibody Hydraulic System Simulations within ADAMS
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Figure 8
The interface is designed to work with Adams and AMESim running simultaneously. The multibody system is changed within Adams and the 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 graphical interface for constructing the system, changing data and displaying results. This is shown diagrammatically below.
Running Combined Multibody Hydraulic System Simulations within ADAMS ® 10/14
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Figure 9
To run the combined system it is necessary to set simulation parameters very different from the default settings. The most consistent algorithm 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 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 occasionally 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 established that the combined simulation produced results that were significantly different from the single domain results. In particular :
1. the hydraulic system was not perfect and the jacks were not well synchronized
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2. the external forces used in hydraulic simulation were not representative of the forces 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 simulation has pointed this out it is 'obvious'. Unfortunately, it is not obvious before the simulation! Figure 10 to Figure 13 correspond to figs. 4 to 7.
Figure 10
Figure 11
Figure 12 Figure 13
Running Combined Multibody Hydraulic System Simulations within ADAMS®12/14 It is clear that the assumptions of each of the single domain simulations are unacceptable. The combined simulation highlighted problems which were missed completely by the single domain runs.
Looking 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 values 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 enable the system to be tuned much better before the combined simulation was performed.
6. The problems
6.1 Numerical problems
It 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 differential algebraic equations. Many other domain specific software also create models governed by equations 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. However, 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 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 be not too small --- this –
Running Combined Multibody Hydraulic System Simulations within ADAMS®13/14 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 derivative 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 :
Ä The Adams integrator is not happy with hard discontinuities. This is not a serious problem as only very few AMESim component 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 the step to a ramp makes this a discontinuity in the second derivative.
Ä 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 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.
Ä To run the combined simulation using the Adams integrator, it is necessary to select the stiff integrator and set initial and minimum step sizes to values much smaller than is normal with multibody systems.
We know of one company that has experimented with the interface in the opposite direction. They have used the callable Adams facility to export the Adams multibody model and then import it into AMESim. It is not known how well this interface worked.
The 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 –
Running Combined Multibody Hydraulic System Simulations within ADAMS®14/14 clear how the communication interval should be set. These problems become severe if three or more software are involved.
6.2 Diversity of skills
Good multibody simulation specialists are relatively rare. Good hydraulic simulation specialists are equally rare. What chance is there of getting a single specialist in both activities?
The solution is probably that domain specific experts must learn to talk to one another. There is still value in performing the multibody and hydraulics simulations separately. When these studies are reasonably mature, the two specialists must get together and organize a combined simulation.
7. The future
Already serious multi-domain simulation is possible. It is clear that an increasing number of interfaces will become available. This is the only way to achieve a virtual prototype.
Numerical integrators exist which, in the hands of a sympathetic operator, can provide successful multi-domain simulation. However, to improve the performance of these interfaces, a new generation of numerical integrators is desirable. These will have to be successful in a broad variety of domains.
With the proliferation of these interfaces, there must come a time when some standardization is essential. At present, most interfaces are specific to the two software packages involved. The alternative is to define a model exchange standard suitable both for components and sub-systems. This would allow a diverse collection of sub-systems to be assembled into a combined system. One suggestion is to use the electronics standard VHDL-A as a more general standard. However, it is not yet clear what, if any standard, will emerge.
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