Multi-Body System and Contact Simulation within the Design Development of Planetary Surface Exploration Systems Andreas Gibbesch*, Rainer Krenn*, Florian Herrmann*, Bernd Sch¨afer*, Bernhard Rebele*, Elie Allouis**, Thomas Diedrich*** *DLR - German Aerospace Center, Institute of Robotics and Mechatronics, Weßling, Germany e-mail:
[email protected] **EADS Astrium Ltd, Stevenage, UK e-mail:
[email protected]
***EADS Astrium GmbH, Bremen, Germany e-mail:
[email protected] Abstract Multi-body system simulation is capable of mechanical design optimization and evaluation of different congurations as well as simulating autonomous recongurable robotic systems. Within the current ExoMars project the presented simulation environment is successfully used for rover performance analysis on typical rough and sloped terrain. For the presented contact simulation methods correlations and validation during the rover design and evaluation phase efforts have been undertaken within the ExoMars study. The exploration system Marco Polo Surface Scout containing a hopping mechanism shows the capabilities of applying the simulation to non-wheeled robotic systems. Within the MoonNEXT study recongurable rover designs are investigated with multi-body simulation tools and sophisticated contact model approaches. The actuator models are integrated in the simulation tool chain. The performance evaluations within the multi-body system approach is conducted by means of sophisticated contact models for hard and soft soil with almost arbitrarily shaped contact surfaces.
mobility systems have to be investigated within a wide range of environmental conditions regarding the mechanical contact with the planetary surface. That can be accomplished by means of multi-body simulation tools and sophisticated contact mechanical approaches. Simulations are conducted for rover performance on typical planetary terrain with rocks and soft soil. Therefore mobility aspects are taken into account like slope and rock climbing capabilities. These results are compared to laboratory measurements of the ExoMars BB2 rover within an in-house testing environment. Within the very early design phase multi-body simulation shows its capabilities for system design and performance evaluation. During the recently completed MoonNext phase A study a four wheeled recongurable rover shows its mobility performance on rocky sloped terrain. A totally different mobility approach is shown in the Marco Polo Surface Scout study. The contribution to the currently on going phase A study from the simulation point of view is the performance evaluation and design support for this hopping system. Multi-body simulation furthermore gives support to the actuator trade-off.
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Introduction
For robotic exploration systems and mission planning the investigation and prediction of mission scenarios or tasks and the required operational performance on planetary surfaces is of major interest and can be conducted by means of mechanical simulation approaches. Not only the mechanical behavior of the robotic systems has to be taken into account but also the interaction with the environment. For mobility aspects the contact mechanics of planetary rocks and soil with rover wheels and other mobility mechanisms like legs or special hopping systems play an important role. This has to be taken into account by the used simulation environment. Therefore robotic
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Multi-body system simulation
The modeling approach is based on a full 3D multibody system (MBS) simulation model [1]. Therein integrated are sophisticated contact mechanics modules capable of multi-point contact calculation on arbitrarily shaped surfaces. The contact mechanics module is divided into an approach for hard and soft soil contact. For hard soil contact a polygonal contact (PCM) approach is used [2]. For soft soil contact an extended approach for soft soil contact (SCM) modeling is used [3]. With these tools dynamic as well as quasi static simulations are undertaken. The possibility of integrating an highly efficient elastic/ plastic contact calculation into the MBS algorithm gives the
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opportunity of investigating different design concepts of planetary surface exploration systems combining the mechanical system with the rocky and sandy environment.
steps. First a collision detection algorithm determines if the contact pairing is in touch. If no collision is detected, the algorithm returns zero force and torque vectors and the analysis is nished. Otherwise PCM constructs in the second step the intersecting areas of the surfaces and discretizes the corresponding contact patches. Finally the contact force of each contact element is determined and the resulting contact force and torque of all contact elements is calculated.
Figure 1. Different planetary surface exploration systems: MBS simulation models
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Figure 2. PCM rover wheel and rocky terrain contact mesh surfaces
Efficient MBS contact simulation for hard and soft surfaces
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Proling of SCM Code
For the specic problem of mobility simulation on sandy, planetary terrain (see application in Fig. 3) a new modeling and simulation technique called soil contact model (SCM) has been developed. It makes the classical, semi-empirical terramechanics theory of Bekker [5] and Wong [6] compatible with the specic requirements of multi-body system simulation regarding modularity, delity and computational efficiency. The efficiency of SCM is mainly based on the empirical-phenomenological modeling approach that provides an adequate level of abstraction for the implementation of complex dynamics problems like soil contact within multi-body systems. From the object structured modeling point of view SCM can be characterized as a specic force object for computing full 3D contact forces and torques between arbitrarily shaped contact objects and a terrain of plastically deformable soil. In the sample MBS of Fig. 4 the SCM object is highlighted in orange. Other contact dynamics objects like the polygonal contact model (PCM) [7], which is a modeling technique for rigid body contact dynamics (e.g. wheel-rock interaction), may optionally work in parallel with SCM without any need for model modication.
The typical planetary terrain looks like the sandy, rocky laboratory setup shown in Fig. 3. Of course there are many different variations possible for grain size, rock surface and consistency and therefore different mechanical behavior of the soil itself. But with the absence of liquid water on Moon and Mars the terrain to cope with is mainly dry sand and rocks. This leads to two different approaches for contact mechanic description between the surface exploration system and the surface itself. For almost arbitrarily shaped surfaces the PCM and SCM modules for MBS simulation are introduced for elastic and elastic/ plastic contact calculation. With respect to the presented applications both contact modeling descriptions are introduced in the following. For a deeper understanding of the contact models several papers has been published [2] and [4].
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The hard soil contact of the wheels is modeled by means of the polygonal contact model approach. Typical contact surfaces of the wheel and the terrain are shown in Fig. 2. For the body geometry denition a polygonal representation is used and the contact forces are determined by means of the elastic foundation model. This makes it possible to dene contacts between almost arbitrarily shaped surfaces and also to calculate multi-point contacts. The PCM contact analysis task consists of three
In order to evaluate the compatibility with MBS requirements and the computational efficiency the proling of the SCM code is a useful method [4]. It helps to identify time-critical sections of the code and potential resources for model optimization. A representative prole of the
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Figure 3. ExoMars BB2 rover in sandy, rocky laboratory terrain
Figure 5. Proling of SCM Code rover can be seen in Fig. 3 and Fig. 7. The rover is equipped with 18 independently controlled actuators in total: • 6 drive actuators • 6 steering actuators • 6 deployment actuators For all joints force/ torque sensors are applicable. For the presented testing scenarios 6 force/ torque sensors are mounted at once. This opens the opportunity for correlation with simulation data from tracking and joint position data to force/ torque data. Three different scenarios shall be evaluated:
Figure 4. Object Based Structure of a MBS Model of Wheeled Rover Chassis SCM code running on a standard desktop PC is presented in Fig. 5. It is recorded for a single wheel-soil interaction application: 0.25 m wheel diameter, 0.1 m wheel width with 16860 vertices per wheel and 40401 vertices per soil DEM. However, in terms of computation time the topology of the bodies and the numbers of vertices are only relevant till the contact detection is nished. Then, for the rest of the SCM code only the number of detected soil contact nodes inside the contact patch affects the computational load. Accordingly, the time prole is plotted against the number of contact nodes. The prole in Fig. 5 presents the accumulated computation time after each major step of the SCM code, which are listed in the legend from top to bottom.
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• coping with an obstacle (see Fig. 6 and Fig. 7.) • deployment on a plane (see Fig. 11)
ExoMars BB2 rover
Figure 6. ExoMars BB2 rover traversing obstacle
Within the ExoMars study the DLR Institute of Robotics and Mechatronics conducts hardware tests with its own powerful testing environment [8]. This includes digital elevation modeling (DEM), pose tracking and the ExoMars BB2 rover equipped with several force/ torque sensors. The testing environment with the ExoMars BB2
With this testing environment exact position tracking of the rover and force/ torque measurements are available. As testing is on-going and not all rover joints are equipped with force/ torque sensors a selection is chosen for correlation results.
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Simulation and correlation of obstacle negotiation
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The measurement and simulation setup of the rover traversing an obstacle consists of a Martian soil simulant and a cuboid obstacle of 20cm height. The measurement and simulation setup can be seen in Fig. 3 and Fig. 7.
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Figure 7. Simulation of ExoMars BB2 rover traversing obstacle
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Simulation results of the contact forces for the left and right hand side of the rover wheels (lhs, rhs) are shown in Fig. 8 and Fig. 9. The visual correlation of both results shows very good compliance for the chosen maneuver.
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Figure 10. Simulation of ExoMars BB2 rover traversing obstacle: correlation of contact force
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The ExoMars BB2 rover has 6 actuators implemented for deployment. With the proper actuator sequence these deployment actuators are also used for a special wheel walking mode of the rover. In Fig. 11 the ongoing deployment of the ExoMars BB2 rover can be seen. The actuator sequence moves the vertical rover legs to a horizontal position as show in Fig. 11. In Fig. 12 and Fig. 13 measurement and simulation results are shown. The deployment torque of a full deployment sequence with the rover going down and up again. In Fig. 13 a correlation of the folding part of the sequence is shown. Despite of the sensor noise the correlation of measurement and simulation results is very good.
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Figure 8. Simulation of ExoMars BB2 rover traversing obstacle: lhs contact forces In Fig. 10 the contact forces of a single wheel of the rover traversing the obstacle are shown in comparison of measurement and simulation results.The characteristic of the obstacle traversing can be seen in both graphs although the measurement shows peak values the simulation cannot reect.
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MoonNext rover
Within the MoonNEXT phase A study recongurable rover designs are investigated by means of multi-
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Figure 11. ExoMars BB2 rover deployment sequence in the plane
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body simulation tools and sophisticated contact model approaches PCM and SCM. The actuator models are integrated in the simulation tool chain. In Fig. 16 the MBS rover model is shown on a typical rocky terrain with the possibility of autonomous reconguration. The rover design allows a reconguration of the chassis and boogies respectively. Simulation results are presented for rover performance on typical terrain with rocks and soft soil. Therefore mobility aspects are taken into account like slope and rock climbing capabilities. Static stability of different scenarios and congurations of the recongurable rover system are investigated and evaluated. In Fig. 14 the possibilities of the MBS simulation tool chain in conjunction with DEM terrain model generation is shown. With the chosen contact model approaches PCM and SCM an almost arbitrarily shaped contact surfaces can be taken for wheel soil/ rock contact calculation.
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and suspension adaption on left and right side separately. The rst step of rover performance analysis is the stability analysis on a tilted plane. MBS simulation has the advantage of easily implementing different congurations of the lever arm and the chassis suspension. As an example a parameter conguration of the slope angle is shown in Fig. 15. With the contact force almost zero the stability region of the rover is almost reached.
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Simulation results of rock negotiation and reconguration
Stability analysis of the rover on a at inclined plane is only interesting for rst performance analysis and system correlation. More important are investigations on typical terrain the rover should cope with. The simulation result shown in Fig. 16 depicts the rover with an adapted bogie congurations for a rocky terrain with 30◦ slope.
Stability analysis and reconguration
The MoonNext rover concept consists of several reconguration options. This includes an actuated lever arm
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low forces acting on the contact bodies. The main task to fulll for the MASCOT concept is turning the box around so that the side with the scientic instruments is upright.
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Figure 16. MoonNext rover on rocky slope
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MASCOT
Marco Polo Surface Scout (MASCOT) is from the mechanical point of view a very simple robotic system. But the application on the rarely known surface of an almost zero gravity asteroid is a highly demanding challenge. The system shall be able of hopping of about 50m and uprighting on the surface. The target of the mechanical system is an asteroid similar to the Itokawa with a very low gravity constant of about 0.0001m/s2 . This leads to very low contact forces and actuator torques for any mobility concept. The main body is roughly a box of about 10kg including an actuator mechanism that has to be evaluated by means of simulation. The two different congurations for the actuator concepts are and shown in Fig. 17 and Fig. 18. The presented simulation tools are of great benet due to the possibilities of the integration of a low gravity eld and the use of advanced contact simulations for hard and soft contact surfaces. Again, PCM and SCM show their capabilities of contact calculation not only for wheels on ground but for nearly arbitrarily shaped objects like the MASCOT box and the actuator levers. The contact force calculation is crucial especially due to the very
Figure 18. MASCOT eccentric tappet conguration
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Design congurations
The rst design conguration for the MASCOT mobility concept is a lever arm system shown in Fig. 17. The lever gives the opportunity of a simple motor design and a simple and easy way for motor controller implementation. The only constraint is to rotate the lever slowly and no specic velocity prole is required. A disadvantage is the critical contact of the lever on the surface and the possibility of getting stuck. The second design conguration shown in Fig. 18 is an eccentric tappet mechanism that has the advantage of no mechanical part outside of the box. Therefore this concept is expected to be very robust and insensitive for any surface condition. Next, especially on soft terrain the larger contact area of the whole box will bring mobility advantages.
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Simulation results
Simulation results are presented for both design congurations the lever arm and the eccentric tappet concepts. The focus within the phase A of the MASCOT study is the design and evaluation of the hopping mechanism for mobility aspects of locomotion and turning the system around. The results in Fig. 19 to Fig. 24 show the actuator torques, rotation angle α and contact forces.
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Conclusions
For the ExoMars BB2 rover the presented MBS simulation tools and contact model can prove their validity for the shown correlation scenarios of obstacle coping and deployment. The possible application there could be mission planning and performance prediction with respect to specic optimization objectives like energy consumption and slip control with consideration of sensor data fusion. The MoonNext rover conguration shows the possibilities of MBS simulation for stability analysis beyond much simpler static tools. As well as the simple quasi static analysis is possible and more efficient for these over determined rover systems multi-body simulation shows with powerful contact dynamics simulation scenarios in typical planetary terrain. This gives a much more realistic view of the future rover capabilities and valuable information for the design engineer with respect to locomotion performance. For the MASCOT study MBS simulation in conjunction with powerful contact modeling approaches shows the possibilities of design support in the very early con-
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Figure 20. MASCOT arm conguration: actuator torque The results of the arm concept shown in Fig. 19 to Fig. 21 shows good performance for the actuator selection due to a very low actuator torque peak value of about 0.024 Nm. Considering the package constraints of the system an actuator concept with direct gear would be possible. The results of the eccentric tappet concept shown in Fig. 22 to Fig. 24 show even better performance. The actuator torque peak value in Fig. 23 is even lower to the arm concept but correlating the contact forces and the time needed until the mobility system comes to rest again the eccentric tappet mechanism has a great advantage over the arm concept.
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[4] Krenn, R. and Gibbesch, A. Soft soil contact modeling technique for multi-body system simulations. In 1st International Conference on Computational Contact Mechanics - ICCCM09, Lecce, Italien, 16.-18. September 2009.
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[5] Bekker, M. G. Introduction to Terrain-Vehicle Systems. The University of Michigan Press, Ann Arbor, 1969.
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[6] Wong, J. Y. Theory of Ground Vehicles. Wiley, New York, 4. edition, 2008.
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Figure 23. MASCOT eccentric tappet conguration: actuator torque
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[7] Hippmann, G. Modellierung von Kontakten komplex geformter K¨orper in der Mehrk¨orpersimulation. PhD thesis, TU Wien, 2004. [8] Apfelbeck, M., Kuß, S., Wedler, A., Gibbesch, A., Rebele, B., and Sch¨afer, B. A novel terramechanics testbed setup for planetary rover wheel-soil interaction. In 11th European Regional Conference of the International Society for Terrain-Vehicle Systems, ISTVS, Bremen, Germany, 5. - 8. October 2009.
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cept phase. Simulation can give support in the design of the complete mechanical system with actuator trade of power-train design. The example of the MASCOT system shows the importance of proper contact simulation for the design and evaluation of mobility systems.
References [1] Sch¨afer, B., Gibbesch, A., Krenn, R., and Rebele, B. Planetary rover mobility simulation on soft and uneven terrain. Journal of Vehicle System Dynamics, Special Issue, 48(1):149–169, 2010. [2] Hippmann, G. An algorithm for compliant contact between complexly shaped surfaces in multibody dynamics. In Ambrosio, J. A., editor, Multibody dynamics 2003, Lisbon, Portugal, July 2003. [3] Krenn, R., Gibbesch, A., and Hirzinger, G. Contact dynamics simulation of rover locomotion. In iSAIRAS 2008, Los Angeles, California, February 2008.
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