Multi-Body Simulation of Earthmoving Equipment using MotionView / MotionSolve Amit Srivastava Manager Larsen & Toubro IES Knowledge City, NH8 Vadodara 390 019, India Gopikrishnan. M Assistant Manager Larsen & Toubro IES Knowledge City, NH8 Vadodara 390 019, India Abbreviations : MBD: Multi Body Dynamics Keywords : Multibody Simulation, MBD, Excaavtor, MotionView, MotionSolve Abstract Excavators are mobile machines that are moved by means of either crawler track or rubber-tired undercarriage. Excavator digs, elevates, swings and dumps material by the action of its mechanism, which consists of boom, arm, bucket and hydraulic cylinders. The design of various components of an excavator is complex due to the fact that the reaction forces, displacements and stresses vary with time throughout the full cycle of operation. Conventional methods of design in this case results in overdesigned models since the designer is forced to use a higher factor of safety due to lack of availability of data for full cycle of operation. However, Multi-Body Simulation in MotionView/MotionSolve facilitates system level kinematic and dynamic analysis of the whole excavator, making it possible to study the reaction forces and stress variation in every component of the system throughout the cycle of operation. It helps in predicting the maximum stress at each location during the cycle and thus offers a way to optimize the design. The scope of this paper is to simulate a full cycle of operation of a 30 ton excavator using Multi Body simulation capabilities of Altair MotionView/MotionSolve and compare the results with available test data. Pressures of hydraulic cylinders, Strokes of hydraulic cylinders, Bucket load and excavated mass of soil are applied as input. The full cycle of operation consists of three major phases, Digging, Swing and Dumping. By defining the component under study as a flexible body, it is possible to retrieve stress values at gauge locations used in actual testing, which can be used to calculate fatigue life of welds which is of utmost interest to the designer. Furthermore, it is possible to generate load cases for detailed static analysis from the reaction force output. The stress results at gauge locations are compared with both actual testing and static FEA by other commercial software. It is also worth noting that in the current simulation, many assumptions in similar previous simulations are updated with actual values from field tests, like bucket tip force. Introduction An excavator or earth moving machinery in general, is a very good example of a mechanical system which can be considered a multi body system. A multi body system consists of rigid bodies and ideal joints, which facilitates relative movement between individual components. The primary concern for designers in a multi body system is the reaction forces at these joints, which eventually leads to the design of individual components. Traditionally, analytical calculations are used to find out the reaction forces and other parameters related to a mechanical system. But this approach takes time and there is a limitation of the complexity of the mechanism under study. This is where computer simulation codes specifically designed for multi body dynamics come to the fore. 1
The advantage of computer simulations performed MBD simulation tools is that they allow one to predict the kinematic and dynamic behavior of all types of multi body systems in great detail during all the design stages from the first design concepts to the final prototypes. The analyst is interested in visualizing a whole set of successive responses of the multi body system, with a simulation of its behavior and operation over all the workspaces and over a certain period of time necessary to obtain a real-time response. MotionSolve is system level, multi-body solver that is based on the principles of mechanics. There are various programs available to simulate the kinematics of various real time applications. But the modeling and simulation tools in MotionSolve enable to create realistic, physics-based simulations of complex mechanical systems. The interesting part of MotionSolve is to help the designer to visualize the response of system for the complete operating cycle in all directions. MotionView is a graphical pre-processor to create complex models that can be solved using MotionSolve. Robust integration with popular Altair products such as HyperMesh is an added bonus. This paper focuses on the problem definition, solution and result correlation of a Hydraulic Excavator. The methodology used to approach the problem is explained in detail, and in the subsequent section, result correlation with experimental data is presented. Finally, the conclusions as derived and future scope of work are mentioned. Problem Definition The working cycle of an excavator consists of digging, swinging, dumping and swinging back to the initial position. The objective is to simulate the entire cycle of operation. A 30 ton hydraulic excavator is used for this particular study. Final output desired is the reaction forces at all joints and to evaluate the stresses at all gauge locations. Input available 1. 3D Geometry or mesh data of every link to be modeled in the assembly. 2. Mesh data of parts to be analyzed for stress (flexible body). 3. Pressure on cylinder side and piston side of all cylinders during cycle of operation. 4. Stroke of all cylinders during cycle of operation. 5. Swing motor rotational speed during cycle of operation. Output required 1. Reaction forces at each joint across the whole timeframe of analysis. 2. Load case generation for detailed FE Analysis. 3. Animation of action and reaction forces at all times. 4. Displacement and Stress plots of flexible link(s) of the assembly during the complete cycle. 5. Severe position based on stress levels at specified strain gauge locations. 6. Trace path of bucket tooth tip. 2
Methodology An excavator consists of many links and joints (Fig.1). The analyst has to segregate the mechanism into different links and decide what kind of joints these links are connected with. The procedure to set up the problem is explained as follows. All the links of the mechanism are imported and assembled with appropriate joints in MotionView11.0. The lower frame is grounded. Both geometry and mesh files can be imported for rigid body simulation, but in case of flexible bodies, mesh files are required. Input data that are available, are applied in the next step. This mainly consists of strokes of cylinders and rotation of swing motor with respect to time for the whole cycle of operation. Cylinder pressures are converted into forces and applied at respective bodies. Flexible bodies are imported by using the Flextools>flexprep environment in MotionView. Here, various parameters such as synthesis type, Interface node list are selected. Craig-Brampton method was used for this particular simulation. Then, the outputs that we require from the simulation are specified. Quasi static analysis is used for simulation taking into consideration the fact that this is not a problem involving high velocity or acceleration, which might call for transient dynamic approach. The next step is to solve the model in MotionSolve11.0. Output frequency is chosen as 10Hz. During the solution, solver calculated the DOF of mechanism and performed number of iterations to calculate equilibrium conditions at all time steps. The next section contains discussions on Results obtained. Fig.1 Various parts of an excavator Results & Discussions The output of the solver is post-processed using HyperView11.0. Reaction forces are generated at each joint due to application of cylinder displacements and pressures. Based on stress levels, severe position of both Arm and boom is found out as shown in Fig.2 and Fig.3 respectively. The reaction forces are plotted for magnitude and direction as shown in Fig.4 for this worst position of boom. These reaction forces can be used for further detailed FE analysis of boom. These reactions 3
are validated by plotting summation of forces acting on boom at all time steps, which suggested that the Boom is in equilibrium throughout the cycle. Another form of output provided by HyperView is the trace path of a point during the entire simulation. This helps in validating the mechanism and also helps in ascertaining the working space of the machine. Fig.5 shows the trace path generated at the bucket tip. Reaction forces and were also compared with Static FEA using another commercial FEA software and analytical calculations. The comparison of reaction forces at various joints are given in table1. Fig.2 Worst case position of Arm 4
Fig.3. Worst case position of Boom. Fig.4. Reaction forces visualized in HyperView 5
Fig.5. Trace path of Bucket Tip for entire cycle Joint Analytical (KN) FEA (Static) (KN) MBD (KN) C 302 291 308 B 296 289 306 F 558 573 591 H 466 474 488 J 576 595 592 5 838 528 525 Table 1. Reaction forces comparison. Fig.6. Joints of an excavator 6
Stresses at gauge locations in Arm and Boom were extracted using Tensor plots in HyperView and compared with static FEA results from another commercial software, and field test values. Table 2 and table 3 give a sample result comparison, taken at a gauge location on Boom (gauge B1). Time (s) Field Test (MPa) MBD Result (MPa) Difference (%) 2 32.86 37.10-11.42 3 54.92 58.36-6.27 4 58.29 64.14-10.03 5 66.85 73.64-10.15 6 59.08 63.98-8.30 7 46.17 53.47-15.81 8 70.34 51.06 27.41 9 30.50 23.92 21.57 Field Test (MPa) Table 2. Stress results comparison Field and MBD MBD (MPa) Static FEA (MPa) Difference (MBD- Field) (%) Difference (Static FEA- Field) (%) 65.96 75.36 69.00 12.470 4.405 Table3. Stress results comparison Static FEA, field and MBD Stress variation at gauge locations in Arm and Boom throughout the cycle of operation was also studied. This was compared to Stress variation observed during actual Field testing. Fig.7 and Fig.8 shows variation of stress at gauge location A1 on Arm, in MBD and Actual field test respectively. Fig.7. Stress variation at Gauge location A1 MBD output 7
Fig.8. Stress variation at Gauge location A1 Field test output Benefits Summary Multi Body Simluation from HyperWorks helps the designer in many ways 1. Kinematic validation of mechanism in initial design phase. Animations, trace paths and other tools can be used to effectively validate design. 2. Iterations can be done very fast to reach the final kinematic design before proceeding to individual component design. 3. The output from multi body simulation (Reaction forces) can be used in detailed design of individual components. 4. Stress results of flexible bodies help in design iterations of individual components of the mechanism. 5. It is easy to identify the high stress locations on individual components during the entire cycle. 6. Weld fatigue life calculation for the entire cycle of operation can be done using stress output. Conclusions 1. Multi body Simulation using MotionSolve / MotionView is a very effective tool for kinematic design. The designer can carry out iterations on the mechanism to achieve desired results. 2. Prototype costs are considerably reduced by using Multi body simulation 3. The stress results on flexible bodies are satisfactorily matching the actual test results. These can be used as input for design changes to individual components of the mechanism. 4. The stress variation with actual tests and static FEA is about 20%, which may not be adequate for some applications, where higher accuracy is desired. 8
Future Plans Future scope is to consider more real world factors in the analysis and extending the application of flexible bodies since stress output is available across the timeframe of the entire cycle, unlike static FEA. 1. Currently the lower frame is fixed, while in actual scenario, it can move slightly. 2. Comparing transient dynamic analysis results with quasi-static analysis and hence conclude whether is cost effective considering the accuracy changes observed. 3. The bucket tooth experiences forces during soil excavation. Coupled Eulerian Lagrangian capabilities of Radioss can be used to find out the variation of Forces experienced by the bucket tip, and this can be used as input for Multibody simulation. This more closely resemble actual conditions. [1] HyperWorks11.0 User s Guide REFERENCES [2] Joseph Shigley, Gordon Pennock, John Uicker, Theory of Machines & Mechanisms Third Edition [3] Frankel Joe, Backhoe Kinematics & Dynamics, Georgia Institute of Technology, 20 August 2003. 9