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6. Dynawiz Features The basic capability of Dynawiz is extensive. It simulates a variety of machines and mechanisms with their control systems. By proper use of the contact constraints, it also models systems with closed mechanical loops. Still, it does more. Prescribed Motion This is a form of constraint where a designated hinge angle (or displacement) is required to follow a particular time function. For example, to model an angular disturbance at hinge j, one would set the angle and angular rate of that hinge to some starting value. From then on, the user supplies the acceleration that corresponds to the angular disturbance. Dynawiz would integrate that accelertion and supply the rate and angular data over time for that hinge. Meanwhile, Dynawiz computes the motion of the rest of the machine given the prescribed acceleration for the designated hinge. Generally, you can have prescribed motion on multiple hinges. Inverse Dynamics The problem is to find the actuation forces in the mechanism that produce the desired motion at designated hinges. One simple version of this problem is to view it as a multiple prescribed motion problem. As Dynawiz solves them, it computes the constraint forces that deliver the prescribed accelerations at selected hinges. You can request the constraint actuation forces as data elements in the Y_FILE. Those data could be used for controlling the actual plant. Multi-Rate Discrete Control System This capability automatically comes with the Matlab/Simulink packages. The Simulink resources allow you to build multi-rate sample data systems in its work space. We also provide that capability with the Fortran and the C++ packages. In the latter packages, Buildx would build the proper program structure to handle multi-rate discrete processes in its control subroutine template creation. You just have to supply at designated subroutines the code defining what your discrete processes must do. During the simulation, Dynawiz would adjust its step size to integrate exactly to the system sample times. Event Based Switching Often one must initiate or disable processes because a particular event occurred. If that event can be described by the condition that a scalar function f(x,t) = 0, then Dynawiz can be programed to detect the event occurance to within a user-specified tolerance and activate appropriate processes at that time. Our satdemo.dat example in the XSV package illustrates this feature well. In this case, the satellite is deploying its solar arrays, where the outer panels of the north and south arrays are unfolding from their respective stowed positions. The unfolding is considered complete when the unfolding panel hinge angle (initially at 180 degrees) reached 0 degrees. The switching function is just the hinge angle of the unfolding panel. In this case, on detecting full deployment, Dynawiz would compute the proper constraint torque at the deployed panel's inboard hinge to keep the hinge angle at zero. Loads Calculation You can request the load forces or torque that appear at hinges. By load forces, we mean the shear forces at a prismatic hinge orthogonal to the free axis. Similarly, by torque load we mean torsional load that acts on a 1 dof rotational hinge orthognal to the free axis. You may prescribe a hinge to zero motion and obtain both the shear and torsional load at the hinge point in three axes. You can specify these loads as data elements in the O_File specification. (i.e. use Edit_Ofile.) Restart Capability To save time for simulations that run for a long time, it is often efficient to save the state of the simulation a several intermediate time points. This lets you simulate from the saved starting time points later. This way of saving the state of the simution is the restart capability. BuildX lets you specify the files to store the model file valid at specified times of the simulation. For example, if you wanted two model files defined at 500 seconds and 1000 seconds into the nominal simulation, you would request two restart files using the BuildX's Edit_Restart Menu. Let's call the two files Model500.dat and Model1K.dat. These two files would be the input model files that has all the vehicle mass properties, constraints, position and velocity data that are in effect at t = 500 and t = 1000 second points. To run a simulation starting at t = 1000 seconds with respect to the original simulation, you only need to specify in the current.dat file that your model is now Model1K.dat. Flexible Body Dynamics Running a Dynawiz simulation with flexible bodies requires some additional effort in preparing the input data. You are required to provide a flexible data file that contains the following information for each flexible body in the plant:
Some other data must be included also. For instance, if a body is connected to a flexible body, then it must include the mode slope and mode shape of the parent body at the hinge attachment point. Similarly, if a wheel, a position marker or a directional marker is defined on a flexible body, then the flex-data-file must include the modal slope and mode shape of the parent body for these items at the attached point or the definition point. Let's say that this file is called flex.dat. Then, register this file as the flexible body file with BuildX using the Timing & Control Edit Menu. Go to that menu, and set the flex file name to flex.dat. That's all you have to do to run a simulation with flexible bodies. The rest of the simulation acitivities is exactly like those in running rigid body dynamics simulations. Built-in Oribit Computations For satellite dynamics simulations, Dynawiz has a built-in Keplerian orbit calculation to initialize the vehicle position in the inertial frame. You can enable or disable this feature by toggling the enable-orbit flag in the Timing & Control Menu. In the same menu, you can specify the Keplerian elements to locate the vehicle position and velocity in the inertial frame. If this feature is enabled, Dynawiz would automatically compute the gravitational forces on each component body of the spacecraft over the simulation. Another convenience is that the initial Direction Cosine Matrix that you specify for the Bus or the reference body (body 1) is the vehicle attitude relative to the local vertical orbital frame. ( In a non-orbit mode, it would be the bus attitude in the inertial frame.) For attitude control, you can request the small angle rotations of the Bus with respect to the local vertical frame. You can also request the Euler angles for the Body 1-to-orbit attitude for several Euler sequences in you plot data list. This concludes our introduction to Dynawiz. You are always welcome to visit this site and our main CDI site. Thank you for coming! |
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DynaWiz, BuildX, XAL, and QX3D are trademarks of Concurrent Dynamics International.
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