This tutorial is meant to be an introduction to trep by walking the reader through extended examples. This tutorial will use a Python shell known as IPython for illustrating the organization of trep's components. It is recommended that IPython is downloaded and installed, although the tutorial is still useful without access to IPython.
Let's begin by creating the trep equivalent of "Hello World".
Let us begin by importing the trep module into Python.
./code_snippets/trepHelloWorld.py
After importing trep if you type trep followed by pressing tab in IPython a list of all of the methods that are available for the trep module are displayed. As you can see that trep.System is listed, which instantiates an empty trep system. This is used in the next section.
>>> trep. trep.CONST_SE3 trep.RZ trep.constraint trep.ryXRT trep.Config trep.Spline trep.constraints trep.rz trep.Constraint trep.System trep.finput trep.save_trajectory trep.ConvergenceError trep.TX trep.force trep.spline trep.Force trep.TY trep.forces trep.system trep.Frame trep.TZ trep.frame trep.tapemeasure trep.Input trep.TapeMeasure trep.load_trajectory trep.tx trep.MidpointVI trep.WORLD trep.midpointvi trep.ty trep.Potential trep.config trep.potential trep.tz trep.RX trep.const_se3 trep.potentials trep.util trep.RY trep.const_txyz trep.rx trep.visual
Let us create a new instance of a trep system.
./code_snippets/trepHelloWorld.py
This creates a new instance of a trep system. Typing system. followed by pressing tab will list all the methods and properties of the system object.
>>> system. system.L system.get_frame system.L_ddq system.get_input system.L_ddqddq system.get_potential system.L_ddqddqdq system.hold_structure_changes system.L_ddqddqdqdq system.import_frames system.L_ddqdq system.inputs system.L_ddqdqdq system.kin_configs system.L_ddqdqdqdq system.lambda system.L_dq system.lambda_dddk system.L_dqdq system.lambda_dddkdq system.L_dqdqdq system.lambda_ddq system.add_structure_changed_func system.lambda_ddqddq system.configs system.lambda_ddqdq system.constraints system.lambda_dq system.ddq system.lambda_dqdq system.ddqd system.lambda_du system.ddqk system.lambda_duddq system.dq system.lambda_dudq system.dqd system.lambda_dudu system.dqk system.masses system.dyn_configs system.nQ system.export_frames system.nQd system.f system.nQk system.f_dddk system.nc system.f_dddkdq system.nu system.f_ddq system.potentials system.f_ddqddq system.q system.f_ddqdq system.qd system.f_dq system.qk system.f_dqdq system.resume_structure_changes system.f_du system.satisfy_constraints system.f_duddq system.set_state system.f_dudq system.t system.f_dudu system.test_derivative_ddq system.forces system.test_derivative_dq system.frames system.total_energy system.get_config system.u system.get_constraint system.world_frame system.get_force
Some examples:
- The current system time is
>>> system.t 0.0
- The current inputs are (the system has no inputs)
>>> system.u array([], dtype=float64)
- The current generalized coordinates are (the system has no configuration)
>>> system.q array([], dtype=float64)
All of the properties and methods for the system object are documented in the System <trep_system> documentation. We will explore more of these properties and methods later in the tutorial.
Let us visualize the system.
./code_snippets/trepHelloWorld.py
Below is what you should see.
This brings up the trep visualization of the mechanical system. Since this system has no components nothing is shown. The VisualItem3D method takes in three arguments: system object, time vector, and configuration vector. In this example the time vector and configuration vector are set to empty vectors. If these vectors were not empty the visualization would play a movie of the system going through the configurations at the given times.
Below is the entire script used in this section of the tutorial.
./code_snippets/trepHelloWorld.py