$\newcommand{\deriv}[2][]{\dfrac{\partial #1}{\partial #2}} \newcommand{\tderiv}[2][]{\tfrac{\partial #1}{\partial #2}} \newcommand{\fderiv}[2][]{\dfrac{d #1}{d #2}} \newcommand{\derivII}[3][]{\dfrac{\partial^2 #1}{\partial #2 \partial #3}} \newcommand{\tderivII}[3][]{\tfrac{\partial^2 #1}{\partial #2 \partial #3}} \newcommand{\derivIII}[4][]{\dfrac{\partial^3 #1}{\partial #2 \partial #3 \partial #4}} \newcommand{\tderivIII}[4][]{\tfrac{\partial^3 #1}{\partial #2 \partial #3 \partial #4}} \newcommand{\derivIV}[5][]{\dfrac{\partial^4 #1}{\partial #2 \partial #3 \partial #4 \partial #5}} \newcommand{\half}{\tfrac{1}{2}} % Macro for 1/2 \newcommand{\ud}{\mathrm{d}} \newcommand{\op}{\circ} \newcommand{\dq}{\dot{q}} \newcommand{\ddq}{\ddot{q}} \newcommand{\dt}{\Delta t}$

Trep “Hello World”

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”.

Import the trep module into python

Let us begin by importing the trep module into Python.

import trep

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

Create a new instance of a trep system

Let us create a new instance of a trep system.

system = trep.System()

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 documentation. We will explore more of these properties and methods later in the tutorial.

Visualize the system with trep’s visualization tools

Let us visualize the system.

trep.visual.visualize_3d([ trep.visual.VisualItem3D(system, [], []) ])

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.

trepHelloWorld.py code

Below is the entire script used in this section of the tutorial.

# trepHelloWorld.py

# Import the Trep module into python
import trep
# Create a new instance of a trep system
system = trep.System()
# Visualize the system with trep's visualization tools
trep.visual.visualize_3d([ trep.visual.VisualItem3D(system, [], []) ])