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If using controls from CMC, the model specification for the Forward Dynamics Tool must match. Visit __40__ Computed Muscle Control for more information.

Specifying Initial and Final Times

The properties <initial_time> and <final_time> specify the time interval over which a forward simulation is to be run. As explained in __40__ How Forward Dynamics Works, the initial time may be adjusted to the nearest time for which state values are available, as these state values (typically from CMC) will be used to initialize the forward simulation.

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If using controls from CMC, as explained in __40__ How Forward Dynamics Works, setting <use_specified_dt> to true (which is recommended) ensures that the Forward Dynamics Tool uses the same integrator time steps as that for CMC. It does this by matching the time steps found in the <initial_states_file>. In this case, the values set for the remaining properties (<maximum_number_of_integrator_steps>, <maximum_integrator_step_size>, <integrator_error_tolerance>, and <integrator_fine_ tolerance>) are ignored. If <use_specified_dt> is set to false then the integrator properties are used to determine the adaptive time step sizes (see Chapter 4 on CMC chapter for how these are used).

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Running a forward simulation requires that the states be set to initial values at the beginning of the simulation, and that the actuator control values (e.g., muscle excitations) are updated throughout the simulation. Both of these are optional with default behavior as explained in __40__ Getting Started with Forward Dynamics. Often, the initial states and the control values used to drive the forward simulation are those computed during the CMC step, in these cases the <initial_states_file> property should be set to the states output from CMC (an .sto storage file), and <controls_file> should be set to the controls output from CMC (the .xml file, not the controls .sto file that is also written by CMC). The <initial_states_file> is used to (a) determine a valid initial time for the forward simulation (it has to coincide with a time for which state values are available), (b) supply the initial state values, and (c) if <use_specified_dt> is set to true, then it is also used to determine the integration time intervals used by CMC (in order to match CMC results as much as possible).

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