rigid_body_assembly Class Reference

Multi-stage rigid body model. More...

#include <rigid_body_assembly.h>

Inherits dsim_model.

Public Member Functions
	rigid_body_assembly (dsim_model_setup *setup)
	Constructor.
	~rigid_body_assembly ()
	Destructor.
void	initialize_data ()
	Initialize outlets and output variables.
void	initialization_complete ()
	Finish initialization.
void	initialize_timestep ()
	Computation done prior to numerical integration.
void	complete_timestep ()
	Computation done after numerical integration.
void	rhs (double t, double jd)
	Computation done during numerical integration. More...
dsim_value	handle_message (const std::string &sender_path, const std::string &message_name, const dsim_value &argument)
	Handle force and torque messages. More...
void *	describe_setup (void *ds)
	Describe the setup function.
bool	parse_setup (const char *command, const char *data)
	Parse the setup. More...
void	pre_calculate ()
	Computation done before rhs is called.

Protected Member Functions
double	get_adj_mass ()
	Check for changed mass of children and recompute new adjusted mass. More...
ml_matrix	get_adj_center_of_mass ()
	Recompute new adjusted center of mass. More...
ml_matrix	get_adj_inertia ()
	Recompute new adjusted inertia matrix. More...
ml_matrix	get_adj_inertia_derivative ()
	Recompute new adjusted inertia derivative. More...
double	get_adj_mass_derivative ()
	Recompute new adjusted mass derivative. More...
ml_matrix	get_adj_center_of_mass_derivative ()
	Recompute the derivative of the center of mass. More...
ml_matrix	get_jet_damping ()
	Compute the jet damping vector. More...

Protected Attributes
dsim_variable	spacecraft_name_dsim
	Spacecraft name.
dsim_variable	spacecraft_type_dsim
	Spacecraft type.
ml_matrix	force
	The external force on the spacecraft.
ml_matrix	torque
	The external torque on the spacecraft.
dsim_variable	totalMass_dsim
	total mass, including mass of children
dsim_variable	center_of_mass_dsim
	Center of mass of combined stages and children.
dsim_variable	accelTranslational_dsim
	Linear acceleration in body frame.
dsim_variable	accelAngular_dsim
	Angular acceleration in body frame.
dsim_variable	position_dsim
	object position
dsim_variable	velocity_dsim
	object velocity
dsim_variable	quaternion_dsim
	attitude quaternion ([scalar;x;y;z])
dsim_variable	bodyRate_dsim
	body rotational rates (rad/s)
dsim_variable	inertia_dsim
	inertia about the center of mass (kg-m2)
dsim_variable	mass_derivative_dsim
	Mass derivative of connected stages.
dsim_variable	inertia_derivative_dsim
	Moment of inertia derivative of combined stages.
dsim_variable	center_of_mass_derivative_dsim
	Center of mass derivative of combined stages.
dsim_variable	frame_dsim
	frame for initial conditions
dsim_variable	ang_momentum_dsim
	Angular momentum.
dsim_variable	angular_acceleration_dsim
	Angular acceleration.
dsim_variable	parent_com_dsim
	Center of mass of parent.
dsim_variable	parent_q_eci_to_body_dsim
	Quaternion of the parent.
dsim_variable	parent_position_dsim
	Position of the parent.
std::string	parent_path
	Path of the parent.
dsim_variable	stagingCommand_dsim
	Staging command.
dsim_variable	torque_out_dsim
	torque
std::string	componentList [100]
	List of the components of the assembly.
ml_matrix	connectionVector
	Stage connections.
int	nComponents
	Number of connected components;.
ml_matrix	quaternionArray
	Stage attitude array.
bool	child_obj
	Flag if child.

Detailed Description

Multi-stage rigid body model.

Rigid body dynamical model applying both force and torque to the spacecraft stage. Models offset center of mass and changing inertia properties.

Integrates the position, velocity, quaternion, and body rates. Allows the mass of the spacecraft to change. Children can use the apply_torque or apply_force messages.

Setup:

The rigid_body_assembly requires initialization of position, velocity, quaternion, body_rate, center_mass, mass_base, and inertia just like rigid_body, but also some additional parameters:

• stageImpulse: The impulse applied at the staging event to separate the stages.
• currentStatus: Sets the initial status of the stage: 0 = inactive, 1 = active stage
• stageConnection: Vector from the origin of this stage's reference frame to the next stage's reference frame
• stage_no: Number of this stage on the stack (1 = first stage to fire)
• stage_count: Number of total stages on the stack

• frame_dsim: Spice frame number of the initialization reference frame

  rigid_body_assembly also handles the mass properties of child objects. In order for the mass properties to be calculated correctly, all children with mass should have the following variables:

• center_mass: The center of mass of the child in the parent's body axes
• inertia: The inertia matrix of the child in the parent's frame with respect to the child's center of mass

• mass: The total mass of the child

  For children with time-varying mass properties, the following should also be included:

• inertiaDerivative: The time derivative of the inertia matrix

• massDerivative: The time derivative of the mass

  Use:

  rigid_body_assembly integrates the state of the active stage with all of the mass properties of the inactive stages added to it. In complete_timestep, the state of each stage is calculated from the result of the active stage. The model accomodates a changing center of mass by re-adjusting the center of the coordinate frame at each time step, and then translating back to the original frame after the integration. Staging events are controlled by the commmand stageCommand. If this variable is set to 1 for the active stage, it will separate and the next stage will become active. Separated stages are then treated as single rigid body objects, and their states continue to be integrated.

Member Function Documentation

rhs()

void rigid_body_assembly::rhs	(	double	t,
		double	jd
	)

Computation done during numerical integration.

Quaternion kinematics and orbit derivative.

The coordinate frame is previously translated to the center of mass, so simple rigid body dynamics are appropriate. The results are translated back to the original frame. Uses qi_to_b_dot from the SCControl framework.

handle_message()

dsim_value rigid_body_assembly::handle_message	(	const std::string &	sender_path,
		const std::string &	message_name,
		const dsim_value &	argument
	)

Handle force and torque messages.

rigid_body_assembly supports the following messages: 'apply_force','apply_torque','stage_event','add_stage','set_state_to_stage' apply_force - Adds the argument to the force vector applied to the rigid body.

Argument should be in the ECI frame. apply_torque - Adds the argument to the torque vector applied to the rigid body. Argument should be in the body frame. !

parse_setup()

bool rigid_body_assembly::parse_setup	(	const char *	command,
		const char *	data
	)

Parse the setup.

Processes AddComponent commands.

The data is the following:

• name of object (as recognized by DSim)
• position (vector to component's body frame origin)
• quaternion (assembly-to-body rotation of the child)

get_adj_mass()

double rigid_body_assembly::get_adj_mass ( )

protected

Check for changed mass of children and recompute new adjusted mass.

Add up the mass of this object and any children that have a mass variable.

get_adj_center_of_mass()

ml_matrix rigid_body_assembly::get_adj_center_of_mass ( )

protected

Recompute new adjusted center of mass.

Determine the center of mass of this object and any children that have a mass variable.

get_adj_inertia()

ml_matrix rigid_body_assembly::get_adj_inertia ( )

protected

Recompute new adjusted inertia matrix.

If the child has an inertia matrix, rotate and translate and add to parent inertia If the child has a mass value, but not inertia, treat as a point mass and add to parent inertia if a center_mass value is available.

get_adj_inertia_derivative()

ml_matrix rigid_body_assembly::get_adj_inertia_derivative ( )

protected

Recompute new adjusted inertia derivative.

Determine the time derivative of the inertia matrix.

get_adj_mass_derivative()

double rigid_body_assembly::get_adj_mass_derivative ( )

protected

Recompute new adjusted mass derivative.

Add up the mass of this object and any children that have a mass variable.

get_adj_center_of_mass_derivative()

ml_matrix rigid_body_assembly::get_adj_center_of_mass_derivative ( )

protected

Recompute the derivative of the center of mass.

Determine the center of mass derivaitve of this object and any children that have a mass variable.

get_jet_damping()

ml_matrix rigid_body_assembly::get_jet_damping ( )

protected

Compute the jet damping vector.

Get the jet damping term for the rhs.

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The documentation for this class was generated from the following files:

• rigid_body_assembly.h
• rigid_body_assembly.cc