Module: MRP_PD

Executive Summary

This module provides a MRP based PD attitude control module. It is similar to Module: MRP_Feedback, but without the RW or the integral feedback option. The feedback control is able to asymptotically track a reference attitude if there are no unknown dynamics and the attitude control torque is implemented with a thruster set.

Message Connection Descriptions

The following table lists all the module input and output messages. The module msg variable name is set by the user from python. The msg type contains a link to the message structure definition, while the description provides information on what this message is used for.

../../../../_images/moduleIOMrpPd.svg

Figure 1: MRP_PD() Module I/O Illustration

Module I/O Messages

Msg Variable Name

Msg Type

Description

outputDataName

cmdTorqueBodyIntMsg

Commanded external torque output message

inputVehicleConfigDataName

vehicleConfigFswMsg

Vehicle configuration input message

Detailed Module Description

This attitude feedback module using the MRP feedback control related to the control in section 8.4.1 in Analytical Mechanics of Space Systems:

(1)\[{\bf L}_{r} = -K \pmb\sigma - [P] \delta\pmb\omega + [I](\dot{\pmb\omega}_{r} - [\tilde{\pmb\omega}]\pmb\omega_{r}) +[\tilde{\pmb \omega}_{r}] ] [I]\pmb\omega - \bf L\]

Note that this control solution creates an external control torque which must be produced with a cluster of thrusters. No reaction wheel information is used here. Further, the feedback control component is a simple proportional and derivative feedback formulation. As shown in Analytical Mechanics of Space Systems, this control can asymptotically track a general reference trajectory given by the reference frame \(\cal R\).

Module Assumptions and Limitations

This control assumes the spacecraft is rigid and that the inertia tensor does not vary with time.

User Guide

The following parameters must be set for the module:

  • K: the MRP proportional feedback gain

  • P: the \(\pmb\omega\) tracking error proportional feedback gain

  • knownTorquePntB_B: (Optional) the known external torque vector \({}^{B}{\bf L}\). The default value is a zero vector.

Functions

void SelfInit_MRP_PD(MRP_PDConfig *configData, int64_t moduleID)

This method initializes the configData for this module. It checks to ensure that the inputs are sane and then creates the output message

Return

void

Parameters

  • configData: The configuration data associated with this module

  • moduleID: The module identifier

void CrossInit_MRP_PD(MRP_PDConfig *configData, int64_t moduleID)

This method performs the second stage of initialization for this module. Its primary function is to link the input messages that were created elsewhere.

Return

void

Parameters

  • configData: The configuration data associated with this module

  • moduleID: The module identifier

void Update_MRP_PD(MRP_PDConfig *configData, uint64_t callTime, int64_t moduleID)

This method takes the attitude and rate errors relative to the Reference frame, as well as the reference frame angular rates and acceleration, and computes the required control torque Lr.

Return

void

Parameters

  • configData: The configuration data associated with the MRP Steering attitude control

  • callTime: The clock time at which the function was called (nanoseconds)

  • moduleID: The module identifier

void Reset_MRP_PD(MRP_PDConfig *configData, uint64_t callTime, int64_t moduleID)

This method performs a complete reset of the module. Local module variables that retain time varying states between function calls are reset to their default values.

Return

void

Parameters

  • configData: The configuration data associated with the MRP steering control

  • callTime: [ns] Time the method is called

  • moduleID: The module identifier

struct MRP_PDConfig
#include <MRP_PD.h>

Module configuration message definition.

Public Members

double K

[rad/sec] Proportional gain applied to MRP errors

double P

[N*m*s] Rate error feedback gain applied

double knownTorquePntB_B[3]

[N*m] known external torque in body frame vector components

double ISCPntB_B[9]

[kg m^2] Spacecraft Inertia

char outputDataName[MAX_STAT_MSG_LENGTH]

The name of the output message.

int32_t controlOutMsgId

[] ID for the outgoing body accel requests

char inputGuidName[MAX_STAT_MSG_LENGTH]

The name of the Input message.

int32_t guidInMsgId

[] ID for the incoming guidance errors

char inputVehicleConfigDataName[MAX_STAT_MSG_LENGTH]

The name of the Input message.

int32_t vehicleConfigDataInMsgId

[] ID for the incoming static vehicle data

BSKLogger *bskLogger

BSK Logging.