thrFiringRemainder

Executive Summary

A thruster force message is read in and converted to a thruster on-time output message. The module ensures the requested on-time is at least as large as the thruster’s minimum on time. If not then the on-time is zeroed, but the unimplemented thrust time is kept as a remainder calculation. If these add up to reach the minimum on time, then a thruster pulse is requested. If the thruster on time is larger than the control period, then an on-time that is 1.1 times the control period is requested. More information can be found in the PDF Description. The paper Steady-State Attitude and Control Effort Sensitivity Analysis of Discretized Thruster Implementations includes a detailed discussion on the Remainder Trigger algorithm and compares it to other thruster firing methods.

Message Connection Descriptions

The following table lists all the module input and output messages. The module msg connection 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.

Figure 1: rwNullSpace() Module I/O Illustration

Module I/O Messages

Msg Variable Name	Msg Type	Description
thrForceInMsg	THRArrayCmdForceMsgPayload	thruster force input message
onTimeOutMsg	THRArrayOnTimeCmdMsgPayload	thruster on-time output message
thrConfInMsg	THRArrayConfigMsgPayload	Thruster array configuration input message

Module Description

This module implements a remainder tracking thruster firing logic. More details can be found in Reference.

Module Input and Output Messages

As illustrated in Figure [fig:moduleImg], the module reads in two messages. One message contains the thruster configuration message from which the maximum thrust force value for each thruster is extracted and stored in the module. This message is only read in on reset().

The second message reads in an array of requested thruster force values with every Update() function call. These force values \(F_{i}\) can be positive if on-pulsing is requested, or negative if off-pulsing is required. On-pulsing is used to achieve an attitude control torque onto the spacecraft by turning on a thruster. Off-pulsing assumes a thruster is nominally on, such as with doing an extended orbit correction maneuver, and the attitude control is achieved by doing periodic off-pulsing.

The output of the module is a message containing an array of thruster on-time requests. If these on-times are larger than the control period, then the thruster remains on only for the control period upon which the on-criteria is reevaluated.

reset() Functionality

• The control period is dynamically evaluated in the module by comparing the current time with the prior call time. In reset() the prevCallTime variable is reset to 0.

• The thruster configuration message is read in and the number of thrusters is stored in the module variable numThrusters. The maximum force per thruster is stored in maxThrust.

• The on-time pulse remainder variable is reset for each thruster back to 0.0.

Update() Functionality

The goal of the update() method is to read in the current attitude control thruster force message and map these into a thruster on-time output message. Let \(\Delta t_{\text{min}}\) be the minimum on-time that can be implemented with a thruster. If the requested on-time is less than \(\Delta t_{\text{min}}\), then the requested thruster on-time is clipped to zero. In the following algorithm unimplemented fractional on-times less than \(\Delta t_{\text{min}}\) are tracked and accumulated, providing additional pointing accuracy. For example, if the minimum on-time is 20 milli-seconds, an on-time algorithm without remainder calculation would create a deadband about the 20 milli-second control request. With the remainder logic, if 5 milli-second on-time requests are computed, these are accumulated such that every 4\(^{\text{th}}\) control step a 20 milli-second burn is requested. This reduces the deadband behavior of the thruster and achieves better pointing. In this example the 5 milli-second un-implemented on-times are accumulated in the variable \(\Delta t_{\text{partial}}\).

If the update() method is called for the first time after reset, then there is no knowledge of the control period \(\Delta t\). In this case the control period is set to 2 seconds, unless the module parameter defaultControlPeiod is set to a different value. If this is a repeated call of the update() method then the control period \(\Delta t\) is evaluated by differencing the current time with the prior call time. Next a loop goes over each installed thruster to map the requested force \(F_{i}\) into an on-time \(t_{i}\). The following logic is used.

• If off-pulsing is used then \(F_{i}\le 0\) and we set

  \[F_{i} += F_{\text{max}}\]

  to a reduced thrust to achieve the negative \(F_{i}\) force.

• Next, if \(F_{i} < 0\) then it set to be equal to zero. This can occur if an off-pulsing request is larger than the maximum thruster force magnitude \(F_{\text{max}}\).

• The nominal thruster on-time is computed using

  \[t_{i} = \dfrac{F_{i}}{F_{\text{max}}} \Delta t\]

  .

• If there un-implemented on-time requested \(\Delta t_{\text{partial}}\) from earlier update() method calls, these are added to the current on-time request using

  \[t_{i} += \Delta t_{\text{partial}}\]

  After this step the variable \(\Delta t_{\text{partial}}\) is reset to 0 as the remainder calculation is stored in the on-time variable \(t_{i}\).

• If \(t_{i} < \Delta t_{\text{partial}}\) then on-time request is set to zero and the remained is set to \(\Delta t_{\text{partial}} = t_{i}\)

• If \(t_{i} > \Delta\) then the requested force is larger than \(F_{\text{max}}\) and the control is saturated. In this case the on-time is set to 1.1\(\Delta t\) such that the thruster remains on through the control period.

• The final step is to store the thruster on-time into and write out this output message

Module Functions

• Read in thruster configuration message: This is used to determine the number of installed thrusters and what the maximum force is for each.

• Convert thruster force requested into an on-time request: Knowing how strong the thruster is, the on-time is scaled such that the effectively applied force is equal to the requested force.

Module Assumptions and Limitations

The module assumes that the incoming forces \(F_{i}\) can be both positive or negative, depending if an on- or off-pulsing mode is being implemented. The particular mode is set through baseThrustState.

Class ThrFiringRemainder

class ThrFiringRemainder : public SysModel

Top level structure for the sub-module routines.

Public Functions

ThrFiringRemainder() = default

Constructor.

~ThrFiringRemainder() = default

Destructor.

void reset(uint64_t callTime) override

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.

Parameters:

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

Returns:

void

void updateState(uint64_t callTime) override

This method maps the input thruster command forces into thruster on times using a remainder tracking logic.

Parameters:

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

Returns:

void

void setThrMinFireTime(double thrMinFireTime)

Setter for thrMinFireTime variable.

Setter method for thrMinFireTime.

Parameters:

thrMinFireTime –

Returns:

void

double getThrMinFireTime() const

Getter for thrMinFireTime variable.

Getter method for thrMinFireTime.

Returns:

const double

void setThrustPulsingRegime(ThrustPulsingRegime thrustPulsingRegime)

Setter for thrustPulsingRegime variable.

Setter method for thrustPulsingRegime.

Parameters:

thrustPulsingRegime –

Returns:

void

ThrustPulsingRegime getThrustPulsingRegime() const

Getter for thrustPulsingRegime variable.

Getter method for thrustPulsingRegime.

Returns:

ThrustPulsingRegime

void setDefaultControlPeriod(double defaultControlPeriod)

Setter for defaultControlPeriod variable.

Setter method for defaultControlPeriod.

Parameters:

defaultControlPeriod –

Returns:

void

double getDefaultControlPeriod() const

Getter for defaultControlPeriod variable.

Getter method for defaultControlPeriod.

Returns:

const double

Public Members

ReadFunctor<THRArrayCmdForceMsgPayload> thrForceInMsg

The name of the Input message.

Message<THRArrayOnTimeCmdMsgPayload> onTimeOutMsg

The name of the output message, onTimeOutMsg.

ReadFunctor<THRArrayConfigMsgPayload> thrConfInMsg

The name of the thruster cluster Input message.

BSKLogger bskLogger = {}

BSK Logging.