Module: ReactionWheelPower

Executive Summary

This module evaluates the power draw of a Module: reactionWheelStateEffector. It reads a message with the RW states containing the wheel speed and motor torque value. These are used to evaluate the power required to change the wheel speed. In addition, there is a base power draw of the RW device that accounts for the electrical power required for the RW device just to be on.

This module is a sub-class of Module: powerNodeBase class. As such it has the common properties:

1. Writes out a powerNodeUsageSimMsg describing its power consumption at each sim update based on its power consumption attribute

2. Can be switched on or off using an optional message of type deviceStatusIntMsg

Message Connection Descriptions

This module uses the input and output messages of the Module: powerNodeBase base class. The following table lists all the module input and output messages in addition to the base class. The module message variable name is set by the user from python. The message type contains a link to the message structure definition, while the description provides information on what this message is used for.

Figure 1: fswModuleTemplate() Module I/O Illustration

Module I/O Messages

Msg Variable Name	Msg Type	Description
rwStateInMsgName	rwConfigLogSimMsg	RW state input message to provide the reaction wheel speed and motor torque information.

Detailed Module Description

Let \(p_{\text{RW}}\) be the net electrical power required to operate a reaction wheel device. This power requirement is computed from the base power requirement to operate the device and the mechancial power requirement to torque a spinning disk. Let \(p_{\text{base}}\) be the base RW device power requirement. This is a positive value indicating what power is required to just turn on the RW device, never mind control the wheel speed.

To compute the power required to change the energy state of a RW device the work/energy principle is used as discussed in Analytical Mechanics of Space Systems. The mechanical power required to change the wheel speed relative to the spacecraft body is

(1)\[p_{\text{mech}} = \Omega \cdot u_s\]

where \(u_s\) is the RW motor torque.

Case 1: No Mechanical Energy Recovery

The most basic RW power behavior assumes that it takes electrical power to both accelerate and decelerate the RW spin rate. Let the efficiency factor of converting electrical to mechanical power be given by \(\eta_{e2m}\). The net RW power is then

(2)\[p_{\text{RW}} = p_{\text{base}} + \frac{| p_{\text{mech}} |}{\eta_{e2m}}\]

Case 2: Recovering Partially Mechanical Energy

The next case considers the RW device ability to partially recover the mechanical energy when breaking the RW speed. Let the mechanical to electrical conversion efficiency factor be \(\eta_{m2e}\). If the wheel speed is being made larger then \(p_{\text{mech}} > 0\) and Eq. (2) is used to evaluate the net RW power requirement.

If the wheel speed is being made smaller and \(p_{\text{mech}} < 0\), then if \(\eta_{m2e} >= 0\) the model assumes that breaking the wheel generates some power and thus decreases the net RW power requirement. This is evaluated using:

(3)\[p_{\text{RW}} = p_{\text{base}} + p_{\text{mech}} \cdot \eta_{e2m}\]

Note that \(p_{\text{RW}}\) could become negative in this situation, illustrating power being returned to the spacecraft power system.

Case 3: No Power Requirement for Breaking

If the user wants to model a case where breaking the RW speed requires no power, then simply set \(\eta_{m2e}\) = 0.

Module Power Output Evaluation

Having computed the net RW power need \(p_{\text{RW}}\), next the module power outpute \(p_{\text{out}}\) must be determined. Note that \(p_{\text{RW}}\) is typically a positive value, indicating it takes power to run this RW device. Thus, the power draw on the network is simply

(4)\[p_{\text{out}} = -p_{\text{RW}}\]

Module Assumptions and Limitations

See Module: powerNodeBase class for inherited assumption and limitations. This RW power module assumes a positive RW power requirement manifests as a negative power draw on the spacecraft power system. A negative RW power requirement \(p_{\text{RW}}\) means the devices is converting mechanical energy back to the power grid.

User Guide

Inheritance

This module inherits the user guide from the Module: powerNodeBase base class. See that documentation for features common to that base class.

Minimum Module Setup

The following code illustrates the minimum module setup within Python assuming the module is connected to the first RW (thus the 0 label):

testModule = PowerRW.PowerRW()
testModule.ModelTag = "bskSat"
testModule.basePowerNeed = 10.   # baseline power draw, Watts
testModule.rwStateInMsgName = testModule.ModelTag + "_rw_config_0_data"
unitTestSim.AddModelToTask(unitTaskName, testModule)

The user needs to specify a base power consumption \(p_{\text{base}}\) through the variable basePowerNeed. This should be a positive value to reflect the power required just to turn on the RW device, even without any motor torque commands being applied.

You also need to specify the RW state message with the module variable rwStateInMsgName.

This setup will evaluate the RW power using Eq. (2) where 100% efficiency is assumed in converting electrical to mechanical energy with elecToMechEfficiency = \(\eta_{e2m}\) = 1, and no electrical energy is recovered from breaking the wheel speeds with mechToElecEfficiency = \(eta_{m2}\) = 1.

Accounting for Non-Ideal Power Conversion

For example, if 10W of electrical power does not lead to 10W of mechanical power, then this is modeled by setting the module variable elecToMechEfficiency to a strictly positive value less than 1. The value of 1 represents 100% conversion efficiency and is the default value for this parameter.

To account for harvesting mechanical power during the RW speed braking process, converting mechanical to electrical power, then the variable \(1 \ge\) mechToElecEfficiency\(\ge 0\) must be set to a positive value. The value of 1 again 100% conversion efficiency (not realisitic). Typically this is a smaller percentage.

To account that breaking does not require any electrical power, then simply set mechToElecEfficiency to 0.

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class ReactionWheelPower : public PowerNodeBase

Public Functions

ReactionWheelPower()

Constructor

~ReactionWheelPower()

void customCrossInit()

Custom message subscription method.

This method subscribes to the RW state message.

Return

void

void customReset(uint64_t CurrentSimNanos)

Custom reset method.

This method is used to reset the module. Here variables are checked for correct values.

Return

void

bool customReadMessages()

Custom read method, similar to customSelfInit; returns true by default.

This method is used to read incoming RW state message.

Return

void

Public Members

std::string rwStateInMsgName

Reaction wheel state input message name.

double elecToMechEfficiency

efficiency factor to convert electrical power to mechanical power

double mechToElecEfficiency

efficiency factor to convert mechanical power to electrical power

double basePowerNeed

[W] base electrical power required to operate RW, typically a positive value

BSKLogger bskLogger

BSK Logging

Private Functions

void evaluatePowerModel(PowerNodeUsageSimMsg *powerUsageMsg)

Computes the RW power load. Determines the netPower attribute in powerUsageSimMessage.

Private Members

int64_t rwStateInMsgId

Message ID for the RW state input message.

RWConfigLogSimMsg rwStatus

copy of the RW status message