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This is the page for the SEDSAT-2 Power subsystem team. Subsystem design notes are filed in Category:SEDSAT-2 Power.

Create new design note: SEDSAT-2 Power Design Notes 20081008

[1] Power schematic 2.

Contents 1 NEW 2 System Overview 2.1 Initial Aims and considerations: 2.2 System components: 2.3 Basic functionality of components used: 2.4 Preliminary Power Availability Estimation 3 Requirements From Other Subsystems 3.1 Payload 3.2 Communications 3.3 Command and Data Handling 3.4 ADCS 3.5 Structures 3.6 Thermal 4 Active Members 4.1 Interface Tables

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Design report-2[2]

System Overview

Introduction:

The primary objective of the power system is to supply energy to other subsystems as per their requirements. For this purpose, a mechanism has to be formulated to harness the naturally available solar energy and energy in Earth’s albedo ,convert it into electrical energy and pass it on to the other systems with minimum loses incurred. This calls on for the functions of power production, distribution, storage and management and proper switching mechanisms. Keeping this in mind, we strive to build a system which does the above functions efficiently and at a minimal cost. The size of cubesat places enormous constraints on the power production capability of the satellite. Due to the limited size available, the solar panels that can be fitted would be small and so would be the power produced. Care must be taken that the energy consumption rates of other subsystems do not outweigh the energy production capacity of the panels. Further, the limited space and weight allocated for the subsystem also constricts the range of components that can be used. Another major challenge in the design of the subsystem is the thermal management required, especially in sensitive areas like the batteries which have lower operating ranges.

Major Functions: The major functions of the power system can be classified as below. 1) Power production 2) Proper power distribution 3) Controlling the supply to different subsystems

For power production, we opted for solar panels to be the source as the other sources are too complex or too risky like the tether systems. The power produced is stored in batteries to be used in the dark periods. Power has to be supplied to the subsystems as per their voltage and current ratings which may differ from subsystem to subsystem. So, power distribution calls on for proper buses to deliver power at different voltage levels to different systems and there must be possibility to switch off the power supply if the subsystems fail or are using power exceeding their rating. There are many places in the power system where a variety of the components’ parameters have to be monitored periodically and the collected data has to be sent to the command and data handling system for proper interpretation. Also, the C&DH may generate some control signals that has to be accepted by the power system to make suitable changes in the system operation.

Initial Aims and considerations:

The power system is allotted around 225 gms of mass. We have a preliminary aim to produce about 2W of power continuously. We intend to take up 2 PCBs exclusively for this purpose along with some other volume for keeping the batteries. The issue of placing the batteries has to be considered seriously as we donot intend to disturb the system’s stability by altering the center of mass thereby producing undesired torque.

System components:

After careful analysis of the system’s requirements, we see the need for the following components:  Solar panels (10 no.s)  Maximum power point tracker with built in DC-DC boost convertor (1 no.)  Battery charger ICs(2 no.s)  Li ion cells (2 no.s)  Battery protector IC (1 no.)  Reverse current blocking diodes(13 no.s)  Temperature sensors (7 no.s)  DC-DC buck boost convertors (5)  Power distribution switches (5 no.)

Basic functionality of components used:

A detailed analysis of the components is provided later but their basic functionality is presented here for the reader to understand the sections that follow this. The solar panel is basically a current generator where incident optical energy produces electron hole pairs in the material chosen. These pairs have to be collected by proper reverse bias which results in current flow. A maximum power point convertor is a scanner varying the voltage level of the solar panels to the point where the current production capacity becomes the highest. The maximum power point tracker has a DC-DC boost convertor built with it to boost the output voltage level to the battery charging voltage. The battery charger performs the safe charging of the batteries attached to it taking them through trickle charge, constant voltage and constant current modes. The charger also disconnects the batteries from the charging circuits in case of lower power availability or battery malfunction. A battery protector IC is used to ensure that the battery discharges properly and switches off the discharge circuitry in case of emergencies and periods of battery drain. The reverse current blocking diodes are ordinary low power consumption diodes used to ensure that current flows only in one direction along the wire. The temperature sensors are used to take the readings of temperatures near the solar panels and the batteries and report these values to the C&DH to generate proper signals for thermal control. DC-DC buck boost convertors are used to step down or step up voltage levels to the desired range of operation of other subsystems. Load protector ICs are interfaced with other subsystems to switch them off in case of excessive current drain through them. Power distribution switch is also used for the same purpose but this is interfaced with the microcontroller to switch back the power under normal conditions and to account for the priority of power supply to be described later.

Preliminary Power Availability Estimation

Note: The following calculations have been done using the solar panels supplied by a company ‘EMCORE’ which claims to be supplying the best space grade solar panels.IF we decide later on to go for spme other panels,this estimation is subject to changes.

Panel specifications: Panel junctions: InGap, InGaAs, Ge Density: 84 mg/cm2 Substrate thickness: 140microm. Solar cell area: 27.47cm2 (A) Voc=2.7v Isc=455 mA Efficiency=28.5 %( BOL) and 27 %( EOL) @280c

Flux densities available: Solar flux density in visible region=1353 W/m2 (F1) Solar flux density in IR region=237 W/m2 (F2) Earth’s albedo radiation flux density=406 W/m2 (F3)

Considering the scenario where one side of the cubesat faces the Sun and one side faces the Earth, We can get an estimate of the solar power that can be produced. The dimensions of the solar panels are 7.61cmX3.61 cm. As such, we can put up two panels on each side.

This configuration leaves out a lot of space for other possible surface mounts. If this turns out to be too inefficient, we can get custom sized solar panels depending on how much we are ready to shell out of our pockets. Assuming a circular orbit for the cubesat at 650 Km altitude, The orbital velocity, V= (mu/R) 1/2 =7.4987 Km/sec Time period of satellite=2xpixR/V =98.84 minutes All further calculations are based on one orbital time period. Assuming there are solar panels on 5 sides of the cubesat (one side dedicated to payload and one side dedicated to Comm.), there are 10 panels in total. The mode of connection among the panels is still undecided but the most probable configuration would be connecting the panels on each side in series and connecting panels on different sides in parallel configuration. The net power incident on panels= (F1+F2+F3)*A*2= (1353+237+406)* 0.002747*2 =10.966 W (p) [since there are two panels on each side] Assuming EOL condition, net power produced by panels on one side=p*0.27 =2.96 W

Keeping in mind that there are situations where two or three sides of the cubesat are under Sun’s influence, a procedure can be devised to find the average power output form the panels. This can be done by finding the vector projections of the area of the sides on to the normal to the Sun. A simple MATLAB program is coded to get the results. The program is raw and may undergo some modifications in the future. For now, the average net power estimated is 2.87 W without considering any losses like the diode losses and losses in convertors. Considering a 75% efficiency for the whole system, the output would be around 2.15 W. click here for the MATLAB code image-->[[3]]

The shadow time for the satellite comes out to be 35.4 minutes which means the sunlight falls on cubesat for 63.43 minutes. Thus the energy available per revolution of satellite is 63.43x60x2.15=8183.476J.

Note: In the above program, nu and phi are the angles of cubesat sides with respect to imaginary Cartesian grid in the space assuming the Sun light falling normally parallel to one of the axes.

Requirements From Other Subsystems

Payload

1. Power specifications of payload unit

2. Difference in power consumption when in idle mode and in image processing mode and the time per orbit of each of the above modes.

Communications

1. Power specifications of all components being used

2. Difference in power consumption when in idle mode and in transmit/ recieve mode and the time per orbit of each of the above modes.

Command and Data Handling

1. Power specifications of all components being used.

2. Power consumption during image processing and idle modes and duration of each mode per orbit.

3. Duration of boot up of on board computer after launch and power required for that purpose.

4. Interrupts that the C&DH requires from the Power microcontroller as well as the interrupts OBC will be giving to the Pwer MCU.

ADCS

1. Power specifications of all components being used.

2. Energy required when in detumbling mode after launch.

3. Duration of detumbling mode.

4. Difference in power consumption when in idle mode and active mode.

Structures

1. Placement of deployment switches and kill switch - what sort of mechanical-electrical interface will be used in the deployment switches?

Thermal

1. Power specifications of any electronics being used.

Active Members

1. Phani Kiran (Team Lead), India

2. Pavan Kumar, USA

3. Shreya Kumar, India

4. Himanshu Misra, India

5. Ajay Kumar P., USA

Interface Tables

Inputs edit inputs

• To: Power - A medium to send housekeeping signals to the ground

-To get the critical functions of power MCU redundantly implemented in CDH's MCU to take over control in situation of power MCU failure or malfunction Outputs edit outputs

• From: Power -

- 5.0 and 3.3VDC standard voltage for logic devices

- 3.3V DC and 150mA for the camera unit - 2.8VDC 10 mA 28 mW (for 3 axis magnetometer)

- 2.8VDC 55 mW (for attitude control VGA sensor)

-To communications:Trasceiver(transmit:1.2A@3.7V for 10 min per orbit,receive:60mA@3.7V for 90 min per orbit),MCU and TNC each(10mA@3.3V for 90 min orbit)

--Other subsystem may add the voltages, amps and at what tolerance they need in their own input table and it will be added as a Power output here.-- Environments edit environments

• On: Power -

Space-grade solar panels should work in the space environment and temperatures.

Lithium-ion batteries operating tempuratures- charge: 0-45C, discharge: -20-60C, storage: -20-45C.

Circuit board should work under normal satellite operating conditions.