power latest work summary

8/4/2019 Power Latest Work Summary

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Power Generation Subsystem

Objectives:

The major functions of the power system can be classified as below.

1) Power production2) Proper power distribution3) Controlling the supply to different subsystems

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 do not intend to

disturb the systems stability by altering the center of mass thereby producing undesired torque.

System Components:

After careful analysis of the systems requirements, we see the need for the following components:

Solar panels (12 no.s) Maximum power point tracker with built in DC-DC buck-boost convertor (1 no.) Battery charger circuit Li ion cells (2 no.s) Reverse current blocking diodes(13 no.s) Temperature sensors (3 no.s)

Current Status:

The power subsystem prototype is nearing completion with the exception of the buck-boost converter. The

battery charger circuit used in the system is shown below. The switching between the primary source (Solarpanels) and the auxiliary source (Li-ion batteries) is done using a microcontroller which also performs theswitching action for the buck-boost converter. There is a pre-defined priority within the microcontroller code for

the loads(subsystems) connected to the voltage bus and these are switched on and off as per power availability

using relay switches. When the input voltage from the solar panels falls below a certain threshold; the controller

automatically switches on the batteries. Similarly, the batteries are charged from the panels when their voltage

falls below a set threshold.

Battery charger circuit:

This circuit was build to charge a couple series Lithium cells (3.6 volts each, 1 Amp Hour capacity) installed in

the subsystem as an auxiliary source of power.

The charger operates by supplying a short current pulse through a series resistor and then monitoring the battery

voltage to determine if another pulse is required. The current can be adjusted by changing the series resistor oradjusting the input voltage. When the battery is low, the current pulses are spaced close together so that a

somewhat constant current is present. As the batteries reach full charge, the pulses are spaced farther apart and

the full charge condition is indicated by the LED blinking at a slower rate.

A TL431, band gap voltage reference (2.5 volts) is used on pin 6 of the comparator so that the comparator

output will switch low, triggering the 555 timer when the voltage at pin 7 is less than 2.5 volts. The 555 output

turns on the 2 transistors and the batteries charge for about 30 milliseconds. When the charge pulse ends, the

battery voltage is measured and divided down by the combination 20K, 8.2K and 620 ohm resistors so that

when the battery voltage reaches 8.2 volts, the input at pin 7 of the comparator will rise slightly above 2.5 volts

and the circuit will stop charging.


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System Block Diagram:The power flow diagram for the subsystem is shown below

Solar panels

Maximum

power point

tracker +

Buck- Boost

convertor

Battery pack

Battery charger circuit

Power distribution switch (priority

control)

Load Load Load

Microcontroller


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Control of Power Supply:

Solar cell output is constantly monitored by MCU. The Maximum Power Point Tracker (MPPT) circuit regulates the input power from solar panels. Similarly charge level of batteries is monitored. Short circuit protection is provided for in form of switches.

Low output detection of panels by MCU switches on the batteries. The number of loads running on battery power are reduced to prolong battery life. The MPPT algorithm shown below has been implemented in the circuit.

power latest work summary

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