Wednesday, October 1, 2008

Photo Roller n Photovore

This is in regards is what to be told to the biggest solar bots competition in India , this is for all those who did not attend the costly workshop conducted by robosoft in co ordination with iit bombay.

The two solar bots competition photovore n photoroller , uses only solar enrgy as the power source over here the power being given by a 500 w bulb .
In photoroller which is the prelims of techfest known as nexus ,u need to travel 1 meter straight .
The problems faced by me were i didnt buy proper dimensions solar cell which gave me lot of problems , like voltage supply n dimensions.

First thing to keep in mind when buying the solarcell is to fit in proper dimensions , n mind the weight of it .Next coming to the ratings of the solarcell , thinking taking higher voltage rated solarcell wuld give me more power then have in mind tat it completely depends on intensity of light falling on the solarcell n the area of the solarcell. I took a solarcell of 6v 120 mA having a dimension of 16x8cm , which was very big n had a problem in fitting it on to the bot.
By looking at other peoples bot , i decided to use a 4v 90mA rated solarcell having a dimension of less than 100cm^2 . The position of the solarcell has to be considered also , like tilting it towards the soalrenergy wuld give more power , bcz in the round of photovore the solarlamp is not perpendicular so there wuld be a problem over there.

Checking the solarcell:
When u first buy a solarcell of ny rated voltage try to hold in it in the sunlight , n check it out wth the help of a 2v rated motor , prefarable timing wuld be in the afternun when there is more sunlight. It wont move wth very high speed but still u will understand some thing.

As the output power tat comes from the solar cell is of arnd 4 v it wont be able to drive the regular 12v motor, not even an inch, the only motors over here tat u can use it is the 9v 10 mA rated taperecorder motor, though 9v it can run at 1.5v wth gud speed.The speed of this motor wuld be arnd 1200rpm which means u will get zero torque.
The final thing on motors is use a taperecorder motor which runs easily at 2 v also , and try to remove the outerlayer of the motor this reduces the weight of the motor.

The general method which people build for solarbots is they hang thier motors at an angle of 45degrees , wth shaft touching the ground as seen in this picture .
The shaft of the motor has been wounded round by a rubber so there is a litle bit of friction to move.

But i used a toy which i got it for 50 rs it had a gear mechanism which made the output speed to come close to around 300 rpm , which also means i will be getting higher torque now . As one wld need to win the competition , one needs gud speed n not doing the traditional solarbots way try to use the gearbox of toycars only , which will give gud speeds n torque.
Now coming to make the motors run with the help of the solarcell . First thing is solarcell doesnt store charges n hence you cant directly connect the terminals of the solar cell to the motor, though it runs sometimes (only under sunlight n tat too at noon) . You wuld be needing a component which will store this charges n send them out instantaneuosly n hence you use a capacitor over here . The general rating of the solarcell is 4700 uF capacitor (25v ) . The capacitor stores charges upto the max limit of it n then discharges out only when u connect a motor, IT WILL DISCHARGE OUT ONLY WHEN U CONNECT THE MOTOR , CAPACITOR ACTS AS OPENCKT WHEN THERE IS NO LOAD. So u cant directly connect the motor to the capacitor bcz u wont be removing one terminal of the motor n then fixing it back . General type of ckts tat people use are :

How it Works(The FLED SE)?

First of all you should know these facts

1. In a NPN transistor current flows from emitter to collector. For this N(emitter) should be negative, P(base) should be positive and N(collector) should be negative.

2. In a PNP transistor current flows from collector to emitter. For this P(emitter) should be positive, N(base) should be negative and P(collector) should be positive.

3. Certain components like Flashing LED's , LED's etc. let current flow through them only at a particular voltage across their terminals. Let us call these components trigger elements.

In Detail

The need to use a capacitor in the solar engine arises out of the fact that the solar cell doesn't generate enough current or charges at an instant to overcome the resistance of the motor and run it efficiently. So sufficient charges are stored in the capacitor and are then discharged to the motor whenever required.

As you can see the capacitor is charged by a solar cell and continues charging till the maximum voltage of the capacitor or the solar cell (whichever is minimum) is reached, if no connections are made to the capacitor.

Let us assume that the motors used in the solar cells work most efficiently when supplied with 3 volts. So the solar cell must be capable of generating minimum 3 volts and the capacitor must also be able to store charges up to more than 3 volts.

If the motor is directly connected across the capacitor the stored charges are immediately discharged to the motor and hence the capacitor has no significance. Hence we need a circuit that will automatically discharge the energy in the capacitor to the motor when the charge in capacitor has reached the required level to run the motor efficiently.This is accomplished using the transistors, FLED and the resistor.

Charges build up in the capacitor starting from 0 volts. The base and the emitter of the PNP are positive (emitter +ve directly through the positive terminal of the cap and base through the resistor and the motor). Since both base and emitter are +ve the PNP transistor doesn't work (current doesn't flow from collector to emitter).

The PNP doesn't work until the voltage across the cap equals trigger voltage of the FLED or LED or diode. At this voltage the current flows through the trigger element. Since current flows through the trigger element ,not to the base of the PNP, the base of the PNP becomes negative. Hence the PNP conducts and current flows from the collector to emitter

Similarly the NPN doesn't conduct as the emitter(N) is negative( through the -ve of the cap) and also the base is negative. For the NPN to work current must flow from emitter to collector. For this the base must be positive. Such a situation arises only at the trigger voltage of the diode when the PNP conducts and thereby makes the base of the NPN positive. At this point current from the capacitor flows through the NPN to the motor and causes it to rotate.

Even if the voltage across the cap falls to less than the trigger voltage and the base of the PNP becomes positive, the motor continues to rotate.The rotation continues until the motor resistance becomes high enough to prevent further discharge of the capacitor. Now the voltage in the capacitor again rises until it reaches the trigger voltage of the trigger element and the above cycle repeats.

Upon notice, the starting of the motor is controlled by the trigger voltage of the trigger element and the stopping of the motor is determined by the resistance of the motor( 2 independent factors). Hence the solar engine acts like a silicon controlled rectifier (SCR).

Some links :