Solar Race Car - Please Help

[zaksmack]

Hello forum,

I am in a competition for an electronics course to design and build a completely solar powered race car. I have a very basic knowledge of motors and micro controllers so I am seeking some wisdom from more experienced ones! I am an electronics enthusiast and very interested in the learning this competition is going to result in so I'm not looking for someone to do ALL the work for me but i appreciate any help you can give.

To begin with the components we have to use are:

1 - solar cell (4.2V 22mA)
7 - 4700uF, 6.3V capacitors

That is the solar cell we have to use and that is the maximum amount of capacitors we are allowed to use. Lower capacitance capacitors can be used with approval. Any other components are allowed to be used. I haven't SEEN the race track yet but after doing some careful questioning I believe it is about a 50 metre two lane design with lights over the entire length of the track to power the cars. The cars usually move in short bursts, alternating between charging the caps and discharging until the fastest car passes the finish line. Round robin tournament style with best 2 out of 3 advancing.

So I have no experience in electronic race cars or harnessing solar energy and there is so much information online I have fruitlessly waded through that I have decided to come here! Some questions I have are:

Race Car Body
What kind of materials are best? Balsa wood? Sytrofoam?
I assume the cars will be moving too slowly for aerodynamics to make much of a difference. Can anyone verify this?
What are some things to be careful for when building the car? ex. wheel alignment, axle friction, gear ratios, belt slippage etc. and does anyone know exactly how much these things affect speed or know where I could look for the information?

Electrical Design
Which kind of motor should I use? There are so many different kinds online I have no idea what kinds of voltage, torque or current requirements I should be looking for.
Should I be going for high acceleration and coasting? Or try to run the motor as long as I can on a charge?
Are smaller or less caps beneficial at all?
What kinds of microcontrollers are typically used in this situation? Which program would I need to program it?
What exactly does a MPPT do? How would I incorporate this?
Are there any tips or tricks anyone knows to get a bit of extra speed?

I welcome any and all responses.

 

[(*steve*)]

I haven't SEEN the race track yet but after doing some careful questioning I believe it is about a 50 metre two lane design with lights over the entire length of the track to power the cars. The cars usually move in short bursts, alternating between charging the caps and discharging until the fastest car passes the finish line. Round robin tournament style with best 2 out of 3 advancing.

How are the cars guided? In a wire, or by "bouncing" off the edge of a walled track. (or something else)

Each will introduce friction and waste energy. You should think about how you can use the track to steer your car in a way that generates the least possible losses.

So I have no experience in electronic race cars or harnessing solar energy and there is so much information online I have fruitlessly waded through that I have decided to come here!

The main thing is energy efficiency. You want to make the most of all you have and to waste the least in friction and other losses.

Some questions I have are:

Race Car Body
What kind of materials are best? Balsa wood? Sytrofoam?

Presuming you're not at risk of spinning your wheels, shouldn't you go for the minimum weight? You might use a mixture of these two and maybe others (even paper)

I assume the cars will be moving too slowly for aerodynamics to make much of a difference. Can anyone verify this?

Don't assume. Can you shave off some of your body to reduce weight and improve aerodynamics as much as possible? How fast will your car go? run tests.

What are some things to be careful for when building the car? ex. wheel alignment, axle friction, gear ratios, belt slippage etc. and does anyone know exactly how much these things affect speed or know where I could look for the information?

All of these are ways of losing energy, or not delivering it as efficiently as possible to the wheels.

Also note that if you're envisaging a situation where the car will run, then charge, then run, ... you need to ensure that it can coast well. Also beware of any hills in the track. You don't want it to start coasting backwards.

Electrical Design

Which kind of motor should I use? There are so many different kinds online I have no idea what kinds of voltage, torque or current requirements I should be looking for.
Should I be going for high acceleration and coasting? Or try to run the motor as long as I can on a charge?
Are smaller or less caps beneficial at all?

These are quite specific. I think you should do your own research.

What kinds of microcontrollers are typically used in this situation?

Use something you are familiar with and/or seem easy to use.

Which program would I need to program it?

depends on the uC. There are always free options and you probably won't need anything else.

Remember the uC requires power to operate. Maybe something that can run from a low voltage is the best, and remember it will have to be powered from the solar panel as well. Low clock speeds are less power hungry.

What exactly does a MPPT do? How would I incorporate this?

Google will answer this

Are there any tips or tricks anyone knows to get a bit of extra speed?

Aside from cheating?

I'd say that attention to detail and thinking outside the box are the key.

If you have access to several solar panels, try each one out and pick the one with the highest power output. They may vary by a few percent and that may be greater than the difference between the best designs.

 

[CocaCola]

Some tips, carve the cars body from polyurethane foam, it's light weight and quite strong for what it is... You can use bamboo skewers if you want to build a wire frame like vehicle... This is the pink or blue foam found in hardware stores for foundation insulation... For the front axles use the smallest/thinnest you can find (sewing pins work well) wheels should be very thin as well, thin buttons maybe or small plastic disc...

As for your motor well you will have to test things, one thing to note you can use very large back wheels think, CD or even 12" vinyl records if you can get enough power to get the car rolling with 12" records you will be amazed at how long it will roll on it's end without the need for another boost of power... Muscle wire with a simple clutch might be perfect to get bit wheels rolling vs a traditional motor, a simply snap pull and let the wheels do the rest...

Google 'mousetrap car using vinyl records' to get an idea of how to optimize roll with little power...

 

[Laplace]

Look for a low-voltage low-current motor as used in battery powered equipment. I had just torn apart an old digital camera and removed the motor. It is 0.44" in diameter and 0.6" long. Hooked it up and at 1.3 V it drew 40 mA and felt like it had sufficient torque to power a small toy car. Since the solar cell is 4.2 V at 22 mA, there would need to be a power converter to match the cell with the motor for highest efficiency. I would start off by designing a test jig for the motor to characterize electrical power vs motor speed vs torque as a starting point for designing the gearing. I might start off with charged capacitors to initially get the car rolling but switch out the caps and put full power continuously to the motor to keep the car always rolling. Use hard wheels and ball bearing construction to reduce rolling friction. If this is a slot car, see if the guide can be made with rollers to reduce friction in the slot. There will be some air drag, but due to the small size and slow speed I doubt you'll be able to do much about it. If there is a level run and constant illumination along the run, there is a single power point, then what is the need for MPPT? Is the power consumed by a micro-controller worth the function it would accomplish? What function would that be? In summary, I would focus most of my attention on reducing mechanical friction, and maximizing efficiency in the power converter and matching it to the motor. You may not be able to purchase the best motor; it might need to be salvaged from other equipment. Again, test your motor(s).

 

[zaksmack]

Thank you for all the speedy responses. Some really great tips and starting points to do some research.

Laplace
I would like to experiment with some low voltage motors and see how it would handle the load of my car (which i'm aiming to be no more than 1kg). Do you or anyone else know of a site that sells small hobby motors individually? I have found some sites that sell minimum 100 pieces which won't do.
And pardon my green-ness but if compared to the motor from your camera, more current draw from a motor would produce more torque and a high voltage motor more rpm?
It isn't a slot car, there are guard rails on both sides so every effort is going to be made to make the device go straight and avoid energy drops from scraping on the side of the track.
And do you know the name of what power coverter you think could be used for this application so I can look some up and research their operation? My plan so far would be to use a couple transistors and a voltage detector to turn on the motor when the caps charged enough. I'm not sure how the DC to DC converter works or where to find one.

CocaCola

Thanks for all of your great ideas about design. I kind of thought about using a foam chassis already. The small forward wheels and large back wheels are exactly the idea I was looking for. I have seen this design before but wanted some verification that it was effective. I'm confused what you mean about the muscle wire and clutch technique though. The mouse trap vehicle looked very interesting, especially since one burst of power got the car so far. Any kinds of potential energy are not allowed in this particular race though so I'm not sure how I would do it.

Steve

You have given me a lot of things to think about. Especially about how all of the things can influence energy loss. I'm going to be very conscious of this in construction.

 

[CocaCola]

I'm confused what you mean about the muscle wire and clutch technique though. The mouse trap vehicle looked very interesting, especially since one burst of power got the car so far. Any kinds of potential energy are not allowed in this particular race though so I'm not sure how I would do it.

Google muscle wire... It should answer your questions...

As to where to get motors, look up micro helicopter motors, or vibration motors found in pagers, phones, playstation & xbox controllers... Lots to be found all over Ebay and other hobbyist sites... Pay attention the current drain if that is specified they can be all over the place but many are very low current...

 

[(*steve*)]

which i'm aiming to be no more than 1kg)

Eek! under 100g would be better!

Any kinds of potential energy are not allowed in this particular race though so I'm not sure how I would do it.

I'm not saying this would work particularly well, but you could have a motor winding up one end of a rubber band, and the other end of the rubber band used to power the wheels.

I think you need some form of steering that will gently guide the vehicle away from the walls if it touches them. This is extremely tricky, because if you do too little the vehicle will hug the wall and may eventually come to a halt. Too much and it will oscillate back and forth basking into both walls alternately. Even if it goes fast, it might simply have to travel further!

A simple solution may be to place small horizontal wheels on springy wire pointing out from the front corners of the vehicle.