So I'm a high school student doing some competition (a wallet-sized one, unfortunately) and I'm supposed to make an electric car. Think an RC-car, (about that size) without the remote control. I want to make one from scratch, so I decided on using an arduino for a micro controller. Unfortunately, I don't really have an adviser that's particularly knowledgeable about this stuff, so I decided to ask here. (brace yourself for quite a few possibly dumb and vague questions. I do understand this question will likely be edited/closed for being too vague, but if, by any chance, someone has answers, I'd also like explanations -- mostly because I want to learn something.)

I've decided on using an aluminum chassis and soft wheels, since the competition wants a car that can go as fast as possible. I've chosen soft wheels because that should give me higher friction, and an aluminum chassis because I want to reduce weight. Now, here comes the first problem. The competition specifies a maximum of a 9v battery to power everything, including the arduino and the motors. Now, of course, we only need to go about 10 meters (I just want sub 3s times). My original design involved having 4 separate dc brushed/brushless motors, and then connecting each dc motor directly to the wheel. The dc motors could be held on with brackets, and then connected to an Arduino connected to a MOSFET, at which point I could then use a motor encoder to see how far to go.

I now have a couple of questions:

  • Should I try having a 9v to 12v converter? or a 9v to 24v converter? That might cut the current significantly, but I'd also be able to use a much more powerful motor. The problem I have with this is that it might be adding a lot of potential weight -- as I understand it, I can't convert directly from DC 9v to something else.
  • I've decided on having a rear wheel drive rather than a 4wd -- but is it worth getting a gearbox and an axle and just having one motor?
  • Should I use a gearbox at all? Or should I try to change the speed based on the size of the wheel I use?
  • Would I try to maximize RPM or torque by getting a geared-down brushless?
  • If I have a 9v battery, can I have 2 9v motors in parallel? Or is that unsafe because 9v is only nominal?

If anyone has tips on how to improve the question, I'd be glad to incorporate them into my edits.

some info, by the way:

  • looking for sub3 second times
  • using banebots 2-7/8 inch diameter wheels
  • hard floor
  • will need to back up after 9m
  • only going straight, doesn't need to be able to turn
  • thinking of a PID loop with a physical rotary quadrature encoder if one axle. thoughts?
  • the car will be ~1kg and I won't be carrying anything else.
  • 2
    $\begingroup$ As you guessed, this question is far too broad. That being said, I'd say most of the individual questions are perfectly reasonable. Try posting them separately and they should be fine and much easier to answer. You can even link between them so a potential answer can be aware of the different dimensions you need to take into consideration. $\endgroup$
    – Wasabi
    Feb 2 '16 at 9:32

Answers to your specific questions:

  • No, voltage converters won't help, at best they will waste 10% of your power. Think about this : they involve transformers. two different windings on a magnetic core to translate one voltage to another. That might help you understand that any given motor (which is just a winding on a magnetic core to translate voltage into motion) can be tuned to run off any voltage by changing the winding. You'll see the same RC car motors with 13,15,17 turns to tune performance.

Instead, learn the relationship between voltage and speed (Kv, rpm/volt, the relationship is simply Speed = Kv * voltage) and current and torque (Kt, Newton-metres/amp). A high speed motor (high Kv) gives little torque, and a low speed motor gives higher torque. So with direct driven wheels you want a LOW speed motor.

You have a mass (1kg), a distance (10m), and a time (3s). If you've done basic high school physics you know how to get speed and acceleration from distance and time. Work out 2 cases : linear acceleration from 0 to twice the average speed, and covering the distance at average speed.

I'll recommend you stick to metric, it makes some of the math easier. If your motor Kv is specified in rpm/volt, you have to convert that to radians/second/volt. In metric units, the torque relationship Kt = 1/Kv, so torque is Kt * current, or 1/Kv * current.

If you think about electric power, P = V * A and mechanical power = Nm/second = torque * rotation rate in radians/second, and the law of conservation of energy, you'll see why Kt = 1/Kv and understand how (high speed, low torque) and (low speed, high torque) can provide the same power.

Now acceleration and mass give you force, and force * wheel radius gives you the torque required for acceleration.

Also multiply mass * gravity(9.8N/kg) * coefficient of friction to get the force required to push the car at constant speed, and convert that to a torque. (You don't know the coefficient of friction : try 0.2 for a perfectly smooth floor and 0.6 for carpet)

Now you know the torque required just for acceleration, and for travelling across the floor. The actual torque you need is the sum of these. (adding a safety factor like another 25% isn't a bad idea).

Given the speeds you need, in m/s, and a specific wheel radius, divide speed by radius to get rotational rate in radians/second. (Convert that to RPM if you want).

Now that you know the speed (rad/sec or rpm) and torque (Nm or Newton metres) you can go back to selecting a motor. Can you find a motor with the right Kv, where 9V * Kv is anywhere close to your desired rotation speed? What current do you need to achieve the torque? If it's over 0.5A you'd better forget that little 9V battery, and that's being optimistic...

I'm guessing not, and that's where you need a gearbox to translate from (high speed, low torque) to (low speed, high torque). So if your motor speeds are 10x too high, use a 10:1 gearbox, multiply your required speed by 10, and divide the required torque by 10. Can you meet your speed and current requirements now?

All this may look daunting but each bit is relatively simple maths. If there are specific sticking points, think about them for a bit, and ask if you need to.

Having laid that groundwork, the rest of the questions should be easy...

  • If you have 4 motors, each delivers 1/4 the torque and consumes 1/4 the current, so there's not much to choose between 1 motor and 4 in overall efficiency, but if one small motor can't supply the torque you need, 2 or 4 might work.
  • The huge chunk of maths above will tell you if you need a gearbox. You probably do.
  • You don't want to maximise RPM OR torque, you want just enough of both to cover the requirements and a bit (50% or so) to spare if possible. (I haven't mentioned inefficiencies like friction that eat away at performance little by little. This is long enough without that!). Brushless motors aren't a whole lot better at delivering torque, where they win is by running much faster, so you'd need to gear them down more. Probably not worth it here.
  • You can run motors in parallel, but remember their currents add up, and your little battery can't supply much current. (Good batteries have datasheets which show how much current they can supply, for how long. In this case, 0.5A for much less than an hour, down to 4.8V)

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