Is my custom rc car setup feasible?

Other Hardware Motors, Mechanics, Power and CNC
1

I want to build my own RC car, but I need some advice, especially concerning motors, motor control, and batteries.

I have a 7.2V 2000mAh RC battery pack I picked up from radio shack. After fully charging it, I measured 8.1V as its voltage using my multimeter.

I plan to purchase a dual gearbox that includes two motors:

I think I will swap out the motors included with the gearbox for these two. The original motors are rated at 1.5-3V, so using these 6V motors would be easier, given my battery's voltage, right? If I'm mistaken, let me know.

I also realized that I would need H-Bridges for bidirectional motor control and decided to just buy a motor driver. Would this one be alright? I've also heard that there is a voltage drop across motor drivers. I tried to find more info about this specific motor driver, but I couldn't determine what the voltage drop would be, assuming there is one.

This is the part I'm really confused about. Do I need a step-down voltage regulator? If I do, will this one be sufficient? I'm worried that it only supports up to 300mA. There's another one that costs $2 more, which supports 600mA.

Oh, I'll probably use a caster for the front wheel. Should I buy a 3/8inch or 1/2inch caster wheel? Or a different size altogether? That, or I'll buy 2 gearboxes, one for the front, the other for the back. Any advice regarding this?

Also, this is off topic, but is it difficult to wire up an xBee to a standalone atmega328P? I'm considering buying two xBees - one for this car, the other for it's controller - and the multitude of pins on the sides of each Xbee intimidates me? I think I'll end up purchasing
two of these:

I don't need to spend money on actual Arduino boards to use Xbees, right?

Last but not least... are there any issues that standout concerning the compatibility of the parts I selected? For example, is my 7.2V battery pack appropriate for my motors, or is it overkill (and overweight)? Did I overlook anything that might cripple my car?

For anyone interested, here is an image outlining my plans so far:

Any advice will be appreciated. Also, if anyone has suggestions about better parts or ways to save a little money, feel free to voice them.

Thank you, very much.

2

There's not much difference in the stock Tamiya motors and the Pololu motor; Pololu claims their motor is better (which I won't argue). Given the Pololu motor is only $1.79 probably just accept their recommendation, but leave the stock motors in while you're testing so you don't blow up the "good ones" :wink:

The motor driver DRV8833 you have linked is appropriate for your application.

You don't need or really want a step down regulator. Just adjust your code and set maximums on the PWM signals to the driver if you feel the motors are getting too hot (too hot = too hot to hold your finger on). It's not a matter of the motors instantly blowing up but rather their lifetime that you're worried about.

An XBee requires a 3V supply, GND, TX and RX -- just four wires. The Adafruit board will handle the regulation for you for the 3V power and the level shifting between the ATMega's 5V and the XBee's 3V. And of course it breaks the pins of the XBee out to the .1" standard pin spacing. In that respect the Adafruit board would certainly be recommended.

3

Thanks for the info.

So, I can eliminate the step-down regulator simply by using PWM to control the voltage applied to each motor?

4

eejdoowad:
So, I can eliminate the step-down regulator simply by using PWM to control the voltage applied to each motor?

It's actually controlling the average current, but yes.

I should have said earlier that you should start with a low limit on your PWM maximums and then gradually increase it. Probably not good advice to suggest running it full power first then lowering it later :wink:

5

Chagrin:

eejdoowad:
So, I can eliminate the step-down regulator simply by using PWM to control the voltage applied to each motor?

It's actually controlling the average current, but yes.

I should have said earlier that you should start with a low limit on your PWM maximums and then gradually increase it. Probably not good advice to suggest running it full power first then lowering it later :wink:

That's helpful to know, but now I'm slightly confused.

Does that mean a dc motor rated for a certain voltage can be powered by a higher-voltage power-source as long as PWM is used?

Or, did you mean that voltage of the battery I have falls within the voltage range of the motor I chose, so using that battery is acceptable even without a voltage regulator for my specific case?

Basically: Is PWM strictly used to control the current without changing the voltage? And why don't I need a step-down voltage regulator?

6

Yes, it is common to power a motor with a higher voltage than rated. With stepper motors, notably, it's typical to drive a motor rated for, e.g., "5V" at much higher voltages to gain an increase in speed... but that diverges from your questions.

When a DC motor reports a voltage it's stating that if you drive it at that voltage that its internal resistance will limit the amperage without any additional current limiting requirement. An "additional current limiting requirement" could be either a PWM drive like yours or an external resistor. It's basically an Ohm's law thing: If the motor has a 5V rating and a 1A limit, then you know it has a (R = V/I or 5V/1A) 5 ohm resistance. If you drive that motor at 10V then you would be pushing it to (I = V/R or 10V/5ohm) 2A of current. That 2A would cause the wires to overheat and burn off their insulation so you have to PWM or "chop" the current to the motor to prevent that from happening.

Of course this doesn't mean you can run a motor at "unlimited" voltages; there are still limits to the insulation on the motor coils (~500V), the sparking/burning of the brushes, limits caused by the inductance in the coils (~40V?) and back EMF. That's not something you want to learn about yet :wink:

So to your last question, hopefully I've explained why you don't "need" a step down regulator. What you should also remember is that you don't "want" it anyway -- you don't want it because it costs money, it adds complexity, and also because it's only 80% efficient (in the case of the one you linked to).

7

Hi,

Here is a small simple RC Robot I build from an RC Transmitter and receiver and an Arduino to convert from the RC Signal into separate motor control signals for the left and right tracks -

There is also a follow up post on the blog with code you could use if you wanted to take this approach. At some point I will add some sensors so that the robot can switch between autonomous and remote control.

Duane B

8

Thanks a lot, Chagrin. Your explanation was very helpful.

One last (i hope) question to anyone:

Is it a good idea to use a separate battery for my atmega328, or will it be okay to just hook it up to my 7.2V battery, along with everything else?

9

Hi,
In my RC Car projects I often run my Arduino from the 7.2 or 8.4 volt batteries in the cars. In my current project I have a 7805 voltage regulator to step the 8.4 volts down to 5 for the Arduino, I then have another regulator to step the 5 volts down to 3.3 for one of the sensors I am using. You can connect your Arduino to 7.2 volts through vin, but if you are just using an ATMega328, you will need a 7805 or similar regulator to bring input voltage down to 5 volts - the Arduino has one built in (two if you include the onboard 3.3 volt supply).

Duane B

10

Ok, thanks DuaneB.

11

I tried to find more info about this specific motor driver, but I couldn't determine what the voltage drop would be, assuming there is one.

I'm also considering using Polulo's 8833 board and can't find the voltage drop if any, documented. I know the 298 based driver board which I currently have chomps volts big-time, and gather that the technology in the 8833 is somewhat superior.

So, is there a voltage drop across the 8833 and if so can someone tell me what it is? Is it a fixed value or a proportion of the input?

(It's certainly a nice small board, and doesn't have a heat sink the size of a truck.....)