Noob Questions about 5" chassis

I’ve spent days on your site reading manuals and forum threads. Boy, does this noob have questions! (Disclaimer: I am NOT an electronics savant so please type slowly so I understand.)

One particular site photo (https://www.pololu.com/picture/view/0J1449) looks very much like what I need to build. I like the use of the Arduino simply because I have a tiny bit of experience with it and its sketch writing. Also, I know it can control a servo exactly as needed. Close examination of the photo reveals missing elements so here starts the questions:

It appears the battery pack is directly connected to the Arduino. Wouldn’t best practice be to use a regulator package like on the 3pi? I’m thinking the 2.5-9.5V regulator (https://www.pololu.com/catalog/product/791) would hit the Arduino’s sweet spot for input voltage. This is assuming using the same 6-cell battery pack as pictured. Am I on the right track? Would this provide steady voltage for the Arduino AND the motors?

My project does not require high speed or extreme steering. In fact, it’s all about finely controlled slow speed (from barely moving to about 1ft/sec) and reliable steering. That’s why I figure I’ll need motors with encoders (that’s what eliminated the 3pi). I’ve read much on the site about the motors and gearboxes but I’m still not sure which set fills the bill with a max voltage of 9.5 and 42mm wheels. Also, would a low current motor be best or not?

Speaking of the motors, the mock up doesn’t appear to have any motor connections. I don’t believe the Arduino can directly drive a motor so which controller best fits the bill and how many Arduino outputs are needed to drive it?

There doesn’t appear to be any power connected to the sensor array. The PDF doc shows it needs 3.4-5V so would it be best to power it from a separate regulator adjusted to 5V?

My apologies for all the questions. I’ve already blown close to $200 on one failed prototype. I hope to be more sure I’m heading in the right direction with this one. I’ll need to build 5 of these once I lock in a design so any and all tips are really appreciated.

Mike

Hi, Mike,

Welcome to the forum. I would use 50:1 or 100:1, non-HP motors, which will make control easier. I also wouldn’t bother with the boost regulator: it’s a cool feature for the 3pi, but it does add complexity that doesn’t need to be there, especially if you’re going to use encoders. You can also easily add it later once everything else is working.

For motor control, I’d use either the TB6612 controlled directly from your AVR or our qik 2s9v1 motor controller, which uses the same motor driver but allows you to send serial commands instead of generating the PWM yourself.

The sensor array draws about 100 mA. If you can spare that on your Arduino, you wouldn’t need another regulator. The extra 5V regulator is a good approach, too, though I wouldn’t bother with an adjustable one unless that’s what happened to be lying around.

- Jan

Jan,

Thanks for the quick reply. I must be misunderstanding something. If the #992 motor (https://www.pololu.com/catalog/product/992) is rated at 6 volts then isn’t the #410 controller (https://www.pololu.com/catalog/product/410) the only controller that will work? The controller comparison chart appears to indicate that the other controllers’ low end is 4.5 volts. The #410 seems to be the only one that can control within the motor’s operating range of 3-9 volts. Reminder, I need good slow speed control!

I’m also confused by your recommendation to skip the boost regulator to get up to 9 volts for the motor. If a 4-cell battery pack is used that’s only 6 volts max. Even a 6-cell pack might just hit 9 volts but wouldn’t stay 9 volts for long so… What am I missing?

Thanks again,
Mike

All the motor controllers we have operate by pulse-width modulating (PWMing) the output to the motor, which means that the full voltage you apply to the controller goes to the motor, with speed control achieved by varying the percentage of the time that the motor is on.

You should probably use rechargeable batteries, which will give you around 1.2V per cell. I recommend a 6-cell pack, which will give you around 9V fully charged and drop to around 6 before you have to charge it again. You don’t need 9V for the motor; you just need to make sure your system will work over the 6-9V input range. For instance, the qik will work from 4.5V to 13.5V, so it works fine with a 6-cell battery.

- Jan

Thanks for the good explanation of PWM. If I understand correctly, a 100% PWM cycle is equivalent to connecting the motor directly to the battery pack. Therefore, to have a full 9V available even if the 6-cell battery is partially depleted then isn’t the boost regulator required? I want to be sure the bot can hit very specific speeds at specific locations regardless of charge level.

New question: Do you happen to carry the capacitors for connecting across the motor leads? It would be handy to order them along with my main order when I place it. I see that Radio Shaft has them but their quality is sh… well, lacking. Besides, many stores have quit stocking “hobbyist” components.

Mike

Yes, 100% duty cycle is like connecting straight to the battery (there will be a bit of a loss in the driver, though).

We don’t have the capacitors up yet, but they should be available in the next few days.

You’ll need some sort of regulation if you want a constant voltage for your motors. However, I think it’s not that necessary, especially if you can add it later. For instance, if you decide the performance at 6V is good enough, you can measure the battery voltage and compensate for it by not giving the motors full power if the voltage is higher. But if you really want to have the boost regulator, go ahead; just keep in mind that it does mean more complexity.

- Jan

I suppose it’s a matter of perspective. My POV is having a constant voltage means less complexity in the program. If I can count on the voltage then I don’t need the program (sketch) complexity of checking voltage levels and adjusting motor speeds to compensate.

It’s a matter of skill level. I’m trying to avoid biting off more than I can chew.

Thanks as always for your advice.

Mike

Hello.

We have made 0.1 uF capacitors available.

- Ben