Arduino project, struggling to decide between motors

Hi, this is my first post here. I have been working with arduino a few years now, some spare time and some school related. We made an autonomous car last semester and I want to expand on the project during my free time.

One big drawback on the project was that the motors were struggling with all the additional weight (batteries, additional servos, Processor etc).

I was hoping someone more experienced could guide me towards the correct motors for the driving mechanism. I'm open to both cheap and expensive options, my only requirement is that an arduino motor shield will control them (undecided which shield, will choose depending on motors).

Here is a link for the motors used in our project: 6v-180-rpm-micro-dc-geared-motor

Here is a motor I consider upgrading to: 168-rpm-gear-motor

When I found these motors I felt pretty competent about my knowledge, but I got really confused when comparing these. The 168 rpm motor had way higher torque with only twice the voltage, yet it used a mere 500mA current at stall compared to 2.8A on the 180rpm motor.
I dont really see how a motor can require less power while still outperforming by a solid amount.

Any recommendations and advices are appreciated, my main goal is to upgrade from the current motor used in the project to a stronger one. It needs higher torque while maintaining about the same speed.

Let me know if I forgot some essential details :slight_smile: The wheels I consider using are 85mm diameter.

This site has lots of helpful information on robot construction, including how to make intelligent choices for robot platform motors.

Determine the torque and power you need and avoid wasting your money on inadequate choices.

Thank you, these links proved to be very useful for my needs!
I would still love some sort of explanation of the logic behind the 2 motors I compared, if you wouldn't mind.

Motor 1: voltage: 6V | RPM: 180 | Stall current: 2.8A | Stall torque 11.11 oz-in.
Motor 2: voltage: 12V | RPM: 168 | Stall current: 500mA | Stall torque 27.3 oz-in.

As you can see here, the RPM is quite even, but the torque difference is huge.
How can this be when the weaker one has a much larger effect (P=U*I)?

From my understanding, if a motor has a larger effect, then it would be stronger at the same RPM. This does not seem to be correct.

There are many complexities to motor design, but in this case, I would imagine that the permanent magnet field strength in the two motors is very different.

With weak magnets you don't get much torque regardless of the motor winding current.

Awesome, I suspected that it had to be something with the magnetism, I just didn't know enough to come to a conclusion. Thank you for the clarification, I didn't want to decide between motors before understanding what separated their performance.

I have another question about the motors, by all means you dont need to answer, I understand if I can be a bit annoying with all these questions :slight_smile:

I read about spur vs planetary motors, planetary seem to outshine spur if your project require a bit high torque. The downside was that planetary is more expensive.

To my surprise, I found a planetary motor with higher rpm and much higher torque for about the same price as a spur motor. The way I see it, the only downside is that the planetary will draw much more current from the batteries, thus reducing the length before a recharge is required.

Could you please check out these two and explain why I would choose the spur over the planetary beside the battery-time downside? spur motor, planetary motor.

Thanks in advance.

If you want to delve into the details, the relevant equation from physics is F = ILB, where:

F is the force on a straight piece of wire in Newtons,
I is the current in the wire in Amperes
L is the length of the wire in meters
B is the uniform magnetic field strength in Tesla (e.g. coming from a permanent magnet)

assuming that the magnetic field is perpendicular to the wire.

I don't know what would determine the price of a particular motor.

For help in estimating battery life, you will need to post complete information about your design choices. Have you tried the "energy calculator" on the robot site linked earlier?

Thanks for providing the formula :slight_smile: I barely touched the electro-physics when I was learning about physics in school.

I haven't used the energy calculator yet, just glanced at it. I thought I'd save it for when I've decided on which motors to buy. I was just afraid I missed some vital detail, because for me, the planetary motor seemed like an obvious choice.

This is the way to design a mechanical system powered by a DC electric motor,
assuming wheel-driven:

  1. Figure out the total torque needed at the driven wheels to move the vehicle in the worst case,
    being the maximum of the steepest slope, harshest acceleration, plus frictional losses if important.

  2. Decide on the maximum speed needed.

  3. The mechanical power needed is the product of torque and power, always work in SI units
    to make this easy and error-free.

  4. You now know the power, speed and torque needed at the wheels, the next step is decide on a gearing
    ratio that will match typical motors (which can be 3000 to 8000 rpm typically). Gearing systems add
    power loss, so you have to compenstate for this by upping the power requirement.

  5. Choose a motor capable of the torque into the gear system and the speed needed - upgrade the torque
    to account for power losses in the gears.

  6. Choose a motor driver that can drive that motor (or motors).

So what is the top speed, mass, max gradient, etc you require for the vehicle? Wheel radius? Any existing gearing?

MarkT, thank you for this wonderful design recipe!

I realise it is not easy to help me given that I dont have much details in place.
One of the big challenges is to estimate a total weight of the car, because it will be a learning project. Meaning that it will most likely keep growing, continuously gaining more weight. For example, I will most likely add more batteries to get a longer lasting session between each recharge. I also want to add a storage box on the back which will be filled using a new lifting mechanism attached to the car.

After quite a bit of consideration I think I might select this spur motor. It has almost 3 times higher torque than the old motors and about the same RPM. I think going higher than 160-180 RPM would be bad, because I need to be able to toggle the speed down to drive slow, if the RPM is too high, I suppose I wont be able to go slow enough.
I was considering this planetary motor aswell, but the torque seems a bit overkill, and if thats the case, it would empty the batteries much faster for no reason.

The radius for the wheels I intend on using is 42.5mm. I most likely will be using this car for flat terrain only as I want it to navigate autonomously doing whatever I demand from it. The purpose is not off-road driving, if my vision changes later, I might have to order new motors for that purpose when the time comes. There are no existing gearing, I'm starting from scratch and plan on making the chassis and most other things myself except gearing etc which I want to keep inside the manufactured motor.

Top speed is something I've been thinking quite a bit about. I want the car to go fast, but it is much more important to be able to reach slow speed for functionality. The image processing for automation might need some time to handle new information so if it can't go slow, this might be problematic.

Here is a picture of the previous car (note that the new one will have much more components):