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Brushless or brushed motor for autonomous rc-car

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Joe_Lowtech
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Brushless or brushed motor for autonomous rc-car

Hello,

I am part of a team, which is working on an autonomous RC-Car. Im trying to figure out, if we should use a brushed or brushless motor. The car we are using is the MIT-Racecar. It comes with an VESC 4.12 and a sensorless brushless motor. The problem we are facing is the control at start up and low speeds.

I have spent some time to get myself familiar with e-motors and the vesc, but I am still discovering new things every day about the issue, which makes the decision even harder the more I discover about motors and esc laugh. As mentioned above, we want a car, that can be controlled at start up and at low speeds(for parking and other slow stuff).

This is how I see the situation:

  • How the behavior of the motor will be, depends on the motor itself, the controller and the configured software.
  • The choice of the motor can be brushed or brushless. Brushed has better low speed-control and smoother start up, but I need an additional encoder to get the necessary odometry.
  • With a sensored brushless motor, I can get almost as good as with a brushed motor at start up and low speeds, but it is more complex on the software side. The upside is, I don't need additional sensors( if the sensors are good enough). Here is a video of a sensored brushless motor at very low speeds, just to give you an idea of what I have in mind:https://www.youtube.com/watch?v=0Vv_fWZATPk

Additional things to consider for the brushless motor (in my opinion):

  • The torque should be high.
  • It should be a 4 Poles motor( 4 Poles referring to the magnets on the rotor) for smoother rotations at low speed
  • The more steps the better for smoother rotations at low speed

Questions:

  1. Are my assumptions correct?
  2. Did I miss some points I should consider? (Like what are additional benefits when using brushless vs brushed)
  3. How difficult is it to configure a brushed vs. a brushless motor with VESC?
  4. Should we get the newer VESC Version?
  5. Can I use external odometry in VESC in combination with a brushed motor?
  6. Can I get a result like in the video with FOC? ( I am guessing, they are using the same technology)

Any input you can contribute would be appreciated greatly :)

Thanks for reading and answering.

Joe_Lowtech

wdaehn
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You can do everything with a brushless motor that a brushed motor can plus you have a lot of advantages

  • lots of more power: 900W for a 150g brushless motor, 150W for a brushed of same size
  • significantly less weight: above 150g and 150W brushed motor can be replaced with a 60g brushless motor with 150W
  • no electronic noise from sparks at the brushes
  • maintenance free
  • way more efficient

So the first question will be, how important is power/weight ratio for you. 

The downside of brushless motors is the price for the ESC and the motor itself (for these small motors. For 2kW motors the brushed motor gets even more expensive.)

One thing I can tell for sure is that either way the motor cannot be used as odometer. Way too much slippage on the wheels for that. The motor encoder is used as a mean to drive the motor smoothly and efficient, especially at low speeds. Nothing else. I mean, of course you can try but you will get to the same conclusion eventually, I guess.

The main problem of a brushless system is the complexity of the control but that the ESC handles. As long as it is not too expensive.

 

I am a bit reluctant to write here, but the VESC has certain advantages but the odriverobotics ESC has other advantages. Maybe you are better off with that? I use both.

 

Regarding your questions, I would argue you have to start from the other direction. What voltage, what power what max rpms do you need? And from that you can calculate back the motor kv value, which is proportional to the pole pairs.

Example: Your application requires 8000rpms top at 12V ==> 700kv is what you need. A four pole motor would rather go into the 2400kv or 30'000rpms.

TechAUmNu
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For low speed operation you want more poles, 14 or 28 is quite common on larger brushless motors.

ameyhande
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Hello Joe_Lowtech,

I also working on MIT-Racecar model and for autonomous ackermann-planner based driving I need lowest rpm possible. With current sensorless motor (Traxxas Velineon 3500) it's not possible. I had searched for sensored brushless motor online, but couldn't find anyone satisfying Shaft Length as well as Shaft Diameter requirements in order to get just motor replaced (i.e. without changing other parts). If you or someone else can guide me that, how you had appoached this problem?

Thank you,

Amey

Amey Hande

Joe_Lowtech
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Hey Amey,

I am currently using  https://www.hacker-motor-shop.com/CARLINE/CARLINE-Motors/Skalar-10-1-10-Onroad-1-10-Offroad-2WD-4WD-2WDSC-/SKALAR-10-21-5-BL-Motor-sensored.htm?shop=hacker_e&SessionId=&a=article&ProdNr=71221500&p=5502 .

It is an alternative to the one, I originally wanted to use : https://www.teamtekin.com/roc412.html . But it has not arrived yet, so I am using the Hacker to learn :) It is compatible to the Traxxas motor mounting plate and should fit.

Regards

Joe_Lowtech