Motor Selection - Self Driving RC car - MIT racecar clone


#1

Hey!
I’m working on building the MIT Racecar. It uses a Traxxas Slash 4x4 chassis. The car has a 360deg LIDAR, an RGB Depth camera (zed camera) and an IMU as sensors. The goal is to teach it to drive autonomously indoors and outdoors. One crucial element for me is accurate odometry.

The MIT team replaces the stock ESC with a VESC for better control. I’m thinking if the ODrive might be suitable too?

Right now, I plan to use sensored BLDC motors for accurate low-speed control of motors. I’ve shortlisted 3 from castle creations:

Could you guys give me your opinion on if these motors will drive a 10lbs car on a 2S battery? I’m looking at a top speed of 15kmph.

I plan to use the VESC and do a comparison with the ODrive when I get all my stuff in the following weeks. Maybe I can help add some sensored motor logic to ODrive…


#2

Cool, this sounds like a great project. What is the gear ratio from motor to tires of that car?

We once had a hall sensor decode logic proof of concept, but we didn’t integrate it, so it’s not supported right now. But we know it should be possible to read the hall sensors on the 3 gpio pins that are for the encoder, and use that. If this is what you mean when you say add sensored motor logic, then please by all means, that would be great.


#3

The gear ratio is 13/54. Yes, the sensored motors have hall sensors. The industry standard here is a 6 pin connector. Not sure yet what the configuration of the sensors is like.

This is indeed what I would like to add.

So, while we’re using an external encoder, does it serve the same purpose as hall encoders for you? Would being able to get the readings from hall sensor help to improve the FOC?


#4

Do you think it’s possible to approximate the torque of these motors given the information?


#5

I think the pinout follows this convention:

Yes if the encoder is attached to the motor, the hall sensors will be redundant, at least once the encoder is calibrated against the motor. I guess if you are using an encoder without index pulse, the hall pulses could help with calibration; but I think an encoder with index pulse will be better.

Yes the torque should be given by this forumla: 8.27 * motor_current / kv. That means for higher torque you want high allowed motor current, and low kv. On your the product page of your considered motors they don’t specify a current spec, so it’s hard to compare. If in doubt, just get the lowest kv motor of the same size.


#6

Thanks for the inputs!