Hello! Our team is working to develop a semi-autonomous Mars rover capable of a number of tasks including autonomous navigation, presence of life detection, equipment servicing, and more.
Currently, we are designing our drive system including the motors and the accompanying electrical hardware to control them. The ODrive 24V motor controllers caught our eye, but we had some questions about the controllers and motors for our case. Our specs are as follows:
The battery is made of lithium-ion cells in a 6s22p configuration outputting 22.2V nominal, 25.2V max, 77Ah, and 220A max
We estimated we need a total torque of about 2.35 Nm per motor and we are planning to use six motors
Our tentative wheels have a diameter of 8.5 in and we would look to be going around 5 mph. Our rover would be about 50kg.
Questions:
We were planning to purchase your 24V motor controllers as our battery has a nominal voltage of 22.2V. However, we are having trouble finding 24V motors to meet our specs. So we have been considering a higher voltage motor that will be overkill for our needs at its max voltage, but we think it should satisfy our needs at 24V. In general, how would supply 24V to motors that are rated as 48V work or impact the motor output? The links below list out some of the motors we are looking to purchase that are higher than 24V. Would these motors be compatible with the 24V ODrive controllers? The motors listed below mention that they require an ESC. Do the ODrive controllers have an ESC built-in? If we were to go with the ODrive controller and one of the motors below, is there anything else on the drive side of things that we would need?
We have looked over the ODrive motor guide to find compatible motors with the 24V and 2.35 Nm requirements, but were unable to find any optimal choices. We were wondering if there are other motors you could recommend that would work with the 24V controllers that would satisfy our specs listed above?
From the research we have done it doesn’t look like it should, but would the motors linked above being belt motors meant for electric skateboards make a difference in anything?
Any advice, guidance, or recommendations would be much appreciated.
Don’t bother with the 24V version, use the 56V even if you have 24V motors and a 22V battery. The 24V ODrive can be easily destroyed by overvoltage e.g. back-emf and braking current from the motor.
Plus, for fast control, you need some voltage headroom to be able to quickly change the torque.
For that reason, I’d recommend using a 48V supply even with 24V motors.
With the 56V board you can run them at a lower voltage and have the option of upgrading the battery later on if you need to. With the 24V ODrive you don’t have that option.
Yes, the ODrive replaces the ESC.
I like these motors, they have a lot of torque (about 0.8 Nm/A) and are the right current rating to work well with ODrive. They only go to 120 rpm/V - I don’t know how fast your mars rover needs to move.
“Hoverboard wheels” also work well for this kind of application.
The specification you should be interested in when choosing a motor are power rating, maximum continuous torque (which will be thermally limited), and maximum speed at a given voltage.
Thank you for getting back to me. It sounds like that if we use a motor around/over 24V that we could use the 24V ODrive controllers, but that we run the risk of easily destroying due to overvoltage.
“I’d recommend using a 48V supply even with 24V motors” - By this do mean that you suggest using a 48V battery instead of the 24V? Do you think the 220A max, ~77Ah capacity would still be satisfactory for our uses?
You will never need 220A from the battery, that will be more than sufficient even for starting torques where the combined motor current could exceed this.
(motor current is always higher than battery current, especially with higher voltage batteries - the ODrive acts like a SMPS with the motor as the inductor)
Okay, thanks. As we presently have the battery built at the specs listed above (22.2V nominal, max 220A, with 77Ah), our options seem to be:
Reconfigure the battery to max it 44.4V nominal, max 110A, with 38.5Ah (difficult, may not be feasible)
Purchase + add more cells to make the battery 44.4V nominal, max 220A, with a capacity 77Ah (ideal, costs lots of money and time)
Run a 48V motor at 24V from the battery (potentially increasing ODrive controllers to 56V for leeway)
Find and use a 24V motor
At this point, I think our best option is just finding a 24V motor and using that with our current specs and a 24V motor controller. However, we have been having a really tough time find 24V motors to meet our needs which is why we are considering moving up in voltage. What would you advise we do?
We plan to have a max speed for the rover of about 5 mph. So accounting for our wheels, we are looking at an axle rpm of 200 rpm, maybe a little less.
Through basic equations/estimations, we calculate we need 0.662 Nm per motor. But we are looking at motors capable of 2.0 - 2.5 Nm to provide leeway and to account for error/losses (is ~3-4x multiplier too much?). We also will be having 6 drive motors
Any advice/recommendations would be appreciated and as you have gauged we need to learn more about motors and motor controllers in general.