Current limits of a ThunderBorg

Submitted by furusato on Wed, 03/24/2021 - 00:16

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Hi,

I'm currently planning a rocker-bogie style robot, so I'll need independent control over six motors. I've been successfully using ThunderBorgs for awhile now and this robot would use three, since each controls two motors.

When looking at the 12v Pololu 25mm motors, they sell both a medium and a high-power version. I'd likely be purchasing the 210-220rpm motor. These are listed at 25D Metal Gearmotors.

The high power has more than twice the torque of the medium power motor, so I'd really like to use the high power version if possible. The rating of the high power motors is 5.6A stall current, where the ThunderBorg is rated at "Current Max 5A per channel continuous." I have considered adding something like an Adafruit INA260 for each motor, but that adds quite a lot of complexity, e.g., parts count, wiring, software, etc. but is certainly possible.

So long as the motors don't sit in a stalled condition for a long time, would it be okay to use the Pololu 25D high-power motors with the ThunderBorg?

Thanks!

piborg's picture

piborg

Thu, 03/25/2021 - 18:59

While it would not be our recommended usage, you should be okay with those motors given certain restrictions.

The board can handle a peak current of 6A for short bursts. Short bursts would be 1 second or less typically, plus at least a few seconds below 5A to recover. Under these conditions everything will be working within specifications.

If you draw more than 5A for longer then the drive chips become reliant on their over-current / over-temperature protection to "throttle" the output to a safe amount. In our experience this works well running slightly high (like you want), but it is not ideal to be relying on the protection circuitry on a regular basis.

The two states that will draw the most current are power when stalled / stationary and sharp changes in power (especially changing direction). Limiting the rate at which you change power output (either in code or by control) will reduce how much current is actually drawn in practice and would likely keep you within the 5A limit in most cases.

Finally make sure you do not exceed the 12V power output if your batteries are higher than 12V. The amount of current the motor will draw in the stalled state is proportional to the voltage applied, so increasing it will make it draw even more current. For example if you have a 14.8V battery limit your output to 81% (12 / 14.8).

furusato

Thu, 03/25/2021 - 20:10

Hi,

Thanks for the quick response. That seems to answer my questions.

I'm writing the Python code for my robots myself so I've got control over what goes to the motors. To that end I've added a slew limiter to control how quickly the power ramps up. I'm using Makita power tool batteries, both their 18v (actually 19.75 up to 21v in practice) as well as their 12v (about 12.75v) and since I want to use the same code on both robots I've added an automatic (dynamic) setting of the maximum (100%) value sent to the motors based on the current battery voltage.

https://github.com/ifurusato/ros/blob/master/lib/motor_configurer.py

I'm also likely to have some notion that if power is being sent to the motors but the encoders aren't indicating movement that I'd shut things down or alter behaviour. I could also rely on the IMU for that. Haven't got there yet but that's in the plans.

Cheers

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