Power delivery question

Our team wants to use power expanders, but we are unsure if we should group the motors of each mechanism, or if we should distribute the load across the 3 PTC’s. My concern with distributing the load is that a difference in battery level would inturn suply unequal amounts of power to each motor respectively, which would make one motor (whichever receives more power) to work more than the other (making that one motor heat up more than the other).

I would suggest putting an entire mechanism on the expander. Like you said you could overheat a motor if you put a partial mechanism on the power expander.

On the contrary, I would recommend separating motors for the same mechanism. The Cortex and power expander have their own PTCs in addition to the ones in the motors, and multiple stalled motors can trip them. The Cortex has two PTCs (ports 1-5 & 6-10) and the power expander has one. My team always tries to separate motors as much as we can.

Spread power hungry mechanisms across the three PTCs and both batteries. Frequently changing both batteries at the same time with freshly charged ones will help give more balanced voltage. The advantage of getting the most legally allowable power out of the system outweighs the small boost in balancing from forcing high power mechanisms to draw current from the same battery or PTC.

Uneven motor RPM might be a bigger concern than uneven motor load and heating. Voltage isn’t the only cause of uneven performance. Individual motors may have a significant variation in their RPM from the nominal value (I’m guessing +/- 5 to 10%). Small differences in friction can also cause otherwise identical mechanisms to run at different speeds. In extreme cases all of this combined can cause drivetrains to curve even when commanded to drive straight, and cause mechanisms like lifts to lean and malfunction.

Battery management, good design, and careful assembly will improve the balance. Control loops with feedback from sensors can compensate for any remaining effects that cannot be solved in hardware.

Thanks for the replies!