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NEED MORE POWER! TMC5041-BOB mods

A project log for HaRoCo @ The DesignLab

Emotive Support Robots

richardrichard 09/25/2018 at 19:230 Comments

We assembled the fur and head silicone recently, and discovered that the little plastic pieces that connect the stepper motors to the eyebrows actually rub quite a lot, robbing them of much-needed power. Even with a lot of trimming, there wasn't enough surplus force to be certain that it wouldn't shift and stall.

This lead me deep back into TMC5041 documentation and register land to try to juice more power out of it, and try to produce better dynamics when it did drag. Hours of trial-and-error later and it was a little better, but not enough.

Inspection with power supply and multimeter makes me think that it's not really cramming enough amps into the stepper motors, despite the specs suggesting otherwise. How does a stepper driver decide how many amps to provide? It measures the drop across a current sense resistor, one per coil. 

The official breakout boards that I've been using, the TMC5041-BOB, come with a 0.27 ohm sense resistor. Yep, that's about a quarter of an ohm. If I reduce this value, the current supplied to the motors increases. Our local Fry's stocks resistors at 0.22, so off I went to buy 1/2 watt wire-lead resistors (huge) to sub in for 1206 SMD resistors (not so huge).

Here's the result:

Cosmetically a bit Enterprise-ish, but sure does make more power! I've just received real 1206 resistors of the right values (going to go even lower!) and so today I'll be swapping in properly sized components.

Bonus! When temporarily removing a lot of jumper wires, I really like to use one of these tweezers to hold them all in the right order... and also take a photo just in case. You can also see the original resistor that was blown while still extremely hot into the plastic header, and embedded there.

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