[TriEmbed] Designing a brushless motor controller during the chip shortage

[Charles West]

Hello all!

For the past 5 years or so, I've been working on a open source low cost
sidewalk delivery robot.  The current draft (prototype picture:
http://goodbot.ai/lib/exe/fetch.php?media=mk3draft1.jpg) has 4 hoverboard
style motors in a skid steer arrangement.  Each robot will need 4 motor
controllers, which is one the big cost drivers right now (even prior to the
covid shortage, the controller cost more than the motors).

My last stab at designing a brushless motor controller failed spectacularly
and the covid parts shortage has also made the central parts of that design
completely unavailable.  In addition, the bulk price for the motor
controllers I've been using jumped from $56 per to ~$100 per.

Given that, I thought it might be time to get back to the drawing board.
However, I am not an expert at this sort of thing, so I thought it might be
good to post what I am thinking and see if I am completely off the
reservation.

I'm currently using off-brand VESC motor controllers (schematic for normal
ones here:
https://vesc-project.com/sites/default/files/Benjamin%20Posts/VESC_6.pdf).
They are great but do way more than I need.  They can handle sensored and
sensorless motors and be reconfigured with MANY different options using a
desktop application.  I don't need to make something that is easy to make
work for lots of different motors and I don't need to support sensorless
operation.  I just need to make something that works for my motors.

The approach that the VESC takes is a central microcontroller which talks
with a (not currently available) DRV8301 chip.  This chip in turn senses
voltage/current across each motor coil and drives 6 N-channel mosfets to
control the motor (3 high side, 3 low side).  It does FOC control, which I
believe is state of the art in terms of smooth motor control. It also
integrates the 3 halls sensors that sensored motors have if they are
available.

I'm having trouble finding any of these 6 mosfet motor controller optimized
gate drivers, so I am thinking that I will just get some high side/low side
mosfet drivers (like maybe these:
https://www.mouser.com/ProductDetail/STMicroelectronics/SRK2000A?qs=WHlX%252B%252B9%2FRwCG%2FkukabfLqA%3D%3D)
and some nice mosfets.  Particularly without dedicated chips being
available, I'm not going to try to do current or voltage sensing.  I am
thinking that I would just PWM the 3 half bridges and try to do sinusoidal
motor control using a STM32 microcontroller to drive it and the hall
sensors in the motors to estimate position relative to the coils.

If I may ask, does this sound reasonable or am I totally off base?

Thanks,
Charlie
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