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

R Radford rradford at mindspring.com
Tue Oct 5 11:45:58 CDT 2021


Carl, Charles is looking for a brushless controller, not just a DC motor
driver. Brushless motors are closer to a stepper motor than a regular DC
motor, but the 'steps' are controlled by sensing hall effect sensors to
know when to step to keep the motor running smoothly.

They are great as they have less mechanical issues over time (no brushes to
replace) and also produce less electrical noise.

The circuit you show is a nice DC to PWM circuit that is similar to one I
did many years ago, but now most (all?) microprocessors have built in
hardware assist PWM so that part is already handled. The first time I used
a similar circuit was controlling a robot from a 286 PC based robot where I
used the parallel port to drive an R2R D/A circuit comparing against a
triangle wave similar to your circuit. It worked great at the time, but now
would not be needed. I still have that old hardware somewhere - perhaps I
should pull it out and upgrade it.

On Tue, Oct 5, 2021 at 12:02 PM Carl Nobile via TriEmbed <
triembed at triembed.org> wrote:

> Charly,
>
> There are a few solutions to the controller problem. I designed an analog
> PWM circuit using op-amps and comparators that works great. You would also
> need an H-Bridge you would then need just one MCU board to control the PWM
> circuits then then control the H-Bridge circuits.
> My design is at: https://github.com/cnobile2012/VoltageControlledPWM
> My motors are fairly low current and voltage so the H-Bridges I'm using
> won't work for you, however MPJA.com has one in the link below, that will
> handle higher currents.
> https://www.mpja.com/Dual-H-Bridge-DC-Motor-Driver/productinfo/35567+MP/
> It will handle two motors and is only $22.95 ea. They give full docs on
> their site.
> My PWM board shou interface with it fine.
>
> ~Carl
>
> On Sun, Oct 3, 2021 at 11:16 PM Charles West via TriEmbed <
> triembed at triembed.org> wrote:
>
>> 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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>
> --
> --------------------------------------------------------------
> Carl J. Nobile (Software Engineer/API Design)
> carl.nobile at gmail.com
> --------------------------------------------------------------
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