This sounds doable as long as there is space on the fpga for a lookup-table
and you can get the commutation tuned correctly (I guess the
tuning/alignment process is already required and works ok for the current
bldc implementation).
Could you somehow verify that the non-optimal behavior or problems you are
seeing actually are because of commutation?
OR, would it be possible that simple PWM ("voltage"?) control with a
PID-loop that gets feedback from the encoder is quite hard and doesn't work
well at high RPM even with a 250us servo-thread?
I once tried using current-sensing chips from IRF that produce PWM-output -
this could be one option if you want to try a current-loop.
Anders
On Wed, Jul 10, 2013 at 12:05 PM, propcoder <marius.alks...@gmail.com>wrote:
> Last month I tried running bldc motors (8 poles, 3000rpm max) with
> LinuxCNC and MESA 5i23, 7i43 and 7i39. I am quite happy now, but I see
> the limitations of this set: sine commutation is rough at higher speeds.
> Motors do run on 5i23 at 200-250us servo period, but not very smooth and
> such period is too short for 7i43.
> I think even 7i43 would be capable of running such motors smoothly at
> max speed if sine lookup and commutation would be made in FPGA by hardware.
>
> I am thinking about new, corrected firmware for BLDC motors, which would
> be superior over current LinuxCNC/hostmot2/bldc solution. Now I am
> talking about "qi" mode only. And it is the only mode I would need.
>
> What I would like it to be is similar functionality, just more tasks
> done by FPGA itself. The main point is to transfer sine commutation. I
> understand these things have to be changed:
>
> 1. Encoder to electric angle conversion by FPGA (configurable
> parameters, such as scale and encoder offset);
> 2. computing or table-lookup of sine function in FPGA;
> 3. remove 3pwmgen.nn.A-value, B-value, C-value and similar pins from
> hm2 component / driver.
>
> LinuxCNC would pass only value (amplitude) to every axis every servo
> period only. This way commutation would be much faster and would require
> less host CPU power.
>
> What do you think about all this?
> What are the possibilities? Who is competent enough to make firmware,
> drivers? I am programming LinuxCNC components by myself, but never
> touched FPGA or similar programming.
>
> Marius
>
>
>
>
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