On 7/16/25 12:23, Chris Albertson wrote:
In the old days there were huge differences in how they were controlled. But
today we use microcontrollers for everything. SO there is some convergence
but still I think we can see differences in the physical motors
1) Stepper is just a BLDC motor is “dozens” of poles and perhaps two or three
phases. Traditionally there were controlled with square waves, But now we we
some “closed loop steppers” controlled with analog sine waves.
2) BLDC is typically a three-phase motor that in the past might have been run
with square wave generated by switches. But today we have Bosch more
sophisticated “FOC” controllers even if some still do use Hall effect switches.
3) A three-phase motor that even in the old days was powered by three-phase AC,
Now that BLDC motors can be controlled this way too I wonder two things (1) are
they really different and (2) how were they traditionally controlled before we
had microcontrollers that could generate waveforms in software?
Which it s/b be said are still learning to walk. FOC to this Certified
Electronics Technician is just an advertising buzzword, and ANY attempt
to synth a sine wave is actually done by shaping the on/off cycles of
the voltage feeding the motor coils by duty cycle modulating thr PWM at
a frequency much faster than the motors inductance allows. And there is
no getting away from that fact. It absolutely has to be done that way
in order to reduce the power dissipation of the driver transistors
involved. They don't heat when turned off, and they must be turned on
hard enough to reduce the effective ohmage of the path thru that
transistor to the .001 ( or less) ohms of a completely on condition. Any
such transistor being used to limit this is going to get hot and can
quickly overheat and destroy itself. We've had transistors capable of
doing this for several years. PWM switching rates in the 35 to 50
kilocycle range are used. The upper limit will be advanced only by the
use of high voltage, high amperage transistors that can be switched at
nanosecond speeds and that limit currently is in the driver transistor
controlling the vfet power transistor due to the gate capacitance that
must be charged to turn the vfet on, or discharged to turn the vfet
off. That capacitance can be .05 uf or more in high current vfet's.
But they are not the speed limit. The driver transistor controlling
them is. I have used these vfet types as finals in cb radios, switching
at 27.5 megahertz, many times faster than whats in these motor controllers.
And a side comment on the encoders used, a motor with a hall effect
encoder can be very accurate when stopped, but due to the time lag of
making the analog output of a hall encoder into a quadrature signal that
can then be used to control the motor to quite small position errors.
BUT because of that conversion lag, cannot be moved slowly AND smoothly.
Optical encoders cost more, but they can be moved slowly as the steps
are generally so fine they are not detectable. So stay away from the
smaller 42C motors.
Of the above for my interrest ig robotics the exciting kind is the BLDC
controled by FOC. These give the best power to weight and best control over
acceleration and position. If you see a humanoid robot, it is because of these
motors and the huge energy stored in lithium batteries.
How would you like to write the “g-code” for this machine (28 axes).
https://youtu.be/I44_zbEwz_w
That is above my pay grade. I'd expect that is actually the g-code
output of a much more complex language. Maybe 50 years ago, but then
the tools didn't exist anywhere in this planetary system. In 1979,
working on a video tape machine controller, I was still looking thru the
RCA programmers manual for the next command I wanted to use, (no
assembler) and entering that opcode in a hex monitor running a cosmac
super elf, had an 1802 mpu. Essentially the same mpu that flew the space
shuttle. CMOS, built in radiation hardened. That was quite successful,
becoming part of the tv stations daily commercial prep for an automatic
station break controller that was still in daily use in the last of the
90's when the whole thing burned to the ground. I still have paper and
audio cart copies and enough schematics I could build another if asked
and I could still buy the parts, but today I've have to scour the net,
paying collectors item prices for some of them. But it would take about
a year I may not have at 90 yo. Today I'd probably put it on a pi
clone. Then a 4k static ram kit on a vector S-100 board was $400. Now I
can buy 32 Terabytes for that same $400 on Amazon.
Jul 16, 2025, at 4:56 AM, Stuart Stevenson <stus...@gmail.com> wrote:
What is the difference between:
1: stepper/servo (Hanpose, et al)
2: BLDC servo
3: A/C servo
It seems to me to be describing the same technology.
Thanks
Stuart Stevenson
4638 Farmstead Ct
Bel Aire, Kansas 67220
316 258 0953
stus...@gmail.com
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