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Stepper motor driver with less noise?

J

John Popelish

Jan 1, 1970
0
john said:
[...]
Thanks John. Good words.
I've enough now to try out on my stepper driver model.


Another note.

I think the switch in my design was driven by something that
was effectively a D or J-K flip flop (though there were two
of them inside an integrated dual H bridge chip), clocked at
the peak of the sawtooth clock waveform, with the D or J and
reset driven by the current comparator. So each clock
period, if the comparator showed current below reference,
the clock edge would start a power pulse, and when the
comparator showed the current above the reference, the reset
would terminate the pulse, and that decision was irrevocable
till the next clock edge began another pulse. The clock
circuit consisted of an external RC, in parallel to ground
with a fast charge and an RC time constant discharge. I
added a bit of that waveform to the current reference input
to make the sawtooth version of the current reference.

I wasn't micro stepping the driver, but operating in half
step mode. I also varied the average reference level
between two different values, depending on whether one or
two windings were being powered to produce a uniform break
away torque, regardless. This took a lot of rotational
torsional vibration out of the rotation.
 
R

Rene Tschaggelar

Jan 1, 1970
0
Joerg said:
Hello Folks,

Fixed a noise problem at a client last week. When I arrived at a stepper
motor driver I could not believe what I saw. It was a A3984
microstepping driver chip from Allegro with the two H-bridges built in.
Fixed off-time, but the on-time was flailing about at a jitter of at
least 30%. This caused very wideband phase noise. Their layout is pretty
good, the supply is clean. Ok, we were able to reduce that jitter down
to 15% by stretching the off-time but the datasheet has no spec how far
you can stretch and eventually it'll quit. I was not too enthused about
the specsmanship, to say it mildly. Asked app engineering about the
off-time limits and they couldn't tell ....

Long story short this gets designed out and we'll roll our own, as
usual. Curiosity question: Are there any microstepping drivers that are
not that noisy?

At what power level ? A current low noise microstepping
design of mine employs a pair of linear audio fullbridges.
Since the rotational speed as well as the torque is
close to zero, a single chip stereo amp that can be
operated DC coupled was choosen as optimal solution.

Rene
 
J

Joerg

Jan 1, 1970
0
Rene said:
At what power level ? A current low noise microstepping
design of mine employs a pair of linear audio fullbridges.
Since the rotational speed as well as the torque is
close to zero, a single chip stereo amp that can be
operated DC coupled was choosen as optimal solution.

About five watts. We can't go linear because of battery operation. Every
milliwatt counts. We'll just roll our own PWM, then there will be peace
and quiet, plus one less part to buy for the purchasing guys ;-)
 
J

john jardine

Jan 1, 1970
0
[...]
I wasn't micro stepping the driver, but operating in half
step mode. I also varied the average reference level
between two different values, depending on whether one or
two windings were being powered to produce a uniform break
away torque, regardless. This took a lot of rotational
torsional vibration out of the rotation.

Nice idea.
I naively started out thinking these motors were just like a DC motor except
they stepped. Little realising that any number of techniques and
'approaches' can make a world of difference to their performance.
Once I'd moved from full to half to micro step, there was no turning back :)
 
J

John Popelish

Jan 1, 1970
0
john said:
[...]
I wasn't micro stepping the driver, but operating in half
step mode. I also varied the average reference level
between two different values, depending on whether one or
two windings were being powered to produce a uniform break
away torque, regardless. This took a lot of rotational
torsional vibration out of the rotation.

Nice idea.
I naively started out thinking these motors were just like a DC motor except
they stepped. Little realising that any number of techniques and
'approaches' can make a world of difference to their performance.
Once I'd moved from full to half to micro step, there was no turning back :)

The extension of my current modulation scheme to
micro-stepping would be to include a map of torque versus
position per amp for your particular motor design, to
approximately unravel the nonlinearities of the magnetic
structure. Stepper motors are not very good sine wave
generators. They have all sorts of cogging (salient poles)
and harmonic factors in their operation.
 
J

Joerg

Jan 1, 1970
0
John said:
john said:
[...]
I wasn't micro stepping the driver, but operating in half
step mode. I also varied the average reference level
between two different values, depending on whether one or
two windings were being powered to produce a uniform break
away torque, regardless. This took a lot of rotational
torsional vibration out of the rotation.


Nice idea.
I naively started out thinking these motors were just like a DC motor
except
they stepped. Little realising that any number of techniques and
'approaches' can make a world of difference to their performance.
Once I'd moved from full to half to micro step, there was no turning
back :)


The extension of my current modulation scheme to micro-stepping would be
to include a map of torque versus position per amp for your particular
motor design, to approximately unravel the nonlinearities of the
magnetic structure. Stepper motors are not very good sine wave
generators. They have all sorts of cogging (salient poles) and harmonic
factors in their operation.


One method to improve the situation is to minimize the noise coming from
the motor but that requires somewhat luxurious software handles for
tweaking. Ideally the motor should just release a very faint hiss, not
the usual "eeeep". However, once they asked me to introduce some "eeeep"
again because they wanted acoustic feedback indicating that it's
turning. Shook my head but did it. The client's wishes must be my wishes ...
 
M

Michael A. Terrell

Jan 1, 1970
0
john said:
Once I'd moved from full to half to micro step, there was no turning back :)


That sounds like a problem with your software. ;-)

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida
 
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