Re: Stepper motor driver with less noise?

bill.sloman@xxxxxxxx wrote:

On Jul 9, 7:15 pm, Joerg <notthisjoerg...@xxxxxxxxxxxxxxxxxxxxx>
wrote:

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 data*** 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?


The data *** talks about regulating the on and off times with a one-
shot. All the monostable circuits I know about involve some kind of
comparator looking at an analog ramp, and the comparator is obviously
going to be sensitive to noise on the ramp and the voltage reference.
You can minimise the external contributions to this noise, but with a
single chip solution you are stuck with the chip-designers layout
inside the chip - a chip that is switching the same drive current that
is running through the motor.


Yup, that's about how they all operate. It can work but only if the chip designer is very experienced with noise mechanisms. Some things have to be done differentially and I am afraid they weren't. The mono-stable seems kind of ok but the comparators are probably the ones where their thresholds are flopping about.


I'd be much happier with a chip that used a digital delay
generator ... Naturally, I've never heard of one.


On that project we'll have one pretty soon. It'll be quantized but that is much better than such gross jitter. I guess that once the client has seen the first "homemade" controller work they'll kick out all the other microstepping driver chips as well. All you really need is an FPGA with some space left on it and a few vacant pins. Or a uC that has an unused timer with at least two CC registers.

--
Regards, Joerg

http://www.analogconsultants.com
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