On Friday 07 December 2007, Tony Bussan wrote:
>> >I cruised some of the Honeywell sensor pages and the Digikey catalog
>> >page with the 1GP4001 on it. I had studied the previous Digikey page
>> >because I was interested in the Honeywell 103SR13A-1 which are installed
>> >on my Hardinge lathe. I am guessing that for rigid tapping that the
>> >sensor will need the zero speed feature. It did not seem obvious to me
>> >which sensors where zero speed capable. What should I look for that
>> >would indicate this?
>>
>> First rule of magnetics is that a coil as and inductance is a velocity
>> sensitive device, so there is a minimum speed below which it doesn't work.
>
>Zero speed capability has to do with the signal conditioning circuit. Some
>sensors use an edge detect, some use a level comparator with hysterisis, and
>some actually have an A/D and digital processing to constantly calculate
>switch points. There are probably more that I am not thinking of right now.
>
I possibly didn't word that as precisely as I could. But at zero speed, there
truly is no signal to condition. Only if its moving can it generate a
signal.
>> As far as I know, all Hall and GMR devices are amplitude sensitive, not
>> velicity. Even a reed switch fits this category if one doesn't mind the
>> mechanical lag which could be in the milliseconds range.
>
>Be careful analyzing Magneto-resistive, it is amplitude sensitive until
>saturated, and it is sensitive to the angle of the flux vector.
It could be that the internal steering of the field to get maximum gain is a
ferrous material, in which case its inductance will slow things down. That
would be one possible explanation for the speed loss.
Also, I believe here is a difference between Magneto-Resistive and the
so-called GMR, which takes place on a physical scale in the disk heads that
is in the sub-micron range. GMR was only discovered maybe 15 years ago, and
its taken all this time to get it into disk drives, in a head that flies
maybe 2 microns above the disk surface. Part of that time was in developing
a write head that put the magnetic domains on the disk so they were
vertically aligned, which raised the data density per square inch by a factor
of about 100 compared to longitudinal recording methods used previously.
>> I'm still puzzled by the 10 kilohertz response listed for this device.
>> WTH?
>
>The upper limit on frequency is most likely due to the signal processing
>circuit. I would bet that it has a low pass filter for noise immunity. You
>really don't want the sensor that sensitive to mechanical, magnetic or
>electrical noise. 10 kHz is pretty fast in mechanical terms.
Not for watching a 50 tooth gear on a headstock spindle turning 3k rpms. That
would be a 150 kilohertz signal, so this particular device then would at the
end of its range at 200 rpm in that scenario and it still wouldn't have but a
7.2 degree resolution. That to me is not a very useful upper speed limit,
nor truly usable resolution for high precision threading.
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Unix is a Registered Bell of AT&T Trademark Laboratories.
-- Donn Seeley
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