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Participant

Join Date: Feb 2010
Location: Bintulu
Posts: 4

Speed Sensor Differences

02/02/2010 9:23 AM

Hi whats the diff between magnetic pickup speed probe & eddy current keyphasor. my understanding is that both works with magnetic field with help from notch/groove on the shaft / surface to get the desired pulse/voltage..

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Guru
Engineering Fields - Instrumentation Engineering - New Member

Join Date: Dec 2009
Location: San Antonio, TX USA
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#1

Re: speed sensor difference

02/02/2010 10:08 AM

Here's a Hewlett-Packard application note that should answer your question. In short, you're right!

http://cp.literature.agilent.com/litweb/pdf/5962-7276E.pdf

Also, this baby is about 88 pages, so watch where you stick it.

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Participant

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Location: Bintulu
Posts: 4
#2
In reply to #1

Re: speed sensor difference

02/02/2010 10:33 AM

Thanks. so i assume there is no difference between those 2 sensor technology ? but why for keyphasor principle they always mentioned eddy current since we instrument are only interested with AC Voltage produced by change of magnetic flux? tq

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Anonymous Poster
#3
In reply to #2

Re: speed sensor difference

02/03/2010 2:21 AM

hi, magnetic sensors cannot pickup high frequency signals (high speed machine) with larger gap between sensor and the target. eddy current probes can sense with larger gap between the sensor and the object(gear wheel or a key slot etc). hence for high speed machines eddy current probes are preferred.

seetharaman

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Guru

Join Date: Aug 2007
Location: Indiana, USA
Posts: 579
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#5
In reply to #1

Re: speed sensor difference

02/03/2010 9:16 AM

Great Link!

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Hobbies - HAM Radio - New Member

Join Date: Oct 2008
Location: Houston, Texas
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#4

Re: Speed Sensor Differences

02/03/2010 8:30 AM

A magnetic speed probe is essentially a permanent magnet alternator. It only puts out a frequency proportional to speed of the shaft and the number of disconuities (teeth, holes, key slots). The amplitude of the magnetic speed probe signal depends upon the surface speed and the gap between the pickup and the sensed surface. If the shaft is not turning, the magnetic speed probe cannot detect it. The eddy current probe is excited externally and basiclly produces a signal proportional to the gap between the probe and the sensed shaft. The shaft doesn't have to be moving to be detected by the eddy current probe. These two sensing technologies are completely different. You can however measure a shaft's speed with either of them. The mag pickup's signal becomes vanishly small as the shaft rotation approaches zero speed. The eddy current probe could be used to determine the shaft speed even if it were less than 1 revolution per year. I hope this helps! TT3

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Commentator

Join Date: Dec 2007
Location: texas
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#6

Re: Speed Sensor Differences

02/03/2010 9:53 PM

Hi,

A keyphasor probe can not detect rotation movement, however it detects distance between the tip of the probe and the shaft.

For example if a keyphasor probe is set at 10 mils away from a solid shaft without a slot, rotating at any speed fast or slow the keyphasor's output voltage will not change.

Now if we have the same probe but this time the shaft has a slot or hole that passes in front of the probe at every revolution then the probe is going to see a change in distance when the hole is in front of it, therefore it's output voltage is going to "spike" at every revolution.

X amount spikes(peaks) per minute equals the same amount of revolutions per minute.

Thank you

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