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OLi

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Reply with quote  #1 
We found a site where they position the tach by meauring DC on the sensor side of the proximitor. In my world that can't work unless you read the freq. or am I wrong? Proximitor say it  can be used with both 4 and 8 mm probes or something like that.
We get a signal of +-0.5 V at -17V from a 10mm notch and I think that is due to the wrong procedure that is placing the sensor way out.

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John from PA

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Reply with quote  #2 
Quote:
Originally Posted by OLi
We found a site where they position the tach by meauring DC on the sensor side of the proximitor. In my world that can't work unless you read the freq. or am I wrong? Proximitor say it  can be used with both 4 and 8 mm probes or something like that.
We get a signal of +-0.5 V at -17V from a 10mm notch and I think that is due to the wrong procedure that is placing the sensor way out.


It isn't clear to me what you mean by "measuring DC on the sensor side of the proximitor".  The proximitor terminals would be on the monitor side of the proximitor.

If by "position" you mean establishing the physical gap between the probe tip and the target, then "yes" you can measure the DC voltage at the proximitor terminals; in fact that is probably the preferred method of gapping probes.  On a -24 vDC system, which virtually all are these days, you shot for a voltage around -10 vDC.  This will put you around the midpoint of the linear range (typical curve attached).  I personally like to gap tach probes on the short side, say around -8 vDC as that insures a healthy spike at the buffered outputs.  One also has to be sure if gapping "blind" you aren't positioned over the notch.  In large machines that might have large bearing clearances, it might also be advisable to take into account the effect of anticipated shaft movement on voltage change.


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OLi

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Reply with quote  #3 

Yes that would be the normal way, due to cable length or whatever we got the impression that yes they would measure on the sensor side where you only have a coax but who knows it may also be a language barrier since I thought it would be crazy doing it that way. Since the target is like 10 mm hi they gap it on the target just to be safe also for thermal expansion and the result is anyway tiny and DC levels -17VDC and -19VDC are indicating that the sensors are way out from the shaft so there are room for improvements.
We may put together a basic platform so you can have vectors and FFT from 2 signals and 1 tacho per module so in this case you hook 2 modules to the Bently panel since it is a 2 shaft machine and could get the basic data you need for balancing and possibly basic analysis thru internet back home and then may limit the visits required to Libya or Somalia and by that account the north sea. Not saying that those are difficult places but at times there are places that have difficulties. What do you think? Anything more you need part from a mechanic applying the weights but that can be solved.

 


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fburgos

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Reply with quote  #4 
Agree with John, they need to get closer to the shaft,

Bias voltage is the common procedure to set any proximity probe right??
OLi

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Reply with quote  #5 
Yes they tell me they do it sort of that way and I agree measurements do say that it is to far out so good we can agree and I was thinking as you but the mounting and adjustment can be improved. Good, many Thank's!
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John from PA

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Reply with quote  #6 
Quote:
Originally Posted by OLi

Yes that would be the normal way, due to cable length or whatever we got the impression that yes they would measure on the sensor side where you only have a coax but who knows it may also be a language barrier since I thought it would be crazy doing it that way. Since the target is like 10 mm hi they gap it on the target just to be safe also for thermal expansion and the result is anyway tiny and DC levels -17VDC and -19VDC are indicating that the sensors are way out from the shaft so there are room for improvements.
We may put together a basic platform so you can have vectors and FFT from 2 signals and 1 tacho per module so in this case you hook 2 modules to the Bently panel since it is a 2 shaft machine and could get the basic data you need for balancing and possibly basic analysis thru internet back home and then may limit the visits required to Libya or Somalia and by that account the north sea. Not saying that those are difficult places but at times there are places that have difficulties. What do you think? Anything more you need part from a mechanic applying the weights but that can be solved.



I don't know for certain that the DC voltage related to gap at the target is even available on the sensor side, and of course you would have to damage the coax cable to get an access point.  The purpose of the proximitor, sometimes termed an oscillator demodulator, is to supply an AC voltage of very high frequency to the coil in the probe.  As best I recall the frequency is in the 2 to 3 gigahertz region and can vary depending on certain probe types.

By the way it is perfectly fine to establish the gap at the front panel buffered outputs with a DMM. 

From your statement "Since the target is like 10 mm hi they gap it on the target just to be safe also for thermal expansion and the result is anyway tiny and DC levels -17VDC and -19VDC" it sounds like they are using a protrusion as opposed to a notch.  The problem with a voltage at -17 to -19 vDC is small voltage spike is obtained as the target passes the probe.  As an example, the maximum voltage is about -20 vDC, so if gapped at -19 vDC somewhere around 1 vDC is the maximum spike you will get.  That may not be enough for some external instrumentation.  If gapped (over a protrusion) at -15 vDC, then the spike could be 5 vDC.  As you can see by the sample chart I provided earlier, a voltage of about -12 vDC is a physical gap of about 1.8mm.

Since notches are far more common than protrusions, when I see gaps of -17 to -19vDC on a tach channel, the first thing I do is rule out that the probe isn't positioned over the notch.  That is about the region of voltage that would be obtained when reading the notch.











OLi

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Reply with quote  #7 
Well normally the transducer side are parts of the oscillator, probe+cable and those I have seen is around 1MHz with variation depending on probe distance to target, Proximitor is just the rest of the oscillator and converter to the signal output in relation to oscillator freq. I don't know if DC signal is seen on the sensor side, I doubt it. You can do a breakout connector to attach at the sensor/extension cable connectors so you don't need to break the sensor but I don't think the gapping works any good that way. This likely explain the signals we got and it is about the levels you describe and yes we got some problems to trig and we solved that now when we know what we are working with. I was just asking here if this procedure described was some known trick way I didn't knew of of but it seems if it is the way they do it a way to put the sensor a bit too far out. Machine is made in Sweden so naturally they did not follow any US or Bently standard when the tacho trigg thingy was designed some 30+ years ago.... so yes there is a thing pointing up from the shaft on these apprx 10mm hi, 20mm wide.
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