Magnetic Pickup output signal problem

I am observing the voltage output of a magnetic pickup transducer. When I maintain a constant rpm (speed) it seems that the signal shows that the voltage varies alot in between peak points. Is there a reason for a voltage in the signal and how can I solve this problem.

Reply to
lissy6098
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This question really belongs in news:sci.electronics.basics or

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How are you reading the voltage? If it is with a digital meter, the sample rate will cause the meter to read at different points of the waveform generated by the sensor. You need a scope to see what the waveform looks like, and to compare the waveform. Also, what are you sensing? a single point, or gear teeth/multiple points?

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Reply to
Michael A. Terrell

The most common reason for mag pickup signal amplitude variation is run out in the speed wheel (tone wheel excitation ring etc.) This effectively changes the gap which is relatively proportional to the signal amplitude. If you look at the signal on a scope set with the sweep equal to one revolution, you can see the signal amplitude variation for one revolution. Other physical factors can cause amplitude variation, like inconsistant tooth profile, damaged or missing teeth. Usually the rotation speed is determined by frquency to voltage converter measuring the time between signal zero crossing, or averaging of fixed size pulses, derived from each cycle of the signal.

Reply to
rnscrs

"A lot" means what? How precise IS the mechanical distance from pickup to 'points'? BTW what did you expect, and what setup you provided?

When "testing" it is very good to rely on known AND steady settings.

HTH

Stanislaw.

Reply to
Stanislaw Flatto

The gear teeth (or other objects being sensed by the pickup) are not well centered. Some are passing closer to the sensor than others are.

Reply to
John Popelish

On Jul 22, 8:34 pm, Stanislaw Flatto wrote: > lissy6098 wrote: > > I am observing the voltage output of a magnetic pickup transducer. > > When I maintain a constant rpm (speed) it seems that the signal shows > > that the voltage varies alot in between peak points. Is there a reason > > for a voltage in the signal and how can I solve this problem. >

I work on Sony analog Betacam tape machines that use Hall sensors to read the tape reel rotation info. When operating properly, the variations are less than 10% but as the reel tables age and the adhesive that hold the magnet onto the wheel shift a bit, the variations can get up to 50%. Replacing the reel turntable corrects this. Output is a pair of quadrature sines that get squared up in comparators.

GG

Reply to
Glenn Gundlach

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The voltage output goes directly into a Labview DAS. Once we have the data in the computer, the data is plotted using Matlab. Where sensing the voltage vs time at a constant rpm. Every time one of the teeth passes the pickup a pulse is outputted, and it should be a sinusoidal waveform but instead of a perfect sinusoid we get one that in between peaks there are a variety of different peaks. Is there anyway to insert a picture of the waveform, this is my first time on here

Reply to
lissy6098

Like I said, you need to look at it with a (Preferably a triggered analog) scope to see the real waveform. I doubt that the shape of the tooth will allow a clean sine wave. The precision of the machining of both the teeth and of the sensor will affect the amplitude and waveform, but unless you have a very high sampling rate, you can't really see the true waveform. You will have aliasing unless the rate is high enough.

Pictures are not allowed on this newsgroup. We use a binaries newsgroup, that you can't access through Google. Put it on a website and post the link.

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Reply to
Michael A. Terrell

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You would need a proper tooth profile to approch a sinusoidal waveform. In most cases the output is quasi sinusoidal, with all kinds of distortions due to tooth profile and other factors. If your DAS is sampling fast enough, you maybe able to determine zero crossing with enough resolution to measure signal crossing intervals or crossings per time. With the crossing interval method, a rolling average of succesive intervals will stablize the reading and provide more acurracy. Counting crossings per time is good when the speed is stable and you can count for 1 to many seconds.

BobS

Reply to
rnscrs

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