MAX9924 configuration and use cases

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Fred
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Re: MAX9924 configuration and use cases

Post by Fred »

He does have the wires reversed, yes! Huff, see how the two larger squares are one pos, one neg? if the wires were reversed you'd have one big one in one direction and nothing in the other.
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Re: MAX9924 configuration and use cases

Post by TonyS »

Hi Fred,
I am getting pretty close to understanding the whole picture. There is only one more thing, that because of my inexperience, isn't perfectly clear to me.
I still have not found / heard / seen, a digram that correlates the VR waveform with the actual position of the sensor relative to the gear.

I believe / know that (with a properly wired sensor) the zero crossing on the falling slope of the waveform corresponds to the moment when the sensor and gear tooth are exactly aligned. But other than that, can I assume that the zero crossing on the rising slope is the point where the sensor is aligned with the middle of the valley? And what sensor / gear alignment accounts for the peaks in the waveform (positive and negative).
Thanks,
-Huff
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Re: MAX9924 configuration and use cases

Post by jbelanger »

The VR sensor reacts the movement of ferro-magnetic metal: move the metal towards it and voltage rise; move it away and it goes down; leave the metal at the same place and you get no voltage. Relate that to moving teeth across the sensor and you have your answer.

Jean
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Re: MAX9924 configuration and use cases

Post by EssEss »

TonyS wrote:... the zero crossing on the falling slope of the waveform corresponds to the moment when the sensor and gear tooth are exactly aligned. ...
checkout my mini whiteboard:
Image

like jean says, as the tooth approaches the pickup, you get a rise (assuming correct polarity) and a fall as the tooth departs the face of the pickup.
'pip' shape & magnitude is determined by pickup bandwidth and ferrous properties of the tooth.
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Re: MAX9924 configuration and use cases

Post by TonyS »

Hi Guys, I have been meaning to come back and summarize my thoughts about this issue, but have been busy with my new job.
jharvey wrote:If we assume the VR signal is a sine wave that runs at the max allowed noted 100vp-p, the RMS voltage would be .707 of 100, so 70.7v, not 100. It's not a sine, and by eyeballing the curves I've seen, I think it's much closer to .5 then it is to .7. The power at 70.7v would be (70.7^2)/20,000 = .249 watts.
Jared, since I have no previous experience with VR sensors or much knowledge on the subject, I have had to try and find accurate information about VR sensor technology and the MAX992x sensor interface
As far as the maximum voltage that the design should accommodate, it would appear that 200-300 Vpk-pk is not out of the ordinary. I am basing this on the information I was able to find from various sources, including the information contained in the documentation for the MAX9926U Evaluation Kit. The kit is claimed to be capable of evaluating VR signals from 50mV to 300Vpk-pk and is built using (2) 5k, 1/4W resistors to get the 10k input resistor values. There is also the resistor derating that tends to come into play above 70C to consider. I think that using the rms value of the pk-pk value to calculate dissipation is probably a good idea as depending on the gear / sensor, the signal can look pretty much like a sinewave.
jbelanger wrote:The VR sensor reacts the movement of ferro-magnetic metal: move the metal towards it and voltage rise; move it away and it goes down; leave the metal at the same place and you get no voltage. Relate that to moving teeth across the sensor and you have your answer. Jean
EssEss wrote:like jean says, as the tooth approaches the pickup, you get a rise (assuming correct polarity) and a fall as the tooth departs the face of the pickup.
These explanations are pretty close, but not entirely accurate. The voltage will climb as the tooth approaches it, BUT starts to fall even before the center of the tooth is aligned with the center of the VR sensor. This appears to be caused by the fact that the amount of “change” in the magnetic field is reduced as the amount of “change” in the tooth metal is decreasing. I finally found this to help me understand –
http://books.google.com/books?id=_wlZ5L ... &q&f=false
jbelanger wrote:Tony,You need to have a better look at the datasheet. It uses the down going zero crossing as the trigger. Look at figure 1.Jean
Now I think I understand how the MAX9924 works. The MAX9924 output is normally an open-collector “high”. When the input signal crosses the threshold voltage on the positive slope, it “arms” and the output goes “low”. When the input signal crosses the “zero” point on the negative slope, the output goes “high” letting the system know that the sensor and tooth are aligned.
EssEss wrote:A classic indicator of 'insufficient bandwidth' is demonstrated in that turbomiata forum posting earlier where the null throws the next following tooth off. It never goes sufficiently negative to trigger the conditioner. In that case, it's usally easier to go to hall - the bandwidth on those things are huge (and you get free conditioning :) ).

I'll try to go lookup that picture I'm talking about.
here: http://img401.imageshack.us/img401/9299 ... 66rws8.gif
Not quite sure how you are using the term “bandwidth” here EssEss. Bandwidth usually has something to do with the frequency. The waveform shown on the link you posted seems to have the same period (frequency) among all the cycles. My guess is that the signal did not go sufficiently POSITIVE and did not exceed the arming threshold.

Thanks,
- Huff
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Re: MAX9924 configuration and use cases

Post by Fred »

Re inaccuracies, you have to look in context. If you had a step that never ended, the V would fly up as approaching, and then gradually sink down over time due to losses and one day reach zero. Likewise if you ahve an infinitely thin tooth, it will behave exactly as Jean said. I'd say that a normal tooth is a closer approximation to infinitely thin, than infinitely long ;-)

EssEss meant band, as in voltage band, band can also be used to describe oxygen "wide band" etc. Bandwidth isn't purely frequency.
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Re: MAX9924 configuration and use cases

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TonyS wrote:The kit is claimed to be capable of evaluating VR signals from 50mV to 300Vpk-pk and is built using (2) 5k, 1/4W resistors to get the 10k input resistor values.
Awesome. That's good content, thanks for posting. I'll keep an eye on Marcos and make sure this happens in the next spin.
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Re: MAX9924 configuration and use cases

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TonyS wrote:Not quite sure how you are using the term “bandwidth” here EssEss
as in the very classical sense - http://en.wikipedia.org/wiki/Bandwidth_ ... cessing%29
yes, the primary period of what you can observe easily in the time domain dominates, but there's is more to it than that.
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Re: MAX9924 configuration and use cases

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Fred wrote:...as approaching, and then gradually sink down over time due to losses... ;-)
Fred, I'm pretty sure that the decay is not due to "losses", but due to the smaller changes to the magnetic field (from what I have read on the internet : ). Whether I, or you, fully understand what is responsible for the decrease in signal amplitude prior to the alignment of the sensor to the tooth, the important point is that the tooth is aligned with the sensor at the zero crossing on the negative slope of the waveform.
Another thing that I now realize, is that the shape of the waveform is very dependent on the tooth size relative to the size of the VR sensing element and also the distance between teeth. You can get pretty much a sine wave from closely spaced teeth, to "spikes" when you have teeth spaced widely apart.
EssEss wrote:as in the very classical sense - http://en.wikipedia.org/wiki/Bandwidth_ ... cessing%29
yes, the primary period of what you can observe easily in the time domain dominates, but there's is more to it than that.
EssEss, I'm sorry but I can be pretty dense sometimes (just ask my wife : ).

Can you please elaborate because I just am not understanding how you are using the term "bandwidth" in regards to the waveform you provided.
Is it like Fred said, that you are referring to the magnitude of the signal?

Thanks,
-Huff
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Re: MAX9924 configuration and use cases

Post by Fred »

TonyS wrote:
Fred wrote:...as approaching, and then gradually sink down over time due to losses... ;-)
Fred, I'm pretty sure that the decay is not due to "losses", but due to the smaller changes to the magnetic field (from what I have read on the internet : ).
No no, I'm right, if you have a continuous piece of steel, or a continuous gap, and there is a voltage on your output at an instant, it wili decay over time, pretty quickly, to nothing, due to the current dumping into the impedance that the sensor is driving, and a lack of change of mag flux to cause it to be maintained. It's a purely hypothetical comment on my part as no such infinite piece of steel exists, at all, let alone on an engine, it was meant to illustrate the extremes of operation, and the fact that a change causes an induced voltage, not a static chunk of steel and/or air.

Re the shape/size thing, depth is a BIG factor too, if the slots are too shallow, the sensor will still be "seeing" the steel at the bottom and the signal will be weak and shit, again, because of lack of change.

Fred.
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