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Frequency measurement adc11-12

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Frequency measurement adc11-12

Postby BentRods » Sun Dec 04, 2011 3:41 pm

Hi
typically what sort of signal is best for the freq measurement of approx 0 to 100Hz with the adc11 and PLW logging every 1 sec ?
I have been using a signal is conditioned using a 74HCT schmitt trigger with a square wave output with an amplitude of 2.5v with mixed results, I dont know if this was a SW problem way back then as its been a few years but thing's seem to have been more reliable of late and I have been busy with the assembly of a few pcb's to scale and condition the signal down to 2.5v

I was also thinking about using a ncv1124 dual variable-reluctance sensor interface as this also has a sq wave 0-5v output.

what the ideal signal that gives good results ? apart from using a FtoV convertor

appreciate the help

Thomas
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Re: Frequency measurement adc11-12

Postby Martyn » Mon Dec 05, 2011 4:16 pm

Picolog calculates the frequency by taking the volts versus time data and counting the number of times the signal 'crosses' the mean value in a given time period. Provided the signal has reasonably clean crossings around the mean value then it will work successfully, square waves obviously fit this criteria.
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Re: Frequency measurement adc11-12

Postby BentRods » Tue Dec 06, 2011 1:35 am

Hi,
OK thanks for the reply, one thing that happens a lot is that when the signal stops the freq / rpm will show a high value as if there is interference of course 50Hz is a problem when using laptops and the AC power supply. I have always tried to observe sheilding and attention to grounding.
Mostly the setup used is a photoelectric sensor that has NPN switching through the 74HCT schmitt trigger, should the output be tied low or high perhaps with a 10k resistor ...will this help
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Re: Frequency measurement adc11-12

Postby BentRods » Fri Dec 09, 2011 5:28 pm

I've used a NCV1124 from ON semi that has a VR sensor input and outputs a square wave, I've scaled this output from 5.0v down to 2.2v with a resistor divider. The problem is when the gear stops rotating there is no input and picolog displays Hz / rpm that's unrealistic.
Here is a little trick that I used that maybe helpful to others... (I got the idea from a thermistor lookup table)

Parameter scaling> Scaling method > lookup table, with the following values
0 0
200 12000
250 0
251 0

I no longer have values that are unrealistic and it shows 0 when the gear stops, I'm sure there are other ways to control this and tweaking the values to suit the input freq range.

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