Problems meassuring rmp

[ddresg5]

Hi!

I have to measure the rpm of a propeller using a Brushless RPM Sensor, and I have two questions:

1- I have a NI 9401 which allows me to measure it, but I wonder if I could measure it with another NI 9205, and the PF0 pin that it has, I can configure it but it doesn't work, I am selecting the daq_ctr0 as counter, and the 9205 PF0 as frecuency terminal.

2- I get a graph where I can see the pulse train who is generated by the rpm sensor, but I can't get the frecuency, I haven't found any tutorial about how to get the frecuency of a pulse train. I also have problems with the number of samples, because if I select a low number, it doesn't work when the propeller is at maximum P, and if I chose a bigger one, it doesn't work with the lowest rpms.

Thank you very much, I have attached the code.

[AJost]

Hi ddresg5,

1) the 9205 PF0 should be able to read out a the digital signal of the sensor.

What is the voltage level you expect from the sensor? The 9205 uses 3.3V digital signals, while the 9401 works with 5V.
For the rest, they connect both to the same counters. Can you specify what you mean with "it doesn't work?".

2) The CI Freq task will give you directly the frequency not the pulse train which is generated by your sensor. Can you post a picture of the data you receive from the sensor?

The number of samples only determines how often the chart is updated and can be adjusted dynamically either by a control or automatically depending on the measured frequency if you want to have a more constant update rate. You could also set it to -1 if you want to read out all available samples at each loop iteration.

Again: what do you mean with "it doesn't work"?

Here is also a good example with explanations for frequency measurements.

NI-DAQmx: Continuously Measure Counter Frequency (Buffered-Large Range)

http://www.ni.com/example/30013/en/

Andreas Jost
Applications Engineer

National Instruments

[ddresg5]

Thank you for your help,

1) I am working with less than 1 V, I attached a screenshot with the "Highlight Execution", the program stop at the daqmx reader waiting for data, but the rpm sensor is working. 

2) I thought so, but when I put a numeric indicator it gave me different numbers, from 1000 to 8000 with the propeller at the same power, this is because of the graphic, so I put a graph to watch what I was getting and , I attache a screenshot of the graph.

9205 is with the Highlight and the other one the graph.

[AJost]

I think in both cases the problem is the 1V. They are 3.3V or 5V modules respectively, so you cannot expect them to work with less than 1V.
If you look at the manuals for both modules they both need a high of at least 2V to function properly. 

NI 9205: http://www.ni.com/pdf/manuals/378020a_02.pdf

NI9401: http://www.ni.com/pdf/manuals/374068a_02.pdf

I cannot say in detail why they behave differently in your set-up without a proper analysis but the problem is:

9205: the module never recognizes a rising edge and therefore waits indefinitely

9401: for some miraculous reason it recognizes most of the edges and gives correctly 8000Hz, but misses a couple from time to time where the measured frequency drops to 1000Hz. With a numeric indicator you miss these points easily, but with a graph they are nicely visible.

So if you want to measure this RPM meter, you either need to get the voltage level higher for the modules to recognize them properly or you need a different measurement module.

[ddresg5]

Good afternoon, I have already solved this problem, there was a problem with the rpm sensor, and now I can measure the frequency.

I would like to know if the NI 9205 is able to measure digital and analog signals at the same time, because when in addition to the frequency, I try to measure some voltages, I only get noise like rpm. I attached is two screenshots of both cases.

Thank you.