Frequency measurement with External Trigger

[Marcelo Fernandes]

Hello:

I

[Marcelo Fernandes]

Hello:

I have a NI-5112 and using External Channel to trigger the waveforms acquisitions of other channels CH0 and CH1.

I want to measure the frequency of this external trigger. Is it possible ?

Thanks.

[Brooks W.]

Hi Marcelo,
I will present three different options to you.  I am assuming that you are doing a multi-record acquisition (one trigger per record).

1. If you have no additional hardware and you have used both of the channels on the 5112 then you could read your data (on channels 0 & 1) as a waveform data type.  The waveform data time will have a To(initial time stamp), dt (time between samples), and Y (amplitude) components.  Knowing this you could look at the To and dt values of the first sample of each record and calculate the frequency of the trigger.  This will be the most difficult way of determining frequency.

2. If you have an addition 5112 channel (on another board or on the same board) that you can route the trigger into, then you can calculate frequency using the NI-Scope functions.  Please look at the example program "niScope EX Advanced Measurement Library.vi" to see an example of this.

3. If you have a mutlifunction DAQ board or a counter/timer board then you can route the trigger into one of the counters on these boards to measure frequency.  Look at  the example program "Meas Dig Frequency-Low Freq 1 Ctr.vi" for an example of this.

Hope this helps!
Brooks W.
National Instruments

[Marcelo Fernandes]

Hello Brooks:

Thank you for attention !

I will try to explain my application:

I need to measure a signal of a speed/position sensor. This sensor works with hall effect principle and is placed in front of a wheel with some teethes. Is used for automotive purpose in CAM and CRANKSHAFT applications. So, I need to measure period, duty cycle, rise time, fall time, Vpp, Angles for each teeth x encoder, everything from 40 to 7500 rpm.

I'm using CH0 to measure a signal of this speed/position sensor.

I'm using CH1 to measure a signal of an encoder connected to wheel.

I'm using EXT Trigger to sync acquisition with encoder's INDEX signal.

I need to measure the frequency of EXT trigger to know the instantly speed of the wheel.

So, If I know that my encoder has 2048 pulse/rev, I can measure FFT_FREQUENCY of CH1 with scalar measurement functions and calculate the speed !

The NI-Scope is wonderful because it can measure a lot of things automatic and fast, like period, frequency, duty cycle, rise/fall times ...

Now I have one more question:

I want to use 10:1 attenuation but I can't use a common probe. I want to connect the CH0 to sensor with a special connector.

I've tried to make the same circuit with 9MOhms and capacitor like explained on site, but I reached bad results. The industrial probes has special characteristics for capacitance and noise rejection.

Do you know how can I solve it ?

Thank you.

[Brooks W.]

Hi Marcelo,

I would really like to help you with this issue but I have no training in probe design.  I am trained in NI hardware and software and I will be happy to help you with this.

Best Regards,
Brooks W.
National Instruments