Using one signal to trigger two separate measurement task: voltage and current

[LeonFluid]

Hi, all,

I am currently measuring voltage from force sensors and current from laser sensors simultaneously. Both current and voltage measurements are triggered by the same signal from 'PFI0'. The labview programme is attached. I set two separate Start VI - Read VI - Close VI routes in one Labview programme.

However, I am wondering if there is any time lag between these two measurements? Ideally both current and voltage measurement should start together, but when I process the data I notice that there might be a small time lag between signals. Is there any method to check this?

Many thanks for your help,

Leon

[JÞB]

Inline a snippet of the vi not a partial picture.  Would you bring a photo of your face to a cosmetic surgeon?

You also forgot to show any data that you are questioning the interpretation of. 

We also need to know what is supplying the power you are monitoring and the characteristics of the load it is applied to.  If there is a reactive component in the impedance the Voltage and Current will be out of phase and your vi is fine but your interpretating the phase angle incorrectly as a delay.

[LeonFluid]

Hi, Jay,

I am sorry for the inconvenience. The labview programme is attached below.

The current signal and voltage signals are from different sensors, current from lasers and voltage from force sensors. They are supplied by different power sources.

Hope this info helps.

Leon

[Kevin_Price]

Some other relevant questions:

What DAQ devices are you using?   Devices based on delta-sigma converters will induce a delay from the trigger to the first sample.  Force sensors are often interfaced to a bridge-based device and NI's either tend to use (or perhaps always use -- I don't happen to know of any exceptions) delta-sigma converters.

What controls the timing of when the PFI0 triggering signal gets asserted?  Can you be sure both tasks are started and waiting for it before it gets asserted?

Are you sampling for a long time?  If so, what keeps the 2 devices in sync with one another?  This will be automatic if in the same cDAQ or PXI chassis, but not if they are PCIe or USB.   Then, typical timing accuracy specs would allow them to skew from one another by ~3 msec/min (based on 50 ppm spec).

-Kevin P

[LeonFluid]

I installed 2 NI-9205 modules on the cDAQ-9188 chassis. One module is used for collecting current signal at 1.5 kHz, and the other one is collecting voltage signal at 120 kHz (Force sensors).

The trigger signal is monitored by the 'PFI0' node on the 9205 module, which is also used for collecting voltage signal. Every time I pressed a physical red button will set a falling-edge signal to 'PFI0' and both 2 measurements will start. I am not sure whether these two measurements start at the same time since it happens so fast. So I am thinking if there is time delay between them but I do not know how to check it.

The measurement only lasts for 5 secs.

Thank you for your kindly help. Hope this information helps.

[Kevin_Price]

Sounds promising so far.  Please do the following:

1. After one of your runs, go to the "Edit" menu and select "Make Current Values Default".

2. Go to the "File" menu and select "Save for Previous Version".  Choose something like LabVIEW 2016.  A lot of us folks that can help probably aren't working with 2021 yet.  I know I'm not.

3. Post the back-saved vi and any output file(s) created during that run.

-Kevin P

[LeonFluid]

Hi,

Please find the attachment. It was saved for Labview 2016.

Kind Regards,

Leon

[JÞB]

Kevin,   I'm working with a phone so I'll let you take lead on this.

Hint: the 9205 only has 1 ADC with a MUX input.  We also have two different sample rates and a shared sample clock trigger.  The convert clock is going to offset between channels on the two devices.   You can add demos.  I would need something more powerful than an android. 

[Kevin_Price]

I see no problem in the data acq config, considering that you have manual control over the triggering signal and don't assert it until both tasks are armed and waiting for it.

I don't know anything about your system and what should be expected, but I see that the sudden response of the laser precedes the sudden response of the force signals by right around 110 msec.  From where I sit, I'd speculate the following:

1. The DAQ config and reported timing are correct

2. The delay between the onset of the laser signal and the observed force response is due to some other system lag.

What causes the observed force to develop?  Is the laser signal an indirect "cause"?  What else is there between the two, i.e., what is the chain of events from one to the other?

100+ msec would be fairly unusual for signal lag, but not so uncommon for system lag.

-Kevin P

[LeonFluid]

Hi, Kevin,

I am measuring an extremely quick force; the duration being less than 0.05 sec. The laser is used for measuring the velocity of a falling object. It is expected to get a large force when the velocity reaches the maximum value.

If you identified a 110 msec time delay between 2 measured signals may cause a difference when post-processing and analysing the data. May I ask how you check the time difference between the two signals? Is there any way that we can output their start time independently?

Kind Regards,

Leon