Re : Possible to use digital line in of a 6024E to sync measurements on LabView?

[gbugnard]

Hi

I want to use a PCMCIA 6024E DAQ card with a laptop to sync. measurements in LabView with an external signal. I want that LabView waits until the card receive a pulse (frequency of this pulse will be 25Hz or 30Hz) and then measure the value of one analog input and grab a picture from a FireWire camera. I have written a basic block diagram as graphical interpretation of what I want to do (see the two attached images). I have LabView 8.2.1 with the necessary modules.

Is possible to to it with this sample frequency?

Thanks!

Guillaume

Message Edité par gbugnard le 10-10-2007 02:04 AM

[Hemetz]

Hi Guillaume,

an example to measurement an analog signal with trigger would be the "Multi-Function-Ctr Retrigg Pulse Train Generation for AI Sample Clock.vi".
You find this example in the NI Example Finder under "Hardware Input and Output >> DAQmx >> Synchronization >> Multi Function". This would be work without any problem with your signal rate, but i think a problem could be the camera.
What specs has this camera? Is it possible to grab 30 single pictures? The most cameras could capture a video with 30 frames per second but not 30 pictures per second.

Stefan

[gbugnard]

Hi Stefan

Thanks for your answer. I will have a look at this example.

The sync signal I want to use to trigger LabView comes from the camera itself. What I plan to do is to separate the video signal with the LM1881 chip from National Semiconductor (http://www.national.com/mpf/LM/LM1881.html) and take the Odd/Even output signal to trigger LabView (the level of the signal is high for the during the first field of the frame and low during the second field of the frame). So I thought that LabView could grab a picture and measure the pressure when the signal change from even to odd field.

I don't know a lot about camera (I am mechanical engineer) but I thought that a frame would corresponds to a picture...

The camera I use is a KS-600 from NET-GmbH with a 1/6'' camera head. It supports standard NTSC and PAL and has a S-video and a Composite-video output. I grab the video signal via a video to FireWire converter from TheImagingSource. I know, the system sounds quite complicated. But unfortunately, I cannot use a standard PCI framgrabber, because I use a laptop and I want that the system remains as compact as possible... I did not find any camera with remote 1/6'' CCD sensor and FireWire connection.

[Hemetz]

Hi Guillaume,

yes that could work if the Odd/Even would be a TTL signal but it isn't. As I see in the datasheet is the high level 0.8 and referenced to the TTL specification is the range from -0.3 to 0.8 a low value. Also you must trigger on a falling edge because a frame starts with it.

Picture and frames are the same but I explicit mention it like this because it’s the same as with a continues and singel sample measurement. Continues measurement is much faster than single measurement with the same rate, because the device must not send and receive for each sample some commands. The same is with a camera. A video with 30 frames per second is like a continue measurement. You send one command to set the parameters and one to start the video acquisition and than you get the video stream.

You must try if your camera is so fast also by single pictures.

But when not, you could grab the video stream and extract single pictures from it.

Stefan

[gbugnard]

Your solution with the image stream looks nice. But after the first tests I just did, how can be 100% sure that the pressure measurement starts exactly at the same time as the stream recording and how can I be sure that the image 10 of the stream really corresponds to the pressure measurement number 10?

I can tolerate a time shift of maximum 10 ms between the image and the pressure...

[Hemetz]

Hi Guillaume,

you get a trigger (even/odd) signal from the camera and you could use the trigger signal to start the measure and capture a frame.
This schould be really syncron.

Stefan

[gbugnard]

Ok, that sounds good 🙂

You said the Odd/Even signal is not a TTL signal. But would it be possible to acquire this signal with the analog input of the card and instead of watching the level's state of the digital line, make a condition that if the voltage is between 0 and 0.4 it's the even field and between 0.4 and 0.8 it's the odd field?

And what about the amplification of the odd/even signal to get the proper range of a TTL signal?

Thanks

Guillaume

[Hemetz]

Hi Guillaume,

I think an amplifier should be the best solution because the original signal has a too sharp boarder between low and high state.
Your idee with analyse the signal would be ok, but you must analyse the signal very fast so that you get exactly the high satus at that time.
A hardware trigger scan the signal the whole time  for rising edges without cpu power and with an software trigger it would need
more cpu performance to analsyse the signal.

Stefan