trigger selection

[soundofwind]

hi, I am measuring the in-cylinder pressure in an engine and a shaft encoder is coupled with the engine shaft ,used to trigger the data acquisition. when the engine runs for one cycle, the shaft encoder turns two rounds, which creates two pulses,called pulse1 and pulse 2.
1.The pulse can only trigger the data acquisition until the acquisiton stops(different number of data may be required to be recorded), and the next pulse after it stops can trigger a new  one.
2. The engine is running at different mode, so the frequency of the pulses changes.
3. How can I use the pulse 1 or pulse 2 only, as I only need the same pulse each engine cycle, equaling each two shaft encoder cycles?

Thanks a lot

[RER]

Hello there!

This sounds like an interesting and rather complicated scenario. But, I am more than willing to help the best I can. Before we can go any further, I’d like to ask you a few questions...

1. What Hardware are you using? Also, how are you currently reading these encoder generated trigger pulses?

2. Which software are you using? I assume from you post that LabView is on the cards – if so which version?

Finally, could you confirm the following details, to ensure that we understand exactly what you are trying to do.

- Each cycle of the engine will cause the encoder to generate 2 pulses (Pulse1, Pulse2).

- The frequency of the trigger pulse train is going to vary, as is the number of samples required at each acquisition.

- You wish to consistently ignore one of the trigger pulses generated by each engine cycle.

Sorry that this is not a solution post, but it is quite a tricky scenario. The more information we gather now, the more we can help you.

I look forward to hearing from you very soon,

Best wishes

[soundofwind]

Hi, Richard,

thanks a lot.

1. What Hardware are you using? Also, how are you currently reading these encoder generated trigger pulses?

I am using NI PCI-6251 and I am using the external clock.When the shaft encoder turns one revolution, which is 360 degree, 3600 pulses are sent to the card, and 3600 data are recorded.

2. Which software are you using? I assume from you post that LabView is on the cards – if so which version?

Labview 8.0

3. Each cycle of the engine will cause the encoder to generate 2 pulses (Pulse1, Pulse2).

Yes, one engine cycle has two shaft encoder revolution, which is 720 dgree and the shaft encoder gives one trigger signal per revolution.

4.The frequency of the trigger pulse train is going to vary, as is the number of samples required at each acquisition.

Yes, as the engine speed is varying.

5. You wish to consistently ignore one of the trigger pulses generated by each engine cycle.

Yes, I only want the same pulse for each engine cycle.

[RER]

Great, thanks for the speedy reply. That post has given us plenty to work with.

As I am sure you are aware, this is a rather tricky issue. However, myself and my colleague will look into this over the next couple of days.

We will get back to you as soon as we possible... maybe sooner, if we have any more questions 🙂

Thank you for your patience,

[KostasB]

Hi soundofwind,

I have a couple of suggestions. If you want to skip every second trigger pulse, don't skip it but just discard the acquired data. If you want to change the number of samples you acquire per trigger, you can do that

by wiring the correct number to the 'Samples  per Channel'  at the DAQmx Timing VI. If the number of samples you acquire depends on the speed of the engine, you can set a task that will calculate the speed of the

engine, scale it and feed it to the 'Samples per Channel' of the triggered Analog Input task.

I hope that makes sense, please let me know if you need some additional information.

KostasB

NI UK Applications Engineering