Making a digital retriggerable gate pulse on demand

Hello,
I am relatively new to using DAQmx, and I was hoping someone here could provide me with a better solution than I have come up with.
The experiment we are running involves a camera, some image processing and then a laser that requires a gate to run. The way our program is currrently setup is that we loop over this process and after each image processing step decide whether or not to gate the laser with a certain number of pulses with a certain frequency and duty cycle. I have attached an example that would functionally do what I am describing. It is worth noting that our laser requires a digital low to gate and not a digital high.
Currently, our camera is triggered using an external trigger, so the while loop hangs on the IMAQ grab until it is triggered. Then we process the image, decide to shoot the laser (running on its 200kHz internal clock). The current example will hang the code until all of the Digital Output is done which may cause us to miss our next camera trigger. It feels like there must be a more intelligent way of doing this. I am even fine routing our own digital output on our DAQ to trigger a pulse train which is also generated by our DAQ, but ideally we would just send our gate pulses as soon as we know whether or not we want to fire our laser.
For extra reference, we are using a PCI2-1433 framegrabber as well as a PCIe-6363 Multifunction DAQ with a BNC 2110 breakout board. Any suggestions would be helpful. I get the feeling that there are a host of different ways that this could be done using this DAQ and the DAQmx software but like I said, I'm really new with these. I'm betting there are things the hardware can do that I have ZERO awareness of.
Thanks for everones time!
Cole
Attachments:
IMAQ_plus_DAQ_retriggerable_digital_gate.vi ‏30 KB
DAQ_example_retriggerable_gate.png ‏111 KB

Hi ColeVV,
I've taken a look at your code. I would recommend using a state machine architecture.
Do you need synchronization among your events?
Sincerely,
Bogdan Buricea
Applications Engineering
National Instruments
Bogdan Buricea
Applications Engineer
National Instruments

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Retriggerable finite pulse train

I have a problem using a retriggerable finite pulse train as in the NI example Retriggerable_Finite_Pulse_Train. I use ACTOUT to gate the first re-triggerable pulse control and the second pulse control generates the continueous pulse train which is gated by the first retriggerable pulse control. The ACTOUT signal is generated by an AI control which senses a crank trigger (Hall Sensor). The re-triggerable pulse train is used to modulate a fuel injector in sync with ignition timing and RPM. If the period of the ACTOUT signal changes due to a change in RPM, the pulse train is recalculated. It works OK with one hitch. Even at constant RPM, after about 15 re-triggerer pulse trains the final pulse of the train does not complete. This leaves the signal high in-between successive re-triggerer pulse trains. This incorrect high signal between re-triggerer pulse trains means that the fuel injector is incorrectly left on between pulse trains, when it should be turned off. This incorrect high signal goes on for about 10 pulse train events and then returns to normal. I use the ActualPeriod of the first re-triggerable pulse to ensure the pulse train ends correctly within window of the first re-triggerable pulse, but it seem to wander. Is there another way to create a different type of re-triggerable pulse train that overcomes this problem. I may have to use a single re-triggerable pulse instead of a re-triggerable pulse train as this work correctly every time. However, multiple pulses creates a finer mist from a fuel injector and is the correct why to modulate a fuel injector. The

CORRECTION TO PREVIOUS POSTING THERE WAS AN ERROR IN HOW I DESCRIBED THE PROBLEM:
I have a problem using a retriggerable finite pulse train as in the NI example Retriggerable_Finite_Pulse_Train. I use ACTOUT to gate the first re-triggerable pulse control and the second pulse control generates the continuous pulse train which is gated by the first retriggerable pulse control. The ACTOUT signal is generated by an AI control which senses a crank trigger (Hall Sensor). The re-triggerable pulse train is used to modulate a fuel injector in sync with ignition timing and RPM. If the period of the ACTOUT signal changes due to a change in RPM, the pulse train is recalculated. It works OK with one hitch. Even at constant RPM, after about 15 re-triggerer pulse trains the final pulse of the train does not complete. This leaves the signal high in-between successive re-triggerer pulse trains. This incorrect high signal between re-triggerer pulse trains means that the fuel injector is incorrectly left on in-between pulse trains. This incorrect high signal goes on for about 10 pulse train events and then returns to normal. This pattern repeats. I use the ActualPeriod of the second control's continuous pulse train to ensure the pulse train ends correctly within window of the first re-triggerable pulse. This work but with time this pulse train seem to shift slightly. Is there another way to create a different type of re-triggerable pulse train that overcomes this problem?

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CORRECTION TO PREVIOUS POSTING THERE WAS AN ERROR IN HOW I DESCRIBED THE PROBLEM:
I have a problem using a retriggerable finite pulse train as in the NI example Retriggerable_Finite_Pulse_Train. I use ACTOUT to gate the first re-triggerable pulse control and the second pulse control generates the continuous pulse train which is gated by the first retriggerable pulse control. The ACTOUT signal is generated by an AI control which senses a crank trigger (Hall Sensor). The re-triggerable pulse train is used to modulate a fuel injector in sync with ignition timing and RPM. If the period of the ACTOUT signal changes due to a change in RPM, the pulse train is recalculated. It works OK with one hitch. Even at constant RPM, after about 15 re-triggerer pulse trains the final pulse of the train does not complete. This leaves the signal high in-between successive re-triggerer pulse trains. This incorrect high signal between re-triggerer pulse trains means that the fuel injector is incorrectly left on in-between pulse trains. This incorrect high signal goes on for about 10 pulse train events and then returns to normal. This pattern repeats. I use the ActualPeriod of the second control's continuous pulse train to ensure the pulse train ends correctly within window of the first re-triggerable pulse. This work but with time this pulse train seem to shift slightly. Is there another way to create a different type of re-triggerable pulse train that overcomes this problem?

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Message Edited by sdkpark on 10-10-2009 12:19 AM
Attachments:
Create Digital Waveform.vi ‏20 KB
Correlated Dig Write With Counter.vi ‏31 KB

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Hi
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Can I measure the frequency of signal 1 by analog means.? so that I can use two counters for pulse generation. (Signal 1 is a TTL signal).
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I have an external pulse train 1 at ca. 8 kHz (frequency not fully stable). With this pulse train, I want to trigger with each pulse an analog waveform. Using X-series boards, this works perfectly.
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Please have a look at the drawing attached
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I don't think I've ever defined both a start trigger and a pause trigger defined for the same task. Good to know it's allowed.
Given what you've already found, the solution is to control the timing of the end of the pause trigger pulse's active state (shown here as high).
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1. Create the pause trigger pulse with a retriggerable single pulse task. Use a minimal "low time" and "initial delay". Set the high (active) time to be approximately (N+1) periods of your AO sample clock.  Technically, N+1 periods is a bit more than necessary, but it's sure to be enough and doesn't require research into deep details of AO timing.
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   Oh dang! That still leaves you susceptible to a partial waveform since you can't sync the software timing to occur during the desired small fraction of the 8 kHz interval with no AO waveform.
Second approach:
1. Similar to #1 above, but set the high (active) time to cover multiple 8 kHz periods and *don't* make the task retriggerable. To get the timing right in hardware, you'll need to generate a pulse that's *approximately* the requested length, but you'll reserve the right to tweak it so the edges fall in the right place.   You'll also define your pulse in terms of the external 8 kHz signal rather than in terms of internal board time.
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    Suppose the desired pause trigger time is 10.3 msec. Nominally, that's 82.4 intervals of the 8 kHz external signal. Well, just round up or down as you see fit and wire this integer # into the "high ticks" input.
2. AO task is configured to retrigger off the external 8 kHz signal and be pause triggered by #1's counter pulse.
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Deterministic updating of retriggerable pulses, part 2.

Re: Question posted by Vern on 6/27/01 on ?Deterministic updating of retriggerable pulses?.
I?m using LABVIEW in a very similar application to Vern, namely fuel injection control of an engine. I've managed to combine the Pulse Width Modulation VI and Retriggerable Pulse Generation VI, such that a pulse is generated on each occurrence of a pulse at the counter?s gate (which comes from an encoder on the engine). However, the VI executes only once, and the gives ?Error ?10609 occurred at Counter Control?, which is called a ?transferInProgError?. The only way to change the pulse width is to restart the VI, which runs once and then gives the same error. I am using a PCI-6071E DAQ board. Why is this happening?

That is precisely what I tried, prior to posting my question here. I still get the "10609 transfer in progress error".
For testing of the VIs, I am using an electric motor to drive my encoder, whose pulses are routed to the gate pin of a counter. The pulses come every 24msec, and I've set my phase 1 to 1msec (delay), and my phase 2 to be 5msec (pulse). I've modified the sub-vi "calculate pulse specs" found in the PWM vi to accept phase 1 and 2 time duration directly, rather than frequency and duty-cycle inputs.
As you can see, the pulse generation (6msec total) should be finished by the time the new gate pulse arrives. Therefore, switching cycles while a pulse is in progress should not be a problem.
The oscilloscope shows a 5msec h
igh pulse, followed by a 19msec low pulse. The low pulse becomes longer if the motor speed is decreased slightly, proving that the operation is gated. The VI runs once, and as long as the motor is driving the encoder, the pulses appear. However, their duration cannot be changed without re-starting the vi.
I'm attaching the relevant VIs for reference.
Thank you for your help Geneva.
Regards,
John
Attachments:
Pulse_Width_Modulator2.vi ‏129 KB
New_Calculate_Pulse_Specs.vi ‏102 KB

Making a digital retriggerable gate pulse on demand

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