Duty Cycle Measuring with a 6211

I seem to be having a bit of an issue reading a duty cycle from a rotational sensor. The sensor that I am attempting to read from is putting out a 3.5% duty cycle, TTL wave of 0-5, at rest. The sensor can travel to almost 100% duty.
When I connect a function generator (set to 3.5% and 0-5 TTL) to my USB 6211, the vi has no issues and will read the generated wave perfectly. When I connect the sensor to the USB 6211 the vi will read it correctly 80% of the time, but then all of a sudden it will throw out values much higher (i.e. 25% or 60%).
My first instinct was to assume that the sensor had an issue so I hooked it up to an O-scope, but it reads perfectly on the O-scope 100% of the time.
I have tried many of the example vi's online and in the help section of LV. I have included my VI, hopefully someone can see what I am doing incorrectly.
Attachments:
Meas Duty Cycle-Buffered-Finite.vi ‏86 KB

Hi CETGirl,
Looking at your code, it would appear that the issue is coming from the output of the DAQmx Read.vi. Since you said there is never an issue with the function generator, but there is with the sensor, I would like to know how these are different. I would suggest putting an indicator on the output of the DAQmx Read and compare the results of using the function generator and using the sensor.
Also, what sensor you are using?
Robert R. | Applications Engineer | National Instruments

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Duty Cycle measurement

Hello all,
I am attempting to Measure and output duty cycle to a device. I don't have the device in my possession so I have my hardware hooked up to each other. I have NI 9472 output card hooked to a NI 9423 input card. I have found these 2 examples of how to adjust duty cycle and how to read duty cycle. They work great, but the reading of the duty cycle is oppesite of the outputing of the duty cycle. For example if I set my output duty cycle to 60% the reading is 40%. If I set my duty cycle to 30% it reads that it is 70%. I looked thru the block diagram to see if the input was looking for the resting signal to be low or hi. Being a new user it might be right in front of me. Can some one take a peak at them and tell me anything? Thank you.
Attachments:
Modify Duty Cycle while running.vi ‏31 KB
Meas%20Duty%20Cycle%20Cont%20LabVIEW%208.2[1].vi ‏31 KB

Hey greentea2,
You can try taking your waveform data and adding a replica of itself to the end of the data using the Append Waveform VI, so that you have multiple cycles of data to send into the Timing and Transition Measurements Express VI or the Transition Measurements VI for calculating the rise time. I've attached a picture that shows a simple example of this using a sine wave.
If your data does not complete a full cycle that comes back to a similar starting low point, then this example may not work for you or give you the right value. If that is the case, then please respond with a picture of what your waveform looks like, so we can determine how to manipulate it.
Hope this helps,
Attachments:
Append Replica Wave Form Data.png ‏28 KB

Duty cycle measurement using RLP

Hi All,
I 'm using a PCI-6229 in RTX environment and, I would like to measure a duty cycle.
Is there any exemple file?
Your help is welcome
FV1234

Hey,
I would answer this ...
Regards
Richard Keromen
National Instruments France
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High accuracy duty cycle with DAQ-STC

I am trying to measure the duty cycle of a 4 kHz TTL pulse train with high accuracy. I can do this over one cycle but actually need to average over a number of cycles because there is noise. This doesn't seem possible with the stanard VI's? Can anyone help?
Thanks,
Gavin

Gavin,
There is an example on our website that does buffered duty cycle measurement on DAQ-STC devices that should be helpful for you. From www.ni.com/support you can search under Option 3 through Example Code for +duty +cycle +stc to find it or follow this link: E Series Duty Cycle Example
Regards,
Molly K.
National Instruments
Applications Engineering
Molly K.
Web Support & Operations Manager
National Instruments

Measuring PWM duty cycle with Analog Input

Hello folks,
I have a NI 9205 module and was wondering if there was a way to measure the duty cycle of a PWM signal that I am generating with a separate microcontroller? I have found a lot of examples with Digital inputs but not analog.. Is this possible?
Thank you,
Bob

Hi Robert,
I'm not sure what that error is, try searching for it.
If you want to measure a PWM signal you would usually use a counter input. A counter is a dedicated piece of hardware that is used to measure digital signals. Since the NI 9205 is an analogue device and it doesn't have any counters onboard, you will not be able to select counter input.
I would recommend you get the right tools for the job and buy a digital acquisition device with an onboard counter. It will make reading PWM much easier for you. The NI 6008 has a counter onboard and is only £99. It will also offload processing from software to hardware since you don't have to calculate duty cycle.
If you still want to use analogue post back and we can look into why its not working for you. Tips to start you off: Voltage is the correct setting and Look in the manual and make sure you have connected it like you have configured it (differential, RSE, NRSE page 14)
Lewis Gear CLD
Check out my LabVIEW UAV

How can I measure a 0% or 100% duty cycle signal ?

I want to measure the duty cycle of an input signal using a counter of a 6030-E board.
I work with a PXI Rack under Labwindows/CVI.
My program already works properly for duty cycle between 1% and 99%, but it is not able to detect 0% duty cycle (ie : signal to ground), nor 100% duty cycle (ie : signal to power supply).
The problem is I don't understand how the counter works in such a situation (it cannot detect any edge, as long as the signal is continuous 0V or continuous 12V).
Please have a look on the following code :
1- First, the counter is configured
// Reset Compteur
i_Status = GPCTR_Control(iDevice, iCptID, ND_RESET);
//Definition du type de mesure du compteur (mesure temps impulsion)
i_Status = GPCTR_Set_Application (iDevice, iCptID, ND_SIMPLE_PULSE_WIDTH_MSR);
//Definition de la vitesse d'echantillonage (20 Mhz)
i_Status = GPCTR_Change_Parameter(iDevice, iCptID, ND_SOURCE, ND_INTERNAL_20_MHZ);
//Definition du signal a mesurer (defaut)
i_Status = GPCTR_Change_Parameter (iDevice, iCptID, ND_GATE, ND_DEFAULT_PFI_LINE);
// initialisation du compteur
i_Status = GPCTR_Change_Parameter(iDevice, iCptID, ND_INITIAL_COUNT, 0);
// Preparation du compteur avant de l'armer
i_Status = GPCTR_Control (iDevice, iCptID, ND_PREPARE);
2- Then, every 10 ms, the value of the counter is read, so that I can calculate the duty cycle...
i_Status = GPCTR_Watch(iDevice, iCptID, ND_COUNT, ul_val_cpt);
3- ...and the counter is armed again for the next reading
i_Status = GPCTR_Control(iDevice, iCptID, ND_ARM);
When duty cycle changes from x% to 0% or 100%, the counter always keep the value corresponding to x%.
Even if I reset the counter between each measurement, it keeps this value ! I don't understand how it is possible.
How can I know that the counter didn't see any edge, and so that duty cycle is 0% or 100% ?
Thank you in advance for your help !
FraB

Hi,
Just have a look at this link, it can be interesting for you...
http://forums.ni.com/ni/board/message?board.id=40&message.id=2161&requireLogin=False
Regards
DD

NI 5105 - Measure PWM Waveform Duty Cycle

Will the Positive Duty Cycle scalar measurement in the NI-SCOPE software calculate duty cycle correctly on a PWM (pulse-width modulated) waveform? I have a 400Hz signal that is pulse-width modulated with a 40Hz signal that I need to measure the duty cycle for. Since this is not a normal, continuous waveform (like a sine wave or square wave) I don't know if the NI-SCOPE software will correctly calculate the Duty Cycle. Has anyone tried this with success? I am looking at using the NI-5105 because it has 8 channels, but am open to using other hardware and just multiplex my signals to the digitizer.

The positive duty cycle scalar measurement should correctly calculate duty cycle on your pulse. By default, the measurement finds the base and top of your waveform using a histogram of the Y values (there will be peaks in the histogram at the base and top values). Duty cycle is calculated from the 50% level crossings between these two values. You must have at least one complete cycle and only the first cycle is measured.
If that doesn't work for you, there are a plethora of parameters you can change (except for which pulse you measure - it is always the first). For example, you can set the upper, lower, and level crossing values to specific % or voltage levels. See the documentation and examples for details.
If you need to measure the duty cycle of multiple pulses in the same pulse train, there is a LabVIEW function (Pulse Measurements.vi) which allows you to do this. It uses the same algorithms as the internal NI-SCOPE routines.
In either case, the measurement is independent of the hardware. Any NI-SCOPE device will work. If you have any other questions, let us know.
This account is no longer active. Contact ShadesOfGray for current posts and information.

Read duty cycle of a PWM with DAQ

Hi,
I am doing a test system and I have several signals that control motors. They use PWM with a max frequency of 5 kHz.
I would like to know if with a USB DAQ or a PCI DAQ for example PCI-DIO96 could I meassure the duty cycle of these PWM signals. I mean I need to meassure with 10kHz at least.
Which are the read frequency of these systems?
Due to the high number of I/O to test I am also thinking of using Compact-RIO with different I/O modules. What do you think?
Thanks in advance

If your PWM frequency is 5 kHz and you want to measure duty cycle to 1%, then you need timing resolution of 2 microseconds or faster.
The USB-6343 has hardware timed DIO at up to 1MHz, so it might be suitable. There are other devices with hardware timed DIO at rates of 10 MHz or faster as well.
Before you buy, determine how often you need to measure the duty cycle to make sure the data transfer can keep up. USB can sometimes be a bottleneck.
Lynn

How to measure a current signal with NI 6211?

How to measure a current signal with NI 6211?

Hi Clecio,
This is a charge accelerometer. It is different from the IEPE accelerometers that outputs a voltage signal.
You need a charge amplifier. Then according to the charge amplifier output we can select an appropriate module to measure the signal.
It is not possible to measure charge accelerometers with NI 9234 directly.
Best Regards,
Rodrigo Schneiater
Application Engineering
National Instruments Brazil
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Problem to measure 0% and 100% duty cycle of a PWM.

Hi,
I am using counters of USB-6341 to measure duty cycle of PWM just by following the example 'Meas Duty Cycle-Buffered-Finite.vi'. It works almost all the time. However, when the duty cycle of a input pwm is 0 or 100%, the DAQ can't read anything, in another word, it can't measure 0 or 100% duty cycle.
How to modify that?
Thank you very much :-)
Nan

That's the thing about PWMs, you never really know if the PWM is 0% or 100%.
Let's say we have a 50% duty cycle, at 1/2Hz. So low for 1 second, high for one second right? What if we run at 1/20Hz, so low for 10 seconds high for 10 seconds. Or is this a 0% duty cycle for 10 seconds, and a 100% for 10 seconds?
A true PWM read will never be 0% or 100% it will just keep waiting until the trigger is seen, and recalculate the frequency and duty cycle of it. The solution to your problem is for you to define a duration to watch for a change, and if one isn't seen, then you can define 0% or 100% appropriatly.
So I would recommend taking your duty cycle like you normally are, and then at the same time watch the digital signal for however long you want, if the digital line does not change state, then output 0% if it is low and 100% if it is high and ignore what ever duty cycle you read. But if the digital line does change state, then use the read duty cycle as the actual value.
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If 10 out of 10 experts in any field say something is bad, you should probably take their opinion seriously.

Generate and synchronize 6 digital channels with differents phases/frequencies/duty cycles/...

Hello,
For some kind of tests we need to generate, with a 6534 card, 6 signals.
The 6534 has 32 digital E/S and can go up to 20MB/s !
Each channel must be independant so that it can have it's own frequency and duty cycle. But for exemple the second channel can be delayed in reference at first channel. I put an image so that you can understand better what i try to do (3 signals are shown but six are needed).
I see a lot of things on forum that are close to my probleme, but none seems to be as advanced as mine.
http://sine.ni.com/devzone/cda/epd/p/id/4683 -> 6534 doesn't have timers
http://forums.ni.com/ni/board/message?board.id=40&message.id=5167 -> Just a phase probleme
http://forums.ni.com/ni/board/message?board.id=170&message.id=261289 -> Another board
http://forums.ni.com/ni/board/message?board.id=40&message.id=5090 -> Another probleme
I contacted NI support, they send me an exemple on "patterns" but it generates frequencies that are multiple of one with no phase consideration or duty cycle.
Any idea ?
Thanks in advance for your advice and help.
Eric.
Message Edité par Eric31 le 08-10-2007 06:52 AM
Message Edité par Eric31 le 08-10-2007 06:55 AM
Attachments:
Sans titre.PNG ‏8 KB

At best, that's gonna be a pain in the neck with that board. If you need very precise timing for freqs, duty cycles and offsets, it gets worse. If they need to change on-the-fly, it'll be near-impossible.
1. You can only operate 1 hw-timed digital output task at a time. So all 6 signals must be part of that 1 task.
2. Thus, you need to define an output buffer representing the states of those 6 bits throughout your entire generation.
3. There's a bunch of greatest common factor and least common multiple stuff needed to figure out the right output rate and buffer size, assuming you'll want to regenerate repetitively. You first need the gcf of all the transition times to come up with the right output rate since all the transitions need to occur on some integer multiple of the output period.   You'll then need the lcm of the 6 signals' periods, combined with the output rate, to size up a buffer that can be regenerated.
4. Forming the output data array may be quite complex.   Personally, I always do my stuff with raw arrays and I know that interleaving all those signals can be quite tricky, especially if there may be an occasional simultaneous transition.   However, I suppose there's a chance that some of the digital waveform functions *might* make this step a bit simpler. I just haven't played around with that format much.
Editorial opinion: Buy a 6602. You can trigger 6 counters to start with different initial phasing, and will be able to change freq & duty cycle on-the-fly for each one independently.
Editorial opinion #2: For higher channel count apps, I'd also recommend checking out the DIO-64 board put out by Viewpoint Systems, an NI Alliance partner. It gives you much smarter hardware-timed DIO. With the NI board, there can be huge amounts of redundant info in the DO buffer. If you find that your GCF is 10 microsec, but the time between consecutive transitions is 10 millisec, you'll have to stuff 1000 identical values into your output buffer.   With Viewpoint's board, your data acq buffer *only* contains data at the transition times. Timing is controlled by delivering a separate array defining the timestamps that correspond to those transitions.
-Kevin P.

Duty cycle two AO signals with a square wave

Hello,
I am running two devices with sending two AO signals (sine wave) out simultanously in my code. I need to duty cycle my signals. in other words, I want to turn ON and OFF the devices continously.In order to do that, I am using a square wave generator with 50% duty cycle. The devices should be running when the square wave has positive values and should be OFF when it is negative.
The code almost does that, but it does not seem to be as fast as it is supposed to, based on the square wave's frequency. I was wondering if anyone can help me with this problem. In addition, I am not sure if this is the best method to do that, please let me know if there is a better way. I also attached the VI .
Thanks,
Pooya
Solved!
Go to Solution.
Attachments:
2AOsignal_dutycycle.vi ‏47 KB

I am using NI-PCI 6071E with BNC 2120as an interface. I am not sure if the problem is the hardware limits. Attahced please find the subvis which I used in my main VI.
one of the which has a for loop (duty_cycle.vi) works just fine and the other one (duty_cycle(new).vi), which is the one I am using in my VI, does not. Any thoughts?
Attachments:
duty_cycle (New).vi ‏23 KB
duty_cycle.vi ‏25 KB

Generating square wave with variable duty cycle

We like to generate a square wave with variable duty cycle using LabView 6.1 and SCB-68 and this duty cycle comes from some PID controller programmed by LabView by reading a voltage and feeding back this square wave

the story begins with some one called separately excited DC motor
this thing sometimes changes its speed
so we are trying to keep a constant speed
irrespective of the load no matter how it changes
this comes by reading the voltage out from
the tackometer which represents the speed as voltage
then comparing this with a reference set by the labview program
the labview program will send a control signal to some chopper circuit
which is connected to the field of the motor which controls the speed
we are tending to make the labview program as pid controller
that will generate our beatiful square wave with some duty cycle
which will vary to increase or decrease the voltage across the field
we tried "Real-Time PID Control.vi" example and other examples
and we are discussing different solutions for this duty cycle thing
but till now no output
this is our whole story with this DC motor thing

Measuring the changing duty cycle of PWM waveform

I need to measure the changing duty cycle of PWM waveform.
conditions:
1. the period is fixed.
2. but the duty cycle is changing.
goal:
measure the every duty cycle of a period in the waveform.
Can you please give me a help?
Thank you in advance.
the waveform image is attached.

You need to provide more specific information.
What is the period? What is the minimum, maximum, and resolution of the PWM signal? What is the time scale in your image?
How are you measuring or generating the signal you show in the image?
What should the output look like? An array of duty cycles expressed as percentage? Some kind of plot on a graph?
How long will this measurement last?
Lynn

How the duty cycle is calculated by Timing and Measurement.vi

I am using the Timing and Measurement express vi(LabView8.0 is used) to measure the Duty cycle and frequency. I want to know how the same is calculated by the Timing and Measurement express vi. Please reply.

The Timing and Measurement Express VI uses \vi.lib\measure\mascope.llb\Pulse Measurements N chan.vi to do the Duty Cycle and Frequency measurements.
Help says the following about this VI:
Accepts a periodic waveform or an
array of periodic waveforms and returns the period,
pulse duration (pulse width), duty cycle (duty
factor), and pulse center of a selected pulse and period.
Refer to the Pulse and Transition Measurements VI in the labview\examples\measure\maxmpl.llb for an example of
using the Pulse Measurements VI.Maybe NI can explain the algorithm.
Hope that this helps,
Bob Young
Bob Young - Test Engineer - Lapsed Certified LabVIEW Developer
DISTek Integration, Inc. - NI Alliance Member
mailto:[email protected]

Duty Cycle Measuring with a 6211

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