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.

Similar Messages

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

1242ag 100% Duty Cycle

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There is only one SSID in the warehouse. The problems started about the same time the local IT guys replaced the PoE hubs with D-Link PoE switches. What I'm trying today is putting one of my Cisco PoE switches in their IDF and plugging three APs into it to see if the problem recurs with those APs. Its the only common link to all APs in the warehouse. I don't like this line of troubleshooting, but its the only thing they have in common and it started about the time they moved to this hardware.

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.
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Dynamic Duty Cycle Change for PWM

Hello savvy LabView people out there!
I am trying to do PWM for servo control using LabView.
I have been able to manually move my servo with this: http://www.ni.com/white-paper/2991/en
My goal is to automate the servo position as a sinuisoidal movement, and to do so,
I need to automate the duty cycle to oscillate in 5%~10% automatically over time.
I have looked into some stuff as:
http://forums.ni.com/t5/Counter-Timer/PCI-6602-PWM-GENERATION-Dynamic-sine-duty-cycle/td-p/1522442
But it was not too helpful, as I was also getting Error -200301.
I want to be able to move the servo position at ~10Hz, if that is important, and the square wave freq. for the
servo signal (PWM) has to be 50Hz.
Any help? Thank you!
Attached is what I am trying to do. The sine motion of the servo is (supposed to be) actually paired up with
an analog output from a BNC-2120 board. The servo control part is separated on the top portion.
Solved!
Go to Solution.
Attachments:
ServoTest3.vi ‏110 KB

Is something like this for the PWM look like what you had in mind?
(see attached)
This works, but I need something that responds faster.
For example, the PWM needs to be at 50Hz, which means the loop delay should be able to
go as low as 20ms
In the picture, it is currently 200ms, and this does not run. I found that with 50Hz, the minimum
delay that the Error 200301 doesn't pop up is with 500ms delay, at which point the
accuracy of PWM duty cycle in each period is compromised.
Is there a way to have the loop delay be as low as ~20ms?
Attachments:
Servo.jpg ‏52 KB

Problem with faux column and 100% min height

hello
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the column i'm trying to do is the left hand side menu
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i just can't get this to work, so i'm obviousley missing
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malc

malcster2 wrote:
> hello
>
> i',m trying to create a faux column on this
> page:
http://www.malcmabe.co.uk/listtest.html.
> the column i'm trying to do is the left hand side menu
column.
>
> i just can't get this to work, so i'm obviousley missing
something.
>
> also, i've set the wrapper as minimum height of 100%.
this isn't working, so
> my hunch is that it's having a knock on effect to the
measurements of the left
> hand column....but i may be wrong
>
> any help would be appreciated.
>
> the css file is
http://www.malcmabe.co.uk/listtest.css
>
> malc
Malcolm
That's not quite how faux columns work - you'd make an image
the width
of your container with the dimensions you need - perhaps a
line only to
separate a left and right column or something a little more
detailesd -
the image's height can be sliced very small as it will repeat
on the y
axis, attached via css to the container. You can then use
margins or
divs floated left and right to keep your content on either
side.
Something like this I made earlier......
http://www.bealeandpyper.co.uk/
and the background image......
http://www.bealeandpyper.co.uk/p7bpassets/images/p7bp_layout.jpg
btw most of us in here use the faster NNTP for accessing the
newsgroups
- OE, Outlook, Thunderbird and/or dedicated mail/newsreader
apps would
do to access them. Try clicking this link.....
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Detect 0% and 100% PWM Duty Cycles on 660X Card

Hi,
We essentially want to be able to detect 0% (all low) and 100% (all high) duty signals as well as > 0% and < 100% duty. However, I noticed that when performing buffered pulse width measurement, if either 0% or 100% duty occurs, the previous detected duty is stored. It seems as if the next edge or level change isn't detected, then the HWSave or SWSave registers store the last successful count. Is there some flag on a status register for the NI PCI-6602 card that would indicate whether PWM is continuing to be detected?
Thanks,
Jeff S.

Hi Jeff-
You might be able to achieve this operation by setting the Counting_Mode to Synchonous_Source mode. Check out the descriptions for bits 2:0 of the Gi_Counting_Mode register, as described in the 660x RLP manual page 3-22. In that mode, you also must set the Gi_Alternate_Sync bit as described on page 3-21.
Without making the change to synchronous counting, you might also be able to glean some useful information about when an error condition has been met by periodically monitoring the Gi_Stale_Data bits, as described on page 3-13.
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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.

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.
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Attachments:
Append Replica Wave Form Data.png ‏28 KB

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)
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Check out my LabVIEW UAV

Is it possible to measure duty cycle of 6 pwn simultaneously using pci6624 board?

Hi
in my application i need to measure duty cycle of 6 pwm signal simultaneously .
Is it possible to measure duty cycle of 6 pwn simultaneously using pci6624 board?

Here's what I mean...
However, it only sends one command at a time. You didn't say if your servo controller requires all 4 commands at once. If so, then you can remove the case structures and concatenate all 4 slider values into one string.
Message Edited by LabViewGuruWannabe on 11-15-2007 01:05 PM
Attachments:
SliderEvents-FP.PNG ‏31 KB
SliderEvents.vi ‏36 KB
SliderEvents-BD.PNG ‏63 KB

Bad Duty Cycle readings

Hello!
I’m building an application in which I want to measure a duty cycle. I can control the “Duty Cycle In” and the FCM analyzes the info and sends a signal to the Motor. I want to measure the “Duty Cycle Out” of the motor. (I attached diagram) For this measurement I just connected a wire to the negative side of the motor. To calculate this Duty Cycle I am measuring the period and the pulse width. The program works very good when I have a “Duty Cycle Out” from 1% to 99%, but whenever is 0 or 100% it shows wrong info. I would expect that the pulse width at 0% or 100% is cero, but LabVIEW shows a value (1,1E-5 s) and calculates a Duty Cycle of about 23% which is totally wrong. Also, if I run the program for the first time and I measure this pulse width with a “Duty Cycle In” of 0%, pulse width is cero., but whenever I change the “Duty Cycle In” to 10% or higher, and then I go back to a “Duty Cycle In” of 0% it shows a value, and calculates a bad Duty Cycle. I don’t know why this only happens when I put higher “Duty Cycles In”, and then I go back to 0%. I measured the signal with an oscilloscope and there is nothing, not even noise! I don’t know how the card gets these lectures. Do you have any suggestions of what can be wrong?
Appreciated!
Solved!
Go to Solution.
Attachments:
Diagram.JPG ‏33 KB

Hi Elmar
Thanks for the reply. I attached my Code. I am measuring the period with ctr0 and pulse width with ctr1. I divide Pulse Width / Period to obtain the Duty Cycle. To detect 0% and 100% I compare the value of pulse width to zero. I created an additional Digital Input, with this channel I check if the signal is high or low. So if Pulse Width is zero, I read from the Digital Input the level of the signal, and then I know whether is 100% or 0%. My problem is when I have a Duty Cycle of 0 or 100%, the DAQ keeps sending a value for the pulse width, and I cannot apply the method explained before. I think this value should be zero, I don’t know if I am mistaken.
Attachments:
PWM_Measurement.vi ‏61 KB
PWM_Measurement.JPG ‏151 KB

How to convert a number to duty cycle (FPGA)

Hi
I want to convert a numerical number to a duty cycle value in FPGA module so I could use it to generate a PWM signal for the numerical value.
I tried dividing it but it shows an error in FPGA as it would produce a float value. I tried even the example of scaled division but the quotient is rounded of to 0 (I wanted to convert an output by multiplying 10/36).
In my project the 100% duty cycle is 360 degrees and 0% is 0 degrees of the motor shaft rotation.
Please help me I need to use it for my project.
Thank you
Dinesh

I am not exactly sure how your duty cycle is setup, but here is a way we have used a number to create a PWM in our FPGA.
If you have 360 deg. then I would guess you want to create a PWM duty cycle that is some fraction of 360, say "number"/360. You could use a for loop where 360 is wired to the "N" and then your "number" can be compared to the current loop count. That comparison would be true/false which could triger a DIO to be high or low. Therefor, if (loop count) < "number" then DIO is HIGH, if (loop count) >= "number then DIO is LOW. This would send out a PWM that has a duty cycle of "number"/360, (multiply by 100 to get in %)
I hope this helps.
--Alex--

Frquency & Duty cycle Value changes- every time

Hello all
im using pcie 6320 to measure the frequency and the duty cycle of a signal.i have attached my vi & the data measured for reference.The values highlighted in red ,is my actual frequency and duty cycle but values changes for every 1 sec.Please correct me if i have any mistake.
Solved!
Go to Solution.
Attachments:
measure frquency-duty.vi ‏54 KB
HZ-DUTY.JPG ‏37 KB

You'll need to enable digital filtering--insert the following property node into your code (call the property node before starting the task):
Try a "Min Pulse Width" of a few us and increase it if you still are picking up incorrect readings.
The errant numbers are coming from false edges being picked up during transitions, enabling digital filtering will cause the DAQ device to ignore any transitions with a pulse width of less than the set value. Your 475 Hz 72% duty cycle signal should have a high time of about 1.52 ms and a low time of about .59 ms):
656 Hz = ~1/(high time)
1718 Hz = ~1/(low time)
MHz values = multiple edges picked up during a single transition.
Best Regards,
John Passiak

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
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irrespective of the load no matter how it changes
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the labview program will send a control signal to some chopper circuit
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Problem to measure 0% and 100% duty cycle of a PWM.

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