Measuring propagation delay of a signal between two measurement points!

Hello,
I am measuring propagation of an acceleration signal (ACC) between 2 measuring points (sensors).With 51.2 [kS/s] sampling rate the resolution is 19.5 [us]. I can count delay's samples "manually", but can this be automated with sufficient certainty?
Thanks in advance,

golubovski,
with that few information bits you provided Gerd,johnsold and craig already provided a lot of information
Here are some more hints:
Of course you can filter the signals! If you apply the same (linear) filter on both signals the phase delay will be the same, so the delay you will measure between the two signals.
And there are filter with zero phase shift (FIR) or you can apply a filter with phase shift , inverse the data array and apply the same filter again.. -> no phase shift
You can increase the resolution to subsample by fitting a peak on the autocorrelation or
Do an phase delay measurement in the frequency domain... I just did that to measure the dispersion of sound in a titan rod ...
BUT all of that involves a basic understanding of signal theory or to stay with your example: All parts for your (software) Ferrari are there and some hints on how to put it together are given. However without understanding what you do you will crash.
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
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Best way of sending signals between two VIs

Hello all,
I have two VIs. One that generate some complicated signals (sine wave, impulse, etc.). The other one takes manually inputted voltage and pressure send them out to some hardware with a pump, and reads back the output of the pump.
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Thanks.
Solved!
Go to Solution.

nitad54448 wrote:
hi
For exactly this kind of process, I am using queues, FG and (relax ppl, I know..) global variables. If you are writting a global in one Vi and reading it in another (to prevent accidental (fake)updating); globals are easy to implement. Many people hate them (I do, sometimes) but for these kind of cases I think they are well adapted.
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If you need to be sure that the info is passed then Queues are to be used...
N
While you may not run into race conditions using global variables in a write once/read many approach they do not protect you from some part of your application deciding to write to them. FG at least provide some level of protecting access while globals do not. In addition, global variables can create multiple copies of your data. If your data is large, this can be problematic. I much prefered using a defined messaging scheme. The intent is very clear and you can provide safe guards. Even FGs are better. I personally would never recommend the use of global variables even though some cases MAY work out. A defined API is always a prefered approach and helps to keep the code modular and decoupled. This leads to greater reuse. Global variables tie things together and make reuse more difficult.
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Roaming between two WiFi access points fails

Hi...
I just bought a Hawking WiFi range extender...a device that acts like a second wireless access point for rooms that are far from your wireless router. It's also known as a repeater. It has the same SSID (network name) as the one set up by the router.
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Losing signal between WDS stations when Time Capsule backing up

I have the most odd problem and I can't figure it out.
Summary:
I have a WDS network set up using three Apple Airport routers. They work perfectly and internet signal and signal between routers is fine. However, as soon as Time Capsule begins to backup, the wifi signal between two other routers (also within this WDS network) slows to a crawl and essentially disabling me from getting to the internet. This ONLY happens when Time Capsule's internal hard drive is backing up with Time Machine.
*Device Details*
I have three Apple Airport routers:
a) Airport Extreme (the older space ship model that only runs on G or B)
b) Airport Express (the older model that runs only on G or B)
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Problem
This WDS network setup works and I do not have problems accessing the internet until my laptop (MacBook Air) activates Time Machines and backsup on the Time Capsule. As soon as that happens, the speed rate between the Extreme and Express drop dramatically. That's right! The speed rate between the Extreme and Express drops dramatically to a rate of 1Mbps (according to Airport Utility) rendering it impossible to access the internet using this network. Meanwhile, the Time Machine backup is going well at a very fast speed. The signal rate between Express and Time Capsule is normal.
Here is a schematic diagram of the connection that has problem (marked with XX) when the Time Capsule hard drive is running:
*dsl <==> Extreme (main) <XX> Express (relay) <-> Time Capsule (remote) <-> laptop*
I do not know why the speed rate between Extreme and Express would drop. Why should the hard drive of the Time Capsule be affecting the signal rate two routers down? It is not even directly connected to the connection between Extreme and Express.
Anyone have any idea what is going on? Is this an indication that my Time Capsule is dying? Or, that somehow the hard drive when working creates serious interference such that router signal is affected? If so, it is still odd that the router signal between Time Capsule and Express is not affected.
_*Actions Taken*_
I have spent many hours trying to solve this problem. Actions I have tried:
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• decrease signal strength of Extreme to 50%. I was thinking maybe the signal of the Extreme was somehow getting messed up with the signal of the Time Capsule and thus affecting the connection between Extreme and Express.
• set network to run as b/g compatible mode, and g only mode. Makes no difference. Connection between Extreme and Express goes down when Time Capsule hard drive is backing up
• change the name of the WDS network to something different. Makes no difference
• turn off MacBook Air wifi and directly connect it to Time Capsule by USB ethernet. When back up happens, there is apparently no interference and I can continue to use the internet. So somehow, backing using wifi from MacBook Air to Time Capsule is creating interference between the other two routers. Makes no sense, but it is what I observe.
• oh yeah, I've also hit the Time Capsule a few times and banged the Express a couple time in hopes that a little bit of thuggish threat would straighten up these two imbeciles. No go. They won't listen
Okay folks! IF you have gotten through this far in my message, you deserve a gold star and a membership in the Last Supper Club. Please help if you have any ideas what I should try besides throwing the Express and Extreme and replacing them with something that runs on N.
=======================
P.S. I should say that I've used this setup for over a year and never a problem until well, yesterday.
Message was edited by: Conal Ho

Here is another oddity, when my other laptop, a MacBook Pro 15" (first generation) activate Time Machine and backs up to the same Time Capsule, it DOES NOT lead to the degradation of wifi signal! I can still access the internet without problem.
Now I'm wondering if this is something about my MacBook Air.
Note that both machines are running 10.6.2. My 15" MBP is a first generation and so the highest wifi speed it does is G speed.

How do I measure the period of one TTL signal and the delay until a second TTL signal?

Hi,
I have a PCI 6024E board, the SCB-68 terminal box and am running LabView 6.1. If needed I could install 7.1.
I have two Honeywell sensors (HOA7720) which each consist of an infrared transmitter and a infrared reciever. When the beam is broken they output 5V, when the beam is open it outputs 0V. These are sensing the presence of a hole in two discs passing thru the sensors.
So as disc 1 rotates the signal is 5V until the hole passes thru sensor 1, then it drops to 0V until the hole passes. Then it jumps back to 5 V.
Disc 2 is rotating at exactly the same RPM as disc 1 because they are coupled with a spring loaded coupling. There is a delay between the first hole passing thru sensor 1 and the second hole passing thru sensor 2. The delay increases with torque.
Using the oscilliscope function and connecting Signal1 and Signal2 to AIn0 and AIn1, I have verified that the TTL signals are there. So electrically everything is working. I just need to know how to measure the period and delay. I'm familiar with A/D, but not with counters/gates etc and I think that's what is required here.
The first measurement needed is RPM.   Somehow I need to measure the period of the TTL signal.
The second measurement needed is the delay between the 1st falling edge and the 2nd falling edge.
I don't think it's possible to use the millisecond timer for anything, because the accuracy wouldn't be good enough. At 3600RPM the disc will spin 60 revs per second, or one every 17ms.  A resolution of 17 samples per revolution is not good enough.
Help please!

It worked!
I ended up figuring it out today. It's really simple. I went analog in to AIn14 and AIn15 which were free. Then the program does a multichannel scan of the signals (collects array of 2 waveforms), Signal 1 and Signal 2 at some scan rate. I used 100,000 S/s for 10,000 samples. Then I split the array into two separate waveforms. Then I did an edge detect on each one and it returns the position in the array where the falling edge occurs. The difference is the delay (after correcting for the scan rate), unless Sig2 comes first in which case the delay is the period - (or +) the difference
To find the period I used a subset of the Signal 1 waveform from the first falling edge on to the end of the waveform, then did the same edge detect except set the reset setting to true instead of false so it wouldn't detect the low signal which was now at the position 0 in the waveform. So it would ignore the initial low value, wait for it to go high, then detect the falling edge position.
Anyway, it works awesome, seems very accurate and gives very stable readings. 3600rpm no problem. It detects changes of one or two degrees in angular position, with a range from ~145 degrees to ~280 degrees

Measure separation between two signals that may be coincident

Hi
I need to determine the time period between two pulse signals. I'm going to assume for now that I can get these pulses as TTL.
I was initially drawn to the 'Two-Signal Edge-Separation' method using two counter inputs. However, it's quite possible that the signals would be coincident some of the time. Could anyone please answer the following questions:
If my signals were coincident, would the 'two-signal edge-separation' measurement be armed on the edge of signal 1 (= same time as edge of signal 2) and continue counting until the next edge of signal 2 as shown below? Or is it re-armed on the next edge of signal 1?
Is there any other method I could use to log the time between my two pulses? Either as simultaneous counter outputs or something completely different (non-counter method maybe)?
(Probably not relevant at this stage, but I will be using Measurement Studio to implement this - hardware as yet unselected).
Thanks in advnace for any help.
CAS
Solved!
Go to Solution.

Hi CAS,
The count is not re-armed on the next edge of signal 1 using the two-edge separation mode. Once the counter is armed the first time, it will continue counting until signal 2 is detected.
Alternate Approach 1:
If you want the count to reset on every edge of signal 1, you could configure an edge count task (counting the internal timebase) using signal 1 as the count reset terminal and signal 2 as the sample clock. You still would have uncertainty of which signal is detected first if they occur at precisely the same time, so your measured result might be close to 0, or it might be close to 1 full period of the signal in the case that signal 1 and signal 2 are identical.
If you wanted to remove this uncertainty, you can actually delay signal 2 by enabling the PFI filter for the signal 2 terminal. The original intent of this feature was to be able to add a debounce filter to avoid picking up multiple edges on transitions, but a result of the implementation is that the signal is delayed by some amount of time (between the pulse width guaranteed to pass and the pulse width guaranteed to not pass). The best case scenario would be X Series using the 100 MHz timebase, you would add 10 ns of jitter but you could delay the signal by an arbitrary amount. So, you can add the delay and account for it in your reported values, but you would run into problems if the delay caused signal 2 to occur after the 2nd edge of signal 1 (i.e. if the signals were already close to 1 full period apart). You'd have to have an idea of the maximum frequency of the signals as well as the maximum delay between them to determine if this would work or not.
Alternate Approach 2:
You could use two counters configured as edge count tasks. Count the fastest internal timebase. Sample the first counter off of signal 1, and the second counter off of signal 2. If you arm the counters together and ensure that signal 1 and signal 2 start at the same time, then you can simply subtract your buffered samples of counter 1 from your buffered samples of counter 2 to get an array of differences.
There are a number of other ways you could get similar results, but I think the above 2 suggestions are probably the easiest to implement. Alternate Approach 1 has the advantage of still only requiring 1 counter and you don't have to worry as much with arming the counters and starting the sampling together (which could be a problem with Alternate Approach 2 if signal 1 and signal 2 are free-running).
I would recommend X Series DAQ for this task for the following reasons:
The count reset feature mentioned in Alternate Approach 1 is only currently available on X Series and 2nd Generation cDAQ. It will hopefully be added in the somewhat near future to M Series with a driver update but I can't make any guarantees.
The 100 MHz timebase on X Series gives a 10 ns resolution to your measurement. M Series and cDAQ use an 80 MHz timebase (12.5 ns resolution), and E Series uses a 20 MHz timebase (50 ns resolution).
X Series have the most flexible digital filters on their PFI lines and the PFI filters introduce the lowest jitter (compared to M Series and 660x that is--E Series devices do not have digital filters at all).
You didn't mention what frequency you would be using, but X Series have on-board FIFOs which will help you avoid errors from samples being overwritten if your external frequency is relatively fast.
X Series are priced similarly to their M Series equivalents. All X Series have the same counter features with the lowest cost X Series being the PCIe-6320.
Best Regards,
John Passiak

Can counter measure pulse width difference between two signals

I am interested in determining the high pulse width time that two signals have when they are in various phases between eachother.

Why don't you try one of the pulse width examples. I don't know what ADE you are using, but you can search for examples (http://www.ni.com/devzone/dev_exchange/ex_search.htm) with the key words "+pulse +width" and find several examples in different languages. This will measure the pulse width of one signal, but you can program most of the counters to do multiple tasks at the same time. Just copy the code twice and change the counter number to program counter 2 as well. If you want the ORed pulse width time, I would recommend adding a OR gate to your circuit and then feeding that into a counter.
I hope this helps.
Joshua

The measurement of the phase between two sine signals with NI 6251

Hi, I am working with NI 6251. I need to measure the phase between two sine signals. I used "Tone Measurements" on each signal and then substracted there phases, but the result was incorrected. Please, help me to solve this problem.

You should have a look at the Signal Processing VIs
Ton
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How to measure the phase of a sinusoidal signal?

I need to measure the frequency and the phase of a sinusoidal signal. I tried to use the Advanced Single Tone Measurements.vi but the phase measured in each iteration (each second) keeps changing (the measured frequency is not in integral Hz, so the first point of the next iteration is at a location different from the location of the first point of the current iteration...I guess this is the reason). How can really measure the phase of a signal (not always changing with time)? When I change the phase of the input sinusoidal signal, the measured phase should be changed though.
I'm using LabVIEW 7.1 and PCI-6110.
Thank you very much!
Marlon

Marlon,
Without DAQ hardware I cannot run your VI. DAQmx is not supported on my platform (Mac OS X), so I cannot examine your VI in detail.
1. The AI VI will wait until it has the amount of data specified. So if you are collecting one second's worth of data at a time, it will wait one second before completing. The 50 ms Wait will run in parallel, so it has no effect on the timing in this case.
2. Continuous AO is possible, depending on the hardware you have. However, I have no experience with implementing it. Since your frequency is such that you do not end the AO data segment at the end of a signal cycle, you need to be careful to avoid discontinuities in the signal sent to the AO.
3. There is no data dependency between the AI and the AO. It is possible that the AO could run after the AI in any given iteration of the loop.
4. Your phase reference should be the excitation signal. The best method of evaluating the response of the beam would be to use two sensors, one at the shaker and one at the tip. Then measure the phase shift of the signal at the tip with respect to the shaker signal. If two sensors cannot be used, either measure the voltage sent to the shaker or use the simulated signal that you send to the AO as the reference. In either case you would need to compensate for the response of the shaker.
5. Consider the phase shift in the filter. The steady state phase shift is about 14 degrees. The initial transient lasts about 5 cycles of the input waveform.
6. Your simulation sampling rate is 1000 samples/second. While this satisfies the Nyquist criterion for a 379 Hz signal, it does not give you much data to work with for the phase information. If the hardware will handle it, I would go to 10000 samples/second.
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Better estimation of phase difference between two signals with variable frequency!

Hello LabView Gurus,
Being a power engineer and having just a little knowledge of signal processing and labview, I have been pulling my hair out for the last couple of days to get a better estimation of phase difference between two signals.
We have two analog voltage signals; 1. sine wave (50Hz ± 1Hz) and 2. a square wave with exactly half of sine wave frequency at any time.
At the starting point of operation (and simulation/acquisition) both signals will have no phase difference. However, the square wave's frequency changes unpredictably for a just a few millisecond but then it gets synchronized with sine wave's frequency again. This means that the square wave will be phased out from its original position. The task of the labview is to find the phase difference between the two signals continuously.
My approach to determine the phase difference is to measure the time when sine wave crosses zero amplitude and the time when the very next square wave changes amplitude from zero volts to +ve voltage (I have a 0.5volts threshold just to avoid any dramas from small line noise). The difference between these times is then divided by the time period and multiplied by 360 to get this phase difference in angles.
As this part is just a small block of a big project, I can only allow 5000Hz sampling rate each for both signals. I read 500 samples (which means I read data from 5 cycles of sine wave and 2.5 cycles of square wave).
Now the problem is, as long as the frequency of sine wave stays constant at exactly 50Hz, I get a good estimation of the phase difference but when the frequency changes even a little (and it will happen in the real scenario i.e 50Hz ± 1Hz and the square wave's frequency is dependent of sine wave's frequency), the estimation error increases.
I have attached my labview program. From front panel, you can set the phase of square wave (between -180 and 0) and you should see the labview's calculated phase in the indicator box named 'Phase'. Then you can press 'Real Frequency' switch that would cause the frequency to change like it would in real operation.
You can observe that the estimation error increases after you push the button.
All I need to do is to reduce this estimation error and make it as close to the actual phase difference as possible. Any help would be greatly appreciated.
I am using LabView 2009 for this task.
The application is for electric machines and the stability/performance of machines under different faults.
Thank you for reading this far!
Regards,
Awais
Attachments:
v603.png ‏320 KB
v603.vi ‏186 KB

Jeff Bohrer wrote:
Basic math gives me a bit of pause on this approach. You are sampling at 50 times the frequency of interest so you get 50 samples per cycle. your phase resolution is 1/50th cycle or 7.2 degrees +/- noise. You will need to samlpe faster to reduce phase resolution or average multiple readings (at a time cost that is signifigant)
Jeff- (Hardly Working)
I am sampling at 100 times the sine wave's frequency and 200 times the square wave's frequency. Increasing the sampling rate completely solves my problem. But since I am acquiring several other inputs, I cannot afford a sampling rate higher than 5kHz.
F. Schubert wrote:
I'm not a signal processing expert, but here my basic understanding.
If you simulate sampling with 5kHz and a frequency of 50 Hz (and both are 'sync' by design), you always get an exact 5 periods. Any variation of your signals frequency gives you a propability to get 4 or 6 'trigger' events. That's an up or down of 20%!
The one measure to reduce such problems is using 'window functions'. They don't fit your current approach (counting instead of a DSP algorithm), so this needs to be reworked as well.
My approach would be to use the concept of a Locki-In amplifier. You need to phaseshift your ref-signal by 90°. Then multiply your measurement signal with the ref signal and the phase shifted ref signal. The obtained values for x/y coordinates of a complex number. Calculate the theta of the complex number (with the LV prim). Feed this in a low pass filter.
The trick on this is, that the square wave has harmonics in it, in this you are interested in the second harmonic which is the sine wave.
To get rid of the effect that the sync between sampling rate and ref signal frequency gives an error, you then can use the window I mentioned above (place it before the lock-in).
For a design that really plays well, use a producer-consumer design pattern to get the calculations done in parallel with the DAQ.
I suggest you to check on wikipedia for some of the keywords I mentioned. Go also for the external links which lead to great tutorials and AppNotes on the signal processing basics.
Sorry, it's not a simple solution I offer and we will have quite some conversation on this forum if you follow this path. Maybe someone else knows a simpler way.
Felix
www.aescusoft.de
My latest community nugget on producer/consumer design
My current blog: A journey through uml
An interesting view. the sine wave can indeed be looked as a second harmonic of the square wave. I will implement your idea and get back to you as soon as I get some results. But since I have very limited knowledge of signal processing, it might take me a while to get my hear around the solution you mentioned.

How can i compute the time elapsed between two pulses or the period of an encoder signal if I dont have a constant rpm?

I need to compute the rpm of an engine using information from an optical encoder attached to it. the engine is running in transient mode so the encoder pulse doesnt have a constant frequency. I need to get the period betwwen two rising or falling edges using DAQmx.
Any ideas?

Using an M Series or TIO device you can measure the period between two edges (on two different signals) using the two edge separation measurement. I'm assuming that you don't want to use position measurement because the frequency is varying so much? If not, you may be able to set the sample clock period short enough that you get a decent average RPM measurement.
gus....

I still acn't get my picture box to flicker between two grey levels WHILE the DAQ takes a measurement.

I received a suggestion that I contain the two parts (grey level ficker and signal capture) in seperate while loops and use a local variable to pass a control to the measurement part from the flicker, with the control triggering the true statement of a case structure inside of the measurement while loop. In each of the cases (both vi's are listed below) the picture box stops swithing between the two grey levels while the measurement is being taken by the DAQ. In my application, I have a photometer taking continuous measurements and sending a proportional voltage signal to the DAQ input. If the picture box (which is what the photometer is measuring the br
ightness of) does not switch back and forth between the grey levels while the DAQ is sampling the incoming signal from the photometer, then the photometer is putting out no signal. No grey level switching, no signal. So the DAQ ends up measuring a flat line. Does anyone have any ideas as to how I can fix this, please try nd run my vi's before you make suggestions, just to see what is happening. Also, if you have a solution, give it to me in remedial form, if you can. I am new to labview.
Thanks for all your help.
Attachments:
Acq_N_Scans_to_File_(wdt)_with_flicker_2while.vi ‏127 KB
Acq_N_Scans_to_File_(wdt)_with_flicker.vi ‏127 KB

I apologize, one of the vi's that I sent was incorrect. I did, more or less, the same as the example you gave me. Like the example you gave me, it stops switching between grey levels momentarily while the DAQ captures the signal. Is it possible to have the two run simultaneously? I will attch the vi that I intended to send, and would appreciate any further ideas you may have on how to fix the problem.
Attachments:
Acq_N_Scans_to_File_(wdt)_with_flicker_2while.vi ‏135 KB

How to find the phase difference between two signals using Hilbert transform

hi,
I am new to LabView.... I am trying to find phase difference between two signals. I sucessfuly found out the phase difference between two predefined waves using single tone measurement. .... But I really want to know how can I measure phase difference between two signals( not predefined... ie we don't know the initial conditions) using hilbert transform or any transformation techniques (without using zero cross detection).. I tried by using hilbert transform based on algorithm... bt I am getting error.... plz help me
Attachments:
phase_differece.vi ‏66 KB

you could try something similar to this, for each table pair that you want to compare:
SELECT 'TABLE_A has these columns that are not in TABLE_B', DIFF.*
FROM (
        SELECT COLUMN_NAME, DATA_TYPE, DATA_LENGTH
          FROM all_tab_columns
         WHERE table_name = 'TABLE_A'
         MINUS
        SELECT COLUMN_NAME, DATA_TYPE, DATA_LENGTH
          FROM all_tab_columns
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      ) DIFF
UNION
SELECT 'TABLE_B has these columns that are not in TABLE_A', DIFF.*
FROM (
        SELECT COLUMN_NAME, DATA_TYPE, DATA_LENGTH
          FROM all_tab_columns
         WHERE table_name = 'TABLE_B'
         MINUS
        SELECT COLUMN_NAME, DATA_TYPE, DATA_LENGTH
          FROM all_tab_columns
         WHERE table_name = 'TABLE_A'
      ) DIFF;that's assuming, column_name, data_type and data_length are all you want to compare on.

Measuring propagatio​n delay of a signal between two measuremen​t points!

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Measuring propagation delay of a signal between two measurement points!