Duty cycle not being displayed

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D@vid43
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Duty cycle not being displayed

Post by D@vid43 »

Hi to all, i have a issue in that i not getting the duty cycle to show on the screen.
the test i was carrying out was engine fan duty cycle. i had connections correct as i could measure with a DMM.
i have measured duty cycle before successfully, but with the latest software i have not.
Also i tried to measure duty cycle of VVT oil control valves and same result just a distorted pattern.
Q1, have i missed something basic?
Q2, is my scope damaged?
I have tried different leads and different channels (4425) Software Automotive 6.13.14.
Regards David,

martinr
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Re: Duty cycle not being displayed

Post by martinr »

Hello David

You say: “..same result just a distorted pattern.”. Until I read that I was picturing a square wave on the display. Are you nor seeing a square wave? If you’re getting a distorted pattern, I can understand that it’d be difficult for the software to make sense of it to calculate duty cycle. You don’t have access to a signal generator to give you a decent square wave to play with do you?

By the way, you’ve posted in the PicoDiagnostics section. Is the duty cycle you want to measure part of one of the Diagnostic tests or does it relate, as I suspect, to PicoAutomotive software - indeed, you did mention that software version.



Martin

D@vid43
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Re: Duty cycle not being displayed

Post by D@vid43 »

Hi Martin, thank you for your reply, I am not seeing a square wave pattern. the first time i measured duty cycle i was measuring instrument cluster light dimming, I connected the scope and the software automatically set parameters. and the square wave was displayed. The engine fan i used the parameters of the guided test, no pattern, i then tried manually to set parameters and same result.
Unfortunately I do not have access to a signal generator,.
Martin apologies if i have posted this topic in the wrong section/area.
Also i have remembered that the VVT oil control valve and the engine fan wave form patterns were similar in looks.
Regards David,

martinr
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Re: Duty cycle not being displayed

Post by martinr »

Don't apologise, Dave: they won't take your 'scope away for posting here; I was just wanting to make sure I understood what you were doing, and, thinking you might have got more viewings in the other section. No matter at all.

I've installed the 6.13.14 software - so thanks for the prod that there's an update, even though my PicoScope 6 Automotive said I was up to date. I'm now running the same software as you; I've got my square wave generator hooked up and have a decent square wave on the screen. Assuming this is what you did, I went to Measurements (very top toolbar, or, alternatively, if you have it so set up, towards the end of the Trigger toolbar) and added a Measurement on Ch A, Duty Cycle over the whole trace. And, as I adjust the duty cycle on my signal generator I see the measurement change accordingly at the very bottom of the screen. So I can confirm it's working as it should. No glitches in the software.

That confirms what I picked up from your post "just a distorted pattern". Your problem is not the duty cycle measurement but the display of the signal or perhaps I should say the signal itself. I'll send you my email via private message; can you send me the Pico file? ( I'm not sure if Pico files can be uploaded onto the forum.)

In the meantime, you can run a simple test: take say a 9V battery and with a suitable lead (BNC one end and, say croc clips the other), connect one battery terminal with a croc clip and touch and release the other croc clip several times on the other terminal, to create a random square wave. You won't get a meaningful duty cycle of course, unless you could repeat the pattern every time or if you limit the duty-cycle measurement to one cycle, but all you want to see is a square wave. That'll tell you if there's anything wrong with your 'scope, which I doubt.

Or if you can upload screenshots of the “distorted patterns”, perhaps it will prompt ideas and suggestions from others?




Martin

D@vid43
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Re: Duty cycle not being displayed

Post by D@vid43 »

Hi martin, I've sent you the waveform as per instructions,
As for the test i have simulated with the probe when the scope was connected to quickly touch the ground wire on & off to simulate the test you has suggested,
yes i see a square wave. which is confusing/frustrating me as im thinking what am i doing incorrectly now where as the first tome i tried it it showed,
Regards David

martinr
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Re: Duty cycle not being displayed

Post by martinr »

Hello David,
Thanks for the email you sent with the .psdata file. In the email you helpfully mentioned that you were using a guided test. When I read that I had an idea what the problem would be: something we've all done before we learned our lesson - too small a timebase. Here's a screenshot of the file you sent. I've circled both the timebase and Y axis scaling. Your guided test setup had the total time of the waveform as one-thousandth of a second! My guess is that the waveforms you are looking at would switch no faster than ten times a second and possibly much slower
file 1.jpg
And look at the scale of the Voltage axis, plus/minus half a volt! So the waveform is focussed on a tiny bit of time and probably well below the 12V range I imagine your supply is.

Using your .psdata settings but then manually changing the timebase - so I see a total time of 2 seconds and with my signal generator set to 10Hz - I also manually set my Voltage axis to +/-10V. Here's what I get:
square wave 34% duty cycle.jpg
With the setup you had with your guided test, you would have been looking at one hundredth of one of the troughs of one cycle on my waveform.

So, I don't want to insult you by labouring the point, but if you use a guided test - and with practice and confidence you won't need to - give some thought to the timebase and max voltage, and if the setup hasn't given you what you need, manually over-ride it. Always best to set a longer total time and a higher voltage, so that you definitely capture the waveform, and then you can adjust time and Voltage scales accordingly.

By all means use the guided tests but only as a guide. If you figure out beforehand what size, shape, frequency of signal you expect to see, you can then look at what the guided test has set and see if it's reasonable and change the settings if not. And if you shun the guided test, even better, because you'll have to think carefully about setting up the 'scope yourself. To be honest, the only way you'll get to grips with the 'scope is not to use guided tests; you won't, for example, learn about triggers through guided tests.

Martin

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Autonerdz
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Re: Duty cycle not being displayed

Post by Autonerdz »

To be honest, the only way you'll get to grips with the 'scope is not to use guided tests
Totally agree. It will actually impede your progress.

topic15831.html?&p=62441&hilit=automotive+menu#p62441

D@vid43
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Joined: Tue Sep 23, 2014 10:12 pm

Re: Duty cycle not being displayed

Post by D@vid43 »

Martin thanks for the heads up i will definitley give it a try and report back.
the reason i went to the guided test is that the first time i dreamed duty cycle on the Chrysler the scope setup the parameters automatically for me this is where i got confused and went to the guided test,
which i normally dont, none the less i will carry out this test with the info and research you have provided.
i never kick myself more than happy to continue learning. great info guys.

D@vid43
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Posts: 6
Joined: Tue Sep 23, 2014 10:12 pm

Re: Duty cycle not being displayed

Post by D@vid43 »

Hi Martin, just a follow up i could not reproduce duty cycle wave form using manual settings, i only showed you that setting to explain what i was seeing, i am investigating this further as i am suspecting that this fan might not be controlled by PCM i will update when i get more accurate info.
Cheers.

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