USB Digital Storage Oscilloscope Capture Set Up Questions.

[Chris Wilson]

I have just got a UK sourced, (I am in England), Dataman 150 MHz digital USB oscilloscope I am trying to use it for some automotive diagnostics. I have been trying to view a single fuel injector waveform, both from a voltage and current perspective. I *THINK* it has finally dawned on me what I am doing wrong, but would like comments please. I am initially trying to capture the current waveform by using a low amperage clamp around one of the injector leads, the signal lead. All six injectors have a permanent 12 volt supply, they are fired by the ECU grounding them for the required pulse time, in firing order. I have my clamp on the number one cylinder injector. I couldn't get a trace this afternoon, only noise. I have been pondering what I am doing wrong and have hooked up my USB scope to an old signal generator I have. The engine is an inline six cylinder Volvo idling at say, for simplicity, 1000 RPM. So that's about 16.6 revs per second. Each injector is fired sequentially, once every four turns of the engine, so *ONE* of the six is fired about 4.2 times a second at idle of 1000 RPM. The number one injector is therefore only fired about once every .7 of a second. That, digitally, is quite slow? I think I have been trying to use too fast a time base and what I thought was a signal was probably just noise. I had been playing with a time base figure in the region of 8KS/sec to 2MS/sec, which I now think was way too fast. When I set my signal generator to output a sine wave at about 4Hz I had to set the USB scope to its slowest time base of 400 samples a second. But then a capture only occurred after waiting a few seconds. Is this to be expected with a digital storage scope? Or am I doing something wrong with the setup? For anyone interested in the spec of this USB scope it's at http://www.dataman.com/oscilloscopes/usb-oscilloscopes/dataman-526-150-mhz-usb-oscilloscope.html

I have a very basic single channel 20 MHz analogue scope, so I tried that connected to the signal generator, and at anything under about 5 Hz the display pattern was broken up and discontinuous. As an aside is that why an analogue scope is not very good for these sorts of measurements? Or is this where adjustable persistence is needed to view an unbroken trace?

Thanks, hope everyone has a healthy and prosperous New Year.

[Fraser]

Chris,

I had an interest in diagnostics on older technology engines (Mini Moke A series) and discovered that Pico Tech had USB DSO kits specifically aimed at car diagnostics. I used Picoscopes and they performed well. I did not have the more modern models though and no sensitive current clamp.

Take a look around their automotive scope site as, from memory, they had a lot of information on carrying out tests and the results that were to be expected.

These pages may help :

A video for you :   http://www.picoauto.com/automotive-oscilloscope-guide/injectors.html


Note the comment about the directional nature of the curent clamp !

http://www.picoauto.com/tutorials/fuel-injection.html

http://www.picoauto.com/automotive-library.html#tutorials  (Choose tutorials)

http://www.picoauto.com/waveforms/

http://www.picoauto.com/automotive-library.html

http://www.picoauto.com/automotive-library.html#case

http://www.picoauto.com/automotive-oscilloscope.html

Many car electronics engineers use DSO's so they should do what you are attempting OK. The principles and set up should map across to your Dataman  product. Please be aware that the Dataman real sampling rate is 200Ms/s which is about 20MHz maximum waveform frequency if aliasing is to be minimised. The 150MHz bandwidth specification can be a little misleading.

The Pico scope manuals may assist in the set ups for the various tests:

http://www.picotech.com/document/pdf/ecu-5.pdf

http://www.picoauto.com/manuals.html  (select the 2 channel kit for its manual. Others are also here so happy reading)

http://www.picotech.com/document/pdf/qsg212kit.defi.pdf

http://www.picotech.com/document/pdf/qsg3000kit.defien-4.pdf

The Pico Automotive Home page is here:

http://www.picoauto.com/

Video Tutorials are here:

http://www.picoauto.com/automotive-diagnostics-videos.html


Hope this helps

[IanB]

Each injector is fired sequentially, once every four turns of the engine

I think that would be every two turns? (All cylinders fire during two complete revolutions of the crankshaft.)

I was recently using my Rigol DSO to look at a signal with a 2 second period. The slow signal rate made it quite difficult to adjust the trigger level to capture the signal effectively. It was convenient to set the scope into rolling display mode to get a feel for the shape and level of the signal before trying to sample it.

[Chris Wilson]

Thanks to Aurora and IanB, some excellent links and glad I am not alone in having this capture issue. IanB, you are of course quite right, sequential fires an injector every two crank revolutions, must have been a legacy of all the pub time I have enjoyed over the holidays Despite the rain I managed to have half an hour connected to my road car this afternoon, and managed to finally capture some decent waveforms from both a current clamp on a single injector, and a voltage trace from a single injector, via an attenuator. It was surprisingly (to me) tricky and slow to capture a single event, but timebase experiments got me there. I then tried the scope's roll mode function, and found this far better for capturing every event. The "manipulation" options of the completed data run left something to be desired though. I now have another question please? What should I expect to see if I connect my little single channel 20 Mhz analogue CRT scope up to an injector via my current clamp, or direct with attenuator for a voltage trace? As I believe an analogue is showing in real time will it show every injector even reliably on screen, with no fiddling to capture an event, but with no means of capturing it? Can any  analogue scope capture a signal? Is this the advantage of analogue my valve loving mate goes on about? Would a "mixed signal" scope allow both types of measurement with a single instrument? Do I really need two or more scopes? Is this why real electronic "geeks" have loads of the damned things sat on their benches, not just for posing, or due to a beery session on Ebay with their guard down, but due to the fact you ideally need different types for different jobs? There seems a lot more to these things than I could have possibly imagined, and as I learn one aspect I find I have a load more questions, sorry!

Cheers.

[alm]

No experience with automotive signals, so I can't help you with that. An analog scope should show every firing if the repetition rate is high enough to show a stable signal (more than a few Hertz), although there is a limit to how fast it can refresh. A digital storage oscilloscope, aka. digital real time oscilloscope (to distinguish it from the older digital sampling oscilloscope), can do the same. On the cheaper models, the refresh rate is much lower however. Usually tens to hundreds of waveforms per second for the low end up to a million waveforms per second for the Agilent DSO-X 3000 series. But even ten waveforms per second should give you a reasonably responsive picture if the signal is stable. Some people like those analog tube scopes because of the sharp trace which gives it more vertical resolution than an 8-bit DSO with 1 bit of quantization noise.

There were analog storage oscilloscopes that could capture a signal and store it on the CRT by electrostatic means, but these are ancient relics, I don't see any reason to use them these days unless you happen to have one around. Another tactic was to mount a camera on the scope and fire it after the signal has triggered the scope. It would take a picture right after the scope drew the signal on the CRT, and the phosphor would glow long enough for the picture to be taken.

A mixed signal oscilloscope combines analog and digital inputs. I don't see any reason to use digital inputs in your case unless the timing is controlled over the CAN bus.

There are good reasons to keep both a digital and an analog scope around, and some scopes are better suited for certain tasks (eg. portability, high bandwidth or very sensitive and low noise), but I would guess that most people keeping many scopes around do this because they like them, not because they need all of them.

[Chris Wilson]

Happy New Year! Thanks for the replies. I have now got another question. In automotive diagnostics we are often measuring what I call very slow waveforms, often in the sub 800 Hz region, sometimes down to 8Hz. Viewing such slow waveforms and seeing any glitches is proving tricky, so a while ago I bought the Roll Mode upgrade for my USB scope. I have been playing with it using an old signal generator I have, giving a square wave signal at  2 to 600 Hz, and capturing various sets of data at different speeds via the roll mode. By means of roll mode it is much easier to see all of the waveform over extended periods. But what effect does running out of internal memory buffer at these low frequencies have on the stored signal? When the red line has crossed the screen I believe this shows the buffer is full on my scope. After that, what is occurring? Is the buffer plain full, and stays full, and the rest of the data is caught in real time on the PC? Or is the buffer emptied very fast and it refills again? I assume the former as no glitch is evident in the traces. What I am getting at is when the buffer is filled, at these low frequencies will roll mode continue to store accurate traces and be reliable for seeing glitches from missed or corrupted injection or spark events, for example? Apologies if the question is naive, but the more I learn unfortunately the more questions I seem to have. Thanks.

[Fraser]

Hi Chris,

I am no expert on DSO's or your particular unit, but I would expect there to be an overwrite or stop option in the software. It would be worth emailing the manufacturer on this matter if it is not documented. Overwrite gives the benefit of enabling you to keep monitoring a situation until a problem occurs and then being able to stop the capture and analyse the detail. A straight capture until the internal buffer is full is OK provided the event you want to see occurs in that time frame....sods law says it won't.

Sorry I can't be of more help but each DSO software is different and options are therefore variable in their presence, implementation and capability. I would expect your roll mode to have some options regarding the buffer though.

[Chris Wilson]

Thanks again Aurora, just noticed you too are in the UK, I have only just realized the International flavour of this forum, I had assumed it was mainly frequented by the Antipodeans

I have contacted Dataman via their forums and asked the questions there, once again, much appreciative of your assistance, cheers.

[Fraser]

Hi Chris,

Blighty is well represented here 

I'm just North West of Milton Keynes....concrete cow country 

[Chris Wilson]

I am in rural north Shropshire, proper cow country, in fact we have 4 highland cattle as pretty, semi automated lawn mowers I am in the race car game, and need to get up to speed on the electronics side of things as I am of an age where learning new technology is not as easy as it once was. I am enjoying it though, and that's half the battle. We also have a wallaby and a pair of kookaburras, which are considered quite exotic here, but probably a bloody nuisance to those from their home country

[Fraser]

My parents in law live near Shrewbury so I visit that part of the world quite often....lovely countryside.

Milton Keynes has quite a lot of new technology companies based in and around it. We have several F1 teams based here and my neighbour works for Renault on F1 projects 

I am envious of your exotic pets. I lived in Australia (Darwin) and know both the Wallaby and Kookaburras well....not pests at all 
We have a wild population of real living (not concrete) white wallaby's around Milton Keynes ! they escaped years ago from a local animal park.

The story is here:

http://www.dailymail.co.uk/news/article-458473/Strewth-Its-albino-wallaby--hopping-round-English-countryside.html

I would settle for some ducks and sheep but the wife says no 

[IanB]

There are colonies of wallabies living all over England. I think they are practically a native species by now. Another exotic species that has made England home is parakeets. In some places they are so numerous they have become a pest.