Feature Request - using both ac and dc from single channel?

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Feature Request - using both ac and dc from single channel?

Postby FioranoCars » Sat Jan 30, 2016 8:55 am

Hi
Reading the latest newsletter [link to follow!] ... sparked a thought!

Well it was in fact the article by Steve Smith of Pico, part 3 of his Scope School, so blame him!

Especially important for those of us struggling to "get more using less" ... of the limited number of physical cabled channels ... currently 4!

So take the AC DC coupling, and a single lead attached to the alternator positive ... is the "AC/DC coupling" feature a hardware element in the scope or software one? Would it be possible to provide both sets of Data for the user to display at the same time/use in Maths channels? ie
Code: Select all
DC(A)
- to force the data to be DC of Channel A
Code: Select all
AC(A)
- to force the data to be AC of Channel A


For instance, we currently need to do 2 separate tests for the alternator, but with this simple (OK, might not be so simple :D ) change we could capture everything in one go. The second test does not move the leads, merely switches the use of 1 channel to AC.

For our test we use:
ChA - battery positive and our common ground
ChB - Amp clamp typically in 200amp mode, often At the battery for ease of access on the negative cable, but ideally at the alternator.
ChC - the alternator positive post (or as close to it as possible) so the nut, cable tag being 2nd best, or a junction box in the engine bay if the alternator is buried so deep in the V or under the exhaust manifold that it's impossible to access. Remember the further away you are the more detail will be 'damped' and by the time you get to the battery, you might have lost anything of value (we have lots of cars where alternator and battery are at opposite ends of the car)
ChD - alternator unit body (IE ground)

The DC coupled version of ChC gives us:
- alternator output in amps, we also load the alternator with a carbon pile tester to 50amp connected to the battery, or using some Nichrome wire tested for the purpose/rating desired). Often an alternator will give 8-15 amps but fail completely at 30-50amps, so this is key to checking the health of alternator, but also the volt drops below.
- positive volt drop - using a maths channel to calc (C-A) the drop between alternator and battery, especially at 50amps+ output.
- negative volt drop - ditto - but no maths needed
- voltage output, as we use a trigger we can see battery voltage before starting, and alternator output on a single screen.

The second test merely flips ChC to AC and repeats including loading, but rather than maths we look at the AC ripple, again using load to see if the diodes perform when loaded as well as at idle output.

Access to OBD engine revs would be great, as we also take revs to 2500-3000 rpm during this, and right now can't see this (OK, we could calc from the AC ripple, if we could trust it!)

I'll post a sample of the two tests on the same car, but but allowing access to both the AC and DC elements to be used separately from the same capture would allow all this from just one test.

I hope that makes sense and shows a genuine business case for the feature request! No we're not lazy, despite my remote keyboard :D , it gives more focus to analyse 1 capture and see different element correlate better.

Thanks
Richard Lukins
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Re: Feature Request - using both ac and dc from single chann

Postby PhilS » Tue Mar 22, 2016 5:47 pm

Hi Richard

thanks for the call today and good luck with the latest diagnostic challenge car !

Just to confirm - the AC/DC selection on the scope is a hardware selection on the input that is then passed to a single channel of data capture. As such there isn't a hidden (free) channel that you can use for lots of neat tricks. If there was, our engineers would have found a way of turning a 4-channel device into an 8-channel one, or even running PS and PD together on the same signals.

So, you can be either AC or DC on a channel, but not both together, and we cannot change that in software.

Please do keep asking the questions and generating challenging ideas - something will surely result in another improvement, and we all win with that.

Best regards,
Phil
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Re: Feature Request - using both ac and dc from single chann

Postby PhilS » Wed Mar 23, 2016 10:00 am

Hi Richard

Having closed off some of the possibilities, I did just want to suggest to other forum users that they spend a bit of time looking at the maths channels functions - for some applications they are another 'free' channel.

With the "Filters" functions in maths - low pass, high pass, band pass and band stop we can first create a modified channel (e.g. high pass at 1000Hz), and can then perform further maths on that when creating the equation.

There are some restrictions based on the way that the maths channels work, and I have had some discussions to find out what they are. As that was not as simple as I hoped, we will kick something off internally to clarify the uses and restrictions. The simple functions are easy, but some of the nested functions can generate a waveform that does not look the same as you might initially expect.

I think I will 'ask Steve' when he comes back from assignment.

Best regards
Phil
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Re: Feature Request - using both ac and dc from single chann

Postby Alan » Wed Mar 23, 2016 3:35 pm

Assuming you are on the 20V DC range for measuring battery voltage then rather than AC couple for the ripple have you tried just vertical zoom?

You could right click to add another scope view, then in this view right click again and hide the channels you dont want. With just the battery voltage in the 2nd scope window you can then zoom to see the ripple. If its a bit blocky due to too much zoom then use the channel options to up the resolution to 16 bits.

Another option would be a maths channel along the lines of (A-13) * 10 The -13 is to take away the battery voltage and x10 to zoom.

Both the above have the issue that if the battery voltage is say 14V or 10V the zoomed signal may be off the screen. I have a probably crazy idea to get around this so off to try it.
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Re: Feature Request - using both ac and dc from single chann

Postby FioranoCars » Wed Mar 23, 2016 3:45 pm

12 out of 10 for cunning :D

Will give this a go, side by side and compare results, not got a known defective one on site, but we will try a couple anyway.

Can you create and apply a math channel post capture? If not, can you add to the wish list :idea: :D ... Please

Thanks
Richard
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Re: Feature Request - using both ac and dc from single chann

Postby Alan » Wed Mar 23, 2016 4:00 pm

Yes you can add a maths channel post capture to try this out.

I thought I had a brilliant idea using some clever maths channels but turns out it was not as brilliant as I thought.. Not given up yet though.
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