Graphing and Filtering

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Graphing and Filtering

Postby Steve Smith » Tue Jan 30, 2018 3:25 pm

I thought I would post this editorial I wrote some time ago as it has some useful hints and tips with regards to graphing and filtering (Especially RPM from our crank signal)

It is often said that there are two skill sets required with oscilloscope diagnosis, one in the capture and one in the analysis. There is an element of truth in both but the more you familiarise yourself with the features of PicoScope the less daunting it becomes.

Our Guided Tests ( https://www.picoauto.com/library/automo ... ided-tests ) will assist you with settings and probe selection, whilst the included technical information provides a description of your captured waveform. As your confidence grows with data capture you soon hunger for the hidden gems that can simplify waveform analysis.

One such gem is “Graphing” whilst the other is “Filtering”, both of which are invaluable when looking at waveforms captured over longer periods. The waveform below is a typical example where the crankshaft signal has been captured in conjunction with a Digital MAF signal from Idle to WOT, returning to idle. Added to these signals we have cylinder 1 injector current and fuel pressure.

WAVEFORM 1.jpg
Waveform 1


First impression of the waveform is not a favourable one given the huge amount of data spread across the entire screen.

So how do we possibly extract useable data from such mayhem? We could zoom and measure different points of our waveform to determine any change in frequency (Channels A & B) which is fine but time consuming.

Alternatively we can use “maths” to graph our data into something more digestible and filtering to remove intrusive noise.
The math channel function of PicoScope opens up infinite possibilities when analysing data assuming we know the formula to apply.
So what is a “math channel” and how do we simplify the data captured above?

A math channel is an accurate, graphical representation of whatever you wish to display. For example: Assume we have captured battery voltage on channel A and starter motor current on channel B, if we create the math channel A x B (Voltage x Current) we generate and display a third waveform graphing the Power of the starter motor in Watts. (Watts = Voltage x Amps)

So, coming back to our original capture above, to create a math channel graphing the frequency of our crankshaft and airflow meter signals we click on Tools>Math Channels>Create to open the Math Channel Wizard.
The initial step is to enter the all-important formula which is used reconstruct the captured data of your chosen channel.

Looking at Channel A the formula includes a filter and is written as LowPass(freq(A);8) once entered, follow the Wizard to select the Colour of your math channel (it helps to choose the identical colour of the channel your wish to graph) next select the Range and how you wish the Units to be displayed (rpm). Finally click Next and Finish to complete the creation of your math channel and close the Wizard.

Now tick the box adjacent to your created math channel in your math channel library and your graph will appear on screen.

Repeat the above process in order to display the data captured on channel B as a frequency graph in kHz (The formula required is freq(B))

Over and above math channels we have “LowPass Filtering” available under the Channel Options button. Selecting a 20 kHz low pass filter for every channel will remove unwanted noise whilst selecting 1 kHz for low frequency signals (Channel D) will improve resolution dramatically

The waveform below demonstrates how graphing and filtering have brought about clarity in comparison to the initial waveform above (Keep in mind they are the same waveform!)

Graphing and Filtering.jpg
After graphing and Filtering


Channels A and B have been hidden from view by right clicking on screen, select Channels and tick the box adjacent to the channel you wish to hide.

As you can see PicoScope has numerous features to assist with the challenges of capture and analysis

Thank you to Pico Chris for the crankshaft math formula enabling the graphing of rpm without the falling spikes attributed to the missing teeth.

I hope this helps, take care......Steve
Steve Smith
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Re: Graphing and Filtering

Postby ashtray1987 » Thu Feb 01, 2018 6:29 am

Hello Steve,

Possible to get the psdata file for this recording?

Appreciate it.

Many thanks.
Ashley
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Re: Graphing and Filtering

Postby ashtray1987 » Thu Feb 01, 2018 8:31 am

Hello Steve,

Thank you for sharing these functions.

Is it possible that you can upload the .psdata file so that I/we can practice this function on the waveforms that your recorded?

Much appreciated if you can.

Thank you.
Ashley
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Re: Graphing and Filtering

Postby ashtray1987 » Thu Feb 01, 2018 8:55 am

Also, what was the math formula for graphing the rpm from the crank signal?

Thank you.
Ashley
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Re: Graphing and Filtering

Postby Steve Smith » Mon Feb 05, 2018 9:38 am

Hello and thank you for the feedback, sorry for the late reply.

The formula for the crank math channel is LowPass(freq(A);8) for a crank signal utilising a pick up of 60 - 2 teeth on channel A

The LowPass feature allows for the removal of those annoying spikes as a result of a momentary drop in the frequency between the missing teeth of the crank pick up.

For more information on the application of LowPass maths could you take a look here:
https://www.picoauto.com/library/training/filtering

The psdata file can be downloaded form the link below:
https://drive.google.com/file/d/1C57qlJ ... sp=sharing

The math channel below can be imported into your math channel library

LowPass Crank Maths.psmaths
LowPass Crank Maths
(333 Bytes) Downloaded 156 times


Save the above psmath file to your desktop, open PicoScope and select Tools > Math Channels >Import

Locate the PSmath file on your desktop and click on Open

The math channel will now appear ready for use in your math channel library

I hope this helps, take care......Steve
Steve Smith
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Re: Graphing and Filtering

Postby ashtray1987 » Tue Feb 06, 2018 6:46 am

Thank you Steve, that works perfectly!

Regards,
Ashley
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