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Old 11 Aug 2007, 17:07 (Ref:1986141)   #1
jetto
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The General PI help thread

Hi,

I'm stuck creating some math channels. I've sort of got a wheel slip ratio channel sorted but I also want to have an acceleration channel (not the longitudinal G but one from differentiating the wheel speed traces).

How do I go about making the differential of the speed trace in a Pi Math Channel?

Thanks
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Old 14 Aug 2007, 11:02 (Ref:1988266)   #2
tobias_funke
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In Pi Maths make a new channel with whatever channel name and units you want. Set the frequency rate to the same as the 'Speed' sampling rate. The channel 'code' only needs:

deriviate ([Speed] , ignore)

NB: It's case sensitive. And make sure you get the units right. If speed is in km/h you need to divide it by 3.6 to get it to m/s and by 9.81 to get the resulting trace go G.
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Old 14 Aug 2007, 21:51 (Ref:1989163)   #3
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Tobias,

Thanks for that, its plotted a graph now, whats the reason for the "ignore" part? That's what I was missing.

It's brought up a graphical result showing a wavy line (like water ripples) for the acceleration trace. The speed channel its derived from shows a linearly increasing speed however the acceleration trace has these ripples of fluctuating acceleration (goes from about 8 to 4 back to 8m/s^2 and so on for the main part of the data I'm looking at).

What would be the reason for this as to me it should show a more constant acceleration?

Thanks,
Oli
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Old 14 Aug 2007, 22:38 (Ref:1989192)   #4
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To tell you the truth I dont remember the reason for the ignore, I just always do it. I think it has something to do with the signal type but for a logged trace I have always used ignore.

Try applying a 0.1 or 0.2s moving average filter to it and make sure the sampling rates for the deriviative channel are the same as the input channel.
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Old 14 Aug 2007, 22:44 (Ref:1989198)   #5
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Oh yeah the peaks/troughs will probably be gearshifts?
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Old 15 Aug 2007, 00:00 (Ref:1989232)   #6
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The data I'm looking at is for a 1st gear launch start acceleration so no gear changes.

I applied a 0.2s moving average filter and nothing much changed but then at a filter of around 0.4 or 0.5s the acceleration trace ripples dissapear and it gives a tace more like what I'd expect.

What is the moving average filter and what problems do I get with higher values of filter?

Thanks
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Old 15 Aug 2007, 07:35 (Ref:1989329)   #7
tobias_funke
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Essentially moving average filter is a 'smoother' if the trace and is good for damper speed and accelerations which have a very high frequency rate.

Im pretty sure that a moving average filter of 0.1s applies a linear (i think) average to 0.1 seconds of samples for every data point. So if you sample at 10 Hz with 0.3 MAF it will for a data point it will take the average of the actual values for the point you want, the point before and the point after and so on for the next point. If that makes any sense..

The problem with having too high a value for the MAF is you throw the baby out with the bath water. You can miss a heap of information as, with a big enough filter, you end up averaging samples over a long period of time. I'd imagine if you applied a 100s moving average filter to the speed trace of a 100s lap you'd simply get the average speed for the lap.

Also what sampling rate did you log the speed at? If the sample rate is too low it could throw some important data out (aliasing). For looking at normal speed traces 20 Hz is probably enough but to derive it like you have done, you would probably need at least 50 Hz though 100 Hz would be more.

It sounds silly, but its better to log at 200 Hz and filter a trace than it is to log and view at 50 Hz.
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Old 18 Aug 2007, 17:24 (Ref:1991456)   #8
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Quote:
Originally Posted by tobias_funke
In Pi Maths make a new channel with whatever channel name and units you want. Set the frequency rate to the same as the 'Speed' sampling rate. The channel 'code' only needs:

deriviate ([Speed] , ignore)

NB: It's case sensitive. And make sure you get the units right. If speed is in km/h you need to divide it by 3.6 to get it to m/s and by 9.81 to get the resulting trace go G.
The actual syntax is:

derivative([Speed], Ignore)

The Ignore tells Toolbox what to do in the case of a discontinuity in the data. Not so much of an issue in logged data but there may well be missing data points in live telemetry streams for example. In this case it interpolates the missing points and carries on calculating the derivative. Your other options for the second argument are Reset i.e. stop calculating the current derivative and start again when there's a discontinuity or NoData i.e. return no derivative at all when the data is missing but hold the value and continue calculating it when the data "reappears".

Rather than putting "fudge factors" in the channel to account for units, Toolbox can do all that for you. If you open the advanced maths channel options you can tell Toolbox to automatically convert [Speed] into metres per second when it reads the input channel. If you set the Quantity of the math channel to be Acceleration and Unit to be metres per second^2 then all the units issue is taken care of. Then if you want to see it in G for example you can tell Toolbox to display the output of your math channel as a different unit by setting it in the Channel Properties. It is much better to let Toolbox handle your units "under the hood".
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Old 19 Nov 2010, 02:42 (Ref:2792962)   #9
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*Help with Pi Toolbox Maths channels*

Hi,
im looking for a channel in Pi Toolbox that will determine slip ratio for a given wheel i know slip ratio is: (wheel speed-vehicle speed)/vehicle speed, but how do you create this as a maths channel? i assume you use XKart speed for vehicle speed but wheel speed im not so sure?

My aim is to plot a graph of longitudinal force (fx) against slip ratio for different wheel loads, any ideas would be appreciated.
Also how do you export data from a Pi toolbox graph into excel???
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Old 23 Jan 2011, 17:27 (Ref:2819584)   #10
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I guess you will use wheel speed from an inductive sensor and vehicle speed from a GPS antenna, right?

Go to "tools" => "Pi Math" => "edit..."

In the window, click on the "gree plus" and you have created a new channel, name it and in the window you will type the formula, eg:

[FL Wheelspeed]-[Speed]/[Speed]

Then check the units, date rates and you are ready.

NOTE: if you use the "speeds" I mentioned, the calculation may be wrong because of the way wheel speed is calculated (everchanging diameter of the tyre is involved). Anyway, it worths a try
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Old 27 Jan 2011, 04:17 (Ref:2821163)   #11
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Quote:
Originally Posted by Belatti View Post
I guess you will use wheel speed from an inductive sensor and vehicle speed from a GPS antenna, right?
I have found that GPS speed has a definite lag against the physical speed using an MXL.
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Old 18 Apr 2011, 13:58 (Ref:2865735)   #12
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The General PI help thread

Hi folks,

I’m new to Tenth Tens, and was wondering whether I could pick some brains from some of the experienced techs here.

I'm currently in my Final Year Degree studying BSc Motorsport Tech, and am using Pi toolbox as part of my analysis. Basically What I need to do is calculate the WT of the vehicle.

I know how I can calculate it using Pi but what I need to know is it possible to convert Damper Pots that are read as a voltage, and convert them to damper Loads as I need to know the force acting upon them to get any further. Any help would be greatly appreciated!
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Old 18 Apr 2011, 15:44 (Ref:2865810)   #13
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Sorry I meant how do you convert Damper pots which is a measurement of mm to force?
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Old 18 Apr 2011, 19:18 (Ref:2865989)   #14
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Bump!
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Old 18 Apr 2011, 20:10 (Ref:2866029)   #15
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You need to know the spring rate and the damper position (for each damper) that the spring is just about to start to be compressed by the weight of the car. Possible but tricky and will need constant resetting. That would give you static load at any point, is that what you are after?
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