Procedure for calibrating sensor delays ?

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ereynold60
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Procedure for calibrating sensor delays ?

Post by ereynold60 »

Does anyone have a good procedure for calibrating sensors delays amongst acceleration, GPS and OBD? If this has been addressed, please point me to the post on this?

Prior to the inclusion of "sensor fusion" logic in v22, I had adjusted my OBD sensor delay to 1.8 secs, after several events and trial and error, but I was never certain it was in fact "correct".

My thought on a procedure is to run a "test" by going thru a corner to induce lateral acceleration and downshifting with a throttle blip at the same point, in a very crude attempt to align acceleration with GPS and OBD...

Any suggestions welcomed.


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Harry
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Re: Procedure for calibrating sensor delays ?

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I'd suggest to select a recording with reasonable data quality, your usual accessory configuration, and known sensor delays (e.g. defaults) first. Goto this recording's charts (Lap List ‣ the lap ‣ Data Recordings / Preview ‣ Charts). The first chart will show GPS and OBD speed side by side. Use min or max points in time to understand the gap between both. Adjust the OBD delay so it will match the the distance from start when recording next. Calibrating GPS and wheel speed absolute values is a different topic, please search the forum. Next, select the Longitudinal Acceleration chart (usually page #4) and zoom into an area that where acceleration crosses the zero line. At this point, speed development should turn around. To understand the speed slightly ahead and behind the crossing point, use the cursor feature (GrandPrix only). Set the cursor and use the text displayed to understand the speed at this point. Like for OBD, adjust the Accelerator Delay so it will match GPS speed development for the next time you record something.

The last thing to calibrate is the Video Delay. This has been addressed a lot here on the forum, please search for that too. Audio and RPM display is the best indicators for video from my point of view.

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ereynold60
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Re: Procedure for calibrating sensor delays ?

Post by ereynold60 »

Thanks Harry... that is generally what I've done in the past... i.e. to get OBD and GPS to line up. I've calibrated OBD absolute speed to GPS separately, using a long duration steady 60 mph run on highway. (My track tires are smaller, and thus I have a 0.97 factor there).

I've had a lot of experience with the Video Delay for overlays, and I use RPM and sound there as the best indicator.

I'll look into the Longitudinal acceleration to get a handle on acceleration delay... makes sense as you describe.
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Re: Procedure for calibrating sensor delays ?

Post by ereynold60 »

Harry - A quick and possible 'dumb' question -- How do we interpret the use of meaning of the sensor delays? Is the delay parameter added to, or subtracted from the timestamp?

For example, I'm finding my OBD Speed peaks and valleys are consistently ahead of (earlier than) the GPS speed peaks and valleys. Upon analysis via Excel, the OBD Speed data is 2 seconds earlier, ahead of GPS Speed. I currently have the following delay settings:

Consumer GPS = 1.5 secs
External GPS = 0.7 Secs <-- I am using XGPS160
Any OBD/Engine Sensor = 1.8 seconds


Since OBD is "early" on the graphs, do I ADD delay here... or reduce ?
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Re: Procedure for calibrating sensor delays ?

Post by Harry »

The sensor delay is the time a real live event needs to until it is propagated to processing. So it is the signal runtime including event to sensor measurement, sensor processing, data transmission (Bluetooth etc), an d finally data dispatching to app and in app. For GPS, this time is comparable long (satellite signal run time, low update rate 1Hz for internal GPS), for the on-board accelerator it is comparably short. LapTimer uses this runtime assumption to fuse sensor data in case the data does not come with a timestamp for the real live event (like GPS).

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ereynold60
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Re: Procedure for calibrating sensor delays ?

Post by ereynold60 »

Thanks Harry - So, with OBD being ahead of GPS... it is GPS that is delayed, or OBD is early.... Then do I REDUCE the OBD delay from 1.8 to 0.0 since it is 2 seconds early ???
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Re: Procedure for calibrating sensor delays ?

Post by Harry »

Or GPS delayed and OBD early at the same time. :-) If OBD is ahead, you need to reduce OBD. While not limited by LapTimer, a zero delay is certainly impossible. There is always a gap when measuring things. So increasing GPS is probably a better match for reality. Keep in mind that delays are applied during recording. Existing records are unchanged. The only exception is the Video Delay, it is applied when overlaying.

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Re: Procedure for calibrating sensor delays ?

Post by ereynold60 »

OK Thanks.... so for an external GPS (XGPS160), I need to increase that delay, not the "consumer-grade" delay, correct?

What is the default OBD delay, I lost track?

Thanks again...
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Re: Procedure for calibrating sensor delays ?

Post by Harry »

Yes. And the OBD default is 1.0
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Re: Procedure for calibrating sensor delays ?

Post by ereynold60 »

ereynold60 wrote: Mon Apr 01, 2019 7:02 pm ... I'm finding my OBD Speed peaks and valleys are consistently ahead of (earlier than) the GPS speed peaks and valleys. Upon analysis via Excel, the OBD Speed data is 2 seconds earlier, ahead of GPS Speed. I currently have the following delay settings:

Consumer GPS = 1.5 secs
External GPS = 0.7 Secs <-- I am using XGPS160
Any OBD/Engine Sensor = 1.8 seconds

I changed sensor delays as follows. Previously, OBD was 2 seconds ahead of GPS, and now OBD is still 1 second ahead, even though I made changes worth 2 seconds -- I'm thinking one of these delays isn't having an effect. Any Ideas on what to try next?

External GPS = 0.7 Secs --> Changed to 1.7 secs
Any OBD/Engine Sensor = 1.8 seconds --> Changed to 0.8 Seconds


Thanks!
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