Accelerometer calibration, or the lack thereof

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Accelerometer calibration, or the lack thereof

Postby lunat1ck » Wed Apr 17, 2013 7:52 am

I'm about to try switching from Harry's Lap Timer to Trackaddict HD, and I'm doing my homework to make sure I have a successful first run with the software. My track days are precious (only get on track once or twice a month, as a driver anyway), so I'm trying to make sure I've dotted all the i's and crossed all the t's.

Anyway, I am a concerned about the accuracy of the accelerometer data due to how I mount my iPhone in my car. I drive a (very heavily modified) 2009 Mustang GT, and I mount the camera using some velcro on the vertical part of the dashboard, in between the pair of air vents above the main console. This is NOT 100% vertical, however, and the phone leans back (i.e. towards the front of the car) by about 20-25 degrees. BTW, I do NOT use the video camera on the phone, I collect video using 4 or 5 GoPros I mount all over my car, so this was the optimal spot for controlling the application, and checking my lap and sector times on the straights.

As I understand things right now, TrackaddictHD or perhaps the phone itself, will only give correct G-force readings if the phone is held perfectly vertical, or perfectly horizontal. If this is true, then I can't really mount the phone in it's current location and expect good results. I could manufacture an angled wedge to insert between the phone and the dashboard to force it to be vertical, but I am also considering mounting it horizontally, face up, on console behind the shifter, where there's a nice flat spot for it.

I figured that worst case, I could just use my high school trigonometry to calculate the adjustment to the G-force values, and then create custom fields in RaceRender that apply the necessary multipler to the data, but that seems like more work than should be necessary.

Is my understanding correct, or am I thinking too hard (again)?
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Re: Accelerometer calibration, or the lack thereof

Postby RaceRender LLC » Wed Apr 17, 2013 3:24 pm

TrackAddict HD will automatically calibrate for the device's mounting position when you tell it to record, as long as you are stopped (ie GPS speed is zero). In Circuit mode, it wont actually start recording until you start moving, but that's ok... the calibration will take place before that, when you are stopped and it is on the Record screen.

A few tips...

- It will attempt to adjust for pitch and roll angles, but it has no way to detect yaw at calibration time, being that there is no gravity acting on that axis. Try to avoid any left/right angle relative to the vehicle... If the camera preview looks like it's pointed straight, you should be good. It's ok if you want to change the pitch to angle it up or down though. If you're not using the video feature and are mounting the iPhone flat on its back, it's best to use the "portrait" orientation and have the top of it towards the front of the vehicle.

- Don't move or adjust the iPhone after you have entered the Record screen, as this may affect the accelerometer calibration, as well as the camera recording orientation.

- If you enter the Record screen while moving, it will use the last calibration it got, so that may or may not be any good.

- The measurement range provided by iOS maxes out some where after 2 G's, which should be enough for the majority of production cars on race tires.

- Any vibrations / wobble, as well as the general hardware design, will mean that it's not always the more scientific measurement. Some windshield mounts are better with this than others, but even regardless of that, there are going to be some limitations with smartphone hardware that you wouldn't always find in dedicated data acquisition hardware.
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Re: Accelerometer calibration, or the lack thereof

Postby lunat1ck » Thu Apr 18, 2013 5:54 am

OK, so let me make sure I understand this. While I'm getting everything set up in the paddock area, before I even strap in and get rolling, I always verify that my iPhone has connectivity to the XGPS150 and the Kiwi2, so once I have done that, I need to put TrackaddictHD into Record mode as well. Since I do all of this after mounting the phone on the dash, it won't move relative to the car at all. There should usually be a couple of minutes between setting the software to Record mode, and when the car starts rolling, since I get harnessed up after I've done all of that, and that takes a minute or two.

The most lateral Gs anyone's ever pulled in my car was when one of my instructors drove it (that was fun!!), and we hit about 1.2, so the 2.0 limit is WAY more than I'll ever need. If I get to the point where I can drive something that pulls 2 Gs I'll have to invest in a pro device of some kind. That's gonna be a few years :-P

Sounds like I won't have any issues, but one thing I find annoying is that Apple provides NO interface for querying the accelerometer status, or checking the calibration. Harry's Lap Timer has a screen that shows the active accelerometer readings, which is pretty cool. Now, I'm not suggesting you just rip-off features from Harry.... well, actually I am :-) It would be nice if you had a way of showing this information. Just as you display the GPS and OBD device connectivity in a very simple, elegant manner on the main screen, would it make sense to show any information about the accelerometer? Perhaps show feedback on the state of calibration, with a little box that's red or yellow while calibration is taking place, and then green once it stabilized.

I think I'm all set. Now I just have to endure waiting until May 2nd to get back on track and test this all out. I'll post some feedback, one way or the other. I'm thinking of writing up a comparison of the two products as well.
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Re: Accelerometer calibration, or the lack thereof

Postby RaceRender LLC » Sun Apr 21, 2013 10:53 pm

Yes, assuming that you've used the Setup screen to set to Circuit mode and the desired track, and set the OBD-2 mode to WiFi for the Kiwi (under "More Options"), you would just mount it in the car and then press the Record button. It will then do the rest, automatically starting recording once you hit 10 MPH (or manually press the on-screen button to start it sooner), so that it doesn't waste space recording you just sitting in grid. If you have the track's start/finish point set, it should start timing laps once you cross the start line, otherwise it will display a large on-screen button to press as you cross the point.

Displaying the accelerometer reading/status is something that has been considered in the past, but the user interface was a but too crowded at the time. It may be something that can be revisited with the new cleaner interface that was introduced a couple versions ago, if there's a good way to do it simply and effectively. Some of what currently appears to be empty screen space is already reserved for other options, but just isn't obvious for all users or situations, so that comes into consideration when trying to maintain the simplicity and keep things from getting cluttered.
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