Testing iPhone Acceleration Apps

I am going to need to use this iPhone to measure the acceleration. However, I need to check this thing and see how accurate it will be. Experiment time. I know someone must have a nice review of acceleration apps for the iPhone – somewhere. Unfortunately, my quick google search for “iPhone acceleration physics” gave […]

I am going to need to use this iPhone to measure the acceleration. However, I need to check this thing and see how accurate it will be. Experiment time.

I know someone must have a nice review of acceleration apps for the iPhone - somewhere. Unfortunately, my quick google search for "iPhone acceleration physics" gave me a my own post from 2010. Those are still valid, but there are probably more options now. I know there are some other reviews, but I want one by a physicist.

The Plan: Really, it's pretty simple. Use the above high speed video to measure the acceleration of the fan cart. Next, compare this to the value the phone gives. Instead of using an acceleration app to record the acceleration, I am going to monitor the acceleration using my Macbook and AirServer. This program allows you to use the airplay feature on your phone to send the screen to your computer. I think this will be useful (HT to John Burk - @occam98). Oh, in case it's not clear I will use Tracker Video Analysis. It's just what I do.

The first app is just called Acceleration. Here is the data for that run from Tracker Video.

Screenshot 5 15 13 5 50 pm

When fitting a parabolic function to the data, the term in front of t2 is half the acceleration. That means this cart (with my iPhone on it) has an acceleration 0.24 m/s2. But what about the acceleration app? This is what it shows during that same time.

Screenshot 5 15 13 5 55 pm

In this app, the axis along the length of the phone is the "y-direction". I am pretty sure that during the acceleration it varied between -0.03 and -0.04 g's. Assuming a "g" of 9.8 m/s2, this would be 0.294 m/s2 to 0.392 m/s2. This is not as good of a measurement as I would like. Maybe a graph recording would work better. Also, I think part of the problem is that this acceleration was for a rather short period of time. It looks like the acceleration app is averaging over some time interval. That means that for a short time interval, it might be wrong.

Let's look at another app. This one is called AccelMeter. Visually, it is my favorite. Here is the video data for that run that matches AccelMeter.

Screenshot 5 16 13 9 15 am

Odd. The acceleration for this run is a little bit higher at 0.32 m/s2. I'm not sure why. I guess it doesn't really matter what the value is as long as it matches with the accelerometer from the iPhone. Here is a shot from the app during that fan cart motion.

Screenshot 5 16 13 10 00 am

I just realized that this won't be a very good app to use. Why? It gives the net "g-force". If I assume that the gravitational force is in the z-direction and the acceleration due to the fan is in the y-direction, then I can write:

Screenshot 5 16 13 10 18 am

That's just plain wrong. I'm not sure I have the right time interval from the app. Also, the acceleration value was jumping all over the place. I guess I am going to have to try another app.

How about AccelGraph? This one lets you record the data and then email it to yourself. Here is the data from Tracker Video.

Screenshot 5 16 13 11 41 am

You'll notice that this graph is different. It's different because I created it last - I was older and wiser. Here I pushed the cart and let the fan slow it down and change directions. This gives a longer period to look at the acceleration and also a longer period of time on the iPhone. From this, I get a near constant acceleration of 0.268 m/s2.

I don't have a screen shot for this app. Well, I do but it just shows the first part of a data table. If I average the acceleration in the y-direction from the phone, I get about 0.05 g's or 0.49 m/s2. This isn't going as well as I had hoped. Let's move on again to another app.

Actually, let me give AccelMeter another chance. It appears that it can give you just the "relative acceleration". However, this time I am going to get the acceleration with a motion detector instead of a video. That way I can be sure that I am matching up the accelerations at the different times (and it will be easier to collect the data).

Here is the data from Logger Pro along with the view from AccelMeter at the same time.

Screenshot 5 16 13 5 01 pm

It's a little hard to see, but from this Logger Pro says the acceleration is 0.225 m/s2 and AccelMeter says about 0.02 g or about 0.19 m/s2. This isn't too bad. Well, I think my expectations are lower now after my previous failures. Actually, the value from the phone fluctuates between 0.02 and 0.03 - so maybe it is giving the correct value.

Accelerometer Summary

I really am not sure how accurate these built in accelerometers are or can even be. Maybe the problem is that the software just rounds off the values to silly levels. If you round the acceleration to 0.01 g, that is almost 0.1 m/s2. I guess this means that if you have higher values of acceleration it might work better. Of course, higher accelerations means a riskier maneuver for my precious iPhone.

Yes. There is another app that I didn't use. There is PASCO's Sparkvue app. I tried that before and I think it only records acceleration in one direction. I guess I should give it another shot. Also, there is Graphical Analysis by Vernier. I need to look at this one too - but I don't really care about the plotting features. It would be cool if it allowed you to connect your iPhone to Logger Pro running on your computer and use it as a wireless sensor.

This post is way longer than I wanted it to be. I think it is going to have to have a follow up to test Graphical Analysis and Sparkvue in more detail.