What are the ranges of the accelerator on the iPhone? - iphone

Question

I can't seem to find any documentation online about this, and what I am googling is giving me a lot of conflicting information...

Answer 1

From iphonedevsdk.com:
The accelerometers used in the first
and second generation iPhones (and I
assume also the iPod touches) operate
in two modes: +/- 2 g, and +/- 8 g.
(Actually, as you observed, they
report accelerations somewhat outside
the nominal range. Accuracy is not
spec'ed outside that range, though.)
Apple operates them in the +/- 2 g
mode. There is a tradeoff: The current
resolution is nominally 0.018 g,
according to the datasheet (though my
first generation iPhone uses
0.018168604, according to a modified version of AccelerometerGraph). In the
+/- 8 g mode, the resolution would be four times cruder.
I assume Apple decided that the finer
resolution would be more useful than
the wider range. (I'd rather see finer
resolution than 0.018 g. So neither of
us is fully satisfied.)
You cannot change the mode with any
published feature of the APIs. Since
they are passing acceleration as a
double, they could theoretically allow
a change in mode, and simply look at
the mode when rescaling the A/D value,
before reporting acceleration. (The
obvious place to set the mode would be
in the call which sets up the
application to receive accelerometer
information.) However, for backward
compatibility, the OS would have to
set the accelerometer mode to +/- 2 g
at the beginning of the application.
And none of the background processes
could be allowed to set the
accelerometer mode.

Answer 2

The aGauge app displays this raw data coming from the accelerometer as you move the device. It can also help you find the "flip" threshold for your device.

Answer 3

I created the following application to try to test out the ranges...
UIAccelerometer *objAccelerometer;
UILabel *lblxmin, *lblxmax, *lblymin, *lblymax, *lblzmin, *lblzmax;
UILabel *lblxnow, *lblynow, *lblznow;
float xmin = 0.0, xmax = 0.0, ymin = 0.0, ymax = 0.0, zmin = 0.0, zmax = 0.0, xnow = 0.0, ynow = 0.0, znow = 0.0;
- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
//NSLog (#"%f, %f, %f", acceleration.x, acceleration.y, acceleration.z);
xnow = acceleration.x;
ynow = acceleration.y;
znow = acceleration.z;
if (xnow < xmin) { xmin = xnow; }
if (ynow < ymin) { ymin = ynow; }
if (znow < zmin) { zmin = znow; }
if (xnow > xmax) { xmax = xnow; }
if (ynow > ymax) { ymax = ynow; }
if (znow > zmax) { zmax = znow; }
lblxmin.text = [NSString stringWithFormat:#"%f", xmin];
lblymin.text = [NSString stringWithFormat:#"%f", ymin];
lblzmin.text = [NSString stringWithFormat:#"%f", zmin];
lblxmax.text = [NSString stringWithFormat:#"%f", xmax];
lblymax.text = [NSString stringWithFormat:#"%f", ymax];
lblzmax.text = [NSString stringWithFormat:#"%f", zmax];
lblxnow.text = [NSString stringWithFormat:#"%f", xnow];
lblynow.text = [NSString stringWithFormat:#"%f", ynow];
lblznow.text = [NSString stringWithFormat:#"%f", znow];
}
-(void) invokeAccelerometer {
objAccelerometer = [UIAccelerometer sharedAccelerometer];
objAccelerometer.delegate = self;
objAccelerometer.updateInterval = (1.0 / 10.0);
}
// Implement viewDidLoad to do additional setup after loading the view, typically from a nib.
- (void)viewDidLoad {
lblxmin = [[UILabel alloc] initWithFrame:CGRectMake(10, 10, 100, 40)];
lblxnow = [[UILabel alloc] initWithFrame:CGRectMake(100, 10, 100, 40)];
lblxmax = [[UILabel alloc] initWithFrame:CGRectMake(200, 10, 100, 40)];
lblymin = [[UILabel alloc] initWithFrame:CGRectMake(10, 60, 100, 40)];
lblynow = [[UILabel alloc] initWithFrame:CGRectMake(100, 60, 100, 40)];
lblymax = [[UILabel alloc] initWithFrame:CGRectMake(200, 60, 100, 40)];
lblzmin = [[UILabel alloc] initWithFrame:CGRectMake(10, 110, 100, 40)];
lblznow = [[UILabel alloc] initWithFrame:CGRectMake(100, 110, 100, 40)];
lblzmax = [[UILabel alloc] initWithFrame:CGRectMake(200, 110, 100, 40)];
[self.view addSubview:lblxmin];
[self.view addSubview:lblxnow];
[self.view addSubview:lblxmax];
[self.view addSubview:lblymin];
[self.view addSubview:lblynow];
[self.view addSubview:lblymax];
[self.view addSubview:lblzmin];
[self.view addSubview:lblznow];
[self.view addSubview:lblzmax];
[self invokeAccelerometer];
[super viewDidLoad];
}
The problem is, when I rotate my iPod along the horizontal/vertical axes and then flip it over, I get values like:
xmin -1.271802
xmax 1.180959
ymin -1.344477
ymax 1.108285
zmin -2.30713
zmax 2.325581
If I take the iPod and shake the heck out of it, I get...
x -2.325581 to 2.307413
y -2.325581 to 2.307413
z -2.307413 to 2.325581
Any ideas what it's measuring?
The best I've come up with is:
vertical axis
x = -1 if tilted to the left ( <| )
x = +1 if tilted all the way to the right ( |> )
where < is the way the screen faces, and | is the bottom of the iPod
y ~ -1 if screen is facing you, perpendicular to floor ("standing up")
y ~ 1 if facing away from you (and upside down)