Kalman Filters are great and all, but I find the Complementary Filter much easier to implement with similar results. The best articles that I have found for coding a Complementary Filter are [this wiki][2] (along with [this article][3] about converting sensors to Engineering units) and a PDF in the zip file on [this page][1] (Under Technical Documentation, I believe the file name in the zip is filter.pdf); 

PS. If your stuck on a Kalman Filter, [here][4] is some C-syntax code for the Arduino that implements it.


  [1]: http://web.mit.edu/first/segway/#misc
  [2]: http://web.archive.org/web/20091121085323/http://www.mikroquad.com/bin/view/Research/ComplementaryFilter
  [3]: http://wiki.orbswarm.com/index.php?title=IMU
  [4]: http://diydrones.com/profiles/blogs/705844:BlogPost:23188

&lt;a href=&quot;http://www.gamasutra.com/&quot;&gt;Gamasutra.com&lt;/a&gt; ran &lt;a href=&quot;http://www.gamasutra.com/view/feature/1494/wheres_the_wiimote_using_kalman_.php&quot;&gt;an article on using Kalman filters for WiiMote filtering&lt;/a&gt;.

There are some links to C++ source code at the end of the article.

I have problem calling a JavaScript function in an `iframe` from the parent page. Here is my two pages:

**mainPage.html**

    &lt;html&gt;
    &lt;head&gt;
    	&lt;title&gt;MainPage&lt;/title&gt;
    	&lt;script type=&quot;text/javascript&quot;&gt;
    		function Reset() 
    		{
    			if (document.all.resultFrame)
    				alert(&quot;resultFrame found&quot;);
    			else
    				alert(&quot;resultFrame NOT found&quot;);
    
    			if (typeof (document.all.resultFrame.Reset) == &quot;function&quot;)
    				document.all.resultFrame.Reset();
    			else
    				alert(&quot;resultFrame.Reset NOT found&quot;);
    		}
    	&lt;/script&gt;
    &lt;/head&gt;
    &lt;body&gt;
    	MainPage&lt;br&gt;
    	&lt;input type=&quot;button&quot; onclick=&quot;Reset()&quot; value=&quot;Reset&quot;&gt;&lt;br&gt;&lt;br&gt;
    	&lt;iframe height=&quot;100&quot; id=&quot;resultFrame&quot; src=&quot;resultFrame.html&quot;&gt;&lt;/iframe&gt;
    &lt;/body&gt;
    &lt;/html&gt;


**resultFrame.html**

    &lt;html&gt;
    &lt;head&gt;
    	&lt;title&gt;ResultPage&lt;/title&gt;
    	&lt;script type=&quot;text/javascript&quot;&gt;
    		function Reset() 
    		{
    			alert(&quot;reset (in resultframe)&quot;);
    		}
    	&lt;/script&gt;
    &lt;/head&gt;
    &lt;body&gt;
    	ResultPage
    &lt;/body&gt;
    &lt;/html&gt;

(I know that `document.all` isn&#39;t recommended but this page should only be viewed with IE internally and I don&#39;t think that&#39;s the problem)

When I press the Reset-button I get &quot;resultFrame found&quot; and &quot;resultFrame.Reset NOT found&quot;. It seems to have a reference to the frame but can&#39;t call the function on the frame, why is that?

Calling javascript function in iframe

I discovered this pattern (or anti-pattern) and I am very happy with it.

I feel it is very agile:

    def example():
        age = ...
        name = ...
        print &quot;hello %(name)s you are %(age)s years old&quot; % locals()

Sometimes I use its cousin:

    def example2(obj):
        print &quot;The file at %(path)s has %(length)s bytes&quot; % obj.__dict__

I don&#39;t need to create an artificial tuple and count parameters and keep the %s matching positions inside the tuple.

Do you like it? Do/Would you use it? Yes/No, please explain.

Python: is using &quot;..%(var)s..&quot; % locals() a good practice?

I am building a balancing robot using the Lego Mindstorm&#39;s NXT system. I am using two sensors from HiTechnic, the first being an Accelerometer and the second being a Gyroscope. I&#39;ve successfully filtered out noise from both sensors and derived angles for both in a range between -90 and 90 degrees, with 0 degrees being perfectly balanced.

My next challenge is to combine both of the sensor values to correct for the Gyroscope&#39;s drift over time. Below is an example graph I created from actual data to demonstrate the drift from the gyroscope:

![enter image description here][1]


The most commonly used approach I&#39;ve seen to make combining these sensors rock solid is by using a Kalman filter. However, I&#39;m not an expert in calculus and I really don&#39;t understand mathematical symbols, I do understand math in source code though.

I&#39;m using RobotC (which is like any other C derivative) and would really appreciate if someone can give me examples of how to accomplish this in C.

Thank you for your help!



**SOLUTION RESULTS:**

Alright, kersny solved my problem by introducing me to complementary filters. This is a graph illustrating my results:

*Result #1*

![enter image description here][2]

*Result #2*

![enter image description here][3]

As you can see, the filter corrects for gyroscopic drift and combines both signals into a single smooth signal.

**Edit:** Since I was fixing the broken images anyways, I thought it would be fun to show the rig I used to generate this data:

![enter image description here][4]


  [1]: http://i.stack.imgur.com/AHx4r.png
  [2]: http://i.stack.imgur.com/MqRid.png
  [3]: http://i.stack.imgur.com/mePBt.png
  [4]: http://i.stack.imgur.com/hWbBh.jpg

Combine Gyroscope and Accelerometer Data

I am building a balancing robot using the Lego Mindstorm's NXT system. I am using two sensors from HiTechnic, the first being an Accelerometer and the second being a Gyroscope. I've successfully filtered out noise from both sensors and derived angles for both in a range between -90 and 90 degrees, with 0 degrees being perfectly balanced.
My next challenge is to combine both of the sensor values to correct for the Gyroscope's drift over time. Below is an example graph I created from actual data to demonstrate the drift from the gyroscope:
<img src="http://i.stack.imgur.com/AHx4r.png" alt="enter image description here">
The most commonly used approach I've seen to make combining these sensors rock solid is by using a Kalman filter. However, I'm not an expert in calculus and I really don't understand mathematical symbols, I do understand math in source code though.
I'm using RobotC (which is like any other C derivative) and would really appreciate if someone can give me examples of how to accomplish this in C.
Thank you for your help!
SOLUTION RESULTS:
Alright, kersny solved my problem by introducing me to complementary filters. This is a graph illustrating my results:
Result #1
<img src="http://i.stack.imgur.com/MqRid.png" alt="enter image description here">
Result #2
<img src="http://i.stack.imgur.com/mePBt.png" alt="enter image description here">
As you can see, the filter corrects for gyroscopic drift and combines both signals into a single smooth signal.
Edit: Since I was fixing the broken images anyways, I thought it would be fun to show the rig I used to generate this data:
<img src="http://i.stack.imgur.com/hWbBh.jpg" alt="enter image description here">

I have always been a bit stumped when I read other peoples&#39; code which had typedefs for pointers to functions with arguments. I recall that it took me a while to get around to such a definition while trying to understand a numerical algorithm written in C a while ago. So, could you share your tips and thoughts on how to write good typedefs for pointers to functions (Do&#39;s and Do not&#39;s), as to why are they useful and how to understand others&#39; work? Thanks!

Understanding typedefs for function pointers in C

I want to create a C macro that creates a function with a name based
on the line number.
I thought I could do something like (the real function would have statements within the braces):

    #define UNIQUE static void Unique_##__LINE__(void) {}

Which I hoped would expand to something like:

    static void Unique_23(void) {}

That doesn&#39;t work. With token concatenation, the positioning macros
are treated literally, ending up expanding to:

    static void Unique___LINE__(void) {}

Is this possible to do?

Creating C macro with ## and __LINE__ (token concatenation with positioning macro)

If in C I write:

    int num;

Before I assign anything to `num`, is the value of `num` indeterminate?

What happens to a declared, uninitialized variable in C? Does it have a value?

What does the following C++ code mean?

    unsigned char a : 1; 
    unsigned char b : 7;

I guess it creates two char a and b, and both of them should be one byte long, but I have no idea what the &quot;: 1&quot; and &quot;: 7&quot; part does. 


What does a colon in a struct declaration mean, such as :1, :7, :16, or :32?

I&#39;ve noticed that the Linux kernel code uses bool, but I thought that bool was a C++ type. Is bool a standard C extension (e.g., ISO C90) or a GCC extension?

Is bool a native C type?

An interesting interview question that a colleague of mine uses:

Suppose that you are given a very long, unsorted list of unsigned 64-bit integers. How would you find the smallest non-negative integer that *does not* occur in the list?

FOLLOW-UP: Now that the obvious solution by sorting has been proposed, can you do it faster than O(n log n)?

FOLLOW-UP: Your algorithm has to run on a computer with, say, 1GB of memory

CLARIFICATION: The list is in RAM, though it might consume a large amount of it. You are given the size of the list, say N, in advance.

Find the Smallest Integer Not in a List

What are some algorithms which we use daily that has O(1), O(n log n) and O(log n) complexities?

Examples of Algorithms which has O(1), O(n log n) and O(log n) complexities

This question may be old, but I couldn&#39;t think of an answer.

Say, there are two lists of different lengths, **merging at a point**; how do we know where the merging point is?

Conditions:

1. We don&#39;t know the length 
2. We should parse each list only once.

![Example of two merged linked lists.][1]


  [1]: http://i.stack.imgur.com/xjn0S.png

Check if two linked lists merge. If so, where?

I want an efficient algorithm to find the next greater permutation of the given string.

Algorithm to find next greater permutation of a given string

How do you write your own function for finding the most accurate square root of an integer?

After googling it, I found [this][2] (archived from its [original link][1]), but first, I didn&#39;t get it completely, and second, it is approximate too.

Assume square root as nearest integer (to the actual root) or a float.

  [1]: http://nlindblad.org/2007/04/04/write-your-own-square-root-function/
  [2]: //web.archive.org/web/20100330183043/http://nlindblad.org/2007/04/04/write-your-own-square-root-function

Writing your own square root function

I need to add a shake feature that will refresh my Android application.

All I find of documentation involves implementing the `SensorListener`, but Eclipse tells me it&#39;s deprecated and suggest `SensorEventListener`. 

Anybody that has a nice guide to how I go about creating this **`shake controller`**?

How to refresh app upon shaking the device?

Now that iOS 4 is no longer NDA, I would like to know what Gyroscope has to offer over the Accelerometer for developers. Is there a difference in APIs? Other things?

Gyroscope vs Accelerometer?

I have recently come across a few websites that seems to access the accelerometer or gyroscope on my laptop, detecting changes in orientation or movement.

How is this done? Must I subscribe to some kind of event on the `window` object?

On which devices (laptops, mobile phones, tablets) is this known to work?

---

&lt;sup&gt;**NB**: I actually already know (part of) the answer to this question, and I am going to post it right away. The reason that I am posting the question here, is to let everyone else know that accelerometer data *is* available in Javascript (on certain devices) and to challenge the community to post new findings on the subject. Currently, there seems to be almost no documentation of these features.&lt;/sup&gt;

How to access accelerometer/gyroscope data from Javascript?

I need some help understanding the output of the DFT/FFT computation.

I&#39;m an experienced software engineer and need to interpret some smartphone accelerometer readings, such as finding the principal frequencies. Unfortunately, I slept through most of my college EE classes fifteen years ago, but I&#39;ve been reading up on DFT and FFT for the last several days (to little avail, apparently). 

Please, no responses of &quot;go take an EE class&quot;. I&#39;m actually planning to do that if my employer will pay me. :)

So here is my problem:


I&#39;ve captured a signal at 32 Hz. Here is a 1 second sample of 32 points, which I&#39;ve charted in Excel.

![enter image description here][1]


I then got some [FFT code][2] written in Java from Columbia University (after following the suggestions in a post on &quot;[Reliable and fast FFT in Java][3]&quot;).

The output of this program is as follows. I believe it is running an in-place FFT, so it re-uses the same buffer for both input and output.

    Before: 
    
    Re: [0.887  1.645  2.005  1.069  1.069  0.69  1.046  1.847  0.808  0.617  0.792  1.384  1.782  0.925  0.751  0.858  0.915  1.006  0.985  0.97  1.075  1.183  1.408  1.575  1.556  1.282  1.06  1.061  1.283  1.701  1.101  0.702  ]
    
    Im: [0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  0.0  ]
    
    After: 
    
    Re: [37.054  1.774  -1.075  1.451  -0.653  -0.253  -1.686  -3.602  0.226  0.374  -0.194  -0.312  -1.432  0.429  0.709  -0.085  0.0090  -0.085  0.709  0.429  -1.432  -0.312  -0.194  0.374  0.226  -3.602  -1.686  -0.253  -0.653  1.451  -1.075  1.774  ]
    
    Im: [0.0  1.474  -0.238  -2.026  -0.22  -0.24  -5.009  -1.398  0.416  -1.251  -0.708  -0.713  0.851  1.882  0.379  0.021  0.0  -0.021  -0.379  -1.882  -0.851  0.713  0.708  1.251  -0.416  1.398  5.009  0.24  0.22  2.026  0.238  -1.474  ]


So, at this point, I can&#39;t make heads or tails of the output. I understand the DFT concepts, such as the real portion being the amplitudes of the component cosine waves and the imaginary portion being the amplitudes of the component sine waves. I can also follow this diagram from the great book &quot;[The Scientist and Engineer&#39;s Guide to Digital Signal Processing][4]&quot;:
![enter image description here][5]


So my specific questions are:

1. From the output of the FFT, how do I find the &quot;most occurring frequencies&quot;? This is part of my analysis of my accelerometer data. Should I read the real (cosine) or imaginary (sine) arrays?

2. I have a 32-point input in the time domain. Shouldn&#39;t the output of the FFT be a 16-element array for reals and a 16-element array for imaginary? Why does the program give me real and imaginary array outputs both of size 32?

3. Related to the previous question, how do I parse the indexes in the output arrays? Given my input of 32 samples sampled at 32 Hz, my understanding is that a 16-element array output should have its index uniformly spread up to 1/2 the sampling rate (of 32 Hz), so am I correct in understanding that each element of the array represents (32 Hz * 1/2) / 16 = 1 Hz?

4. Why does the FFT output have negative values? I thought the values represent amplitudes of a sinusoid. For example, the output of Real[ 3 ] = -1.075 should mean an amplitude of -1.075 for a cosine wave of frequency 3. Is that right? How can an amplitude be negative?


  [1]: http://i.stack.imgur.com/IHj1I.jpg
  [2]: http://www.ee.columbia.edu/~ronw/code/MEAPsoft/doc/html/FFT_8java-source.html
  [3]: https://stackoverflow.com/questions/3287518/reliable-and-fast-fft-in-java
  [4]: http://www.dspguide.com/pdfbook.htm
  [5]: http://i.stack.imgur.com/MDI8J.jpg

Understanding FFT output

I am developing an Android App to track indoor position. My phone is an Google Nexus S using two sensors, the first being an Accelerometer and the second being a Gyroscope.

My idea is that from a starting reference point (a known GPS position), using the 2 sensors (accelerometer for motion and Gyro for directions) track the phone when moving. And display on a map when the user is going.

But the problem is that i have no idea how to combine both of these sensors to give me an actual position?

I have read some article on internet that talk about &quot;kalman filter&quot; and after some more research i found out that that filter is very very complex /too much for me) especially to code it in java for Android (cpu killer)

Does someone has some experience that he can share with me about indoor positioning system using Gyro and accelerometer?


Content Type	Original Author	Original Content on Stackoverflow
Question	Dylan Vester	View Question on Stackoverflow
Solution 1 - C	kersny	View Answer on Stackoverflow
Solution 2 - C	Vatine	View Answer on Stackoverflow

Combine Gyroscope and Accelerometer Data

C Problem Overview

C Solutions

Solution 1 - C

Solution 2 - C

Python: is using "..%(var)s.." % locals() a good practice?

Calling javascript function in iframe

Attributions