Quadcopter Chronicles - I

So, i've decided to build my own drone from scratch. Here is a picture of the work in progress. The orange PLA chassis was printed on my 3D printer. It is going to be similar to the Crazyflie with a few modifications. So, why not buy the Crazyflie ? well, no fun in that...

So this is the X configuration. The slits have been cut into the motor housing to account for the error in the print caused by the 3D printer. I've observed with my printer that, if i give the hole diameter as 7.03 mm it always turns out to be less by around 0.2-0.1 mm, which is the error. When this happens the motors do not pass through the hole and i have to sit and file it which results in inconsistencies in the 4 holes.

By giving the same value of 7.03 mm is the diameter of the motor and a slit the motors can be accommodated in a tight fit by wedging the slit to allow the motors to pass though, removing the wedge causes the structure to wrap tightly around the motor due to it's elastic nature. 

Also there is something weird in the image below. If you figure it out leave a comment.

The body was designed in SolidWorks as shown below and weighs 9 grams. Yep, that is a lot of green, almost a relation fest, but i am learning to clean that up. Most of it is the symmetry relation due to mirroring the entities.

The motors and props are similar to the one used in the Crazyflie. The next task is to design the control system electronics.


If you would like to download the 3D STL model visit my Thingiverse link below:
http://www.thingiverse.com/thing:1336837

Licensed under Creative Commons

Samsung S-Pen Internals

The S Pen, introduced with the Galaxy Note, is faster and more accurate than conventional stylus pens. It is an input tool that provides a similar feel to an actual pen by detecting the pressure applied on the screen with great accuracy (wait, what....) [Source]

I was lucky to get my hands on a spare S Pen which turned out to be not so lucky for the pen. This post will explore the S Pen internals, crack it open, hook it up to a scope etc.

Cracking the Pen

The S Pen is a very robust design and it's internal circuitry cannot be easily accessed. You will have to use a saw or a dremel to carefully cut it open. I used a dremel to mercilessly slaughter the S Pen. The best position to start sawing would be at a distance of about 80 mm from the tip of the pen. Then slowly working your way up towards the tip till you reach a printed circuit board capped with a small plastic holder as shown in the images below. The pcb assembly slides outwards away from the tip. You will have to remove the plastic button in order to remove the pcb effortlessly.

The S Pen disassembled. Top PCB and bottom side

A close up of the components on the pcb is shown below. Starting from the tip we have the coil assembly consisting of a 0.22 mm dia (approx.) wire wound on a soft iron core which is not cylindrical. The tip is connected to a spring at the base of the assembly which appears to be some kind of switch or a variable capacitor. I believe this assembly consisting of the spring deals with the pressure sensing, either by bringing the coil close to the display surface by compressing the spring or by using the contacts at the base of the assembly as a variable parallel plate capacitor. We then have the push button that is usually used for the air command interface followed by two sensitivity adjustment potentiometers and a bunch of capacitors connected in parallel to the coil thereby forming an LC resonant circuit. The little solder blobs below each capacitor are jumper connections. Therefore as shown below, from left to right the first capacitor and the last capacitor are not connected in parallel with the coil. If you are wondering where is the circuit diagram.....it's coming soon....

Close up of the PCB

Operating Principle

Samsung partnered with Wacom to develop the S Pen. The pen works on the principle of Electro-Magnetic Resonance (EMR). Devices utilizing EMR Technology do not require any cable or built-in battery-based power supply at all. This is because the cellphone incorporates a sensor board beneath the OLED display that detects the pen's movement. Weak energy is induced in the pen's resonant circuit by a magnetic field generated by the sensor board surface. The pen's resonant circuit then makes use of this energy to return a magnetic signal to the sensor board surface. By repeating this movement, the board detects information on the pen's coordinate position and angle, as well as on its general operating condition including speed and writing pressure, etc. A sensor unit is equipped at the side of the sensor board to switch the magnetic field on and off and to receive signals at high speed and detect various kinds of information. In principle, all Wacom's tablet devices are composed of this kind of sensor unit and electronic pen. In addition, the sensor unit itself consists of a sensor board and a control board.

The sensor board is an array of tiny loop antennas arranged in overlapping X and Y directions. The control board switches a high frequency signal into each of the loop antenna coils. This causes them to generate a magnetic field. When the pen passes through this magnetic field, it picks up and stores energy in it's resonant circuit. The control board then instantaneously switches to the receiver and roughly determines the pen's location by scanning all the loop coils. After that, the control board scans multiple loop coils in the vicinity of the pen, and uses the information to calculate the pen's coordinate value precisely. The control board has its own custom DSP circuit performing all the high speed calculations.

For example, let's take the Samsung Galaxy Note 2. The internal view of the Note is shown below. Notice the golden panel. That is the sensor board - the digitizer that interacts with the S Pen [Source]

Hooking it Up

In order to see what is going on inside the S Pen circuitry i connected some wires to the coil pads on the pcb and hooked it up to my oscilloscope. The device operates at a certain carrier frequency as shown below. The signal is power optimized and the data transaction is in some frames that i observed in a repetitive pattern. The same pattern seems to shape-shift when the S Pen is oriented to the display at different angles.Refer to the text below each image for more information. The scope waveform was very jumpy and the values mentioned are close approximate readings off the scope.

Each frame is approximately 15 ms. The above pattern repeats over and over.

Here is the same 15 ms frame but it appears different because the pen was oriented at an angle of around 30 degrees (back end towards the home key) with the surface of the display

Here is the same 15 ms frame but now oriented at an angle of around 120 degrees (back end away from home key) with the surface of the display

Let's turn that time base down

The carrier frequency appears to be approximately 560 KHz

The FFT of the signal gives a strong peak around 560 KHz

Wonder what happens when you push the small button on the S Pen ? The 560 KHz peak shifts to 530 KHz. I guess this frequency shift gets detected as a button press by the sensor board. Pressing the switch must be connecting additional capacitance in the LC parallel circuit thereby changing its resonant frequency. Barely noticeable in the image, but if you click on the image and switch to the image viewer you can alternate between image numbers 8 and 9 using the arrow keys to observe the shift.

This is the frequency response of the LC parallel resonant circuit of the S Pen which was obtained by sweeping a sine wave from 100 KHz to 1 MHz at the coil terminals, with the coil connected to the PCB. The dip is at around 565 KHz.

In a nutshell, the phone containing the "digitizer" or sensor board generates a magnetic field that induces a current in the coil that is connected to the parallel LC tank circuit, the sensor board then switches into receiving mode, letting the resonant energy drain through the S Pen coil thereby detecting it's position, pressure and angle. 

Let's  go back to the first paragraph of this post referenced from another post stating "detecting the pressure applied on the screen". Well, nope. The statement is framed incorrectly leading anyone to believe there are pressure sensors on the screen for the S Pen. But as we found out, it is not so...

The S Pen therefore accomplishes a lot with simple circuitry and basic principles of electromagnetism. Michael Faraday will be proud :P

These observations are based on the S Pen i own using the Digilent Analog Discovery. If you perform the same set of experiments and get to know something new and weird or decode the frame structure if any, comment below. Found a mistake in the text ? comment below...

For information on LC resonant circuits check out the video below:

Wireless RGB Light Control (Adafruit BT + NeoPixels)

This is a massive upgrade to my earlier project that i posted a couple a days ago here. The interface using the RoundButtons from Coding4Fun toolkit looks much cleaner and aesthetic. More features like controlling the light intensity, single tap color change, turning off all the LEDs on the shield, reflection of a hardware reset in the phone UI etc. The paragraphs below briefly detail out the two main components.

The Roundbuttons - Created using the RoundButton class from coding4fun toolkit. These buttons have two event handlers registered in the code, one for Hold and other for a Click. Holding a button for a certain duration causes the color selector window to pop up. This window has a Hexagon color picker interface created using the same coding4fun toolkit. Touching and dragging your finger on this picker will update the selected RGB LED color in real-time. Once a color is selected it can be applied to one or more LEDs by clicking on them, this is handled by the Click event handler.

The ColorPicker - The hexagon color picker has a handler registered to it's Color Changed event. The color data received is framed in a transmission packet and sent over the BT interface to the Arduino board in this handler. Adjusting the slider below the color picker adjusts the brightness of the LEDs. The Turn Off button turns off all the LEDs on the shield and also clears the button background color on the UI.

Implementation

All the round buttons used, follow the xaml template shown below. 40 buttons have been created using the same xaml statement. The only difference is the Name field which is unique for each button. The buttons are placed inside a Grid component therefore the (0, 0) grid position corresponds to the upper left LED near the Reset button on the RGB shield. the buttons have been named in a similar pattern. The button inside the     (0, 0) grid cell is named a_a, the button in cell (0, 1) as a_b, (1, 0) as b_a and so on. All buttons have the same Click and Hold event handler.

When the application is started it establishes a connetion with the Arduino BT shield, which exposes serial ports to the Arduino. (The shield uses the BT Serial Port Profile) The read / write operations on the arduino are done using the softwareserial library at 9600 baud. On the phone side, DataReader and  DataWriter objects are created to read from the Socket (StremSocket class). The socket connects using the ConnectAsync call in the StreamSocket class.

When a button is held for a specific amount of time the Hold Event occurs. This triggers the hold event handler and opens another window that displays the hexagon color picker. The hexagon color picker has a event handler attached to its ColorChanged event. When you touch the color picker and move your finger over it, the ColorChanged handler is continuously called. The OS passes the sender and the Color to the event handler. Inside the event handler, the serial packet to be transmitted to the arduino is built. It has the format as shown below. Hence if i click on button (2, 3) in the grid with full green color selected on the color picker then the packet will be

c_d_0_255_0

As the data is sent as a String the "_" is used as a separator. This command will be parsed on the Arduino. Since the X and Y co-ordinate positions in the command are fixed at offsets 0 and 2 respectively the value can be easily obtained by doing a (int)command[0] - 97. extracting the color values requires a little processing. This value can be then sent to the NeoPixel Library. The intensity adjustment follows a similar format with a smaller packet and a command identifier as 'I'. An alternative method would be to send the numerical values corresponding to the buttons directly. But i could not find any means to figure out which button inside the grid had been clicked without using the Name property....(duh)

When the arduino is reset all LEDs on the phone UI need to be turned off. This is accomplished by sending the string "RESET" from the Arduino to the phone. The phone maintains a journal in an custom LEDManager class about the LEDs that are currently active. When the reset command is received, using the DependencyObject with the grid as the parent and the VisualTreeHelper class, a list of all buttons in the parent is created, then iterating over the List, each button's background color is set the to the OFF color which is black. Similarly when the Turn Off button is clicked on the color picker interface it sends a "OFF" command to the Arduino, turning off all the LEDs and clearing the UI.

Most of the actions are demonstrated in the below video where you can see me messing around with the LEDs.

Some Usage scenarios

1. Arduino Powered Up, Windows phone BT turned ON, Application start

In this case the serial link is established between the phone and the arduino hardware and the data packets are exchanged normally between the two.

2. Arduino Powered Up, Windows Phone BT turned OFF, Application start

In this case the application will open a message box indicating the the phone bluetooth connection is turned off. The application UI will be disabled. Application will have to exit and be restarted once the phone BT is turned ON

3. Arduino OFF, Whindows phone BT turned ON, Applicatino start

- Undefined behavior. Application runs without crashing. Re-connect will not happen even when the board is powered up. The application needs to be restarted.

Future upgrades include hooking up the shield with Facebook API, group control of LEDs, Gesture controls, using sensor interfaces on the WP8 to manipulate LED colore etc. Now this is one hell of a way to learn Windows Phone Programming....

References: 
Bluetooth communication between Arduino and Windows 8.1
Windows Phone API Reference


Where is all the code ? Here is the code running on the Arduino. The Windows Phone code will make this post the longest ever. Therefore i leave it up to you to implement.

Visible Light Communication Chronicles Part II

This is the continuation from Part I on my experiments with visible light communication that i posted sometime ago here. I set up a basic transmitter - receiver circuit, a square wave was given as an input to the transmitter and the output of the reciever was monitored. The transmitter consisted of a N-channel power MOSFET - P55NF06 driving a LED and the receiver consisted of the TEMT 6000 ambient light sensor. The square wave signal was generated using an Arduino MEGA and all the I/O signals were monitored using the Digilent Analog Discovery. Why i chose the above mentioned MOSFET ? because i already had it in stock and decided to use it. The light source is a commercially available 10 W high power LED which i got over here.

Transmitter

The overall circuit is shown below. The gate of the MOSFET is driven by one of the arduino pins. The sketch on the arduino is nothing but the blinky sketch. By adjusting the parameters to the delay function i could generate a square wave from 2 KHz (delay(1)) to 0.5 Hz (delay(1000). The gate resistors Rs and Rg were chosen such thar R1 << R2. From the datasheet, the P55NF06 N-channel mosfet has a drain-source voltage of 60V and a max drain current of 50A.The gate threshold voltage is around 2-4V. The LED forward voltage is around 7V. When turned ON the LED draws approximately 0.5 A which is within the spec of the MOSFET. The transmitter is assembled on a breadboard (yeah kind of not a good choice for a circuit like this, specially considering the mosfet parasitics, you never know...)

Receiver:

The receiver consists of the TEMT 6000 breakout board powered via a 3.3 V supply and it's output fed into the analog discovery. 

The distance between the RXR and TXR is 6 cm. I kept it small for initial measurements. All the signals seen on the scope of the analog discovery are shown below.

The above square wave has a time period of  2 seconds, 0.5 Hz. The blue signal is the input signal and the orange signal has been captured at the output of the ambient light sensor. Note that the rise time and fall time of both the signals appear to be instantaeneous. Also note that when the LED is OFF the TEMT sensor output does not drop to zero but goes to around 48 mV which as i mentioned in an earlier post is the interference from the fluorescent light in my room. The interference is visible in the above image when the input signal is 0V.

The input signal here is of 5 Hz. The output of the ambient light sensor follows the input given to the MOSFET. Note the ripple in the output when input goes to 0V.

In this case the input signal is of 50 Hz. Notice that the output signal starts exhibiting some non-linearity when the MOSFET turns OFF. The interference due to the fluorescent light starts getting negligible. More on this a scroll later.

In this case the input signal has a frequency of 500 Hz. If we continue to increase the frequency the signal at the output of the TEMT 6000 almost disappears and all we get is a DC voltage. with very faint signs of the input signal. After this initial test i decided to give the output of the UART to the TXR circuit and observe the output on the TEMT 6000. Here are the output response of the light sensor to different baud rates

300 Baud.

600 Baud. OFF time non-linearity starts kicking in.

2400 Baud.

4800 Baud. A still noticeable signal that can be recovered using some extra circuitry.

9600 Baud. Beyond 9600 you can only imagine what will happen to the signal at the sensor output.

To investigate on the TXR side. I used a 5V drain-source supply and switched the MOSFET using a 500 Hz gate signal. The following image shows the scope output. The output (orange) was taken at the drain hence the signal inversion. It can be seen that when the gate voltage drops to 0V, turning off the mosfet, the output voltage increases non-linearly with time.(based on the mosfet turn-off delay parameters). The delta is 46.7 usec. The datasheet says that the turn-off delay is around 30 nsec for the specified test conditions.

Allrighty that is a lot of information in this post. Part 3 of this series will elaborate on the analysis of the circuit, MOSFET parameters and the above plots.  Until then, take it easy...

Update

I did one more test in which i plotted the TEMT6000 output over the drain-source voltage Vds.

The orange plot is the output of the light sensor and the blue plot is Vds, which is inverted as i connected the scope to the drain as shown in the schematic. The nonlinearity that i had seen before was therefore confirmed to be due to the TEMTP 6000 sensor. I did a couple of tests with varying input frequencies and found that in all cases the time it took for the sensor outout to go from maximum to 0V was 1.4 msec.which means that it can handle signals from 1 Hz to 714 Hz approximately. This was true as seen in the output plots above. Increasing the Vcc for the sensor to 5V does not help. Time to find a better sendor or fix the output of this one. I will probably have to make a new board and change the 10K resistor to a more feasible value.