Triple Axis Accelerometer-ADXL335

Lead Author: Mauricio Ayllon Unzueta

Triple Axis Accelerometer Breakout - ADXL335

Introduction

Accelerometers measure acceleration caused by motion, mainly. However, when they are at rest with respect to earth’s surface, the acceleration sensed by the accelerator is due to gravity pulling down on it.

A particular aspect of accelerometers of this sort is that they sense rotations. Nevertheless, they can only sense differences in 180 ̊of movement as the other 180 ̊ is just a mirror image of the first. In order to detect a 360 ̊ rotation, the two axes sensors work differently. For instance, when the sensor is being rotated around the X Axis, the sensed values do not change, so it is possible to combine the y and z values to find x using a trigonometric function called Atan2 (ADXL335), which returns values as -180 ̊ to 180 ̊ in radians. [1].

Yaw is the name for rotation around an axis that is similar to spinning a top, or shaking motion sideways. Accelerometers cannot measure this type of motion. However, it is possible to incorporate a gyro or digital compass in order to accomplish this task. This will be done for the present project. [1]

The accelerometer reports proper data when it stands still. When it is in free fall or any other kind of motion, the accelerometer’s reading is no longer purely gravity based. For the purpose of this project, a clean reading during movement is required, and hence, a gyro and an accelerometer working in combination are needed. Together they form what is called an IMU (Inertial Measurement Unit). The gyro is able to kick in where an accelerometer leaves off, and vise versa. In summary, a gyro serves for measuring rotation, and an accelerometer for determining orientation. [1]

The particular type of accelerometer used in this project is the ADXL335, and connecting it to the Arduino is fairly simple. It powers off of the 3.3v, and the x, y, z connectors connect to the 0, 1, 2 analog input pins. This accelerometer outputs an analog voltage depending on the sensed value. [1]

Purpose

The purpose of installing this accelerometer in our payload is to sense sharp changes in wind speed and direction. In particular, we expect to know when the balloon crosses the troposphere into the stratosphere since the jet stream is located within that boundary, and we expect the balloon to experience abrupt movements.

The ADXL335

The ADXL335 is a thin, small, low power, complete 3-axis accelerometer with signal conditioned voltage outputs. It measures acceleration with a minimum full-scale range  3g. This device measures the static acceleration of gravity in tilt-sensing applications and dynamic acceleration resulting from motion, shock, or vibration. [2]

The user selects the bandwidth of the accelerometer using the CX, CY, and CZ capacitors at the XOUT, YOUT, and ZOUT pins. There are few bandwidths that can be selected to suit the task needed. They range from 0.5 Hz to 1600 Hz for the X and Y axes and from 0.5 Hz to 550 Hz for the Z axis. [2]

The ADXL335 contains a polysilicon surface-micromachined structure built on top of a silicon wafer. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces. A differential capacitor, consisting of independent fixed plates and plates attached to the moving mass, measures the deflection of the structure. Acceleration unbalances the capacitor, which in turn, results in a sensor output with amplitude proportional to the acceleration experienced. [2]

·         Mechanical Sensor

The ADXL335 uses a single structure for sensing the three axes. Therefore, the axes’ sense direction is orthogonal and has little cross-axis sensitivity. Mechanical misalignment is the principal source of cross-axis sensitivity. However, this misalignment can be calibrated out at the system level. [2]

·         Performance

Innovative design techniques make sure that high performance is achieved by the ADXL335. For this reason, there is no quantization error or nonmonotonic behavior, and temperature hysteresis is very low (usually less than 3mg over the -25 ̊C to +70 ̊C temperature range) [2]

Testing Procedure

The team tested the accelerometer in cool dry ice at -20 ̊C for over 24 hours. The accelerometer gave constant readings throughout the test. It was tested after complete integration including data logging. The code implemented for the Arduino to correctly read these values was:

/*

 ADXL3xx

 Reads an Analog Devices ADXL3xx accelerometer and communicates the

 acceleration to the computer.  The pins used are designed to be easily

 compatible with the breakout boards from Sparkfun, available from:

 http://www.sparkfun.com/commerce/categories.php?c=80

 http://www.arduino.cc/en/Tutorial/ADXL3xx

 The circuit:

 analog 0: accelerometer self test

 analog 1: z-axis

 analog 2: y-axis

 analog 3: x-axis

 analog 4: ground

 analog 5: vcc

 created 2 Jul 2008

 by David A. Mellis

 modified 16 Mar 2012

 by Capstone420

 This example code is in the public domain.

*/

// these constants describe the pins. They won't change:

// const int groundpin = 18;             // analog input pin 4 -- ground

// const int powerpin = 19;              // analog input pin 5 -- voltage

const int xpin = A2;                  // x-axis of the accelerometer

const int ypin = A3;                  // y-axis

const int zpin = A4;                  // z-axis (only on 3-axis models)

void setup()

{

  // initialize the serial communications:

  Serial.begin(9600);

  // Provide ground and power by using the analog inputs as normal

  // digital pins.  This makes it possible to directly connect the

  // breakout board to the Arduino. 

 // pinMode(groundpin, OUTPUT);

 //

 // pinMode(powerpin, OUTPUT);

  // digitalWrite(groundpin, LOW);

 // digitalWrite(powerpin, HIGH);

}

void loop()

{

  // print the sensor values:

  float xRead = analogRead(xpin);

  float yRead = analogRead(ypin);

  float zRead = analogRead(zpin);

 

  float xg = -(xRead - 352)/73.;

  float yg = -(yRead - 361)/75.;

  float zg = -(zRead - 361)/71.

  ;

  Serial.print(xg);

  // print a tab between values:

  Serial.print("\t");

  Serial.print(yg);

  // print a tab between values:

  Serial.print("\t");

  Serial.print(zg);

  Serial.println();

   Serial.print(xRead);

  // print a tab between values:

  Serial.print("\t");

  Serial.print(yRead);

  // print a tab between values:

  Serial.print("\t");

  Serial.print(zRead

  );

  Serial.println();

 

  // delay before next reading:

  delay(1000);

}

Results

The results obtained were as follows:

Accel X	Accel Y	Accel Z
-0.74	-0.27	-1.76
-0.75	-0.27	-1.79
-0.78	-0.32	-2.27
-0.73	-0.25	-1.72
-0.75	-0.27	-1.75
-0.75	-0.27	-1.76
-0.74	-0.27	-1.75
-0.74	-0.25	-1.72
-0.73	-0.23	-1.73
-0.74	-0.27	-1.75
-0.75	-0.25	-1.45
-0.73	-0.29	-1.76
-0.74	-0.31	-1.76
-0.74	-0.27	-1.75
-0.73	-0.27	-1.75
-0.74	-0.25	-1.75
-0.75	-0.27	-1.75
-0.75	-0.27	-1.76
-0.74	-0.25	-1.73
-0.41	-0.13	-1.77
-0.74	-0.25	-1.75
-0.75	-0.25	-1.72
-0.74	-0.24	-1.72

The values were a little bit off since calibration is still needed. The team will calibrate it right before launch to ensure precision.

Conclusion

In summary, the ADXL335 accelerometer is a small, low profile package, 3-axis sensing device that requires low power (350 , typically) and a single supply operation: 1.8V to 3,6V. It also possesses 10,000 g shock survival, excellent temperature stability, BW adjustment with a single capacitor per axis, and RoHS/WEEE lead-free compliant. [2]

We expect to know when the balloon will pass the jet stream once the payload is recovered.

Recommendations on Test Results

As mentioned before, Calibration is still needed to ensure proper data gathering. However, this will be done right before launch. 

References

[1] (http://bildr.org/2011/04/sensing-orientation-with-the-adxl335-arduino/, “Sensing Orientation With The ADXL335 + Arduino” Friday, April 22 ^nd , 2011 )

[2] (http://www.sparkfun.com/datasheets/Components/SMD/adxl335.pdf, AnalogDevices, 2009)