Free fall detection with ADXL346Z

Dear Community,

I am using the accelerometer ADXL346Z from analog Devices.
I am trying to detect a free fall in order to send an alarm signal.

As a first step, I compiled and checked the two examples provided in the following URL:
https://www.sparkfun.com/tutorials/240

Following a similar scheme respect the advanced example I have built the following code:

#include <SPI.h>
//Assign the Chip Select signal to pin 10.
int CS = 10;

///////////////////////////////////////////////////////////////////////
//ADXL345 Register Addresses
#define  DEVID   0x00  //Device ID Register
#define THRESH_TAP  0x1D  //Tap Threshold
#define OFSX    0x1E  //X-axis offset
#define OFSY    0x1F  //Y-axis offset
#define OFSZ    0x20  //Z-axis offset
#define DURATION  0x21  //Tap Duration
#define LATENT    0x22  //Tap latency
#define WINDOW    0x23  //Tap window
#define THRESH_ACT  0x24  //Activity Threshold
#define THRESH_INACT  0x25  //Inactivity Threshold
#define TIME_INACT  0x26  //Inactivity Time
#define ACT_INACT_CTL 0x27  //Axis enable control for activity and inactivity detection
#define THRESH_FF 0x28  //free-fall threshold
#define TIME_FF   0x29  //Free-Fall Time
#define TAP_AXES  0x2A  //Axis control for tap/double tap
#define ACT_TAP_STATUS  0x2B  //Source of tap/double tap
#define BW_RATE   0x2C  //Data rate and power mode control
#define POWER_CTL 0x2D  //Power Control Register
#define INT_ENABLE  0x2E  //Interrupt Enable Control
#define INT_MAP   0x2F  //Interrupt Mapping Control
#define INT_SOURCE  0x30  //Source of interrupts
#define DATA_FORMAT 0x31  //Data format control
#define DATAX0    0x32  //X-Axis Data 0
#define DATAX1    0x33  //X-Axis Data 1
#define DATAY0    0x34  //Y-Axis Data 0
#define DATAY1    0x35  //Y-Axis Data 1
#define DATAZ0    0x36  //Z-Axis Data 0
#define DATAZ1    0x37  //Z-Axis Data 1
#define FIFO_CTL  0x38  //FIFO control
#define FIFO_STATUS 0x39  //FIFO status
/////////////////////////////////////////////////////////////

//This buffer will hold values read from the ADXL345 registers.
char values[10];
char output[20];
//These variables will be used to hold the x,y and z axis accelerometer values.
int x, y, z;
boolean alarm = false;



void setup() {
  //Initiate an SPI communication instance.
  SPI.begin();
  //Configure the SPI connection for the ADXL345.
  SPI.setDataMode(SPI_MODE3);
  //Create a serial connection to display the data on the terminal.
  Serial.begin(9600);

  //Set up the Chip Select pin to be an output from the Arduino.
  pinMode(CS, OUTPUT);
  //Before communication starts, the Chip Select pin needs to be set high.
  digitalWrite(CS, HIGH);

  //Create an interrupt that will trigger when a tap is detected.
  attachInterrupt(0, fall, RISING);

  //Put the ADXL345 into +/- 4G range by writing the value 0x01 to the DATA_FORMAT register.
  writeRegister(DATA_FORMAT, 0x01);

  //Send the Frefall interrupt to INT1, others INT2
  writeRegister(INT_MAP, 0xFB);

  //Enable only the frefall int
  writeRegister(INT_ENABLE, 0x04);

  //Frefall parameters
  writeRegister(THRESH_FF, 0x05); //free-fall threshold
  writeRegister(TIME_FF, 0x14); //Free-Fall Time


  //Put the ADXL345 into Measurement Mode by writing 0x08 to the POWER_CTL register.
  writeRegister(POWER_CTL, 0x08);  //Measurement mode
  readRegister(INT_SOURCE, 1, values); //Clear the interrupts from the INT_SOURCE register.

}

void loop() {
  //Reading 6 bytes of data starting at register DATAX0 will retrieve the x,y and z acceleration values from the ADXL345.
  //The results of the read operation will get stored to the values[] buffer.
  readRegister(DATAX0, 6, values);

  //The ADXL345 gives 10-bit acceleration values, but they are stored as bytes (8-bits). To get the full value, two bytes must be combined for each axis.
  //The X value is stored in values[0] and values[1].
  x = ((int)values[1] << 8) | (int)values[0];
  //The Y value is stored in values[2] and values[3].
  y = ((int)values[3] << 8) | (int)values[2];
  //The Z value is stored in values[4] and values[5].
  z = ((int)values[5] << 8) | (int)values[4];


  //Print the results to the terminal.
  Serial.print(x, DEC);
  Serial.print(',');
  Serial.print(y, DEC);
  Serial.print(',');
  Serial.println(z, DEC);
  delay(100);

  if(alarma == true){
    Serial.println("ALARMA");
    delay(5000);
    alarma = false;
  }
  
  detachInterrupt(0);
  delay(500);
  attachInterrupt(0, fall, RISING);
}


//This function will write a value to a register on the ADXL345.
//Parameters:
//  char registerAddress - The register to write a value to
//  char value - The value to be written to the specified register.
void writeRegister(char registerAddress, char value){
  //Set Chip Select pin low to signal the beginning of an SPI packet.
  digitalWrite(CS, LOW);
  //Transfer the register address over SPI.
  SPI.transfer(registerAddress);
  //Transfer the desired register value over SPI.
  SPI.transfer(value);
  //Set the Chip Select pin high to signal the end of an SPI packet.
  digitalWrite(CS, HIGH);
}

//This function will read a certain number of registers starting from a specified address and store their values in a buffer.
//Parameters:
//  char registerAddress - The register addresse to start the read sequence from.
//  int numBytes - The number of registers that should be read.
//  char * values - A pointer to a buffer where the results of the operation should be stored.
void readRegister(char registerAddress, int numBytes, char * values){
  //Since we're performing a read operation, the most significant bit of the register address should be set.
  char address = 0x80 | registerAddress;
  //If we're doing a multi-byte read, bit 6 needs to be set as well.
  if(numBytes > 1)address = address | 0x40;
  
  //Set the Chip select pin low to start an SPI packet.
  digitalWrite(CS, LOW);
  //Transfer the starting register address that needs to be read.
  SPI.transfer(address);
  //Continue to read registers until we've read the number specified, storing the results to the input buffer.
  for(int i=0; i<numBytes; i++){
    values[i] = SPI.transfer(0x00);
  }
  //Set the Chips Select pin high to end the SPI packet.
  digitalWrite(CS, HIGH);
}

void fall(void){
  //Clear the interrupts on the ADXL345
  readRegister(INT_SOURCE, 1, values); 
  alarm = true;
}

I have set the minimum values for the free fall threshold and the free fall time.
Everything seems correct to me, but when I test the sketch, wich means throwing down the board with the sensor, no one alarm is showed.

Someone has any suggestion or idea?
Thanks in advance for your time.

Do you get normal readings from the accelerometer when the device is at rest?
i.e. zero in the horizontal axes, and g downwards?

AWOL:
Do you get normal readings from the accelerometer when the device is at rest?
i.e. zero in the horizontal axes, and g downwards?

The readings when the board is on the table are:

10,5,-74
9,5,-75
9,6,-74
9,5,-74
8,6,-74
10,5,-74
9,5,-74
9,6,-75
10,5,-75
9,5,-74
9,4,-74
9,6,-74
10,4,-74
9,5,-74
10,5,-73
9,5,-74

I have used the examples of the link for acquiring the values

wich means throwing down the board with the sensor, no one alarm is showed.

Free-fall means just that; if you're throwing it, that implies an acceleration.
The free-fall may not be long enough.

AWOL:
Free-fall means just that; if you're throwing it, that implies an acceleration.
The free-fall may not be long enough.

Sorry I meant leaving it fall.
I have set the same parameters of the free fall used by the guy of the following video, and as you can see, he only needs a few cm to detect the falling.

when turned 90 deg (so gravity works on other sensor element) u probably see last value close to 0 (within +/- 10)

Detect ALL values < 20… and call that falling.
U cannot expect all values to return zero. Even the slightest spin (any direction) will f… up readings.

knut_ny:
when turned 90 deg (so gravity works on other sensor element) u probably see last value close to 0 (within +/- 10)

Detect ALL values < 20… and call that falling.
U cannot expect all values to return zero. Even the slightest spin (any direction) will f… up readings.

Could you explain this in other words? I cannot understand you.
Thanks for your time

By turning the sensor in different directions u’ll see the numbers for zero g.
Your table shows 2 axis, 8…10 and 4…6
Find the zero g reading for the 3rd axis. I should be well under 20!

You can also see that the axis that experience gravity gives a reading 50+.
If the sensor is dropped, all axis should give low readings.

code: if (abs(xx)<20 && abs(yy)<20 && abs(zz)<20) { action for freefall}

The number 20 is my guess.