Vibration Data logger with ADXL345

Hello everyone!

I am trying to build a vibration datalogger for recording vibrations on a machine. It will be consist of 4 Arduino Uno with ADXL345 sensors and a Raspberry Pi for database.

The Raspberry Pi will read the data coming from Arduinos and it will insert them to a MySQL table. At the moment I am planning to use USB ports for communication (max distance from Raspberry Pi to Arduino will be less than 3 meters).

The required resolution is 4G and data rate is approximately 1000Hz (thus I selected 800Hz).

At the moment I am saving the readings to a file on Raspberry Pi, and my main problem is that I am getting 70-80 readings per second. I am not an coding expert and I have started with ADXL345 basic code which can be found everywhere and done little modifications.

Could anyone can help me to find the problem with data rate?

//Add the SPI library so we can communicate with the ADXL345 sensor
#include <SPI.h>

//Assign the Chip Select signal to pin 10.
int CS=10;


char POWER_CTL = 0x2D;	
char DATA_FORMAT = 0x31;
char BW_RATE = 0x2C;     
char DATAX0 = 0x32;	//X-Axis Data 0
char DATAX1 = 0x33;	//X-Axis Data 1
char DATAY0 = 0x34;	//Y-Axis Data 0
char DATAY1 = 0x35;	//Y-Axis Data 1
char DATAZ0 = 0x36;	//Z-Axis Data 0
char DATAZ1 = 0x37;	//Z-Axis Data 1

//This buffer will hold values read from the ADXL345 registers.
char values[10];

//These variables will be used to hold the x,y and z axis accelerometer values.
int x,y,z;

void setup(){ 
  //Initiate an SPI communication instance.
  SPI.begin();
  //Configure the SPI connection for the ADXL345.
  SPI.setDataMode(SPI_MODE3);
  //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);

  writeRegister(DATA_FORMAT, 0x01);
  writeRegister(POWER_CTL, 0x08);  
  writeRegister(BW_RATE, 0x0E);    

  //Create a serial connection to display the data on the terminal.
  Serial.begin(9600);
}

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.
  x = ((int)values[1]<<8)|(int)values[0];
  y = ((int)values[3]<<8)|(int)values[2];
  z = ((int)values[5]<<8)|(int)values[4];
  
  Serial.print(x);
  Serial.print(",");
  Serial.print(y);
  Serial.print(",");
  Serial.println(z); 
}

//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);
}

Regards

Serial.begin(9600);

9600 BAUD is 960 ASCII characters per second, . As you are printing values, every character is a byte. So while an integer value is 2 bytes in memory, it is converted to characters to print over serial.

Increase the BAUD rate as high as you can without the rPi choking.

It you still cannot get the serial bandwidth as fast as you need, consider creating a struct to hold the data and copying the struct elements to the rPi... There are a few examples of this technique around the forum... Usually for radio control stuff. With struct data, you can move the data with any protocol.

Ray

You can also use Serial.write() to send the raw data as individual or multiple bytes, without formatting into ASCII characters. This is much faster than using Serial.print(). Again, use the highest baud rate possible.
http://arduino.cc/en/Serial/write

Thank you for your replies.

Increasind baud rate helped to get more readings, at 115200 I am getting aprox. 1200 readings per sec, which is enough for my project.

However I realized that there was another problem. If I don't add delay() after print commands I allways get 1200 readings independent from the BW_RATE that I set. I have tried it for 50 to 800 Hz. For 800 Hz I believe that delay should be 1.25, but if I add delay (1.25) I get 600-650 readings. I have also tried it for other frequencies and I allways get 10-15% less readings then it should be.

What could cause that problem?

Regards.

BW_RATE is a power setting for the ADXL. It does not control any actual data rate on the part. You need to not read x,y,z registers any faster that the BW_RATE you have set. Max setting on the ADXL is for 3200Hz. If you are getting 1200 reading per second, make sure the BW_RATE is 0x0e or 0x0f.
Your delay time is added to whatever other overhead you have in your program. That overhead may be enough, alone, to provide a suitable data rate.