Accelerometer giving high Z-axis values

Hi

I've just got myself an ADXL345 (http://www.dx.com/p/gy-291-adxl345-digital-3-axis-acceleration-of-gravity-tilt-module-for-arduino-148921#.VVW02M7Sm9A) but it's reporting high Z-axis values. The values I get are:

  • around 2000 when laid flat
  • around 1700 when rotated by 90 degrees
  • around 1400 when rotated by 180 degrees

The X and Y axis values are fine. I'm using 16G because 2G gives a clipped value of 511 for Z.

I wonder if I'm missing something or mine's a dud! Here's my code:

//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;

//This is a list of some of the registers available on the ADXL345.
//To learn more about these and the rest of the registers on the ADXL345, read the datasheet!
char POWER_CTL = 0x2D;	//Power Control Register
char DATA_FORMAT = 0x31;
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.
int16_t x,y,z;

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);
  
  //Put the ADXL345 into +/- 4G range by writing the value 0x01 to the DATA_FORMAT register.
  writeRegister(DATA_FORMAT, 0x02);
  //Put the ADXL345 into Measurement Mode by writing 0x08 to the POWER_CTL register.
  writeRegister(POWER_CTL, 0x08);  //Measurement mode  
  
  writeRegister(0x20, 0x00);
}

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 = (int16_t) ((uint16_t)values[1]<<8)|(uint16_t)values[0];
  //The Y value is stored in values[2] and values[3].
  y = (int16_t) ((uint16_t)values[3]<<8)|(uint16_t)values[2];
  //The Z value is stored in values[4] and values[5].
  z = (int16_t) ((uint16_t)values[5]<<8)|(uint16_t)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(1000); 
}

//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);
}

Any help will be appreciated.

Thanks

The axis pointing upwards measures the earth gravity.

See this tutorial.

mun35:

  • around 2000 when laid flat
  • around 1700 when rotated by 90 degrees
  • around 1400 when rotated by 1800 degrees

This means an offset of 1700, and a symmetrical sensitivity of 300/g, close to the typical sensitivity of 256/g.

The sensor looks fine to me, but I don't know what eventual problems the high offset could indicate.

But is it typically the sense for the Z-axis to not work at 2G and only work at 16G though?

You may have a defective accelerometer, a defective program, or both.

jremington:
You may have a defective accelerometer, a defective program, or both.

I've bought two and both of them have the same issue. The Z offset is for some reason so high, even though the sensor seems to be detection motion quite fine.

It is extremely unlikely that two accelerometers would be defective in the same way. Therefore, if there is a problem, it is most likely in your program. Work your way carefully through the data sheet and understand and verify all commands and addresses sent to the device. You should also understand the data format that is expected (see below).

  1. Pay particular attention to the DATA FORMAT Register. Note the FULL_RES Bit
    From the data sheet:

When this bit is set to a value of 1, the device is in full resolution mode, where the output resolution increases with the g range set by the range bits to maintain a 4 mg/LSB scale factor. When the FULL_RES bit is set to 0, the device is in 10-bit mode, and the range bits determine the maximum g range and scale factor.

  1. I note that you are reading the data as unsigned integers. The data format, however, is twos complement and you have selected 10 bit mode. That should be a strong hint.

  2. The axes should behave similarly. Orient the device in several different ways (X up and down, Y up and down) and see if this is the case.

Consider other example software, like I linked above.

Hey I got an ADXL345 a month or so ago and it's showing the same issue. Doing a quick Google search I see numerous other individuals reporting the Z axis always showing 511 in 2g mode as well. It appears the manufacturer probably did a big run of defective chips for some customer, and ended up dumping them in the after market.

Same here, we have 1 older ADXL345 which works with the same Arduino Mega2560, same programs, same cables. Then we bought 2 new ADXL345 and both show static Z axis values when the scale is set to 2G.

In case you still have yours, the ADXL on the working breakout has this markings:

345B
#223
0736
PHIL

Edit: just found another "good" one:
345B
#224
7352
PHIL

and the bad ones are both

345B
#119
3748
PHIL

regards,
Joao