are these ADXL345 accelerometers broken or they just suck?

Other Hardware Sensors
1

Hi all,

i recently bought 6 of these:

because they where cheap. Now i'm thinking that broken because i have hooked them up in SPI mode and in the same config, the readings are completely different:

All in horizontal position. According to the data sheet this should result in X = 0G Y = 0G Z = 1G

This is what i get:

accelerometer 1:

acc 1,   G value (xyz): -0.24	-0.35	-0.34
acc 1, raw value (xyz): -31	-45	-43

acc 2:

acc 1,   G value (xyz): -0.71	12.11	0.87
acc 1, raw value (xyz): -91	1552	112

acc 1,   G value (xyz): -0.73	-0.12	0.75
acc 1, raw value (xyz): -94	-16	96

acc 1,   G value (xyz): -0.73	-0.12	-1.00
acc 1, raw value (xyz): -94	-16	-128

acc 1,   G value (xyz): -0.71	4.12	0.75
acc 1, raw value (xyz): -91	528	96

acc 3:

acc 1,   G value (xyz): 0.56	-1.17	-0.03
acc 1, raw value (xyz): 72	-150	-4

acc 1,   G value (xyz): 0.56	-1.17	-0.02
acc 1, raw value (xyz): 72	-150	-3

acc 1,   G value (xyz): 0.57	-1.17	-0.02
acc 1, raw value (xyz): 73	-150	-2

acc 1,   G value (xyz): 0.57	-1.17	-0.02
acc 1, raw value (xyz): 73	-150	-2

and the fourth:

acc 1,   G value (xyz): 0.07	-0.01	0.89
acc 1, raw value (xyz): 9	-1	114

acc 1,   G value (xyz): 0.07	-0.01	0.89
acc 1, raw value (xyz): 9	-1	114

acc 1,   G value (xyz): 0.07	-0.01	0.89
acc 1, raw value (xyz): 9	-1	114

and acc 5:

acc 1,   G value (xyz): -0.09	0.06	-0.84
acc 1, raw value (xyz): -12	8	-108

and 6th:

acc 1,   G value (xyz): -0.26	-0.18	-0.30
acc 1, raw value (xyz): -33	-23	-38

So some are not conistent, it looks like they are moving while being in rest. Others just give too high values.

is it normal that you would need to compenstate or recalculate the correct values? In the tutorial on sparkfun they do not mention any calibration:

http://www.sparkfun.com/tutorials/240

Do i need to calibrate?

2

I had the same thing. It turned out it was the vibration of my computer on the same table.

Although only your fourth sensor has good readings, it is more likely that there is something with your code or connections.
Start with just one ADXL345 on the SPI bus, and try to get it right.

Hardware:
Is it connected to 3.3V ? Do you use 4-wire SPI mode ? How is the 'CS' connected ?

Software:
Do you use an example or library ? Which one ? Can you upload your code ? Are you sure the bytes are in the right order ?

Do you know there are links in the Accelerator section in the Playground : http://arduino.cc/playground/Main/InterfacingWithHardware#motion_accelerometer

I think you need to calibrate the offset anyway. As fas as I know all MEMS-based sensors need to be calibrated. Check the datasheet for "Offset Calibration".

In the end, they can get broken if they are hit too hard. But the ADXL345 can survive 10.000 g shock ! That's more than many other sensors.

3

You have to be careful with most I2C/SPI dual interface accel's and gyros - most are NOT SPI-compatible (nothing else can use the SPI bus). Although its rather slower try using the I2C interface. To use SPI you need CPOL=1, CPHA=1 and you need to bring and hold SCLK high between SPI transactions - hence no other devices can be on the SPI bus.