LIS2DH12 acceleration data not stable

[Manuel Ferrero]

I am using the STM driver to control the LIS2DH12TR accelerometer.

The freeRTOS sensor task is as following (checks on return values and variables definitions are removed to simplify the code comprehension):

 

void MEMS_task(void)
{
 lis2dh12_self_test(&dev_ctx);

 result = lis2dh12_operating_mode_set(&dev_ctx, LIS2DH12_HR_12bit);
 result = lis2dh12_data_rate_set(&dev_ctx, LIS2DH12_ODR_100Hz);
 result = lis2dh12_fifo_mode_set(&dev_ctx, LIS2DH12_BYPASS_MODE);
 result = lis2dh12_block_data_update_set(&dev_ctx, PROPERTY_ENABLE);
 result = lis2dh12_temperature_meas_set(&dev_ctx, LIS2DH12_TEMP_ENABLE);

 for (;;)
 {
 result = lis2dh12_xl_data_ready_get(&dev_ctx, &acceleration_data_available);
 if (acceleration_data_available != 0)
 {
 /* Read accelerometer data */
 memset(data_raw_acceleration, 0x00, sizeof(data_raw_acceleration));
 lis2dh12_acceleration_raw_get(&dev_ctx, data_raw_acceleration);
 acceleration_mg[0] = lis2dh12_from_fs2_hr_to_mg(data_raw_acceleration[0]);
 acceleration_mg[1] = lis2dh12_from_fs2_hr_to_mg(data_raw_acceleration[1]);
 acceleration_mg[2] = lis2dh12_from_fs2_hr_to_mg(data_raw_acceleration[2]);
 }
 }
}​

 

 

If I put my board on a table I would expect to read zero on X ands Y axes and 1g on Z axe, but instead read values that flickers around the expected ones.

Here a few samples:

 

X: 0
Y: -22
Z: 1016

X: -2
Y: -20
Z: 1024

X: -1
Y: -21
Z: 1017

X: 4
Y: -24
Z: 1019

 

Is this normal?

How do I define a position as stable, if acceleration values are so variable?

[Federica Bossi]

Hi @Manuel Ferrero ,

Yes, it is normal. You have to consider the impact of noise.

In addition, test them in controlled environment to minimize ambient noise as well.