Temperature stability of accelerometer FXOS8700CQ

[gertjanhoeve]

Hi,

I am looking for a high accuracy (<0.001g) accelerometer, with limited zero-g offset change in temperature range from 278 to 315 Kelvin ( <0.002g, or as small as possible), to serve as an inclinometer. Currently I have under investigation a FXOS8700CQ sensor on a STBC-AGM01 board, on a K64F development board.

I use the Kinetis Interface Tool (KIT2) and the Quick Read Streaming Commands to watch the accelerometer signal, as that is the only way I managed to establish communication with the board.

The accelerometer offset appears to be very temperature stable, however, about half of the time when the sensor is heating after taking it out of the refrigerator, the offset of all three accelerometers makes a small jump of about 0.002g (all three simultaneously, though not by the same amount). As the signal of the vertical acceleromater also changes by the same order of magnitude, I am pretty sure it is not a small rotation of the sensor. Can anybody please explain what can possibly be the cause? Is the signal perhaps temperature compensated, and that there is a problem with the temperature sensor? Or does the compensation algorithm have a problem with rapid temperature changes? I have to copies of those sensor boards here, and so far I have only observed it on one of them. Anyway, I would like to know if the underlying cause is a structural problem of the sensor, or that there is a problem with the sensor board, so we can continue development.

While browsing through the forum looking for a solution I discovered the MMA8451Q. According to the data sheet the temperature performance is even a little better than the FXOS8700CQ. In fact I could not find any downside on the MMA8451Q (except for the missing magnetometer, but I am not using that one anyway). Any advice please? Is MMA8451Q simply a newer, improved version?

Best regards,

Gert Jan

[TomasVaverka · Accepted solution]

Hi Gert Jan,

I would recommend considering the MMA845xQ family with slightly better temperature performance than the MMA865xQ, FXLS8471Q accelerometers or FXOS8700CQ 6-axis sensor.

As stated in the data sheet, the zero-g level changes typically within ±0.15 mg/°C. This means that if the environmental temperature changes 37 °C, then the zero-g level changes within the range of ±0.15 mg * 37 = ±5.55 mg. 2 mg is therefore hardly achievable unless you measure each part at two temperatures to compensate the TCO_ff.

As for thermal and mechanical stresses during the soldering process, re-calibration is recommended for applications requiring higher accuracy. This involves placing the final product at multiple orientations and calculating additional new calibration parameters which should then be stored in non volatile memory and applied by an external uC. We have an application note AN4399 and example code illustrating how to do this. Most customers do 3 or 6 orientation calibrations, but we recommend the 12 point algorithm that corrects for a 3x3 gain matrix and 3x1 offset vector.

I hope it helps.

Regards,

Tomas

PS: If my answer helps to solve your question, please mark it as "Correct". Thank you.