LIS3DHH accuracy and stability

[Kirill Rzhav]

Hello! I am R&D inclinometer based on LIS3DHH. Firstly I started my works with LIS2HH12 as MEMS accelerometer due to his best accuracy and resolution. I wanted to get accuracy about 1-2 angular minutes. All was good when I turned on almost all internal filters and using software moving average. Even with 7 sec response it was ok for my solution. But after few experiments I discovered problem with temperature stability. I decided use internal temperature sensor to compensate temperature coefficient and started temperature experiment (first with termal camera, but after without, because was a lot noise from camera). And I can't find out temperature coefficient for that. Next figure shows angle for one axis (red), temperature from internal sensor (blue) and reference signal from another special inclinometer. So, angle changed unpredictably on different days.

Next figure shows one angle (green) and temperature (blue). Here some unpredictable overshoot.

In this case I cant solve the problem with temperature and discover new LIS3DHH. I made a new PCB layout for it. There only 2 internal filters and with using moving average. The best result I've got only 5 angular minutes accuracy. Then I start temperature experiment for LIS3DHH and it was better than LIS2HH12.

Can you help me, how I can solve temperature stability problem in LIS2HH12 or accuracy in LIS3DHH. Maybe better to chose another accelerometer for inclinometer solution? Is it possible to have stability angle with 1-2 angular minutes accuracy with MEMS technology?

[Miroslav BATEK]

I have a few questions:

• How do you calculate the tilt angle?
• How did you measure the temperature?
• Is the first chart from LIS2HH12 and second from LIS3DHH?
• Form the first chart, it seems the angle change about more than 10 degrees with very small temperature change. Can you confirm that my understanding is correct?
• For the second chart the angle changed about 5 degrees but in the test you say 5 angular minutes.

[Kirill Rzhav]

Hi! Sorry for delay with reply

• I calculate angle as standart. The same formula was in AN4509.
• I mesured temperature by internal sensor in LIS.
• Yes, you are right.
• No, 10 angular minutes. All angles on chart in angular minutes

[Miroslav BATEK]

May I know, how did you heat up the LIS3DHH sensor? Can you guarantee that it didn't introduced any movement with the sensor?

[alessio31183]

Hi everyone,

I'm working with the LIS3DHH to make a precision inclinometer.

But what I discover is a bad behavior of the sensor around 5°C. There is a sort of unpredictable offset which compare suddenly.

My test chamber is a Parani climatic chamber with a proper internal stabilized plane where I can test the inclinometer temperature changing from 50°C to 0°C.

Actually I'm in contact with some guy of the technical ST staff but I'm still waiting for a reply.

What I know is that in the previous datasheet revision there was the bit 0 of CTRL_REG_4 relative to an "internal temperature compensation" that was not so clear explained.

After the next datasheet revision that bit is specified as "1" and MUST BE SET TO 1 FOR PROPER OPERATION.

Honestly I close to discard this component for this because I cannot trust in it for this weird temperature drift behavior.

Previously my precision inclinometer was based on the ADXL203 that I consider the best of the MEMS accelerometer for tilt measurement (of course the best with an honest price).

Anyway I was looking a digital solution to miniaturize the system as well.

The image attached shows the raw data from the accelerometer without the tilt calculation just to be sure that this behavior was regarding to the accelerometer itself and was not coming from some strange calculation in firmware

[Miroslav BATEK]

@Community member​  is the sensor moving/tilting in the chamber? From you chart is seems it is moving, as the Y axis is in degrees and the change is several tens of degrees.

[alessio31183]

No the movement that you see is only due to the temperature drift because it's not compensated by me during this test just to see the effect of the accelerometer itself.

In red you see the temperature changing from 45°C to 5°C passing through the 25°C

[Miroslav BATEK]

So you did the test with the bit 0 in CTRL_REG_4 set to 0?

[alessio31183]

both, with 0 and 1 and I got the same result.

That jump around 5°C is the same in both cases

[Miroslav BATEK]

I just want to add, that the device has specification of the maximum zero-g level vs temperature 0.4mg/°C. You can calculate the impact on the tilt angle, the maximum is 11 angle minutes/°C!