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]

Yes, I got your point. Let me discuss with sensor specialists.

[alessio31183]

Hi Miroslav, do you have some news?

In the meantime I give yout another example on how data changes dramatically and suddenly without any motivation

 

Moreover, this behavior happens going down only with the temperature and NOT going up

Thanks in advance

[alessio31183]

Miroslav I've found the problem.

THE RESIN PROCESS ON THE ACCELEROMETER MAKES THIS STRANGE BEHAVIOR.

UNBELIEVABLE

Usually I did the process for all other accelerometer that I used in the past

Probably this time the resin was to hard and with a different thermal coefficient so it stressed the accelerometer position makes this sort of offset.

I don't want what to say. Sorry to everyone for the time that I made you lose.

Now I will do some other tests in order to be sure that this is the right solution for my application.

[Miroslav BATEK]

OK, @Community member​ thank you for the information.

[alessio31183]

Hi Miroslav,

I'm just looking carefully to the Recommended power-up sequence explained in the datasheet.

Basically it says that the accelerometer need to connect the 3.3V remotely with a couple of capacitor close to the component.

10uF for the 3v3 (pin8) and 100nF (pin7) in order to make the Twait.

Do you think that this is can involve in some bad behavior if not observed?

Thanks in advance

[Miroslav BATEK]

I don't understand what exactly is your question.

[alessio31183]

Hi,

my question was relative to the bad behavior that I have on some accelerometers.

Anyway I just wanted to ask you if the internal temperature compensation process can be disabled setting the first bit on the CTRL_REG4 equal to "0".

Can I ?

Basically because I use my temperature compensation process in firmware which is perfectly working with the ADXL203.

Many thanks

Alessio

[alessio31183]

Here the thermal tests on 12 boards.

Test are based on 2hour @ 25°C, 2hours @ 45°C, 2 hours @ 5°C and back to 25°C to check if the inclinometer comes back to the initial value.

As you can see someone is simple crazy.

Waiting a reply.

Alessio

[alessio31183]

Updating......

After several tests on 60 pieces I've observed a different temperature behavior on the same accelerometer.

Basically, if I do a thermal test where I get the temperature coefficient for every device in order to perform the firmware temperature compensation, I can have a different value on a different test.

So it doesn't seems constant for the same component. This aspect makes the accelerometer absolutely unless.

My idea is that either probably the ecopack package has a different thermal behavior or the internal temperature compensation works in some way in order to change the output value.

Further I cannot figure out how the temperature compensation can work because each component has a different temperature behavior which require a test on every device.

Anyway I just wanted to ask you if the internal temperature compensation process can be disabled setting the first bit on the CTRL_REG4 equal to "0" and which kind of calculation it performs internally.

Please try to help me because I will switch over on the MMA8451 immediately.

I cannot waste time and effort on this component anymore.