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Noise and Error in Linear Acceleration Readings

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Johnny101

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Hi everyone,

I am trying to measure linear acceleration using an H3lis100dl (www.st.com/resource/en/datasheet/h3lis100dl.pdf) with an arduino. I am measuring high g accelerations thus using the 100g accelerometer. The issue is that whenever I slightly tap the accelerometer it shows that the linear acceleration is around 7-8 g and even sometimes more, which is not possible. So, to check this I hooked up an MPU6050 (16g) along with the h3lis on the same board. And as you can see in Fig 1, on tapping the board slightly, the MPU6050 shows a slight movement suggesting that a low g movement has occurred but the H3lis can peak upto 7-8 g's. The y-axis in the figure is in m/s2. Red - H23lis, Blue - MPU6050.
Fig 1.png

Now if I don't tap the board and only do a rotation or a continuous sweeping sort of movement. I get a bell sort of a curve and in this case both the readings are pretty much in sync (Fig 2 & 3).
Fig 2.png Fig 3.png

So, the 100g accelerometer under a sudden shock situation is not responding in the right way. Also, I get a constant noise in the H3lis signal shown in Fig 4, and also visible in other figures that H3lis readings fluctuate a lot.
Fig 4.png

Any suggestions for figuring out and rectifying the H3lis behavior, under a sudden movement situation and improving the noise response(I am doing a simple averaging to control the noise).

Thanks in advance.
 

Hi,

There are some possible reasons:
* purely electrical, like ESD
* electro-mechanical, like piezoelectric effect of capacitors or wires
* or mechanical, like a loose screw or similar.

Klaus
 

Both devices have user-programmable digital filters with distinct cut-off frequencies, so that reading may not be the same for short-lived impulses. In other words, these accelerators must have the same response for a constant acceleration, but each one will surely exhibit a different reading for events of very short duration. Anyway the apparatus you made is an intresting way to assess performance of each device subjected to the same condition at the same time.
 

You are right about the digital filters and since upon the tap I am getting a sharp peak it had to do with the low pass filter, so I reduced the H3lis cut off to 37 Hz and now the response upon short taps doesn't exceed 3g.

So, lowering the cutoff for H3lis improves the situation, but now the other problem is that the cutoff is not programmable rather tied to the output data rate (ODR) of the sensor. So, inorder to get 37 HZ, the ODR was set at 50Hz where as I was using 400 Hz before that.

Any ideas on keeping the ODR 400 Hz but reducing the cutoff to say 37 Hz. May be implementing a digital low pass filter, I am already doing a simple averaging.
 

Hi,

inorder to get 37 HZ, the ODR was set at 50Hz
I wonder ... then the cutoff frequency is beyond nyquist limit.

Filter:
A simlpe 2nd order biquad is easy to implement and doesn´t need much processing power.

Klaus
 

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