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Noise and Error in Linear Acceleration Readings

Discussion in 'Microcontrollers, Programming and IoT' started by Johnny101, Aug 21, 2017.

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  1. Johnny101

    Johnny101

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    0
    Jul 13, 2014
    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.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,582
    2,360
    Nov 17, 2011
    Why do you think this is not possible? It doesn't take a lot of force to accelerate a 20mg sensor.
    Are the low pass filters in both devices progarmmed to the same cut-off frequency?
     
  3. kellys_eye

    kellys_eye

    4,275
    1,146
    Jun 25, 2010
    What is the x-axis scale?
     
  4. Johnny101

    Johnny101

    43
    0
    Jul 13, 2014
    I am talking about slight taps and nudges and it gets upto 7-8g and even upto 12g. The readings were taken with cutoff frequency of 292 Hz for H3lis and 256 Hz for MPU6050. I have since then tried lowering the cutoffs. So, for 37 Hz cutoff for H3lis there is significant improvement and now it doesn't cross 3g's. For MPU6050, there is not much difference operating at low or high cutoffs.

    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. My sampling rate is 200 Hz.

    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.
     
  5. Johnny101

    Johnny101

    43
    0
    Jul 13, 2014
    The x-axis is the number of samples my sampling rate is 200 Hz.
     
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