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I had a PCB containing a micro usb and a 3 axes accelerometer LIS3DH. I put the PCB at fixed position and the value of Z vary between --77at -83. but when I touch the usb cable with my hand or with a material stuff and without changing the PCB position, I receive big glitches of the values of z (up to 700 on Z value). is that really my hand which cause this fluctuation (knowing that if I dont touch the usb, the value would stay stable (-77 -83for hours)

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5 Answers 5

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The accelerometer has a digital interface. It's fairly unlikely that you are looking at electrical noise or interference, and you're looking at accelerations that the device is experiencing. You should make an effort to put some numbers in real units on your Y-axis to figure out how large the accelerations are.

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You may well have induced-electrical-noise issues.

Test:

  • Place the PCB and cable on a flat surface and

    • Weight the cable with a cup or similar non conductor near the PCB. This allows you to move the cable slightly when you touch it without moving the PCB.

    • Now touch the cable on the non-PCB side of the weight.

If you get similar results it's probably electrical noise.
If so, you need to examine the circuitry where the USB cable interfaces to see if it may be allowing induced signal to interfere with the accelerometer signal.

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    \$\begingroup\$ methinks you mean electrical noise/interference. In accelerometers there is also mechanical noise. \$\endgroup\$ Jun 23, 2016 at 15:02
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    \$\begingroup\$ @placeholder Yes, I meant electrical noise. My clamp on the cable procedure above was aimed at eliminating (or much reducing) mechanical coupling from cable to sensor so that electrical coupling effects can be checked. Olin deals with mechanicaal coupling. \$\endgroup\$
    – Russell McMahon
    Jun 23, 2016 at 18:00
  • \$\begingroup\$ Ummm .. that comment was a hint that perhaps you should clarify your write up. \$\endgroup\$ Jun 23, 2016 at 18:26
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    \$\begingroup\$ @placeholder Anyone following my suggestions (physically or thought-experimentally) and then not understanding the implications of the outcome (of which there are two main probable ones) is unlikely to benefit from any amount of clarification. \$\endgroup\$
    – Russell McMahon
    Jun 24, 2016 at 0:25
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    \$\begingroup\$ @placeholder Tidied formatting. Added a few words. I wot that anyone straining at enough gnats to render " ... without moving the PCB ...may be allowing induced signal to interfere with the accelerometer ..." as relating to mechanical "signal" affects on the PCB probably should be asking the question on a mechanical site and ignoring the fact that electronics exist. \$\endgroup\$
    – Russell McMahon
    Jun 24, 2016 at 0:30
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Always begin your troubleshooting tasks by measuring/monitoring the power supplies. I recommend you use an oscilloscope to monitor the power supply voltage at the accelerometer IC's power pin to see what happens to the voltage when you touch the USB cable. (n.b. You cannot use a multimeter for this test; you must use an oscilloscope.)

Using the oscilloscope, look for an obvious voltage spike on the power bus at the moment you touch the USB cable. If touching the USB cable induces obvious electrical noise on the power supply bus (see also electrostatic induction), then you need to remedy this problem before continuing.

If an obvious voltage spike DOES NOT appear on the power bus when you touch the USB cable, then as others have already suggested, it seems likely you are inadvertently moving the accelerometer IC as you touch USB cable, and the IC is detecting this motion.

And in addition to what @RussellMcMahon suggested, I would orient the accelerometer IC so that one of its three axes (e.g., its Z-axis) is perpendicular to your tabletop, and lightly press the IC (or circuit board) onto the tabletop so that it cannot move perpendicular to the tabletop—e.g., place something onto the IC/circuit board to hold it down (don't press on it with your finger). Now touch the USB cable. If touching the USB cable causes the IC to move, one would expect that the strongest acceleration values would be observed in the two axes that are parallel to the tabletop (e.g., the X-axis and Y-axis directions), and the weakest acceleration values would be observed on the axis perpendicular to the tabletop (e.g., the Z-axis).

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The LIS3DH accelerometer has a built-in thermometer. While the accelerometer may be sensitive to both mechanical and electrical noise, the themrmometer is probably sensitive only to electrically-induced noise. If you monitor the output of the thermometer, you might be able to determine whether your original noise is of mechanical or electrical origin.

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  • \$\begingroup\$ Interesting, but doesn't answer the question. \$\endgroup\$ Jul 13, 2017 at 15:54
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It seems like the accelerometer is picking up small accelerations caused by your hand or this "material stuff" moving it.

A more remote possibility is that there is a bad connection, like a bad solder joint, on the board, and the small motions are making it open and close intermittently.

Try the board in different orthogonal gravity-aligned orientations. If only one axis responds as you describe, then perhaps something is wrong with that axis.

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  • \$\begingroup\$ Hy Olin, I dont think I create accelerations, I ust have touch the usb cable with an unplugged usb male to see the glitches \$\endgroup\$ Jun 23, 2016 at 12:15
  • \$\begingroup\$ olso I noticed that, when there is a glitch, if affected the 3 axes x,y,z simultaneously \$\endgroup\$ Jun 23, 2016 at 12:19

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