What is the source of the noise when approaching?

feklee commented 8 years ago

See attachment. What is the source of the noise?

Note: This is not the 50 Hz electric noise that the setup easily picks up from mains.

Ideas for investigation:

Is the noise also there when just measuring the signal? (it should not)
Is the noise also there when moving the piezo down, but not closely approaching?
Is the noise also there when running the code for moving the piezo down, but without physically moving it? approach_2.pdf

feklee commented 8 years ago

First results:

The noise is not visible when just monitoring the signal.
The noise is visible when moving the piezo down, without being close to the surface (at least one screw turn away).

When not actually changing the position of the piezo (code edited temporarily), noise is visible, but of different quality, more like an offset by 0.5 V). See attachment. Command line (note that piezo position stays at zero):

> piezo-down 20000 2 10
Last signals in reverse (V): 0.57, 0.57, 0.57, 0.65, 0.56, 0.57, 0.57, 0.57, 0.57, 0.55, 0.57, 0.57, 0.57, 0.58, 0.57, 0.57, 0.57, 0.57, 0.57, 0.56, 0.57, 0.57, 0.57, 0.56, 0.57, 0.57, 0.56, 0.57, 0.56, 0.57, 0.57, 0.57, 0.56, 0.56, 0.57, 0.57, 0.56, 0.57, 0.56, 0.57, 0.57, 0.56, 0.57, 0.57, 0.57, 0.57, 0.57, 0.56, 0.57, 0.55, 0.57, 0.56, 0.57, 0.56, 0.56, 0.57, 0.57, 0.57, 0.57, 0.57, 0.65, 0.57, 0.57, 0.57, 0.57, 0.58, 0.55, 0.57, 0.57, 0.57, 0.58, 0.57, 0.57, 0.56, 0.57, 0.57, 0.56, 0.57, 0.57, 0.57, 0.56, 0.57, 0.57, 0.57, 0.57, 0.56, 0.57, 0.57, 0.57, 0.58, 0.55, 0.56, 0.57, 0.56, 0.57, 0.55, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.55, 0.57, 0.57, 0.57, 0.57, 0.56, 0.57, 0.56, 0.57, 0.57, 0.57, 0.65, 0.56, 0.57, 0.57, 0.57, 0.56, 0.55, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.56, 0.57, 0.57, 0.57, 0.57, 0.56, 0.57, 0.56, 0.57, 0.57, 0.56, 0.57, 0.56, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.56, 0.56, 0.55, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.57, 0.56, 0.57, 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Current signal (V): 0.01; Steps moved: 20000; Piezo position (0-65535): 0; Measured bias (mV): 51.51
(0.45 s)

The noise disappears when commenting out SPI communication with the DAC. It also changes depending on which lines of code are disabled. Could the CLK line be causing the noise?

noise_without_positioning_piezo.pdf

feklee commented 8 years ago

See attachment. A delay of 100 µs after SPI communication with the DAC controlling the piezo makes the noise go away.

no_noise_with_delay.pdf

feklee / stm

What is the source of the noise when approaching? #3