nGauge AFM User Manual
17
used. It is also possible that the sample topology is not centered around
the middle of the Fine Z range, which could result in image clipping. The
Approach tab displays a graph with the Fine Z position. If it appears that
most of the image data is at the lower or higher end of the Fine Z range,
you can click
Show Advanced Controls
and increase or decrease the
Coarse Z to center the data. In some cases, you may even need to
Disengage
and then re-
Approach.
PID Controller Tuning
The PI controller measures the tip oscillation amplitude and subtracts this
value from the setpoint to give an error term. The error term is multiplied
by the Proportional (P) coefficient and the Integral (I) coefficient to
generate a response in the Fine Z. If the P and I coefficients are too low,
the controller response will be slow and the tip will not track the sample
topology very well. If the P and I coefficients are too high, the controller
may oscillate and the image will look like noise. One method of tuning the
P and I values is to set them both to zero and increase the P until it begins
to oscillate, then decrease P by a bit. Then begin to increase the I until it
begin to oscillate and decrease I by a bit. For more detail, refer to
https://en.wikipedia.org/wiki/Ziegler%E2%80%93Nichols_method
7.0 Advanced Controls
The main controls that are visible in each screen should be sufficient for normal imaging
conditions. However, each tab in the navigation contains advanced controls that can extend the
functionality or control of the scan. In addition, there is a side menu available by clicking the button on
the top right of the screen that will enable access to all of the controls available.
7.1 Frequency Sweep - Advanced Controls
In the frequency sweep tab, click on
Show Advanced Controls
. These additional controls are for
running a manual frequency sweep and controlling the driving frequency and tip oscillation amplitude.
When the advanced controls are displayed, the drive frequency can also be set by clicking the sweep
graph at the desired frequency.
Start Frequency
The frequency at which you would like to start a manual frequency sweep
Stop Frequency
The frequency at which you would like to stop a manual frequency sweep
Step Size
The step size, in hertz, that the manual frequency sweep will increment. Step Size
determines the number of data points in the results.
Manual Sweep
Manual Sweep will perform a frequency sweep from Start Frequency to Stop
Frequency, incrementing the frequency by Step Size. The results will be displayed
in the main Frequency Sweep plot.
Clear
Clears all the results from the main Frequency Sweep plot.
Amplitude (%)
Controls the amplitude of the driving signal applied to the Z actuator. This can be
thought of as the tip oscillation amplitude. Increasing this value will increase the
tip oscillation amplitude.
