• Proportional Integral Regler
• Scan Characteristics

Control - Scan and Sample

Figure: DSP Control plug-in window

Proportional Integral Regler

Figure: Control circuit - scheme (STM)

All parameters which control the details of the scanning procedure are set in the dialog DSP Control In STM-mode U is the tunnel voltage and I the target tunnel current. In AFM-mode U is the Set-Point - that is the voltage, onto which the z-force-signal coming from the PSD shall be driven to -, I is without function.

The control behaviour - FeedBack - Characteristics - is set thorugh CP (proportional part) and CI (integral part).

Recipe if no sensible values are known:

1. Set CP to zero, CI to 0.01. If you now change the z-offset, the control should follow slowly.
2. Increase CI slowly until the control behaviour begins to become unstable. When you abruptly change z-offset the control oscillates.
3. Now decrease CI again until all oscillations are gone.
4. Set CP to 100% to 200% of CI to stabilize the control.

CS is a slope correction in the case, that your sample is tilted towards the intended optimal foces plane. CS=1 results in the addition of the mean slope to lighten the load off the z-signal-correction. CS=0 prevents this slope correction, CS=0.5 add only the half of the extrapolated height, CS1 add accordingly more than the average slope would suggest.

Scan Characteristics

Figure: Linescan execution

The Control dialog allows the user to set the following scan properties:

A 2D-Scan is defined by some parameters: A size (range X, Y), number of points in X and Y (points x, y). With this information one can calculate the sample dot distance (dx, dy). This dx and dy have to be a multiple of the smallest possible stepwidth (DA-unit) and, as a consequence, the total range has to be a multiple of the small stepwidth (dx, resp. dy).

If the stepwidth is large, it leads to a smoother movement, not to jump from sample point to sample point, but to define some intermediate steps (Inter-steps).

Inter-steps contains the stepwidth of intermediate steps in multiples of DA-units. An entry of 1 means, that the range between two sample points is covered via steps of the length 1xDA-unit. A smaller value is not possible.

During a scan for position changes the Line entry is used, otherwise the Move entry

Additionaly it is possible to stay for some time at any sample point. The textfield FB-Loops contains the number of feedback-loops which halts the scanhead for a limited time. The loop runs with 50 kHz.

The parameter N sets the how often at any position a value is sampled. After these values are measured. The result is their average.

Pre defined the number of steps, which are scanned before any data acquisition takes place.

DPL (dots per line) and dos distance (in DA-units) cannot be set. They are FYI only and set at scan-start