Instruction manual

SPM Fundamentals for the MultiMode
Feedback Gains
36 MultiMode SPM Instruction Manual Rev. B
where is the new average error calculated by adding the old average error to the product
of the integral gain times the error. The running average represented by maintains itself
continually until one or more of the major scanning parameters is changed by the operator.
Whenever major scan parameters are changed (e.g., Setpoint), the error accumulator is dumped
and begins a new running average. With the average error calculated, the feedback system is
prepared to make its final error correction based upon proportional gain.
Proportional Gain
The third and final step in the feedback process uses proportional gain to complete error correction.
Recall that proportional gain responds to error in proportion to how much it differs from the
setpoint. Proportional gain is used to calculate the final correction voltage sent to the Z-axis piezo
according to the relation
As suggested in the equation, by the time proportional gain is figured in, the bulk of error correction
has already been completed. This tends to make Proportional gain a less “touchy” control when
compared to Integral and LookAhead gain. Nevertheless, the system can be driven into oscillation
whenever gains are excessive, including Proportional gain.
REMINDER: Gain values entered on the Realtime > Feedback Controls panel do not directly
translate to any real quantity, but are merely self-referencing; e.g., a Proportional gain value of 2.0
is not
the same as Integral or LookAhead gain values of 2.0.
2.3.9 What Data Type of Image?
SPM technology at Veeco has rapidly grown beyond its scanning tunneling roots to encompass
numerous types of microscopy. This includes: ECSTM, contact AFM, ECAFM, TappingMode in
air, TappingMode in fluids, amplitude and phase magnetic force microscopy (MFM), surface
potential and field gradient electric force microscopy (EFM), lateral force microscopy (LFM), force
modulation imaging, scanning capacitance microscopy (SCM), thermal imaging, and force volume
imaging. In addition, there are numerous variations and combinations of the above; new types of
SPM are added continually as the field expands. Each of these variations reveals something unique
by using Veeco’s feedback system to process and extract signals in slightly different ways.
The NanoScope system allows up to three simultaneous image channels, plus auxiliary channels.
Each of the image Channel control panels (Channel 1, 2, and 3) contains a Data type parameter
specifying the type of image to be shown on that channel. The Data type, in turn, is determined by
the currently selected microscope (Realtime > Microscope > Select) and AFM mode shown on
the Other Controls panel. For example, although Height data can be displayed for most types of
imagery, only TappingMode displays Amplitude data. Similarly, only contact AFM displays
Friction data. Whenever the AFM mode and Data type parameters are changed, some new portion
of the feedback signal is utilized and/or processed differently. Some users tap the NanoScope’s
auxiliary channels to generate new type of images from the feedback system.
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z voltage z
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