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
fi
nal error correction based upon proportional gain.
Proportional Gain
The third and
fi
nal 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
fi
nal correction voltage sent to the Z-axis piezo
according to the relation
As suggested in the equation, by the time proportional gain is
fi
gured 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
fl
uids, amplitude and phase magnetic force microscopy (MFM), surface
potential and
fi
eld 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
fi
eld 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.
z
acc
new
z
acc
old
z
acc
new
z voltage
z
acc
new
error
G
prop
⋅
+
=
