Biomedical Engineering Reference
In-Depth Information
processing (DSP) chips or analogue electronics. This section does not discuss the imple-
mentation, but describes the block functions in the controller. The primary function of the
electronics in an AFM is to:
(a) Generate scanning signals for the x-y piezoelectrics.
(b) Take an input signal from the force sensor and then generate the control signal for
the Z piezo.
(c) Output control signals for X-Y - Z stepper motors.
(d) Generate signals for oscillating the probe and measuring phase or amplitude when
an oscillating mode is used for scanning.
(e) Collect signals for display by the computer.
As mentioned above, these functions may be implemented with either digital or analogue
electronics. In the digital approach, see Figure 2.21, all signals from the stage are digitized,
and a DSP chip takes care of all of the feedback control calculations. Also, the DSP chip
generates the x-y raster scan functions. The advantage of analogue electronics is that they
are typically less noisy. This will generally lead to a lower noise floor of the instrument,
and thus may enable acquisition of higher resolution images. Because the functionality of
a DSP chip is created by a software program, the DSP approach gives more flexibility and
can be changed very rapidly. Instruments with digital electronics might, for example,
allow simple software 'upgrades' to enable new features or acquisition of more data
channels simultaneously. The following sections are a detailed description of the functions
shown in Figure 2.21.
2.3.1
x-y signal generation
The x-y signal generator create a series of voltage ramps that drive the x and y piezoelectric
elements in the AFM, as illustrated in Figure 2.22. The scan range is established by
adjusting the minimum and maximum voltage. The position of the scan is established by
offsetting the voltages to the ceramic. Finally, the scan orientation is rotated by changing
the phase between the signals. It can be seen from Figure 2.22, that the forward and reverse
scan lines do not cover exactly the same topography. However, it is usually assumed that
they are equivalent, and generally, there is no appreciable difference between the two. In
general it is best if the drive signals do not have sharp edges at the turning point. Sharp
edges can excite resonances in the piezoelectric ceramics, and cause them to vibrate. Such
vibrations create unwanted artefacts and 'ringing' in the images. Higher speed scanning
with an AFM in particular is almost always done using rounded signals such as sinc waves
to drive the piezoelectric ceramics. Furthermore, even with slow speed AFM when using
straight-edged signals such as shown in Figure 2.22, the response of the scanner is not
linear at the turnaround points. To overcome this some 'overscan' is typically included in
the scanning, such that only the linear response part of the data is recorded. For example,
to scan a 10
m area, the instrument might really move 12
m in the slow scan direction,
and discard 1
m of the data from either end. In this way, the recorded data does not suffer
from edge artefacts.
The maximum scan range of the AFM scanner is established by the mechanical-elec-
trical gain of the piezoceramics and the maximum voltage they can tolerate before
depolarizing. As an example, the piezoceramics may have a gain of 1
m per volt. If
 
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