Biology Reference
In-Depth Information
The
-piezoactuator is held at both ends with lexures, so that its centre of
mass is hardly displaced and, consequently, no large mechanical excitation is
produced. The
x
-piezoactuator (maximum displacement, 2 μm at 100 V; self-
resonant frequency, 400 kHz) is held only at the four side-rims parallel to the
displacement direction. The
z
-piezoactuator can be displaced almost freely
in both counter directions, and consequently, impulsive forces are barely
exerted on the holder. This holding method has an additional advantage
in that the resonant frequency is not lowered by holding, although the
maximum displacement decreases by half. The
z
x
-scanner is actively damped
either by the previously developed
-control technique
5
or by feed-forward
control using inverse compensation.
12
The
Q
z
-scanner is also actively damped
by the
Q
-control technique. The
z
-scanner bandwidth
f
s
is extended to ~500
kHz, and the quality factor
Q
s
is reduced to ~0.5. Therefore, its response time
τ
s
(=
Q
s
/π
f
s
) is ~0.32 μs.
8.2.2.5 Dynamic PID control
The force reduction is quite important for biological AFM imaging. A shallower
amplitude set point can reduce the tapping force but promotes “parachuting”
during which the error signal is saturated and therefore the parachuting time
is prolonged with increasing set-point amplitude, resulting in a decrease in
the feedback bandwidth. The feedback gain cannot be increased to shorten
the parachuting time, as a larger gain induces an overshoot at up-hill regions
of the sample, resulting in the instability of the feedback operation. To solve
this problem, a novel PID controller named “dynamic PID controller” was
developed.
6
It can automatically change the feedback gain depending on
the oscillation amplitude. Namely, the feedback gain is increased when the
error signal exceeds a threshold level, which shortens the parachuting time
or avoids parachuting. The dynamic PID controller can avoid parachuting in
fact even when the set-point amplitude is increased up to 90% of the free
oscillation amplitude.
8.3 HIGHSPEED AFM IMAGING OF PROTEIN SAMPLES
High-speed AFM is not completely established yet as a tool for routinely
observing biomolecular processes, although the performance of high-
speed AFM has been markedly improved in the last 3-4 years. At present,
it is important to examine whether we can really image biological processes
that have been expected or known to occur. Further, high-speed AFM
has not yet been applied to observe cellular structures because of some
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