Biomedical Engineering Reference
In-Depth Information
2.2.1.3 Piezoelectric displacement sensors
Many types of position sensors may be used for correcting the unwanted characteristics in
piezoelectric materials. The position sensor must be small in size, stable over long time
periods, easily calibrated, have very low noise levels, and be easily integrated into a
scanner. Several types of position sensors are available including light-based sensors [31],
strain gauges, induction sensors, and capacitance sensors. Optical sensors available
include a simple design based on a knife edge attached to the scanner occluding a light
beam. [32] The signal from a photodetector is reduced as the knife edge cuts the beam.
Other types of light-based motion sensors include using a pinhole above a position
sensitive detector and a light lever. Each of these light-based designs requires a high-
gain amplifier. The primary advantage of the light-based position sensors is that the parts
required for construction are relatively inexpensive. There are many disadvantages how-
ever, including the fact that the sensor is not inherently calibrated, misalignments of the
light source cause problems, high noise, and the requirement for a high-gain amplifier. The
light sources also can cause thermal drift in the AFM scan head. Interferometers may also
be used for this function [28, 33], but they tend to be rather bulky and difficult to integrate
into the AFM head. Capacitance-based motion sensors are simple devices that measure the
capacitance between two plates which depends on the distance,
d
(Figure 2.11) between
the plates, and thus can make a highly sensitive position detector. Capacitance sensors are
common primarily because the electronics for capacitance sensors are very sensitive, and
Fig. 2.11. Different approaches to include sensors in AFM scanners. Top: a capacitive sensor,
middle, an interferometer-based sensor; bottom: an inductive sensor.
