E1C01 09/14/2010
15:40:34 Page 3
Sensor and Transducer
An increasingly important area of scientific inquiry is the characteristics of matter at the nanoscale.
Suppose we want to measure the profile of a surface at a nanometer scale. We discover that a small
(very small) cantilever beam placed near the surface is deflected by atomic forces. Let’s assume for
now that they are repulsive forces. If this cantilever is translated over the surface, the cantilever will
deflect, indicating the height of the surface. This concept is illustrated in Figure 1.2; the device is
called an atomic force microscope. The cantilever beam is a sensor, a physical element that employs
some natural phenomenon, in this case deflection under the action of a force, to sense the variable
being measured, in this case the height of the surface.
So, we have a sensor to measure at the nanometer scale. But we have no means of getting an output
from the sensor that we can record. Suppose that the upper surface of the cantilever is reflective, and we
shine a laser onto the upper surface, as shown in Figure 1.3. The movement of the cantilever will deflect
the laser. Employing a number of light sensors, also shown in Figure 1.3, the deflection of the laser can
be sensed and that deflection corresponds to the height of the surface. Together the laser and the light
sensors (photodiodes) form the transducer component of the measurement system. A transducer
converts the sensed information into a detectable signal. The signal might be mechanical, electrical,
optical, or may take any other form that can be meaningfully recorded.
We should note that sensor selection, placement, and installation are particularly important to
ensure that the sensor output accurately reflects the measurement objective. The familiar phrase
Cantilever and tip
Sample surface
Figure 1.2 Sensor stage of an atomic-force
microscope.
Cantilever and tip
Photodiodes
Laser
Sample surface
Detector and
feedback
electronics
Figure 1.3 Atomic-force microscope with
sensor and transducer stages.
1.2 General Measurement System 3
15:40:34 Page 3
Sensor and Transducer
An increasingly important area of scientific inquiry is the characteristics of matter at the nanoscale.
Suppose we want to measure the profile of a surface at a nanometer scale. We discover that a small
(very small) cantilever beam placed near the surface is deflected by atomic forces. Let’s assume for
now that they are repulsive forces. If this cantilever is translated over the surface, the cantilever will
deflect, indicating the height of the surface. This concept is illustrated in Figure 1.2; the device is
called an atomic force microscope. The cantilever beam is a sensor, a physical element that employs
some natural phenomenon, in this case deflection under the action of a force, to sense the variable
being measured, in this case the height of the surface.
So, we have a sensor to measure at the nanometer scale. But we have no means of getting an output
from the sensor that we can record. Suppose that the upper surface of the cantilever is reflective, and we
shine a laser onto the upper surface, as shown in Figure 1.3. The movement of the cantilever will deflect
the laser. Employing a number of light sensors, also shown in Figure 1.3, the deflection of the laser can
be sensed and that deflection corresponds to the height of the surface. Together the laser and the light
sensors (photodiodes) form the transducer component of the measurement system. A transducer
converts the sensed information into a detectable signal. The signal might be mechanical, electrical,
optical, or may take any other form that can be meaningfully recorded.
We should note that sensor selection, placement, and installation are particularly important to
ensure that the sensor output accurately reflects the measurement objective. The familiar phrase
Cantilever and tip
Sample surface
Figure 1.2 Sensor stage of an atomic-force
microscope.
Cantilever and tip
Photodiodes
Laser
Sample surface
Detector and
feedback
electronics
Figure 1.3 Atomic-force microscope with
sensor and transducer stages.
1.2 General Measurement System 3