Page 339 - The Mechatronics Handbook

Micro- and Nanosensors
Microsensors (sometimes also called MEMS) are the miniaturized version of the conventional macrosen-
sors with improved performance and reduced cost. Silicon micromachining technology has helped the
development of many microsensors and continues to be one of the most active research and development
topics in this area.
Vision microsensors have found applications in medical technology. A fiberscope of approximately 0.2 mm
in diameter has been developed to inspect flaws inside tubes. Another example is a microtactile sensor,
which uses laser light to detect the contact between a catheter and the inner wall of blood vessels during
insertion that has sensitivity in the range of 1 mN. Similarly, the progress made in the area of nanotech-
nology has fuelled the development of nanosensors. These are relatively new sensors that take one step
further in the direction of miniaturization and are expected to open new avenues for sensing applications.

Selection Criteria
A number of static and dynamic factors must be considered in selecting a suitable sensor to measure the
desired physical parameter. Following is a list of typical factors:

Range—Difference between the maximum and minimum value of the sensed parameter
Resolution—The smallest change the sensor can differentiate
Accuracy—Difference between the measured value and the true value
Precision—Ability to reproduce repeatedly with a given accuracy
Sensitivity—Ratio of change in output to a unit change of the input
Zero offset—A nonzero value output for no input
Linearity—Percentage of deviation from the best-fit linear calibration curve
Zero Drift—The departure of output from zero value over a period of time for no input
Response time—The time lag between the input and output
Bandwidth—Frequency at which the output magnitude drops by 3 dB
Resonance—The frequency at which the output magnitude peak occurs
Operating temperature—The range in which the sensor performs as specified
Deadband—The range of input for which there is no output
Signal-to-noise ratio—Ratio between the magnitudes of the signal and the noise at the output
Choosing a sensor that satisfies all the above to the desired specification is difficult, at best. For example,
finding a position sensor with micrometer resolution over a range of a meter eliminates most of the sensors.
Many times the lack of a cost-effective sensor necessitates redesigning the mechatronic system. It is, therefore,
advisable to take a system level approach when selecting a sensor and avoid choosing it in isolation.
Once the above-referred functional factors are satisfied, a short list of sensors can be generated. The
final selection will then depend upon the size, extent of signal conditioning, reliability, robustness,
maintainability, and cost.

Signal Conditioning
Normally, the output from a sensor requires post processing of the signals before they can be fed to the
controller. The sensor output may have to be demodulated, amplified, filtered, linearized, range quantized,
and isolated so that the signal can be accepted by a typical analog-to-digital converter of the controller.
Some sensors are available with integrated signal conditioners, such as the microsensors. All the electronics
are integrated into one microcircuit and can be directly interfaced with the controllers.

Calibration

The sensor manufacturer usually provides the calibration curves. If the sensors are stable with no drift,
there is no need to recalibrate. However, often the sensor may have to be recalibrated after integrating
it with a signal conditioning system. This essentially requires that a known input signal is provided to

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