Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 12

2.2 Sensor Characteristics

In this chapter we introduce some of the important characters of sensors. Under-
standing of these is important when choosing the right sensors for your application.

2.2.1 Transfer Function

Transfer function is a mathematical representation of the relation between the input
and output of a system. In terms of sensors it represents the relationship between
the physical signal on the input, and the electrical output signal.

2.2.2 Hysteresis

A sensor should be capable of following changes in the input parameter, regardless
of what direction the parameter changes in (whether there is an increase or a
decrease in value). Some sensors do not return the same value for both directions
of change. Hysteresis is the measure of this property.

2.2.3 Linearity

Linearity of a sensor shows how much the measured curve deviates from the ideal
transfer function. There are several ways to interpret and represent linearity, the
most commonly used being:
End point linearity

•

• Best-fit straight line (BFSL)

• Least squares BFSL linearity

On the Input against Output graph a straight line is drawn from the zero point to the
full-scale output point. The point on the actual measured curve that deviates most
from this line is used to define the linearity of the sensor. This is usually quoted as
a percentage of the full-scale output of the sensor.

2.2.3.1 Best-Fit Straight Line (BFSL)

BFSL linearity is a measure of the most accurate overall results that can be obtained
for a given sensor. Instead of using a line that connects the full-scale point to the
end point, a different line is chosen so that the maximal deviation of the actual
measured curve is minimal. Note that this is simply a means of interpretation of the
results and relies entirely on the system, including the measuring equipment, being
set up to take advantage of this technique.