Page 129 - MEMS Mechanical Sensors

118 Pressure Sensors

6.2.1.2 Linearity

A linear sensor response to pressure over the entire operating range is highly
desirable. This greatly simplifies subsequent signal processing. In practice, this is
unlikely to be the case. Pressure sensors of the MEMS variety tend to be based on
micromachined diaphragms and typically exhibit a declining rate of increased out-
put with increases in applied pressure [2]. Linearity (also referred to as nonlinearity)
can be defined as the closeness to which a curve fits a straight line. There are gener-
ally three definitions of linearity used in the specification of pressure sensors [3], and
these are shown in Figure 6.7:

• Independent linearity: the maximum deviation of the actual measurement
from a straight line positioned so as to minimize this deviation (a best fit
straight line);
• Terminal based linearity: the maximum deviation of the actual measurement
from a straight line positioned to coincide with the actual upper and lower
range values;
• Zero-based linearity: the maximum deviation of the actual measurement from
a straight line positioned to coincide with the actual lower range value and
minimize the maximum deviation.

6.2.1.3 Hysteresis
Hysteresis is a measure of the repeatability of the sensor output over the operating
pressure range after one or more cycles. Elastic behavior at low stresses suggests the
sensor element will deflect by a constant amount for the same pressure after any
number of cycles. In reality, the sensor output as pressure increases from zero to full
scale will be different to the output as pressure falls from full scale to zero. This is
shown in Figure 6.8. The measure of hysteresis is the difference between ascending
and descending readings usually at mid-scale. It is normally expressed as a percent-
age of full scale. It is due to molecular effects such as molecular friction causing the

BFSL
Zero
baseline

Sensor
output
Actual
response

Terminal
baseline

Pressure
Figure 6.7 Linearity baselines.