Page 131 - MEMS Mechanical Sensors

120 Pressure Sensors

6.2.2 Dynamic Pressure Sensing

Dynamic pressure sensing covers applications where the user is interested in moni-
toring changes in pressure over small time intervals. This can provide additional
information such as rate of change and the pattern of change. An example where
such additional information is of use is blood pressure monitoring where it provides
more detailed information about the health of the cardiovascular system. In addition
to the requirements of a static pressure sensor, the frequency response of the
measurement system must be considered. Frequency response is defined as the abil-
ity of a measurement system (the packaged transducer, its assembly, and electronics)
to accurately reflect dynamic pressure changes. All the components of the measure-
ment system must be considered. Within the packaged transducer and its assembly
this includes the response of the mechanical element coupling the pressure to the
sensing mechanism and the response of the pressurized media within the package
and assembly.
The mechanical element will behave like a spring mass system and therefore its
dynamic response will depend upon its stiffness, mass, and the degree of damping
present. The natural frequency of such mechanical elements will be specified by
the sensor manufacturer. Operation close to this frequency must be avoided. In
addition, the correct level of damping must be applied for the transducer to be
suitable for dynamic sensing. Underdamping will cause amplification of the pressure
wave and dynamic error in the measured pressure. Overdamping will attenuate the
pressure wave.
The dynamic response of miniature pressure sensors is discussed in more detail
in Section 6.5.1, but broadly speaking, due to their small size and the elastic proper-
ties of single crystal silicon, resonant frequencies in the megahertz range are possi-
ble. This gives them excellent inherent dynamic response characteristics. Typically,
however, a stainless steel barrier diaphragm is employed between the pressure sen-
sor and the pressurized media to ensure media compatibility. The volume between
the stainless steel diaphragm and the silicon sensor is filled by hydraulic oil that
transmits the pressure to the sensor die. The presence of the barrier diaphragm and
the hydraulic oil will both serve to lower the resonant frequency of the transducer as
a whole. Hydraulic over range protection mechanisms also limit dynamic response
since these tend to overdamp the system rendering the transducer unsuitable for
dynamic pressure sensor applications.
The frequency response of the pressurized media within the fluid channels and
sensor cavity is often the most limiting factor. The natural frequency of such a fluidic
system depends upon the volume of the sensor cavity, the length and diameter of the
channels, and the speed of sound in the fluid to be measured. As with the natural fre-
quency of the mechanical element, dynamic pressure measurements at the natural
frequency of the fluidic system are not recommended. This would cause severe dis-
tortion and amplification of the pressure waveform. The frequency at which toler-
able distortion occurs will depend on the damping in the system. Assuming the worst
case where damping levels are low, as a rule of thumb the maximum usable fre-
quency for any given fluidic system is generally taken to be one-fifth or one-seventh
of its natural frequency.
The electronics associated with a pressure sensor provide power to the
sensing mechanism and perform signal conditioning on the output signal. Signal