THE RESONANT CYLINDER PRESSURE SENSORResonant-cylinder sensors are strain-based, wherein a
structure is caused to resonate at its natural frequency
and this frequency is modulated as a function of the
input parameter.The pressure sensor is the most common adaptation of
the resonant principle, where a flexible metallic bellows
is used to modulate the force applied to the resonant
structure as a function of pressure.The resonant cylinder structure is caused to oscillate at
its natural frequency where changes in the frequency of
oscillation occur due to the pressure-induced hoop-and
axial-strain.
Recent advances in quartz fabrication technology have
resulted in the fabrication of a new generation of Double
Ended Tuning Fork (DETF) resonant structures that are
being successfully applied in the fabrication of inertial
grade miniature accelerometers.
For the resonant-cylinder sensors, the structure must
be driven into resonance by either electromagnetic or
piezoelectric methods.The resonance of any structure is the frequency at
which maximum mechanical output occurs with a
minimum energy input. For this reason, the total energy
required is small. Resonance is therefore, the frequency
of motion at which maximum efficiency results for any
structure.Modern quartz crystal wrist watches contain a
single-ended-tuning fork assembly resonating at typically
32,768 Hertz as the time base for the watch circuitry.
Since a quartz watch crystal oscillates for several years,
accumulating almost 2 million flexural cycles per minute,
on the energy contained within a watch battery, resonant
frequency must therefore represent a highly efficient
operating frequency!Resonant cylinder systems are normally configured to
allow a high-quality internal vacuum to exist around the
resonant structure, thereby eliminating the viscous
damping effects that an internal gas environment would
present to the resonating structure, and to reduce the
drive power requirements. The internal vacuum also
prevents ideal gas thermal expansion forces that would
act upon the resonant structure and the large variable
effects that airborne moisture would cause. The use of
high-elasticity, low-creep, and low hysteretic materials
in the fabrication of the resonant structure results in a
highly-stable and high-resolution measurement method.
The structural resonance of the cylinder is driven by a
feedback-controlled oscillator circuit configured to maintain
the resonant structure at its most mechanically-efficient
frequency or “maximum-Q” response point. Counter
circuitry then counts the oscillator output over some
defined time-averaging window.The frequency response of the resonant sensor is
therefore a direct function of the number of time
-averaged samples provided per second and is generally
low. Alternatively, the frequency of the resonant
structure can be measured utilizing a period
measurement system to provide a much wider
measurement bandwidth.Period measurement systems rely upon a second internal
time base operating at a much higher frequency than the
resonant structure to provide adequate period resolution.Naturally occurring electrical noise tends to generate
uncertainty in the turn-on and turn-off points in period
measuring systems resulting in degraded overall
measurement resolution.
Counting many resonant cycles over some defined time
period tends to average circuit noise to zero improving
measurement resolution. For static or quasi-static
measurements, the longer the counting time period, the
higher is the resolution of the system.It is not uncommon for resonant sensors to show 8
decades or more of signal resolution.Resonant-cylinder pressure sensors are sensitive to
media density as the measurand is admitted directly
into contact with the resonant structure. These
sensors are provided with inlet filters to prevent the
ingress of particulate matter.In metallic cylinder structures, the thermoelastic
modulus results in a strong thermal-sensitivity
dependence and these systems are most often
thermally controlled to minimize thermal error.The resonant-cylinder device provides extreme
resolution with excellent linearity but where a single
degree of temperature change can result in error
that is 10 to 100 times greater than the nonlinearity
error.
This article is taken from the Handbook, 'The Art of
Practical and Precise Strain Based Measurement' by
James Pierson
For details of PRESSURE SENSOR Suppliers, click here...
Home - Website - Search - Suppliers - Links - New Products - Catalogues - Magazines
Problem Page - Applications - How they work - Tech Tips - Training - Events - Jobs
