THE RESONANT CYLINDER PRESSURE SENSOR
Resonant-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
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
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
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