QUARTZ
QUARTZDYNE'S PRESSURE SENSOR
is a quartz resonator which changes frequency in response to
pressure. The structure is a thickwalled hollow cylinder with
closed ends. A thickness-shear-mode disc resonator divides the
central portion of the hollow cylinder. Fluid pressure on the
exterior hydrostatically compresses the quartz cylinder, producing
internal compressive stress in the resonator. The vibrating
frequency of the sensor changes in response to this stress.
The sensor is made of  crystalline quartz,
which is naturally piezoelectric. Quartz resonators use the
inverse piezoelectric effect to induce the resonator to vibrate
at its mechanical resonant frequency when electric fields
are applied to its electrodes. An oscillator
circuit supplies the power and allows the frequency
to be measured.
Because frequency (and its inverse, time) can be measured with greater precision
than any other parameter, the sensor's frequency output provides high resolution pressure measurement.
Quartz is a perfectly elastic material, providing sensor stability and repeatability.
The quartz resonator has a high Q, which means that its vibration
can be driven with very little electrical power. This provides
long operating life in battery-powered applications.
THREE-CRYSTAL TRANSDUCER ASSEMBLY
The "transducer" is the entire assembly required to produce the frequency or digital output
signals. Each Quartzdyne transducer relies on three thickness-shear-mode quartz
resonators: the pressure sensor, a temperature sensor, and a reference crystal.
The quartz pressure
sensor changes frequency in response to
changes in pressure. Because the pressure crystal also has
a small change in frequency with temperature, a quartz temperature
sensor is used to provide digital temperature compensation.
We use a quartz crystal temperature sensor because it provides
stability, shock resistance, and high resolution with low
power consumption.
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The reference crystal is the SC-cut crystal discovered by
Quartzdyne co-founder Dr. Errol P. EerNisse. (The SC-cut is
the preferred crystal for use in precision laboratory frequency
counters.) The output signal of the reference crystal is mixed
with the pressure and temperature signals to reduce their
frequencies from the MHz range to the kHz range. (This simplifies
the frequency counting requirements.) At room temperature
and pressure, the frequencies of the pressure, temperature,
and reference crystals are approximately 7.22 MHz, 7.15 MHz,
and 7.20 MHz, respectively, providing pressure and temperature
outputs of 20 kHz and 50 kHz. The pressure signal increases
with pressure and the temperature frequency decreases with
temperature.
The reference signal can also be used to count the two sensor
frequencies. Since the reference signal is digitally-temperature
compensated, it introduces no additional temperature error. Pressure resolution
of 0.01 psi and temperature resolution of 0.001°C are easily
obtained with one second gatetimes.
The metal housing of the transducer contains the high pressure
fluid, and shock mounts the three crystals and the circuit.
The housing design also ensures that the three quartz crystals
are thermally coupled for temperature compensation.
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