Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-05-10
2001-04-24
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000, C324S300000, C073S024060
Reexamination Certificate
active
06222366
ABSTRACT:
BACKGROUND OF THE INVENTION
Bulk acoustic wave (BAW) chemical sensors, including quartz crystal microbalance (QCM) devices, are used to measure the concentration of constituents or analyte in fluids (gases and liquids). These acoustic wave devices are typically constructed of piezoelectric crystals coated on at least one side with a material that has an affinity for the analyte whose concentration is being measured. The device is placed in the fluid stream containing the analyte being measured, and analyte is adsorbed or absorbed onto the coated surface. The amount of analyte adsorbed or absorbed by the acoustic wave device increases the mass of the device and alters the viscoelastic properties at the surface of the device, thereby damping the acoustic wave properties of the device. As a result, the frequency at which the acoustic wave device will resonate is altered (usually lowered).
When the acoustic wave device is incorporated into an electrical oscillator circuit, the change in resonant frequency of the device changes the operating frequency of the oscillator. The concentration of the analyte can be determined by measuring the change in operating frequency of the oscillator circuit over time.
QCM devices require unique analyte-specific coatings to address sensor performance in various operational conditions. Thus, these sensors are designed to operate in specific ranges of environmental conditions, such as temperature (e.g., −10° C. to 50° C.) and humidity (e.g., 0% to 90% relative humidity) and are capable of detecting small concentrations, and small changes of concentrations, of the targeted analyte. However, small changes in analyte concentrations can produce small changes in the resonant frequency of the crystal. In typical environments, concentrations of analyte being measured might, for example, alter the resonant frequency as little as 0.002% of the nominal resonant frequency. Thus, for a crystal having a nominal resonant frequency of 10.000 MHz, a small concentration of analyte being measured might alter the resonant frequency by about 200 Hz. Moreover, QCM devices are capable of detecting small changes in the analyte concentration through small changes in the resonant frequency. Therefore, the detection circuit must be capable to detect the resonant frequency of the crystal quite accurately, often to a resolution within about 5 Hz or less.
However, the viscoelastic properties of the device can be affected by thermodynamic conditions to which the device is subjected. More particularly, temperature and humidity “age” the characteristics of the crystal, causing permanent alteration of the viscoelastic properties of the crystal. This alteration of viscoelastic properties affects the dynamic characteristics of the device, and hence the velocity ofresonance in the crystal forming the device. Alteration of the resonance properties of the crystal often creates inharmonic responses, which generates noise in the operating frequency of the oscillator circuit. Therefore, it is important to eliminate the effects of noise in the detection circuit.
The aforementioned Dilger et al. application describes apparatus for measuring changes in the resonant frequency of the sensor to a resolution of about 0.1 Hz. More particularly, Dilger et al. describe a QCM sensor exposed to an analyte to generate a resonant frequency representative of the instantaneous analyte concentration in the fluid. A first counter samples the resonant frequency over a test period to supply a coarse count. The coarse count represents a frequency that is lower that the resonant frequency of the sensor by an amount based on the resolution of the coarse count. The coarse count is converted to a signal frequency which is digitally mixed with the resonant frequency from the sensor to produce a pulse width modulated signal. The low frequency component of the pulse width modulated signal is representative of the difference between the constructed coarse frequency and the resonant frequency of the sensor. The pulse width modulated signal is filtered to remove the high frequency component, and the resulting low frequency component establishes a sample period during which a high frequency clock operates a second counter to derive a count representative of the difference frequency. The result is combined with the coarse count from the first to derive an absolute digital representation of the resonant frequency of the sensor, which is representative of the concentration of analyte. Changes in concentration can be identified from changes in the digital representation of the resonant frequency of the sensor.
The invention described in the aforementioned Dilger et al. application is effective in accurately measuring analyte concentration in fluids. More particularly, small changes in analyte concentration cause small changes in the resonant frequency of the sensor which are detected by the relatively large change in difference frequency count based on the filtered pulse width modulated signal. While the Dilger et al. apparatus is highly effective, the resolution often exceeds the requirements and capabilities of the equipment and conditions being monitored. Moreover, the apparatus requires significant computational resources and power. Accordingly, there is a need for a simpler system that does not require such extensive computational resources and power.
BRIEF SUMMARY OF THE INVENTION
This invention utilizes time domain signal processing to increase the signal-to-noise ratio of resonating QCMs.
In one form of the invention, the analyte concentration in a fluid is measured by placing a quartz crystal microbalance sensor device in the fluid. A quartz crystal microbalance reference device provides a resonant frequency representative of the resonant frequency of the sensor device when the sensor device is not exposed to the analyte. The resonant frequencies of the sensor device and the reference device are summed to provide a difference signal frequency representative of the difference between the resonant frequencies of the sensor device and the reference device. A predetermined number of cycles of the difference signal frequency is counted to identify a sample time period. A clock provides a clock signal frequency whose cycles are counted for the duration of the sample time period to derive a count representative of the difference signal frequency. Small changes in the difference frequency due to small changes in the analyte concentration sensed by the sensor device are sensed with respect to the difference frequency.
In another form of the invention, a circuit is provided for measuring the analyte concentration in the fluid. A quartz crystal microbalance sensor device is connected to a first resonator circuit and is exposed to the fluid containing a concentration of the analyte. The sensor device provides a resonant frequency representative of the analyte concentration in the fluid. A quartz crystal microbalance reference device is connected to a second resonator circuit to provide a resonant frequency different from the resonant frequency of the sensor device before the sensor device is exposed to the analyte. A digital differencing circuit is connected to the first and second resonant circuits to provide a signal representative of a sample time period that represents the difference between the resonant frequency of the sensor device and the resonant frequency of the reference device. A counter is connected to the digital differencing circuit and is responsive to the clock signal to advance the count during the sample period. The resulting count in the counter is representative of the difference between the resonant frequencies of the sensor device and the reference device.
In a preferred embodiment of the circuit, the digital differencing circuit includes a summing circuit connected to the first and second resonator circuits to provide a signal having a frequency representative of the difference between the resonant frequency of the sensor device and the resonant frequency of the reference device. A second counter i
Fisher Controls International Inc.
Kinney & Lange , P.A.
Oda Christine
Shrivastav Brij B.
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