Photoacoustic detection system

Abstract

A photoacoustic detection system (20) includes a detector (22) having a chamber (24), a pulsating light source (26), a piezoelectric tuning fork (28), and a photoelectric sensor (30). The chamber has an inlet and an outlet for the flow of an analyte. The pulsating light source is adjacent to the chamber and is operable to emit a light beam along a path through the chamber. The tuning forks are arranged along the path, and each of the tuning forks is operable to emit a first sensor signal. The photoelectric sensor is arranged along the path and is operable to emit a second sensor signal. A controller (38) is connected to receive the first and second sensor signals. The controller is configured to determine whether a target species is present in the analyte based on the first sensor signal, and to determine whether the target species is present in the analyte based on the second sensor signal.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Application No. 62 / 670,188, filed May 11, 2018. Background Art

[0003] For security purposes, etc., the detection system can be used to identify smoke or chemicals for early warning of threatening events. As an example, the system can be designed to identify trace amounts of smoke particles in a building as an early warning of a fire, trace amounts of a target chemical as an early warning of the toxicity of the environment, or to identify trace amounts of airborne substances during security screening of people, luggage, packages, or other objects.

[0004] One type of detection system is a photoacoustic detector. Generally speaking, these types of systems rely on the interaction between light and a target substance to produce an acoustic response that can be measured and used for detection. A photoacoustic detector may include a chamber, pulsed light of a preselected frequency, and a microphone. Ambient air circulates through the chamber. Light is pulsed through the chamber, and the microphone listens for the acoustic response. The wavelength or spectrum of the light coincides with the absorption wavelength of the spectrum of the target substance. If the target substance is present in the air, the substance absorbs a portion of the light during the light pulsation. When the substance absorbs light, it heats up and causes a local pressure increase in the air. Between pulses, the substance cools, thereby producing a local pressure drop in the air. The alternating pressure increases and decreases appear as an acoustic signal that can be heard by the microphone and used to identify the presence of the substance in the air. Summary of the invention

[0005] A photoacoustic detection system according to an example of the present disclosure includes a detector having a chamber and a pulsating light source adjacent to the chamber, the chamber having an inlet and an outlet for the flow of an analyte. The pulsating light source emits a light beam along a path through the chamber when operated. A plurality of piezoelectric tuning forks arranged along the path are operable to emit a first sensor signal. A photosensor is arranged along the path and is operable to emit a second sensor signal. A controller is connected to receive the first and second sensor signals. The controller is configured to (a) determine whether a target species is present in the analyte based on the first sensor signal, and (b) determine whether the target species is present in the analyte based on the second sensor signal.

[0006] In a further embodiment of any of the foregoing embodiments, the controller is further configured to: when there is a determination that the target species is not present in the analyte based on the second sensor signal, identify that the determination that the target species is present in the analyte based on the first sensor signal is a false positive.

[0007] In a further embodiment of any of the preceding embodiments, the controller is configured to trigger a notification in response to the false positive.

[0008] Further embodiments of any of the preceding embodiments comprise a beam splitter in the path, the beam splitter being operable to split the light beam into first and second secondary beams along respective first and second branch paths.

[0009] A further embodiment of any of the preceding embodiments comprises a surface plasmon resonance sensor arranged along the first branch path, the surface plasmon resonance sensor being operable to emit a third sensor signal.

[0010] In a further embodiment of any of the foregoing embodiments, the controller is further configured to: when there is a determination that the target species is not present in the analyte based on the third sensor signal, identify that the determination that the target species is present in the analyte based on the first sensor signal is a false positive.

[0011] In a further embodiment of any of the preceding embodiments, the controller is configured to distinguish a chemical identity of the target species based on a distinct signature across the first, second, and third sensor signals.

[0012] In a further embodiment of any of the preceding embodiments, the controller is configured to trigger a notification in response to the false positive.

[0013] In a further embodiment of any of the preceding embodiments, the plurality of piezoelectric tuning forks comprises first and second piezoelectric tuning forks, the first piezoelectric tuning fork being arranged on a first side of the path and the second piezoelectric tuning fork being arranged on a second opposite side of the path opposite the first piezoelectric tuning fork.

[0014] In a further embodiment of any of the preceding embodiments, the controller includes a differential amplifier connected to receive a first sensor signal of each of the first and second piezoelectric tuning forks. The differential amplifier subtracts the first sensor signal of the second piezoelectric tuning fork from the first sensor signal of the first piezoelectric tuning fork. The first sensor signal of the second piezoelectric tuning fork is out of phase with the first sensor signal of the first piezoelectric tuning fork to generate a differential sensor signal.

[0015] In a further embodiment of any of the foregoing embodiments, the controller is further configured to determine whether the target species is present in the analyte based on the differential sensor signal.

[0016] In a further embodiment of any of the foregoing embodiments, the controller is configured to: when there is a determination that the target species is not present in the analyte based on the differential sensor signal, identify that the determination that the target species is present in the analyte based on the first sensor signal is a false positive.

[0017] In a further embodiment of any of the preceding embodiments, the controller is configured to identify whether the plurality of piezoelectric tuning forks are faulty based on the differential sensor signal.

[0018] A photoacoustic detection system according to an example of the present disclosure includes a detector having a pulsating light source operable to emit a light beam along a path and a plurality of piezoelectric tuning forks arranged side by side to form a resonant cavity along the path. Each of the piezoelectric tuning forks has a corresponding unique resonant frequency, and each of the piezoelectric tuning forks is operable to emit a sensor signal indicating a photoacoustic response of an analyte to an interaction of the analyte with the light beam. A controller is connected to receive the sensor signal.

[0019] In a further embodiment of any of the foregoing embodiments, the path defines a central axis, and each of the piezoelectric tuning forks has first and second tines that straddle the central axis.

[0020] In a further embodiment of any of the preceding embodiments, the first and second tines comprise gold and quartz.

[0021] In a further embodiment of any of the foregoing embodiments, each of the piezoelectric tuning forks has an opening defined between tips of the first and second tines, and each opening is oriented in a clocked position about the central axis.

[0022] In a further embodiment of any of the preceding embodiments, the controller is configured to determine whether a target species is present in the analyte based on an intensity of the photoacoustic response.

[0023] In a further embodiment of any of the preceding embodiments, the controller is configured to determine a characteristic of a target species present in the analyte based on a spectrum of a photoacoustic response across the piezoelectric tuning fork.

[0024] In a further embodiment of any of the preceding embodiments, the unique resonant frequencies fall within a range of 4 kHz between a maximum one of the unique resonant frequencies and a minimum one of the unique resonant frequencies.

[0025] In a further embodiment of any of the preceding embodiments, the plurality of piezoelectric tuning forks includes first and second piezoelectric tuning forks, and the controller includes a differential amplifier connected to receive a sensor signal of each of the first and second piezoelectric tuning forks. The differential amplifier subtracts the sensor signal of the first piezoelectric tuning fork from the sensor signal of the second piezoelectric tuning fork. The sensor signal of the second piezoelectric tuning fork is out of phase with the sensor signal of the first piezoelectric tuning fork to produce a differential sensor signal, and the controller determines whether a target species is present in the analyte based on the differential sensor signal.

[0026] A photoacoustic detection system according to an example of the present disclosure includes a series of piezoelectric tuning forks arranged side by side along an axis, each of the piezoelectric tuning forks having a unique resonant frequency, and each of the piezoelectric tuning forks having first and second tines across the axis to form a resonant cavity.

[0027] In a further embodiment of any of the preceding embodiments, the unique resonant frequencies fall within a range of 4 kHz between a maximum one of the unique resonant frequencies and a minimum one of the unique resonant frequencies.

[0028] In a further embodiment of any of the foregoing embodiments, each of the piezoelectric tuning forks has an opening defined between tips of the first and second tines, and each opening is oriented at a clocked position about the axis. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description.The drawings that accompany the detailed description can be briefly described as follows.

[0030] Figure 1 An example photoacoustic detection system is illustrated.

[0031] Figure 2An example arrangement of tuning forks in a photoacoustic detection system is illustrated.

[0032] Figure 3 A graph of the difference between the sensor signals of two tuning forks is shown.

[0033] Figure 4 Another example photoacoustic detection system is illustrated.

[0034] Figure 5 Pictured Figure 4 The surface plasmon sensor of the system.

[0035] Figure 6 An example piezoelectric acoustic sensor is illustrated.

[0036] Figure 7 Pictured Figure 5 The sensor is a piezoelectric tuning fork. DETAILED DESCRIPTION

[0037] Figure 1 An example photoacoustic detection system 20 ("system 20") is schematically illustrated. As will be appreciated from the examples herein, system 20 can rapidly perform detection with enhanced sensitivity and reliability by reducing false positives.

[0038] The system 20 includes a detector 22 consisting of a chamber 24, a pulsed (modulated) light source 26, a piezoelectric tuning fork 28, and a photoelectric sensor 30. The chamber 24 has an inlet 24a and an outlet 24b for the flow of an analyte (such as but not limited to ambient air). As an example, the system 20 is an aspirating system that uses a fluid mover (e.g., a fan, blower, or compressor) to collect the analyte and deliver the analyte from the area of ​​interest to the inlet 24a. Optionally, one or more filters can be used upstream of the inlet 24a to remove particulates.

[0039] The pulsating light source 26 is adjacent to the chamber 24. The light source 26 emits a light beam B along a path 32 through the chamber 24 when it is operated. Optionally, a resonator tube 33 may be arranged along the path 32 to amplify the resonance of the acoustic response described later in this article. The light source 26 may be controlled to emit light within a range of discrete wavelengths and / or frequencies. As an example, the light source 26 is a light emitting diode (LED) or a laser that can emit a light beam of a wavelength that can be changed in a controlled manner across a range of wavelengths and a range of frequencies. As used herein, "light" may refer to wavelengths in the visible spectrum and near infrared, infrared, and ultraviolet regions. For example, the light source 26 can scan across a range of wavelengths and / or frequencies. In another example, the light source 26 can generate broadband light or can generate narrowband light. In another example, the light source emits light within a wavelength range of 1000 nm to 25000 nm. The wavelength range can be adjusted by a filter, or the light source can be selected to generate light with a spectrum width of 10 nm or less that falls within the wavelength range. The light source can also be controlled to generate multiple discrete wavelengths matched to the target species to improve sensitivity and selectivity.

[0040] The piezoelectric tuning fork 28 is arranged along the path 32. In this example, there is a first piezoelectric tuning fork 28a and a second piezoelectric tuning fork 28b having equal resonant frequencies. The piezoelectric tuning forks 28a, 28b may be made of quartz, lead zirconate titanate (PZT), Rochelle salt, or other high voltage electrically responsive electrical materials. The first tuning fork 28a is arranged on a first side 32a of the path 32, and the second tuning fork 28b is arranged on a second opposite side of the path 32 opposite the first tuning fork 28a.

[0041] In the absence of a resonator 33, the optimal geometry occurs where the beam B passes through the legs of the piezoelectric tuning forks 28a, 28b, where the piezoelectric tuning forks 28a, 28b will be in line with the beam B. When a resonator 33 and two piezoelectric tuning forks 28a, 28b are used, as Figure 1 As shown in the example in , it is geometrically advantageous to align them just outside the center of the resonator 33. When the resonator is used with a tuning fork, the tuning fork is placed between the two halves of the resonator and the light passes through the legs of the tuning fork. However, if two tuning forks are placed between the two halves, the two halves of the resonator are further apart, which reduces coupling and causes a loss of sensitivity. Figure 2As shown in , the light beam B passes between the piezoelectric tuning forks 28a, 28b. Alternatively, the piezoelectric tuning forks 28a, 28b may be aligned with the light beam B so that the light beam B passes through both of the piezoelectric tuning forks 28a, 28b, for example, the light beam passes through the fork of each of the tuning forks 28a, 28b. It is advantageous to use two piezoelectric tuning forks 28a, 28b (or more) instead of just one, so as to be able to achieve the suppression of surrounding vibrations in the case where differential detection techniques can be used. The signals from each piezoelectric tuning fork 28a, 28b may be out of phase with each other so that when the signals are subtracted, the noise is subtracted, but the photoacoustic signal is not subtracted. For example, the signals may be out of phase by 180 degrees, in which case the signal will be amplified by a factor of 2 (and the background noise will be reduced) compared to one tuning fork.

[0042] The piezoelectric tuning forks 28a, 28b are communicatively connected at respective connections 34, 36 to communicate with the controller 38. It is to be understood that the communicatively connected or communicated herein may refer to an optical connection, a wired connection, a wireless connection, or a combination of connections thereof. The piezoelectric tuning forks 28a, 28b are operable to transmit a first sensor signal, which is received by the controller 38. The use of piezoelectric tuning forks can advantageously reduce the size of the system 20 as opposed to an acoustic transducer such as an electret microphone.

[0043] The photosensor 30 is also arranged along the path 32. In this example, the path 32 is linear, and thus the photosensor 30 is arranged on a straight line away from the light source 26. Alternatively, via the use of a mirror, the path may be non-linear so that the photosensor 30 will be arranged on an indirect line away from the light source 26. The photosensor 30 may be a solid-state sensor such as, but not limited to, a photodiode, a bipolar phototransistor, a photosensitive field effect transistor, etc. The photosensor 30 is communicatively connected to the controller 38 at 40 and is operable to transmit a second sensor signal to the controller 38.

[0044] The controller 38 may include hardware (e.g., one or more microprocessors and memory), software, or both, configured (e.g., programmed) to implement the functionality described herein. In this example, the controller 38 is configured to (a) determine whether the target species is present in the analyte based on the first sensor signal; and (b) identify false positives in determination (a) by independently determining whether the target species is present in the analyte based on the second sensor signal (i.e., redundancy). These two determinations may be referred to herein as presence determinations and false positive determinations, respectively.

[0045] The controller 38 is configured to make a presence determination using the first sensor signal from the piezoelectric tuning fork 28 through the photoacoustic response of the target species. For example, the controller 38 includes a light source modulation unit 38a communicatively connected to the light source 26 at 42. Through the modulation unit 38a, the controller 38 can control the operation of the light source 26 and thereby control the characteristics of the light beam B with respect to one or more of the frequency and the pulse frequency. Control of the wavelength can also be controlled by the modulation unit 38a, but it cannot itself change the wavelength of the light. To change the wavelength, a filter can be used and switched out from a broadband light source or a tunable laser device with narrowband light (e.g., using a Littman configuration). The modulation unit 38a can also use feedback from the photosensor 30 (received through the connection 40) to control the operation of the light source 26 and modulate the light beam B to a selected wavelength and / or frequency.

[0046] In one example, the modulation unit 38a includes an FPGA or microprocessor capable of generating a TTL signal. The TTL signal is used to drive a switch that pulses the light source 26 at the resonant frequency of the tuning forks 28a, 28b and / or the resonator 33 (at a given duty cycle, such as 50%). Additionally, there may be one or more laser driver chips for powering the light source 26, and the TTL signal can also be sent to the laser driver chip(s) to pulse the light source 26.

[0047] The controller 38 causes the light source 26 to emit a light beam B having a frequency of an absorption band of a target species, such as, but not limited to, carbonyl compounds, silanes, cyanates, carbon monoxide, hydrocarbons, and sulfides (e.g., H2S), which may be present in toxic gases, aerosols, particulates, or mixtures thereof. The light beam B is also pulsed at a pulse frequency corresponding to the resonant frequency of the piezoelectric tuning fork 28. For example, if the first tuning fork 28a and the second tuning fork 28b have an equal resonant frequency of 19 kHz, the light beam B can be pulsed at a pulse frequency of 19 kHz.

[0048] If the analyte contains the target species, the target species will absorb a portion of the light beam B and emit an acoustic response which causes the tuning forks 28a, 28b to resonate and thereby cause a change in the first sensor signal above the baseline or background signal.

[0049] The controller 38 includes an analysis unit 38b that is communicatively connected to the modulation unit 38a and the photosensor 30 via a communication connection 40 at 44. As an example, the analysis unit 38b may include one or more algorithms for processing the first sensor signal and making a presence determination. For example, if the first sensor signal exceeds a threshold value above the background signal, the analysis unit 38b makes a positive presence determination that the target species is present.

[0050] The photoelectric sensor 30 provides a mechanism for independently identifying false positives from the tuning forks 28a, 28b of whether the target species is present in the analyte. As an example, the target species scatters a portion of the light beam B, reducing the intensity of the light received into the photoelectric sensor 30 compared to the baseline or background signal. The analysis unit 38b identifies a reduction in intensity below a predetermined threshold to make a positive presence determination. The intensity below the threshold indicates a positive presence determination. If the second sensor signal indicates a positive presence determination, the positive presence determination is compared to the presence determination made based on the first signal of the piezoelectric tuning forks 28a, 28b to identify whether there is a fault. If there is a negative presence determination from the piezoelectric tuning forks 28a, 28b, but there is a positive presence determination from the photoelectric sensor 30, the controller 38 triggers a fault. If there is a positive presence determination from the piezoelectric tuning forks 28a, 28b, but there is a negative presence determination from the photoelectric sensor 30, the controller 38 triggers a fault and generates a notification signal.

[0051] The analysis unit 38b can also be configured with one or more algorithms to determine the concentration of the detected target species. For example, the magnitude of the acoustic response of the target species is proportional to the concentration of the target species in the analyte. Thus, for example, the analysis unit 38b can use the magnitude of the first sensor signal to determine the concentration by comparing the first sensor signal to a lookup table (or other stored data format) in the memory of the controller 38. The lookup table can have predetermined correlation information that maps the level of the first sensor signal to the concentration level.

[0052] In this example, the controller 38 includes a differential amplifier 38c that is communicatively connected to the respective first and second piezoelectric tuning forks 28a and 28b at 34 and 36 to receive a first sensor signal from each of the first and second piezoelectric tuning forks 28a and 28b. The differential amplifier 38c is also communicatively connected to the analysis unit 38b at 46. The differential amplifier 38c subtracts the first sensor signal of the second piezoelectric tuning fork 28b from the first sensor signal of the first piezoelectric tuning fork 28a. The first sensor signal of the second piezoelectric tuning fork 28b is out of phase with the first sensor signal of the first piezoelectric tuning fork 28a. The differential amplifier generates a differential sensor signal that can be used by the analysis unit 38b of the controller 38 to determine whether the target species is present in the analyte. That is, both tuning forks 28a, 28b are used to sense the analyte (rather than one in a reference environment), thereby increasing sensitivity while subtracting background. In another example, the sensor signal is amplified and then sent to a differential amplifier, allowing each signal to be monitored and checked for contaminated / faulty tuning forks. To identify contaminated or faulty tuning forks, the signal can be monitored over time and checked periodically against a baseline signal level. Deviations from the baseline signal level can trigger a fault.

[0053] exist Figure 3 An example of a differential signal is depicted in the graph in FIG. Line 48 represents the first sensor signal of the first piezoelectric tuning fork 28a. Line 50 represents the first sensor signal of the second piezoelectric tuning fork 28b, and line 52 represents the differential sensor signal. This differentiation removes the background portion of the signal, resulting in an enhanced signal-to-noise ratio, which in turn enables better sensitivity for target species detection. As an example, a target species such as hydrogen sulfide can be detected at a level of 1 ppm.

[0054] Figure 4 Another example photoacoustic detection system 120 is illustrated. In the present disclosure, similar reference numerals designate similar elements where appropriate, and reference numerals with an additional 100 or multiples thereof are understood to be modified elements incorporating the same features and benefits of the corresponding elements. In this example, the detector 122 of the system 120 has an acoustic sensor 60 including a plurality of piezoelectric tuning forks 128. The piezoelectric tuning forks 128 are connected to an analysis unit 138b of a controller 138 at 134a, 134b, 136a, and 136b. Although four piezoelectric tuning forks 128 are shown, there may alternatively be fewer or additional piezoelectric tuning forks 128.

[0055] The system 120 further includes a beam splitter 62 in the path 32. When the light beam B is emitted, the beam splitter 62 is operable to split the light beam B into a first secondary beam B1 and a second secondary beam B2 along the respective first branch path 32c and the second branch path 32d. A surface plasmon resonance based sensor 64 is arranged along the first branch path 32c, and the photosensor 30 is arranged along the second branch path 32d. The surface plasmon resonance sensor 64 is communicatively connected to the analysis unit 138b of the controller 138 at 66 and is operable to emit a third sensor signal. The surface plasmon resonance sensor 64 provides an additional (to the photosensor 30) mechanism for independently determining whether the target species is present in the analyte.

[0056] Figure 5 An example of a surface plasmon resonance sensor 64 is illustrated. The surface plasmon resonance sensor 64 comprises a prism 68 coated with a thin metal film 70 , such as a gold coating, on a first face 68 a. The prism 68 is positioned to reflect a light beam B to a photosensor 72 .

[0057] The metal film is exposed to the analyte in the chamber 24. The light beam B enters the prism 68 through the second face 68b and propagates toward the interface of the prism 68 and the metal film 70 at an incident angle R1. The light beam B excites the surface plasmon polariton in the metal film and is reflected from the interface at a resonance angle R2. If the analyte contains a target species, the binding of the target species to the metal film causes a change in the resonance angle R2 due to a change in the dielectric constant, thereby reducing the intensity of the light beam B. The photoelectric sensor 72 is used to monitor the intensity of the light beam B and transmit a third sensor signal to the controller 138 for analysis. As will be appreciated, surface plasmon resonance sensors and devices are known, and other types of surface plasmon sensors and techniques may be used.

[0058] The analysis unit 138b of the controller 138 processes the third sensor signal to independently identify false positive determinations made based on the piezoelectric tuning fork 128. As an example, if the third sensor signal exceeds a threshold value above the background signal, the analysis unit 138b makes a positive presence determination that the target species is present. This positive presence determination can then be compared with the presence determination made based on the first signal of the piezoelectric tuning fork 128 to identify whether there is a fault. If there is a negative presence determination from the piezoelectric tuning fork 128, but there is a positive presence determination from the surface plasmon resonance sensor 64, the controller 138 triggers a fault. If there is a positive presence determination from the piezoelectric tuning fork 128, but there is a negative presence determination from the surface plasmon resonance sensor 64, the controller 138 triggers a fault and generates a notification signal. Thus, the surface plasmon resonance sensor 64 provides another level of redundancy to the photosensor 30.

[0059] In another example, the controller 138 is further configured to distinguish the chemical characteristics of the target species based on the unique characteristics across the first, second, and third sensor signals. For example, a target species such as, but not limited to, hydrogen sulfide (H2S) may have close chemical analogs that are detected by the piezoelectric tuning fork 128 (or Figure 1 28a, 28b in Figure 1. Similarly, analogs can produce similar but not identical responses in the surface plasmon resonance sensor 64 and the photosensor 30. In order to distinguish between analogs, the controller 138 compiles the responses across the piezoelectric tuning fork 128, the surface plasmon resonance sensor 64, and the photosensor 30 to produce a signature thumbprint for each analog. The signature of the analog can then be compared with a library of signatures to identify which analog the target species is. Additionally or alternatively, the responses across the piezoelectric tuning fork 128, the surface plasmon resonance sensor 64, and the photosensor 30 can be input into a neural network to build a foundation for identifying and distinguishing analogs.

[0060] In another example, the system 120 is also configured for tunable diode laser absorption spectroscopy (TDLAS). TDLAS involves the absorption of a single frequency or wave number in the absorption spectrum of the target species. The controller 138 tunes the light source 26 to emit a beam B having a wavelength of a specific frequency or wave number. The unabsorbed portion of the beam B (i.e., the transmitted portion) is received at the photoelectric sensor 30 and can be compared with a baseline or background to determine the intensity. According to the Beer-Lambert law, intensity is related to concentration. The controller 138 can thus use a lookup table (or other stored data format) stored in a memory to determine the concentration of a given intensity. As an example, the concentration can be used for early warning, for verifying the concentration determined by the acoustic sensor 60, or both.

[0061] Figure 6 Another example of an acoustic sensor 160 is shown, which can be used in the system 20 or 120, respectively, in place of the tuning fork 28 and the acoustic sensor 60. In this example, the acoustic sensor 160 includes a series of piezoelectric tuning forks 228, the piezoelectric tuning forks 228 including piezoelectric tuning forks 228 n To 228 n+x , where n=1 and x is at least 2, at least 5, at least 10, or at least 14. The piezoelectric tuning forks 228 are arranged side by side along an axis A, which is coaxial with the path 32 in the system 120 .

[0062] Figure 7 A representative example of one of the piezoelectric tuning forks 228 is shown. Each piezoelectric tuning fork 228 has a first tine 74 and a second tine 76 extending from a base 78 to respective tips 74a, 76a of the tines 74, 76. In this example, the tines 74, 76 are semicircular. Alternatively, the tines 74, 76 may be square, rectangular, or other geometric shapes, as long as they together form a resonator chamber having a resonant frequency tuned to the resonant frequency of the tines (or the resonant frequency of the tines tuned to the resonator chamber).

[0063] There is an opening 80 between the tips 74a, 76b. The tines 74, 76 span the axis A so as to form a resonant cavity 82 (see also Figure 6 ). The opening 80 is oriented in a clocked position about the axis A so as to maintain the resonant cavity 82. The clocked position is the orientation of the opening 80 relative to the adjacent opening 80. The clocked position can be described by the rotation angle between the two openings 80 relative to a reference orientation. If the opening 80 is not clocked, the resonant cavity 82 will be less sensitive because the continuous cylindrical geometry will be compromised by a large continuous gap. As an additional measure to protect the continuous cylindrical geometry, the opening 80 should be as small as possible and isolated.

[0064] The tuning of the tines 74, 76 can be achieved by varying the length of the corresponding portions of the legs composed of two different materials (conductor and piezoelectric, such as gold and quartz). The resonant cavity 82 has a length L and an inner diameter D ( Figure 6 ). The length L is equal to the sum of the tine widths of the piezoelectric tuning fork 228 and the sum of the gap 90 in the axial direction along the axis A. Each of the piezoelectric tuning forks 228 has a unique resonant frequency. In this regard, each piezoelectric tuning fork 228 has a tine length of the tines 74, 76 extending from the base 78, a tine width between the lateral sides 84, 86 of the piezoelectric tuning fork 228, and a tip gap dimension of the opening 80. The tine lengths, tine widths, and tip gap dimensions between the piezoelectric tuning forks 228 may be different to provide a unique resonant frequency. The resonant frequency may be a first-order frequency, but the piezoelectric tuning fork 228 may also have other unique resonant frequency modes that may be used in addition to or instead of the first-order resonant frequency (i.e., the first-order resonant frequency is part of a unique set of resonant frequencies for each piezoelectric tuning fork 28). As Figure 6 As depicted in FIG. 1 , the piezoelectric tuning forks 228 may be mechanically interconnected via attachment portions 88 on the base 78, but the tines 74, 76 of adjacent piezoelectric tuning forks 228 are separated by gaps 90. As an example, the size of the gaps 90 is equal in the direction of the axis A.

[0065] Although unique, the resonant frequencies of the piezoelectric tuning fork 228 are relatively close. As an example, the unique resonant frequencies fall within a range of 4 kHz (between the maximum resonant frequency and the minimum resonant frequency). In additional examples, the range is tighter, within a range of 1 kHz or within a range of 0.5 kHz. In a further example, the unique resonant frequencies are also at least greater than 18 kHz. For example, if the resonant frequencies are 18.0 kHz, 18.4 kHz, 18.8 kHz, 19.2 kHz, and 19.6 kHz, the range will be 1.6 kHz (the difference between the maximum value of 19.6 kHz and the minimum value of 18.0 kHz), and each of the resonant frequencies will be equal to or greater than 18.0 kHz.

[0066] The unique resonant frequency of the piezoelectric tuning fork 228 provides the ability to capture and analyze a band of photoacoustic responses of the target species rather than just the response at a single frequency point. As described above, if the analyte contains the target species, the target species will absorb a portion of the beam B and emit an acoustic response. The acoustic response may be strongest at a discrete target frequency, but there may also be secondary acoustic responses at off-target frequencies near the target frequency. The secondary acoustic response at off-target frequencies can be used to further characterize and identify the target species or distinguish between analogs. For example, two analogs may have similar responses at target frequencies that are difficult to distinguish. However, the secondary responses at off-target frequencies may be different and can achieve direct distinction or distinction in combination with features across the surface plasmon sensor 64 and the photoelectric sensor 30. The piezoelectric tuning fork 228 can also be arranged in pairs similar to the tuning forks 28a, 28b and used with the differential amplifier 38c to produce a differential sensor signal as described above.

[0067] In another example, the range of unique resonant frequencies of the piezoelectric tuning fork 228 enables the controller 138 to compile a response spectrum of signal strength versus resonant frequency. The response spectrum can then be compared to a library spectrum, a previously collected response spectrum, or a later collected response spectrum to aid in the determination of the presence or chemical identification of a target species.

[0068] Although combinations of features are shown in the illustrated examples, not all of them need to be combined to achieve the benefits of the various embodiments of the present disclosure. In other words, a system designed according to an embodiment of the present disclosure will not necessarily include all of the features shown in any of the figures in the figures, or all of the parts schematically shown in the figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.

[0069] The foregoing description is exemplary rather than restrictive in nature. Changes and modifications to the disclosed examples may become apparent to those skilled in the art, and these changes and modifications must not depart from the present disclosure. The scope of legal protection afforded to the present disclosure can only be determined by studying the following claims.

Claims

1. A photoacoustic detection system, comprising: A detector, the detector comprising: a chamber having an inlet and an outlet for the flow of an analyte, a pulsed light source adjacent to the chamber, the pulsed light source emitting a light beam along a path through the chamber when operated, a plurality of piezoelectric tuning forks arranged along the path, each of the piezoelectric tuning forks being operable to emit a first sensor signal, a photosensor disposed along the path and operable to emit a second sensor signal; and a controller connected to receive the first sensor signal and the second sensor signal, the controller being configured to: (a) determining whether a target species is present in the analyte based on the first sensor signal, and (b) determining whether the target species is present in the analyte based on the second sensor signal; wherein the plurality of piezoelectric tuning forks are arranged side by side along an axis defined by the path, each of the piezoelectric tuning forks has a unique resonant frequency, and each of the piezoelectric tuning forks has a first tine and a second tine that spans the axis to form a resonant cavity, wherein each of the piezoelectric tuning forks has an opening defined between the tips of the first and second tines, and each opening is oriented at a clocked position about the axis, the clocked position being the orientation of the opening relative to an adjacent opening and being described by a rotation angle between the two openings relative to a reference orientation.

2. The system of claim 1 , wherein the controller is further configured to identify a determination based on the first sensor signal that the target species is present in the analyte as a false positive when there is a determination based on the second sensor signal that the target species is not present in the analyte.

3. The system of claim 2, wherein the controller is configured to trigger a notification in response to the false positive.

4. The system of claim 1, further comprising a beam splitter in the path, the beam splitter operable to split the light beam into a first secondary beam and a second secondary beam along respective first branch paths and second branch paths. 5 . The system of claim 4 , further comprising a surface plasmon resonance sensor disposed along the first branch path, the surface plasmon resonance sensor being operable to emit a third sensor signal.

6. The system of claim 5, wherein the controller is further configured to identify a determination that the target species is present in the analyte based on the first sensor signal as a false positive when there is a determination that the target species is not present in the analyte based on the third sensor signal. 7 . The system of claim 6 , wherein the controller is configured to distinguish the chemical characteristics of the target species based on unique features across the first sensor signal, the second sensor signal, and the third sensor signal.

8. The system of claim 6, wherein the controller is configured to trigger a notification in response to the false positive.

9. The system of claim 1, wherein the plurality of piezoelectric tuning forks comprises a first piezoelectric tuning fork and a second piezoelectric tuning fork.

10. The system of claim 9, wherein the controller includes a differential amplifier connected to receive the first sensor signal of each of the first piezoelectric tuning fork and the second piezoelectric tuning fork, the differential amplifier subtracting the first sensor signal of the second piezoelectric tuning fork from the first sensor signal of the first piezoelectric tuning fork, wherein the first sensor signal of the second piezoelectric tuning fork is out of phase with the first sensor signal of the first piezoelectric tuning fork to produce a differential sensor signal.

11. The system of claim 10, wherein the controller is further configured to determine whether the target species is present in the analyte based on the differential sensor signal.

12. The system of claim 11, wherein the controller is configured to identify a determination that the target species is present in the analyte based on the first sensor signal as a false positive when there is a determination that the target species is not present in the analyte based on the differential sensor signal.

13. The system of claim 11, wherein the controller is configured to identify whether the plurality of piezoelectric tuning forks are faulty based on the differential sensor signal.

14. The system of claim 1, wherein the unique resonant frequencies fall within a range of 4 kHz between a largest one of the unique resonant frequencies and a smallest one of the unique resonant frequencies.

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