Abstract:
A process monitoring system determines a spectral response of a process material. This system has a tunable laser for generating an optical signal that is wavelength tuned over a scan band and an optical probe for conveying the optical signal to the process material and detecting the spectral response of the process material. The optical probe expands a beam of the optical signal to a diameter of greater than 10 millimeters. This avoids one of the difficulties with monitoring these process applications by ensuring that the spectroscopy measurements are accurate and repeatable. It is desirable to sample a relatively large area of the processed material since it can be heterogeneous. Additionally the large area mitigates spectral noise such as from speckle.

Description:
RELATED APPLICATIONS 
     This claims the benefit under 35 USC 119(e) of U.S. Provisional Application No. 60/737,506, filed Nov. 17, 2005, and of U.S. Provisional Application No. 60/682,606, filed May 18, 2005, both of which are incorporated herein by this reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Spectroscopy, and specifically near infrared (NIR) spectroscopy, has been proposed as a technique for process monitoring in the manufacture of substances, such as pharmaceutical products. For example, in drying applications, e.g., pill drying, the spectroscopy instruments can be used to monitor the water or solvent content during the drying process. This allows the process to be terminated at the optimal time, saving energy and ensuring product uniformity between batches. In blending applications, spectroscopy can be used to monitor the distribution of the active ingredient in the binder material to insure that the processed material is completely admixed and thereafter terminate the blending process. 
     SUMMARY OF THE INVENTION 
     One of the difficulties with monitoring in these process applications is ensuring that the spectroscopy measurements are accurate and repeatable. It is desirable to sample a relatively large area of the processed material since it can be heterogeneous at small scales. Additionally the large area mitigates spectral noise such as from speckle, which is associated with the use of coherent sources such as lasers. 
     In general, according to one aspect, the invention features a process monitoring system for determining a spectral response of a process material. This system comprises a tunable laser for generating an optical signal that is wavelength tuned over a scan band and an optical probe for conveying the optical signal to the process material and detecting the spectral response of the process material. The optical probe expands a beam of the optical signal to a diameter of greater than 10 millimeters. 
     In embodiments, the diameter of the beam of the optical signal is collimated and greater than 20 millimeters, and even 30 millimeters in diameter and the beam of the optical signal has a divergence angle of less than 4 degrees. Preferably, a window element between the probe and the process material is made of nitrogen impregnated sapphire. 
     The tunable laser preferably comprises semiconductor tunable laser. 
     In one implementation, the optical probe comprises a frame, a projection lens system carried by the frame for receiving the optical signal from a semiconductor tunable laser and expanding and collimating the beam of the optical signal, a detector for detecting light from the process material, and a collection lens system for collecting light from the process material and directing the light to the detector. 
     The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
         FIG. 1  is a schematic view of a material processing system with the inventive spectroscopic process monitoring system; 
         FIGS. 2A ,  2 B, and  2 C are perspective, top plan, and cross sectional views of a first embodiment of an optical probe for the process monitoring system, according to the present invention; 
         FIG. 3  is a schematic view of the optical probe illustrating the relationship between beam size and sample distance/response dependency; 
         FIG. 4  is a ray trace for the first embodiment optical probe; 
         FIG. 5  shows an intensity distribution for the tunable signal from the projection lens assembly of the first embodiment probe; 
         FIGS. 6A and 6B  are perspective and cross sectional views of an optical probe according to a second embodiment of the present invention; 
         FIG. 7  is a ray trace for the second embodiment optical probe; 
         FIG. 8  shows an intensity distribution for the tunable signal from the projection lens assembly of the second embodiment probe; 
         FIG. 9  is a plot of absorbance as a function of wavelength for various chemicals; 
         FIG. 10  is a plot of the second derivative of the spectral response as a function of wavelength for a blending process; and 
         FIG. 11  is a plot of average standard deviation of spectral blocks over the course of a blending process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates an exemplary material processing system  10  incorporating a process monitoring system  100 , which have been constructed according to the principles of the present invention. 
     In more detail, the illustrated blender-type processing system  10  comprises a drum  50 . This contains the process material  54  that is to be blended. In one example, the material includes a binder material or vehicle and active ingredient(s) that must be dispersed evenly throughout the material  54 . This material  54  is contained within the interior  52  of the drum  50 . In one example, the drum  50  includes blades  56  for facilitating the mixing of the substance  54 . The entire drum in the illustrated implementation rotates to thereby create the mixing action. In other examples, a V-blender design is used. Further, in other examples, the drum does not rotate but instead the blending is performed by a rotating auger or mixing blade in the drum or a vibratory action. 
     In other examples, the processing system  10  is a dryer, such as a pill dryer. In such example, the processing is intended to remove water or other solvent. In still other examples, the processing system supports a chemical reaction in a batch process or a continuous process system. 
     The process monitoring system  100  is used to monitor the spectroscopic response of the process material  54  within the blender drum  50 . In the illustrated example, a region of the sidewall of the drum  50  contains a window element  60 . In one example, this window element  60  is made from sapphire, which has a negligible spectral signature in the near infrared. 
     In a preferred embodiment, the window element  60  is nitrogen impregnated sapphire. AlON, with nominal chemical composition of 9(Al 2 O 3 )5(AlN). This is a material having a spinel crystal structure and chemical resistance properties very similar to sapphire (Al 2 O 3 ) but powder processed and mixed with nitrogen (4-5%) to eliminate birefringence (ALON is optically isotropic). 
     A spectroscopic optical probe  110  is secured to the outside of the drum  50  and opposite the window element  60 . The optical probe  110  optically interfaces or couples a source of a tunable signal  200  to the inside of the drum  52  via the window element  60  and includes a detector for detecting the diffusely reflected light from the process material sample  54 , enabling the spectrometer system  100  to determine the spectroscopic response of the process material  54 . 
     In the preferred embodiment, the tunable signal source  112  is a laser is constructed as described in U.S. Patent Application Publication No., US 2006/0215713 A1, published on Sep. 28, 2006, entitled, “Laser with Tilted Multi Spatial Mode Resonator Tuning Element,” by Dale C. Flanders, et al., which is incorporated herein by this reference in its entirety. In other examples the tunable signal source is constructed as described in U.S. Pat. No. 7,061,618 B2, Issued on Jun. 13, 2006, entitled Integrated Spectroscopy System by Walid A. Atia et al, which is incorporated herein by this reference in its entirety. Generally, the tunable signal source comprises a semiconductor gain medium that is optically coupled to the probe via optical fiber, preferably single mode fiber. 
     In the illustrated embodiment, the tunable source or tunable laser  112  and probe  110  are secured to the drum to rotate with the drum. In one example, the spectroscopy system  100  is battery powered and uploads spectroscopy data to a host computer  114  that controls the processing system  10 . 
     In the preferred embodiment, the tunable source system  112  generates a tunable laser signal that scans over a spectral scan band. Typically, there is signature information in this scan band that characterizes the process material  54 , such as the active ingredient(s) or solvent. By scanning the laser signal over the scan band, and detecting the diffusely reflected response in the scan band with the probe detector, the spectroscopic response of the process material  54  is resolved to determine a process parameter such as the level of mixing or moisture of the material  54 . 
       FIG. 2A  is a perspective view of a first embodiment of the probe  110 . It comprises a projection lens system unit  210  that projects the tunable signal from a laser through the sapphire window  60  to the process material  54 . A collection lens system  212  is used to collect the diffusely reflected light from the material  54  and focus it onto a detector. The projection lens system  210  and the collection lens system  212  are located within a housing  214 , which is usually bolted or otherwise secured to the drum  50  of the processing system  10 . 
       FIG. 2B  shows the top of the probe  110  with the projection lens system  210 , the collection lens system  212 , and housing  214 . 
       FIG. 2C  is a cross-sectional scale view of the probe  110  taken along line A-A in  FIG. 2B . The projection unit  210  comprises a cylindrical frame  216 . This holds an outer convex lens  218  and an inner concave lens  220 . An optical input fiber  224  carries the tunable optical signal from the tunable laser  112 . These lenses  218 ,  220  work together to collimate a beam of the tunable optical signal emitted from a fiber endface/connector assembly  222 . 
     In the illustrated example, the combination of the concave lens  220  and convex lens  218  produce a generally collimated, large diameter beam, which is preferably greater than 10 millimeters in diameter. In the preferred embodiment, it is even larger, greater than 20 millimeters or even 30 to 40 millimeters in diameter or larger (diameter being measured from the 1/e 2  point). The large spot size enables the sampling over a large region of the substance  54 , specifically a region corresponding to the volume of a dosage, such as a pill, and to mitigate speckle. Spectral noise from speckle is generally inversely proportional to illumination spot size. 
     In the preferred embodiment, this optical input fiber  224  is single lateral mode polarization maintaining optical fiber or other polarization controlling fiber such as fiber that transmits only a single polarization. The optical fiber  224  ends in a fiber collimator assembly  222 . This is held in a plate  228 , which is secured to a spacer  230  that spaces the collimator assembly from the concave lens  220 . 
     In the preferred embodiment, the optical axis  225  of the projection lens unit is nearly perpendicular or perpendicular to the window  60 . This is done because the spectrometer is preferably a tunable laser with a semiconductor source. As such, it produces highly polarized light. The near perpendicular to perpendicular arrangement minimizes polarization dependent loss (PDL) of the window and any resulting spectral structure due to temperature dependent birefringence in the fiber. 
     Currently the angle α between the optical axis  225  and the window  60  is between 75 and 90 degrees. A slight angling from perpendicular is used to reduce stray light and feedback into the tunable laser  112 ; the range of 80-85 degrees is preferred, with 82.5 degrees currently being used. 
     The collection lens system  212  comprises a series of convex collection lenses, held in a cylindrical frame  217 , that collect light from the substance  54  traversing through the sapphire window  60  over a large numerical aperture. 
     The collection lens system  212  comprises a first outer convex lens  240 . A second convex lens  242  further collects the signal from the sample and directs it to a third convex lens  244 . This focuses the signal from the sample on to a photodetector  248 , which is held on a circuit board  250 . In the preferred embodiment, this photodetector system further comprises a temperature controller, such as by a thermoelectric cooler, in order to control its temperature to improve performance and stability over changes in ambient temperature. 
       FIG. 3  is a schematic diagram illustrating the response as a function of sample to projection lens assembly distance. Specifically, because of the angle between the optical axis  225  of the projection lens system  210  and the optical axis  226  of the collection lens assembly  212 , the signal response varies as a function of the distance between projection lens system  210  and the sample  10 . Specifically, this relationship is generally illustrated in the insert graph  310 . This dependency is mitigated by the increased size W p  of the tunable optical signal beam. Specifically, by increasing the W p  of the beam  312 , the relationship between the change in signal as a function of sample depth is minimized. 
       FIG. 4  is a ray trace illustrating the relationship between the projection lens system  210  and the rays collected by the collection lens system  212 . 
     As illustrated, the high NA lens system of the collection lens system  212  collects signal over the entire expanse of the tunable optical signal beam projected onto sample  10 . 
     In one embodiment, the detector  248  is moved a few millimeters, between 2 and 10 millimeters from the focal point of the last lens  244  to create a defocused configuration. 
       FIG. 5  illustrates the beam intensity distribution of the tunable signal from the projection lens unit  210 . The distribution was derived from a MontiCarlo simulation taken in plane perpendicular to optical axis  225  of the projection lens system  210 . It shows how the Gaussian intensity distribution of the single mode fiber  224  is maintained to the sample  10 . 
       FIG. 6A  is a perspective view of a second embodiment of the probe  110 ′. It similarly comprises a projection lens system that projects the tunable signal from a laser and collection lens system. In this view, they are hidden by the housing  214 ′, which provides an air sealed arrangement so that the optical assembly can be filled with an inert gas to control spectral noise from water vapor, for example. 
       FIG. 6B  is a cross-sectional scale view of the probe  110 ′. The projection unit  210 ′ of the second embodiment comprises a cylindrical frame  216 ′ held in housing  214 ′. This holds outer convex lens  218 ′ and an inner convex lens  220 ′. An optical input fiber  224  carries the tunable optical signal from the tunable laser  112  to the output fiber facet in the connector assembly  222 . These lenses  218 ′,  220 ′ work together to form a collimated beam of the tunable optical signal emitted from a fiber endface/connector assembly  222 . 
     In the second embodiment  110 ′, a reflecting prism  610  is used as a fold mirror between the endface  222  and lens  220 ′ to facilitate the construction of more height-compact optical train. 
     In the illustrated example, the combination of the convex lenses  218 ′,  220 ′ produce a generally collimated, large diameter beam, which is preferably greater than 10 millimeters in diameter. In the preferred embodiment, it is even larger, greater than 20 millimeters or even 30 to 40 millimeters in diameter or larger. The large spot size is important to sample over a large region of the substance  54  and to mitigate speckle. Spectral noise from speckle is generally inversely proportional to illumination spot size. The second embodiment differs from the first embodiment in that the Gaussian beam output from the single mode fiber  224  is converted to a beam with a top-hat intensity distribution measured in a plane perpendicular to the optical axis  225 ′ of the projection lens system  210 ′ by using aspheric lenses  218 ′ and  220 ′. 
     In this second embodiment, the optical axis  225 ′ is nearly perpendicular or perpendicular to the window  60 . This is done to minimize PDL. A probe housing window  612 , which is preferable also AlON, is further used in this second embodiment to enable the airtight sealing of the optics in the probe  110 ′. 
     Currently the angle α between the optical axis  225 ′ and the window  60  and the probe window  612  is similarly in the range of 80-85 degrees is preferred. 
     The collection lens system  212 ′ comprises a series of convex collection lenses, held in a cylindrical frame  217 ′, that collects light from the substance  54  traversing through the sapphire window  60  and probe window  612  over a large numerical aperture. The cylindrical frame  217 ′ is in turn held in the probe housing  214 ′. 
     The collection lens system  212 ′ comprises a first outer convex lens  240 ′. A second convex lens  242 ′, which is preferably biconvex, further collects the signal from the sample and focuses the signal from the sample on to photodetector  248 , which is held on a circuit board  250 . In the preferred embodiment, this photodetector system further comprises a temperature controller, such as by a thermoelectric cooler, in order to control its temperature to improve performance. 
       FIG. 7  is a ray trace illustrating the relationship between the projection lens system  212 ′ and the rays collected by the collection lens system  212 ′ of the second embodiment probe  110 ′. 
     As illustrated, the high NA lens system of the collection lens system  212 ′ collects signal over the entire expanse of the tunable optical signal beam projected on to sample  10  that is located on the outer side of window  60 . 
     In order obtain a uniform, tophat distribution, the projection lens system  210 ′ uses a first molded aspheric lens  220 ′ to bring the light from the tunable laser to a spherically aberrated spot  614 . The expanding beam from lens  220 ′ is collected and collimated by lens  218 ′, which is also aspheric. This yields the large, W p =30 to 40 millimeter, beam having a uniform intensity distribution. 
       FIG. 8  illustrates the beam intensity distribution of the tunable signal from the tunable laser in the second embodiment probe  110 ′. The distribution was derived from a MontiCarlo simulation taken in plane perpendicular to optical axis  225 ′ of the projection lens system  210 ′. The intensity distribution of the single mode fiber is converted to the tophat distribution in which the intensity variation over width W p  is less than 10% to 20% except for a bright ring  710  at the beam periphery. 
       FIG. 9  is a plot of absorbance, arbitrary units, as a function of wavelength. Specifically, in this illustrated example, the scan band of the tunable laser covers approximately 1350 to 1900 nanometers. In this range, common pharmaceutical materials, including sodium saccharin, lactose, monocrystalline cellulose, and magnesium stearate exhibit characteristic optical signatures. 
     As illustrated in  FIG. 10 , a plot of the second derivatives of the response as a function of wavelength between 1350 and 1800 nanometers shows changes over a blending process, including saccharine, lactose, monocrystalline cellulose, and magnesium stearate due to the mixing action. This shows the dependency of the spectrum as a function of time during blending. 
       FIG. 11  is a plot of average standard deviation of spectral blocks as a function of blending time. Specifically, after approximately 1 minute, the spectral response stabilizes until the 16 minute mark when magnesium stearate is added. This leads to a change in the standard deviation of the spectrum which then stabilizes after approximately 17 minutes of blending, showing how the present system is able to monitor the progress of the blending process. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.