ECONOMICAL SPECTROMETER UNIT HAVING SIMPLIFIED STRUCTURE

Abstract: A spectrometer construction is shown in which the chassis (32) of the spectrometer is fabricated by stamping, punching, and bending sheet metal, the mirror supports (58, 66) and other elements are also formed from sheet metal. Precision locations are assured by tab and slot matching of the sheet metal parts. The mirrors in the spectrometer, other than those in the interferometer (36), are secured directly to vertical sheet-metal walls without intervening adjustments. The mirrors (54, 56, 60, 62) have unitary reflection and backing structures formed either by a diamond-cutting lathe, or by precision plastic molding.

Description: EC0N0MICAL SPECTROMETER UNIT HAVING SIMPLIFIED STRUCTURE Background of the Invention: This invention relates to the spectrometry field, and specifically to spectrometers which incorporate interfero¬ meters. Its purpose is to provide a "spectrometer arrange- ment which utilizes important developments in manufac¬ turing techniques to simplify spectrometer fabrication and eliminate parts, while maintaining a high level of spectrometer performance. The spectrometer systems heretofore supplied for laboratory and manufacturing environments have generally used thick aluminum base plates, which are. drilled Co provide mounting holes for the mirror mounts. The mirror mounts are complex assemblies, using springs, screws, and the like, to permit adjustment of the mirror positions. The overall costs of parts fabrication, assembling, and adjusting tend to result in relatively expensive structures . One of the aspects of interferometer spectrometer systems which has not been fully utilized in structural design is the tremendous difference in accuracy require¬ ments between the interferometer portion of the spectrometer system, and the remaining portions of the spectrometer system. Within the interferometer the precision required is orders of mmagnitude greate than that required in the portions of the spectrometer external to the interferometer. This, in part, is the basis for the present bold restructuring of the spectrometer. Summary of the Invention: The present invention makes use of recently developed technologies to: (a) radically reduce the manufacturing costs; ard (b) eliminate mirror position adjustments. Manufacturing of the supporting elements (base, mirror mounts, etc.) is accomplished by computer- controlled stamping machines; i.e., press-formed sheet metal parts are substituted for the supporting elements in the prior art structures. Location of the mirror mounts, and other parts, with respect to the base is accomplished by tab and slot combinations, or hole and fastener combinations, which can be held to very close tolerances. The mirrors are formed by an accurately repeatable process, and are located by the close-tolerance fit of the tabs and slots (or holes and fasteners). No adjustment devices are required for the mirrors, which are directly secured to the sheet metal parts. Initially the mirrors will be formed by a high- precision diamond turning process. Subsequently, for high volume, they may be formed as plastic molded optics. Brief Description of the Drawings : Figure 1 is a plan view of the entire assembled spectrometer, showing some broken-away portions of the cover ; Figure 2 is an elevation view showing one longi¬ tudinal side of the assembled spectrometer; Figure 3 is an elevation view showing the other longitudinal side of the assembled spectrometer; Figures 4 and 5 are sectional views taken on the lines 4-4 and 5-5, respectively, of Figure 1; Figure 6 is a plan view of the sheet metal chassis, which has been formed from a single sheet metal stamping Figures 7A and 7B are elevation " views showing opposite longitudinal sides of the chassis shown in Figure Figures 8 and 9 are front elevation and end views, respectively, of one of two end wall plates, each of which constitutes the fourth wall of either the interferometer section or the detector section; Figures 10, 11 and 12 are bottom, rear elevation, and side elevation views, respecti ely, of one of the two (preferably identical) mirror-mounting brackets, each of which is formed as a sheet metal stamping, and is then secured to the chassis and to one of-the wall plates; Figures 13, 14, and 15 are side elevation, end, and top views, respectively, of a gusset used to rigidify one side wall of the interferome er section; Figures 16, 17 and 18 are plan, longitudinal eleva- tion, and end views, respecti ely, of the channeled supporting member which underlies the spectrometer floor; Figures 19 and 20 are plan and end views, respec¬ tively, of a sheet metal supporting bracket for the interferometer; Figures 21-23 show a sheet metal frame and cover plate used to cover the interferometer section; and Figures 24 and 25 are plan and longitudinal elevation views, respec i ely, of a chassis weldment assembly, which includes all the sheet metal parts after they have been welded to the chassis; and Figure 26 is a cross-section taken on line 26-26 of Figure 24. Detailed Description of Preferred Embodiment: Figures 1-5 show the completed spectrometer structure. As best seen in Figure 1, a unitary sheet metal chassis 32 is provided, which has an open sample area 34 between an enclosed interferometer section 36 and an enclosed detector section 38. The covers of the interferometer and detector sections have been generally broken away in Figure 1 to display the internal units. An interferometer 40 is supported on chassis 32. This interferometer .may correspond to the one disclosed in U.S. Application Serial No. 789,849, filed October 21, 1985, and assigned to the assignee of this application. The interferometer has its beamsplitter at 42, its movable mirror arranged to reciprocate along a left-to-right path (as seen in Figure 1), and its fixed mirror located in lateral arm 44. Access to the adjusting mechanism used for initial position-adjustment of the fixed mirror is permitted by removal of an access cover 46. A radiation source 48 is secured directly to side wall 50 of the interferometer section. Also secured directly to side wall 50, without any adjusting mechanism, is a paraboloid mirror 52, which receives radiation from source 48, and directs a colliuiated beam toward the beam- splitter in interferometer 40. The collirnated beam leaving interferometer 40 is reflected by a flat mirror 54 toward a paraboloid mirror 56. Both flat mirror 54 and paraboloid mirror 56 are supported on opposite side walls of a mirror-support bracket 58, having a substantially channel-shaped horizontal cross-section. The radiation reflected from paraboloid 56 has its focal point at the center of the sample area. Post-sample radiation is recollimated by a paraboloid mirror 60, which directs a collirnated beam toward a paraboloid mirror 62. Radiation leaving paraboloid 62 is directed to a detector 64. The two paraboloids 60 and 62 are supported on opposite side walls of a mirror-support bracket 66, which preferably is identical to bracket 58, for manufacturing economy. Each of the mirrors 56, 60 and 62 is secured directly to the bracket wall, without any adjusting mechanism. Flat mirror 54 also is non-adjustable. It is shown as a glass mirror 68 glued to an aluminum wedge-shaped block 70, which is directly secured to the bracket wall. The two paraboloid mirrors 56 and 60 have relatively long focal lengths, and are identical. The two paraboloid mirrors 52 and 62 have relatively short focal lengths, and are identical. The absence of adjustments for the mirrors (except the mirror inside the interferometer) provides a very significant cost reduction. This elimination of mirror adjustments is permitted in part by the use of a highly repeatable and precise mirror-forming method, and in part by precise, low tolerance location of the mirror- supporting walls. Additionally, as stated above, the elimination of mirror adjustments takes advantage of the very much lower precision requirements outside the interferometer than inside it. Detector 64 is secured directly to a side wall of the spectrometer. . It may be desirable to allow adjustability of the detector position. An enclosed compartment 72 houses a transformer 74 and a laser power supply 76.. This full enclosure, which includes its own cover, avoids shock risk and isolates the noise from the sensitive portions of the spectrometer. An access cover 78 is secured to one side wall of compartment 72. This assembly adds rigidity to the side wall near the radiation source 48 and mirror 52. A strengthening gusset 80 is secured to the floor 82 of the chassis and to its side wall 50, in order to provide additional rigidity for the side wall near radia¬ tion source 48 and mirror 52. Mounting of source 48 on the side wall, which is metallic, permits escape of heat- through the wall and through a finned heat sink 86. As seen in Figures 2-5, the chassis 32 rests on, and is welded to, a unitary chassis support member 88, which is a sheet metal stamping having three longitudinal channel-shaped runners 90 (see cross-section in Figure 4), and four fla.nges 92 welded to the bottom of the chassis. One of the major advantages of the present invention is the use of sheet metal parts. This permits substantial reduction of cost, while maintaining high performance capability. Modern computer-aided manufacturing (CAM) systems are available for sheet metal stamping processes. The position locations which require precision can be controlled to tolerances in the neighborhood of 0.010 in., which is adequate for good spectrometer performance. Most of the precision locations are determined by slots, tabs, and holes formed in the sheet metal chassis as part of the press-forming (punching) sequence. Another advantage of CAM is the ease with which design changes may be made, as experience dictates their desirability. The sheet metal chassis eliminates the former thick aluminum base plate, e.g., one-half inch thick, and eli' inates all casting, machining, drilling and tapping processes . Figures 6, 7A and 7B show the- sheet- metal chassis, which was first punched out of sheet metal stock, as an integral element having a floor section, extensions which will form three side walls of the interferorneter section, and extensions which will form three side walls of the detector section. A large number of rectangular slots and holes have been cut in the sheet metal, whose positions have been held to close tolerances, in order to determine accurate locations for the mirror-mounting structures and other units in the spectrometer . (New numerals will be applied in describing the element-by-element sheet metal forming processes, and the welding and other means used to secure the elements together). The entire chassis stamping is indicated by the numeral 100. The flat floor 102 of the sample area has a plurality of holes 104 formed therein. These holes provide various locations for temporary mounting of what¬ ever sample holding accessory is being used. The holes are adapted to receive PΞM fasteners, some of which (106) are shown in place in Figure 7A. An advantage of PEM fasteners is that their insertion creates some metal flow, which ensures tight .nd accurately located fastener connections . The two side walls 108 and 110, and the end wall 112, of the interferorneter r ection have been bent upwardly from 25 the sheet metal ' LO O T to extend perpendicularly to the floor. The two side walls 114 and 116, and the end wall 118, of the detector section have also been bent upwardly from the sheet metal floor to extend perpendicularly to the floor. Additionally, each of the six vertical walls 108 through 118 has an integral narrow horizontal flange 120 turned inwardly at its upper end. These flanges have 45° edges which abut one another as shown. Four rectangular slots 122 are formed in the floor of the interferometer section to locate one mirror-mounting bracket; and four rectangular slots 124 are formed in the floor of the detector section to locate the other mirror- mounting bracket. Both the i erferometer s'iction and the detector section require a fourth vertical wall, each of which is a separately formed stamping. Two slots 126 are provided to locate the fourth wall of the . interferometer section; and two slots 128 are provided to locate the fourth wall of the detector section. Near each end of the chassis, a group of six laterally spaced slots 130 are shown. These slots receive integral tabs formed as integral projections of the multi¬ channel sheet metal support which underlies the floor of the chassis. Also, a slot 132 is used to locate the gusset which is secured to the floor of the interferometer section and to its side wall 108. Two rectangular holes 134, one in the interferometer section, and one in the detector section, are used to admit interconnecting electrical cables to those sections, the cables con- veniently extending inside one of the channels formed in the support element underneath the chassis. The same holes and channel are used to conduct nitrogen gas into the spectrometer, for purging purposes. Figure 7A shows round holes 136 in the wall 108, which are used to locate the radiation source; and both round holes 138 and oblong holes 140 in wall 108, which are used to locate the pre-interferometer paraboloid mirror. As previously stated, both the radiation source and the adjacent paraboloid are secured directly to wall 108. Holes 142 in the floor of the interferometer section combine with PEM fasteners to secure the interferometer- supporting element. The fully enclosed chamber containing the transformer and laser generator is defined in part by the locations of slots 144 in wall 108. Slots 146 in wall 108 receive tabs on the wall-rigidifying gusset. Figures 8 and 9 show a sheet metal plate 148 which provides the fourth wall for either the interferometer section or the detector section. The two such walls are mirror images of one another. The lower edge of each wall has two tabs 150 which tightly fit into and are therefore located by, either the two slots 126, or the two slots 128, in the chassis floor (Figure 6). The upper edge of wall 148 is bent over to form a narrow flange 152, the edges of which engage the upper flanges on the two adjacent side walls of the chassis. A hole 154 through wall 148 allows radiation to pass through. On the inter¬ ferometer side, the radiation is coming from a paraboloid mirror; and on the detector side, the radiation is going toward a paraboloid mirror. Four slots 156 are provided in wall 148, in order to receive tabs formed on the adjacent mirror-mounting bracket. Figures 10-12 show one of the two mirror-moun ing brackets, which are preferably identically formed, for manufacturing economy. Bracket 158 is a sheet metal stamping having an essentially channel-shaped horizontal cross-section, as shown in the bottom view (Figure 10;. The side v/alls 160 and 162 each have two tabs 164 which fit into slots 156 in wall element 148 (Figure 8). The side walls 160 and 162 also each have two tabs 166 • nicϊi fit into slots in the floor of the chassis (Figure 6). The four tabs 166 of one bracket 153 fit into slots 122 in the interferometer section. The four tabs 166 of the other bracket 158 fit into slots 124 in the itector section. The brackets 158 are each welded both to the floor of the chassis and to the respective fourth vertical wall 148. Two integral,^ horizontally-extending flanges 168, one bent outwardly from the bottom of side wall 160, and one bent outwardly from the bottom of side wall 162, are welded to the chassis floor. Each side wall has two integral, vertically-extending flanges 170, which are welded to vertical wall 148. As previously explained, mirrors are supported on each of side walls 160 and 162 of the bracket. Its third wall 172 has a plurality of openings therethrough, as seen in Figure 11. A large opening 172 is the one through which radiation passes, either entering or leaving the sample area. Three other openings 174 are access openings through which the mirrors can be reached during the process of mounting them on the side walls. As seen in Figure 12, a plurality of slots and holes are provided in each side wall for use in mounting and securing a mirror. Oblong slots 176 are arranged to receive closely-fitting dowel pins, which also enter openings in the rear of the mirror structure. These pins and slots provide close tolerance location of each mirror. Four other holes 178 ' receive fasteners which clamp the mirror structure to the bracket side wall. Figures 13-15 show a sheet metal gusset 180, which stiffens, and guarantees perpendicularity of, that side wall of the interferometer section on which are mounted the radiation source and one of the paraboloid mirrors. Gusset 180 has one downwardly-projecting tab 182 which fits into slot 132 in the floor of the chassis (Figure 6), ' and two laterally-pro ecting tabs 184 which fit into slots 146 in the side wall of the interferometer section (Figure 7). Two horizontally-extending integral flanges 186 are welded to the chassis floor; and t.wo vertically-extending integral flanges 188 are welded to the side wall. Figures 16-18 show a sheet metal chassis-supporting element 189. It is formed with three longitudinally- extending channels having bottom surfaces 190 which rest on a working table, or the like. Between the channels are two flat longitudinally-extending "integral strips 192 which are spot welded to the bottom of the chassis floor. Also two longitudinally-extending side flanges 194 are spot welded to the bottom of the chassis. As seen in Figure 17, the sheet metal support has upwardly-bent end walls 196 at each end of each channel. These end walls (six in all) enclose the channels, and each of them has two upwardly-projecting tabs 198 which fit into slots 130 in the floor of the chassis (Figure 6). A plurality of spaced holes 200, in two rows, are provided for the user's convenience in locating a sample-containing unit in the sample area. Several other holes 202 permit access for spot welding. Figures 19 and 20 show a sheet metal bracket 204, on which the interferometer is mounted. The upper surface 206 of bracket 204 supports the interferometer, and has a plurality of holes 208 to receive fasteners securing the interferometer to the bracket. Two outwardly-extending integral flanges 210 on opposite sides of the bracket are secured to the chassis floor by fasteners extending through holes 212. The interferometer and detector sections each have covers. In order to simplify cover attachment, two sheet metal parts are used to cover each section, a frame and a cover plate. Figures 21-23 show these two sheet metal elements for the interferometer section. A similar arrangement is used to cover the detector section. A frame 214 is shown in Figures 21 and 22. It has a rectangular shape, as seen in Figure 21, which is open at the center. Four flat integral frame sides 216 provide openings 218 for cover-securing fasteners. Each of the four frame sides 216 (which are L-shaped in cross-section) has an integral right-angle downwardly-extending flange 220 which engages one of the walls of the interferometer section. The frame 214 is located just under, and in engagement with, the narrow flanges 120 (Figure 6) and 152 (Figure 9) which are turned inwardly from the upper ends of the interferometer section walls. The frame 214 is spot welded to the section walls and flanges. A flat sheet metal cover 222 is shown in Figure 23. It is placed on the frame 214, and is secured thereto by fasteners extending through countersink holes 224 into openings 218. Figures .24-26 are plan, longitudinal elevation, and end elevation views, respectively, of the chassis weldment assembly, i.e., the sub-assembly which results when all of the sheet metal stampings have been welded to the sheet metal chassis. The parts have been located to close tolerances by the slot and tab alignment technique. The numerals applied in Figures 24-26 are the same as those used in identifying the sheet metal stampings in Figures 6-23. From the part-by-part description, it is clear that a thorough redesign of the spectrometer structure has been accomplished, and that very significant cost reductions can be accomplished in the manufacturing process. As previously stated, another major cost reduction results from the elimination of complex and expensive mirror adjustment devices. In addition to close-tolerance location of the mirror supports, this requires the use of mirror manufacturing processes which provide exact dimen- sional repeatability of the mirror unit. Not only must the mirror face be properly contoured and have adequate reflectivity; but also the distance from the mirror face to the mounting structure must be accurately maintained. The usual mirror-forming process involves grinding and polishing of glass optical ele ents. After forming the reflective surface, the mirror must be secured to a backing element, which in turn is secured by suitable fasteners to a supporting member. The process has two disadvantages. It is not easily, or accurately, repeatable; and the distance between the mirror's reflec- tive face and the supporting member t-ends to vary. For these reasons, it has been customary to provide mirror adjustment parts, and to carefully align each mirror by manipulating such adjustment parts. The present invention eliminates the costs both of the adjustment parts and of the aligning procedure by using a mirror-forming process which provides a precisely- repeatable focal length, and which holds to a close tolerance the distance between the reflecting face of the mirror and its vertical supporting member. At present, diamond-forming of a reflecting surface on an aluminum mirror body is the preferred mirror-forming method. This is a lathe-turning process which can be held to very accurate dimensions. Since the mirror surface is integral with its body, it can be directly fastened to the supporting member. As stated above, dowels are used to align openings in the back of the mirror with openings in the sheet metal walls of the mirror-mounting brackets. Threaded fasteners are then used to clamp the back of the mirror to the sheet metal wall. The mirror surface forming process is referred to as micro-surface generating, single-point diamond machining, or micro-machining. It involves the use of single point diamond cutting tools, and high accuracy, low vibration two-axis lathes. Because the diamond cutting tool leaves a tooling- mark on the mirror surface, this process is not recom¬ mended for mirrors reflecting visible, or relatively short wavelength, radiation. However, it is very satisfactory for mirrors in inf- ared radiation systems. Another potential mirror-forming method is plastic molding. Optical surfaces may be accurately formed by injection molding, using such materials as acrylic, polystyrene, and polycarbonate. Use of this plastic molding method would permit a higher production rate than the diamond cutting process. Although aspheric mold making is very complex, the manufacturing process is relatively straightforward, once the " old has been formed. From the foregoing description, it will be apparent that the apparatus and method disclosed in this applica¬ tion ill provide the significant functional benefits summarized in the introductory portion o f the specifica¬ tion . The following claims are intended not only to cover the specific embodiments disclosed, but also to cover the inventive concepts explained herein with the maximum breadth and comprehensiveness permitted by the prior art.

Claims: What Is Claimed Is : 1. A spectrometer structure comprising: a chassis, formed by stamping sheet metal, and having an interferometer section, a detector section, and a sample area between the interferometer and detector sections; the chassis providing a horizontal spectrometer floor; an interferometer supported on the chassis; a first mirror-supporting bracket located in the spectrometer section, formed by stamping sheet metal, and having its sides pεi endicular to the spectrometer floor; a second mirror-supporting bracket located in the detector section, formed by stamping sheet metal, and having its sides perpendicular to the spectrometer floor; a first unitary mirror-providing element having a parabolic reflecting surface which reflects radiation into the sample area, and a rear surface secured directly and non-adjustably to a side of the first mirror-supporting bracket; and a second unitary mirror-providing element having a parabolic reflecting surface which reflects radiation coming from the sample area, and a rear surface secured directly and non-adjustably to a side of the second mirror-supporting bracket. 2. The spectrometer structure of claim 1 wherein the sheet metal chassis includes: three integral vertical side walls of the interfero¬ meter section; and three integral vertical side walls of the detector section. 3. The spectrometer structure of claim 2 also comprises : a radiation source secured to one vertical wall of the interferometer section; and a third unitary mirror-providing element having a parabolic reflecting surface and a rear surface secured directly and non-adjustably to the same vertical wall as the source, and arranged to receive radiation from the source and to direct a collirnated radiation beam into the inter-ferometer . 4. The spectrometer structure of claim 3 which also comprises: a detector secured to one vertical wall of the detector section; and a fourth unitary mirror-providing element having a parabolic reflecting surface and a rear surface secured directly and non-adjustably to a side of the second mirror-supporting bracket, and arranged to receive a collirnated radiation beam from the second unitary mirror- providing element and to direct a focused radiation beam to the detector. 5. The spectrometer structure of claim 4 which also comprises: a flat mirror non-adjustably supported by a side of the first mirror-supporting bracket, and arranged to reflect a collirnated beam leaving the interferometer and to direct it toward the first unitary mirror-providing element. 6. The spectrometer structur. of claim 4 wherein the parabolic reflecting surfaces ot the first and second unitary mirror-pro iding elements have relatively long, substantially identical focal lengths. 7. The spectrometer structure of claim 6 wherein the parabolic reflecting surfaces of the third and fourth unitary mirror-providing elements have relatively short, substantially identical focal lengths. 8. The spectrometer structure of claim 1 wherein the first and second mirror-supporting brackets are precisely located by integral tabs which extend into closely-fitting slots formed in the chassis floor. 9. The spectrometer structure of claim 4 wherein each of the four unitary mirror-providing elements is an aluminum member whose mirror surface is formed by a single point diamond tool controlled by a two axis lathe. 10. The spectrometer structure of claim 4 wherein each of the four unitary mirror-providing elements is a plastic member formed by injection molding in an aspheric- shaped mold.

Inventors: AUTH GERALD L, AUTH, GERALD, L.
Applicants: MIDAC CORP, MIDAC CORPORATION
Application year: 1987
Main classification: G01B9
IPC codes: G01B9, G01J3, G02B7