Patent Publication Number: US-2023152167-A1

Title: Spinning Flat Plate Calorimeter

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The invention is a Continuation-in-Part, claims priority to and incorporates by reference in its entirety U.S. patent application Ser. No. 17/524,886 filed Nov. 12, 2021 and assigned Navy Case 210335. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention described was made in the performance of official duties by one or more employees of the Department of the Navy, and thus, the invention herein may be manufactured, used or licensed by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND 
     The invention relates generally to laser calorimeters. In particular, the invention relates to rotating flat plate calorimeters for measuring heat flux from high energy lasers. 
     SUMMARY 
     Conventional calorimeters yield disadvantages addressed by various exemplary embodiments of the present invention. In particular, various exemplary embodiments provide a spinning flat plate calorimeter device for receiving and measuring laser energy. The device includes a circular disk, a shaft, a structure and a motor. The circular disk has temperature-detection instrumentation for measuring temperature from the laser energy. The shaft is supported by distal and proximal bearings. The structure supports the disk, shaft and its bearings. The motor turns the shaft and the disk. 
     Other various embodiments additionally provide for the disk further including a flat plate, a yoke wheel and a plurality of spacers. The plate has an obverse face for receiving the laser energy and a reverse face with a spiral groove for attaching the instrumentation. The yoke wheel attaches to the shaft. The thermal isolator spacers mechanically attach the yoke wheel to the flat plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and various other features and aspects of various exemplary embodiments will be readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, in which like or similar numbers are used throughout, and in which: 
         FIGS.  1 A and  1 B  are isometric assembly views of an exemplary flat plate calorimeter (FPC) device; 
         FIG.  2    is an isometric exploded view of components for turning components of the FPC device; 
         FIG.  3    is an isometric exploded view of components of the FPC device; and 
         FIG.  4    is a set of elevation views of the FPC device. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of exemplary embodiments of the invention, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized, and logical, mechanical, and other changes may be made without departing from the spirit or scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     The disclosure generally employs quantity units with the following abbreviations: length in inches (″) or meters (m), mass in grams (g), time in seconds (s), angles in degrees)(°), force in newtons (N), temperature in kelvins (K), energy in joules (J), power in watts (W) and frequencies in gigahertz (GHz). Supplemental measures can be derived from these, such as energy fluence in joules-per-square-meter (J/m 2 ) and the like. 
     The exemplary Flat Plate calorimeter (FPC) has demonstrated its ability to accurately measure the downrange performance of high energy laser (HEL) systems. With increasing operating power of these HEL systems, difficulties arise in producing a power meter that can survive the engagement. By spinning the FPC&#39;s absorber plate, the laser&#39;s energy is spread out over a much larger area, and the power handling capabilities of the sensor are significantly increased. This enables the FPC to be used as HEL systems are improved. 
     Accurately measuring the downrange power of an HEL system is necessary in order to access system performance. The exemplary FPC was developed specifically for this task and has been used to measure HEL power (W). By simultaneously imaging the surface of the FPC, the laser spot size can be measured, which enables the irradiance (W/m 2 ) to be determined. As HEL system performance improves, the power increases and the spot size on target decreases. Both of these improvements will cause the anticipated downrange irradiances to increase. 
     Each material has a fluence (J/m 2 ) limit, beyond which it will suffer damage and/or degradation. In the case of an FPC, exceeding the specific design&#39;s fluence limit can either destroy the surface coating or damage the sensor wire imbedded within the sensor. Reflective surface coatings, such as gold, can serve to increase the sensor&#39;s fluence limit, but these techniques have limitations and also decrease the FPC&#39;s sensitivity (by increased reflectivity and thereby reduced emissivity), as well as increase its noise floor. 
     By spinning the sensor plate, and aiming the laser spot some radial distance from the center of rotation, the laser&#39;s energy is spread out over an annular area that is much larger than the laser spot area. The FPC&#39;s fluence limit is effectively increased by the ratio of this annular area to the laser spot area. Ratios of ten or more are easily achieved. 
     Furthermore, imaging laser spots off of a spinning target has long been known to give the most accurate spatial representation of the spot structure. Any surface imperfections that would cause speckle in the image are effectively averaged out by the moving surface. A spinning FPC enables a single device to be used for both accurate spot size and HEL power measurements. 
     Conventionally, in order to increase power handling, beam splitters are often used to direct some small percentage (i.e. 3%) of the beam&#39;s power onto the power meter. This is a cumbersome procedure that is very geometrically sensitive to the relative location of the laser source to target. Error is also introduced if the exact value of the splitter&#39;s reflection-to-transmission ratio is not known to a high precision. 
     As alternatives, conventional water cooled power meters are also available, but these are cumbersome to use due to the requirement for a pump, hoses, and support equipment. They also have much slower time responses compared to this device and do not enable simultaneous beam imaging. 
       FIGS.  1 A and  1 B  show perspective assembly views  100  of an exemplary spinning FPC assembly  110 . This includes an aluminum base frame  120  that supports distal bearings  130  for a hollow shaft  140 . A gear motor  150  drives a pulley transmission belt  160  at a rotational speed limited by a slip ring  170 . The shaft  140  turns a circular disk  180  along their shared axis perpendicular to the wheel&#39;s periphery. An obverse face  185  of the disk  180  receives radiant energy from a laser (not shown, but featured in application Ser. No. 17/524,886 incorporated by reference). The disk  180  mounts to an upright frame  190  supported by elbow brackets  195  and the base frame  120 . 
       FIG.  2    shows an isometric exploded view  200  of components for the disk  180  and related rotating components of the FPC assembly  110 . The disk  180  includes a circular instrument sensor plate  210  with peripheral attach points  215  and a yoke wheel  220  having a shaft hole  225 . On its reverse face opposite the obverse face  185 , the sensor plate  210  includes a spiral groove  230  extending from center to periphery to receive temperature instrumentation, such as voltage response based on electrical resistance. 
     A set of four spacers  240  connect the wheel  220  to the plate  210  via corresponding screws  245 . A central hub collar  250 , including a sleeve  255  and a radial flange  260  attach the shaft  140  through the axis hole  225  of the wheel  220 . The screws  245  pass through the spacers  240  and into their corresponding attach points  215  of the plate  210  along respective axes  265 . The slip ring  170  passes FPC signal wires  270  inside the shaft  140  from the plate  210  to a stationary data acquisition device (not shown) via electrode prongs  280  that extend beyond the base frame  120 . A breakout board  290  ties the wires  270  from the temperature instrumentation along the groove  230  to the slip ring  170  and attaches to one of the spokes on the wheel  220 . 
     The spinning FPC assembly  110  described is shown in view  100 . The device assembly  110  includes an aluminum frame  120  that supports distal bearings  130  for the hollow shaft  140 . A gear motor  150  drives the shaft  140  via a power transmission belt  160 . The rotational speed of the motor  150  is limited by the capabilities of the slip ring  170  that passes the FPC signal wires  270  from the rotating plate  210  to a stationary data acquisition (DAQ) device (not shown). 
     Details of the spinning assembly are shown in view  200 . The sensor plate  210  absorbs a fraction of the laser power depending on the absorptivity of its surface coating. The absorbed laser energy causes the sensor plate  210  to rise in temperature. The reverse side of the sensor plate  210  (opposite its obverse face  185 ) contains a spiral groove  230  that contains multiple passes of a 30-gage enameled coated copper wire held in place with a rubber O-ring. This sensing wire&#39;s resistance is proportional to the average temperature of the sensor plate  210  and is used to accurate measure the resulting temperature rise. 
       FIG.  3    shows an isometric exploded view  300  of components  310  of the FPC assembly  110 . The base frame  120  includes a square flange  320  that holds the distal bearings  130  and a bearing wheel  330 , which connects to the shaft  140 . A flat plate  340  with a belt-tensioning jack screw  345  also mounts to the base frame  120  that connects to a test platform table, while the upright frame  190  connects to the base frame  120 . The plate  340  with frames  120  and  190 , along with brackets  195  and  350  constitute a disk support structure for attaching to the table. 
     The motor  150  mounts to an L-shape motor bracket  350  and turns a drive pulley  360  connected to the transmission belt  160  that a shaft pulley  370  on the upright frame  190 . Proximal bearings  380  connect the yoke wheel  220  to the upright frame  190 . The shaft  140  can also to pass through the collar  250 , the proximal bearings  380  and the wheel  220 . 
     The sensor plate  210  is mounted offset from the yoke wheel  220  with four spacers  240  composed of high-temperature polyether ether ketone (PEEK) plastic (C 19 H 14 O 3 ) to thermally isolate the sensor plate  210  from the aluminum yoke wheel  220 . The four screws  245  that mount the sensor plate  210  to the wheel  220  are also composed of PEEK for thermal isolation. The wheel  220  bolts to a central hub collar  250  that attaches the entire disk  180  to the hollow shaft  140  for spinning. The collar  250  features a hole in its sleeve  255  that enables the signal wires  270  to pass from the sensor plate  210  through the hollow shaft  140  to the slip ring  170 . 
       FIG.  4    shows elevation views  400  featuring the relative size of the exemplary assembly  110  (omitting the shaft  140 ). In this configuration, a 12-inch diameter sensor plate  210  is used, and the assembly  110  can fit into a 12.5″×12.5″×13″ box. The upright frame is 12.5″ in width and height, and the length from the motor  150  to the sensor plate  210  is 13″. This size is exemplary only. 
     By spinning the FPC as the sensor plate  210 , the fluence limit of the sensor can easily be increased by an order of magnitude. That tolerance improvement can be used to either increase the total power of the laser that can be tested, or increase the duration of the engagement. Furthermore, because spinning targets are already routinely used for beam imagining, the exemplary device  110  can be used for both power measurement and beam imaging thereby eliminating a required piece of equipment. Also, any variation in surface absorptivity will be averaged out by the spinning motion, which leads to more accurate power measurements compared to a static power meter. 
     While certain features of the embodiments of the invention have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fail within the true spirit of the embodiments.