Abstract:
A multi-stage grading system and method of grading products. The initial stage comprises a binary grader grading products into a large size and a small size. The small-size products are subsequently graded into a plurality of small-size size grades by a mechanical grader. The large-size products are subsequently separated and graded into a plurality of large-size size grades by a higher precision weight-based grader, such as a vision-based grader or a checkweigher.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This applications claims the priority of U.S. Provisional Patent Application No. 62/150,415, filed Apr. 21, 2015. The disclosure of that application is incorporated into this application by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates to sizing and grading products. 
         [0003]    Mechanical graders are used to size and sort articles into different size grades. Common mechanical graders use rollers that form adjustable sizing gaps between adjacent rollers or between rotating bars and a flat surface. The width of the gaps between adjacent rollers increases along their length from the upper entrance end to the lower exit end of a roller-type grader and decreases from bar to bar from entrance to exit in a bar-type grader. The largest-sized products are sorted off closest to the exit with a roller-type grader and closest to the entrance with a bar-type; the smallest-sized products are sorted off closest to the opposite ends. 
         [0004]    Shrimp processors often grade shrimp into many size ranges, for example, 15 size grades. A single bar-type grader would require 14 grading bars to sort shrimp into 15 size grades. Such a bar-type grader would have to be long to accommodate so many grades. What is often done to avoid having to use a single long grader is using two bar-type graders. For example, the large shrimp sorted off by the first grading bar of a first 7-bar grader are conveyed to a second 7-bar grader that further sorts the large shrimp into eight grades. The small shrimp not sorted off the first grader&#39;s first grading bar are then sorted into one of seven small grades by the remaining six grading bars of the first grader. Thus, instead of a long 14-bar grader, two shorter 7-bar graders can be used. In this way the shrimp are divided into two equal batches and graded in parallel. A similar parallel approach can be used with roller-type graders. 
         [0005]    Mechanical roller-type or bar-type graders are useful because they can handle bulk flows of shrimp. But they are prone to misgrading. One measure of grading quality is the uniformity ratio, defined as the ratio of the total weight of the N largest shrimp in a graded batch to the total weight of the N smallest shrimp in that batch, where N is an integer representing typically up to 10% of the total number of shrimp in the batch. Uniformity ratios for mechanical (whether roller-type or bar-type) graders are relatively high, reducing their utility for precision grading. 
         [0006]    Large shrimp typically have a higher price differential from grade to grade. And because larger shrimp each weigh more than smaller shrimp, each misgraded large shrimp makes a bigger difference in price than does a misgraded small shrimp. 
         [0007]    Weight-based graders using visioning systems to estimate weight and checkweighers used to measure actual weight are also used to size and grade products. But such precision graders require that the products not be presented in bulk for visioning or weighing. And requiring that products in bulk be separated reduces the throughput compared to that of bulk-flow mechanical graders. 
       SUMMARY 
       [0008]    One version of a grading system embodying features of the invention comprises a mechanical bulk grader grading products into a plurality of small-size size grades and a large-size size grade. A separator separates the products in the large-size size grade into individual distinguishable products. A precision grader determines the weight of each of the products in the large-sized grade received from the separator and grades each of the products into one of a plurality of large-size grades. 
         [0009]    Another version of a grading system comprises an initial grader sorting a bulk flow of products into large-size products and small-size products and a mechanical bulk grader grading a bulk flow of the small-size products received from the initial grader into a plurality of small-size grades. A separator separates the large-size products received from the initial grader into individually distinguishable products. A precision grader determines the weight of each of the large-sized products received from the separator and grades each into one of a plurality of large-size grades. 
         [0010]    In another aspect of the invention a method for grading products comprises: (a) sorting a bulk flow of products into a first size range and a second size range in a bulk grader; (b) sorting the products in the second size range into a plurality of second grades in a bulk grader; (c) separating the first size range of products into a flow of individually distinguishable products; (d) determining the weight or a weight-related property of each of the individual products in the first size range; and (e) sorting the individual products in the first size range into a plurality of first grades based on weight or a weight-related property. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a block diagram of a grading system embodying features of the invention; 
           [0012]      FIG. 2  is an isometric view of a mechanical bar-type grader usable in a grading system as in  FIG. 1 ; and 
           [0013]      FIG. 3  is a top plan view of a vision-based grader usable in a grading system as in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    A block diagram of a grading system embodying features of the invention is shown in  FIG. 1 . The grading system  10  comprises an initial grader  12  that sorts a bulk flow  14  of products into two size ranges of products: large-size products  16  and small-size products  18 . The initial grader  12  may be realized as an adjustable roller-type or bar-type grader, or any equivalent mechanical bulk grader that grades products that are received in bulk rather than individually. The demarcation between large and small products may be set to divide the incoming product flow  14  in any ratio, such as one-half. Or the demarcation may be set to sort products above a certain grade to the large size. For purposes of describing the invention, shrimp will be used as an example product. 
         [0015]    The mass flow  18  of small shrimp is conveyed to a mechanical grader  20  by a conveyor belt, a flume, or a chute, for example. It is also possible for the initial grader  12  and the mechanical bulk grader  20  to be realized by a single grader. In the case of a single bar-type grader, the grading bar nearest the entrance performs the initial binary grading function by diverting the large-size shrimp  20  in a first initial size range off the grader and passing the small-size shrimp in a second initial size range to the grader&#39;s remaining grading rollers or bars. The mechanical bar-type bulk grader  20  then grades the small-size shrimp into N small-size grades S 1 -S N . The initial grader  12  could alternatively be realized as a bulk roller-type grader, in which the roller sizing gaps are constant, but adjustable, along the length of the grader. The sizing gap is adjustable to set the desired demarcation between small-size and large-size shrimp. The small-size shrimp falling through the gaps are then routed to a mechanical bulk grader  20  to be graded into the small-size grades S 1 -S N . A single roller-type grader with an increasing sizing gap width could also be used to both separate out the larger shrimp and grade the small-size shrimp into individual grades S 1 -S N . The remaining shrimp that are not graded into any of the small-size grades S 1 -S N  are the large-size shrimp. So, instead of exiting the bulk grader first as with the bar-type grader, the large-size shrimp exit the roller-type grader last. 
         [0016]    The mass flow  16  of large-size shrimp from the initial grader  12  is conveyed to a separator  22  that separates the shrimp enough for the weight of each shrimp to be determined. The separator  22  may also form the separated shrimp into a single file on a conveyor  24 , such as a conveyor belt, conveying the singulated shrimp from the separator. The conveyor  24  feeds the large shrimp to a precision grader  26 , such as a weight-based grader. The precision grader  26  may be a checkweigher weighing each shrimp individually or a vision-based grader creating a digital image of each shrimp and from that image estimating the shrimp&#39;s weight or a weight-related property of the shrimp, e.g., volume, footprint, or profile, that is functionally related to weight by a predetermined mathematical function. As used in this specification, weight-based grader refers to a grader that is controlled by a system that determines the actual or estimated weight or a weight-related property of individual products. The precision, weight-based grader  26  sorts the separated large shrimp into M grades L 1 -L M . The M grades are relatively precise and can be much finer than the grades for the less valuable small shrimp, resulting in the uniformity ratio of the M large-size grades L 1 -L M  being much closer to unity than the uniformity ratio of the N small-size grades S 1 -S N . And because the precision weight-based grader  26  does not have to grade the small shrimp, fewer grading lanes and sorting ejectors have to be used. So the speed of the conveyor belt can be reduced. 
         [0017]    One example of a mechanical bulk grader using three grading bars is shown in  FIG. 2 . The grader  30  is similar to the Laitram® Model G-8 grader manufactured and sold by Laitram Machinery, Inc. of Harahan, La., U.S.A. (An example of a roller-type grader is the Laitram® Model PRG grader.) The mass flow  14  of shrimp is delivered by a flume  32  to a declining grader bed  34  at its upper end  36 . Water issued from nozzles  38  in a conduit  40  lubricates the declining grader bed  34  and, along with gravity, urges the shrimp down the grader. Diagonal grading bars  42 A-C, rotated by motors (not shown), are spaced above the bed  34  by a distance defining the grading gaps. The gaps get successively smaller down the grader bed  34 . The large shrimp  44 L are too large to pass through the gap under the uppermost grading bar  42 A. So they are directed by the uppermost roller  42 A through an opening  46 A in a side wall  48  of the bed  34 . The small shrimp  44 S pass under the uppermost grading bar  42 A to be graded by the remaining two grading bars into successively smaller-size grades S 1 -S 3 . The shrimp in each small-size grade S 1 -S 3  drop into a container (not shown) for each batch. In this arrangement the uppermost grading bar  42 A serves as an initial grader sorting the bulk flow of shrimp  14  into large-size shrimp  44 L and small-size shrimp  44 S. 
         [0018]    Referring now to  FIGS. 1-3 , the large-size shrimp  44 L are conveyed to the separator  22 . The separated large shrimp are conveyed to a vision system  50 , which produces a digital image of each shrimp. The vision system  50  estimates the weight of each shrimp from its digital image on a conveyor under the visioning sensor, such as a video camera, ultraviolet sensor, X-ray sensor, or laser sensor. Instead of a vision system the precision grader  26  can use a checkweigher to measure the weight of each shrimp directly. In the case of the checkweigher, the shrimp are presented in a single file so that only one is on the checkweigher at a time. Because the vision system can image more than one shrimp at a time, they don&#39;t necessarily have to be in a single file as long as they are separated enough for the vision system to distinguish individual shrimp and produce their digital images. In this example the vision system has three vision stations  50  operating in parallel, but there is no inherent limitation to the number of visioning conveyor lanes  52  or vision systems  50 . The shrimp exit the vision stations on the visioning conveyor lanes  52 . The vision system  50  controls diverters or ejectors (not shown) to selectively divert each shrimp from the visioning conveyors  52  exiting the vision stations onto transverse conveyors  54 , each dedicated to an individual grade L 1 -L 3 . In this example three transverse conveyor  54  output lanes are used, but there is no inherent limit to the number of these lanes  54 . Shrimp and other objects not meeting the grading criteria can also exit onto a reject conveyor  56  for discarding or recirculation back into the input flow  14 . Because the precision grader  26  is not overloaded with the small shrimp, the number of conveyors  52  and vision stations  50  that are required may be reduced, as well as the required speed of the conveyors  52   
         [0019]    Referring again to  FIGS. 1 and 2 , the height of the bar  42 A performing the initial binary grading function can be adjusted according to various criteria. The initial bar&#39;s height can be manually adjusted using methods such as screw-jacks, or the bar can be mechanized and its height remotely adjusted by means such as servo motors. If the bar is remotely adjustable, its height can then be automatically controlled depending on various criteria. For example, a feedback signal  58  to control the initial bar&#39;s height can be provided from the vision system or checkweigher to maintain throughput above or below a desirable threshold, which in turn can be automatically adjusted downward to ensure that the uniformity ratio of the vision-graded shrimp does not exceed a maximum acceptable value. Excessive throughput tends to reduce the grading performance of any grading system, whether mechanical, checkweigher-based, or vision-based. 
         [0020]    Although the invention has been described mainly with respect to one version, other versions are possible. For example, the initial grader could be a manual operation in which human operators sort the shrimp into large- and small-size grades. And the separator can be realized as a conveyor belt with converging side walls, a flume with a tortuous channel, or a V-channel vibratory feed, for example.