Abstract:
A system and method evaluates a characteristic for suitability in classification of subjects. The system generates subject data including characteristic data and class data. The method includes arranging the subject data based on the characteristic data, and identifying the number of class changes from one class to another class in the arranged subject data. The number of class changes represents the suitability of the characteristic for classification of the subjects. The method further includes measuring the interval between class changes in the arranged subject data. The interval between class changes also represents the suitability of the characteristic for classification of the subjects.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of signal processing. More specifically, the present invention relates to signal processing a characteristic signal of a subject. 
     BACKGROUND OF THE INVENTION 
     In industrial automation, signal processing is used to classify an object being manufactured or processed based on a characteristic of the object. For example, an apple might be classified by a weight sensor configured to sense the weight of the apple. If the weight is greater than a predetermined weight, the apple is identified as “good”, and, if not, the apple is identified as “bad”. 
     However, the object can also be classified by other signals. For example, the apple might also be classified by acquiring a color digital image of the apple. If the apple is darker than a predetermined gray scale, or if the apple lacks sufficient red color, the apple is identified as “bad”. The challenge is to determine which characteristic (e.g., weight, color, gray scale, etc.) best classifies the objects into the desired classifications, so that the best characteristic can be used during production to automatically classify objects. 
     A standard method for evaluating the classification of objects has been implemented which assumes a bimodal distribution of the measured characteristic, the distributions assumed to be Gaussian. For example, referring to FIG. 1, this standard method generates a histogram  9  of the frequency of occurrence of different values of the characteristic. The x-axis represents the values of the characteristic (e.g., weight, color, etc.) and the y-axis represents the frequency of objects having that characteristic. A first mode  11  includes objects in a first class (e.g., “bad” objects) and a second mode  13  includes objects in a second class (e.g., “good” objects). According to this method, the mean values  17 ,  15  of each mode are identified, the variances of mean values  17 ,  15  are determined, and the distance  19  between mean values  17  and  15  is determined. The smaller the variances and the greater the interval between mean values  17 ,  15 , the greater is the quality of the characteristic for classification of this object. 
     One drawback of this method is that characteristic distributions frequently are neither bimodal nor Gaussian and, thus, are incorrectly evaluated by this prior method. With reference to FIG. 2, a frequency distribution  21  of another characteristic is shown, in which mode  23  is not Gaussian. Further, mode  23  includes objects in a first class, mode  24  includes objects in a second class, and mode  26  includes additional objects in the first class. An example of such a distribution might be one in which the characteristic is the length of a wooden dowel, wherein “good” dowels must have a length within a certain tolerance. Thus, “bad” dowels have lengths greater than (mode  26 ) and less than (mode  23 ) “good” dowels (mode  24 ). Prior methods will not adequately evaluate the suitability of this characteristic for classification purposes, since the distribution in FIG. 2 is not Gaussian and not bimodal. 
     Accordingly, there is a need for a system and method for evaluating the suitability of characteristics for classification. There is further a need for such a system and method which is applicable to non-Gaussian distributions. Further still, there is a need for such a system and method which is applicable to non-bimodal distributions. There is also a need for such a system and method which is robust against noise. 
     SUMMARY OF THE INVENTION 
     According to an exemplary embodiment, a method of evaluating a characteristic for suitability in classification of subjects based on subject data is provided. The subject data includes characteristic data and class data. The method includes arranging the subject data based on the characteristic data, and identifying the number of class changes from one class to another class in the arranged subject data. The number of class changes represents the suitability of the characteristic for classification of the subjects. 
     According to an alternative embodiment, a method of evaluating a characteristic for suitability in classification of subjects based on subject data is provided. The subject data includes characteristic data and class data. The method includes arranging the subject data based on the characteristic data, identifying consecutive subject data having a class change, and measuring the interval between the two consecutive subject data. The interval between class changes represents the suitability of the characteristic for classification of the subject. 
     According to yet another alternative embodiment, a system for evaluating a characteristic for suitability in classification of subjects is provided. The system includes sensing means for acquiring characteristic data from a plurality of subjects and classification means for classifying each subject with one of a first class and a second class. The system further includes means for arranging the subject data based on the characteristic data and identifying the number of class changes from one class to another class in the arranged subject data. The number of class changes represents the suitability of the characteristic for classification of the subjects. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like elements, and in which: 
     FIG. 1 is a histogram of a bimodal, Gaussian frequency distribution of a characteristic; 
     FIG. 2 is a histogram of a non-bimodal, non-Gaussian frequency distribution of a characteristic; 
     FIG. 3 is a block diagram of a system for evaluating a characteristic for suitability in classification of objects, according to an exemplary embodiment; 
     FIG. 4 is a flow diagram showing steps in a method according to an exemplary embodiment; 
     FIG. 5 is a number ray according to an exemplary embodiment; 
     FIG. 6 is a number ray illustrating a characteristic relatively good for classification; and 
     FIG. 7 is a number ray illustrating a characteristic relatively bad for classification. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 3, a block diagram of an exemplary system is shown. System  10  includes a plurality of sensors  12  and a signal processing circuit  14 , and, optionally, a display  16  and user input device  18 . System  10  acquires characteristic data sets (e.g., an image data set, a sound data set, other one-dimensional or multi-dimensional signal data sets, etc.) of a subject  20  (e.g., an object such as a welding spot, a tile, a motor, etc.) on a conveyor belt or platform  22 . System  10  may be implemented on a multisignal processing system, such as, SIMULTAN, manufactured by Siemens AG, Munich, Germany. The SIMULTAN system can generate many different characteristic data sets regarding a subject and can process a wide variety of signal types such as, images, sounds, vibration, current, force, etc. Thus, the system and method described below selects the best characteristic or characteristics to use for each classification purpose. 
     Sensors  12  (e.g., video cameras, ultrasonic transducers, infrared sensors, microphones, etc.) are configured to acquire characteristic data, for example, in the form of a digital image of subject  20  against a background or platform  22 . In this example, a video camera is configured to acquire an image data set in gray scale and to transmit the image data set to signal processing circuit  14 . Alternatively, the video camera may acquire characteristic data in color and transmit separate subject data sets for red, green, and blue for each image. Alternatively, only one sensor may be available, though multiple sets of characteristic data may be generated or calculated from the output of the one sensor. 
     Signal processing circuit  14  (i.e., a computer) includes a programmed microprocessor (e.g., an INTEL x86 processor, microcontroller, etc.), memory, communication interfaces, etc. Alternatively, signal processing circuit  14  may comprise programmable logic, discrete circuit components, etc. Circuit  14  operates according to a software or hardware program stored in memory (e.g., hard disk storage, firmware, non-volatile memory, etc.), is configured to perform various signal processing functions on the received characteristic data sets, and may further be configured to provide display signals to display  16  (e.g., a cathode ray tube, liquid crystal display, etc.) and receive user input from user input device  18  (e.g., a keyboard, touchscreen, etc.). 
     Referring now to FIG. 4, steps performed by circuit  14  will be described. At step  102 , system  10  is configured to sense, via sensors  12 , characteristic data regarding subject  20 . According to one example, a user places subject  20  (e.g., a metal part having a spot weld) on platform  22  and presses a switch (e.g., button, mouse click, touch screen, etc.) on user input device  18 . In response, signal processing circuit  14  commands sensors  12  to sense one or more characteristics of subject  20 . For example, one of sensors  12  might be a video camera configured to obtain a digital image of subject  20 . Another of sensors  12  might be a weight sensor configured to weigh subject  20 . Yet another of sensors  12  might be a length sensor configured to measure a diameter of subject  20 . Any type of sensor for measuring any characteristic about a subject  20  is contemplated. When the one or more characteristic data sets of subject  20  are acquired, they are transmitted to signal processing circuit  14  which stores the characteristic data in a data structure in memory for further processing. The data structure includes both characteristic data and any other subject data, such as class data (see step  104  below). The user typically processes a number of subjects (e.g.,  10 ,  20 - 30 , etc.) to acquire a statistically significant sampling of the various classes of subjects. 
     At step  104 , the subject data is classified into one of a number of classes. In this exemplary embodiment, the subject data is classified into a “good” class and a “bad” class. Alternatively, subject data can be classified into classes representing various sizes (e.g., short, medium, long), qualities, features, etc. One method of classifying subject data is for a user to press a switch on user input device  18  to indicate whether the subject currently on platform  22  is good or bad. A user puts a plurality of subjects on platform  22  and, for each subject, acquires characteristic data, and associates a class (e.g., good spot weld, bad spot weld, etc.) with each subject to generate the class data. Circuit  14  is configured to store the subject data, having the characteristic data and the class data, in a memory in preparation for further processing. 
     At step  106 , circuit  14  is configured to begin processing of the subject data. Circuit  14  first arranges the subject data based on the values of the characteristic data. More specifically, the characteristic data typically assumes values over a range of values from a minimum to a maximum (e.g., from a deep red color to a deep blue color, from a minimum weight of zero to a maximum weight of perhaps 2-3 kilograms, etc.). As a graphical representation of the result of this arrangement, a number ray  30  is shown in FIG.  5 . Number ray  30  includes a line  32  extending outward from a minimum characteristic value  34 . Each of the subject data is plotted on number ray  30 , such as is shown at points “X”  36  and “O”. The “X” and “O” indicia represent the class data for each data point (e.g., class X, class O), as generated at step  104 . The number ray is merely a representation of the arrangement; typically, in an actual embodiment, circuit  14  is configured to store the subject data in a linear data structure or in consecutive memory locations to form the arrangement. Alternative arrangement methods are contemplated. 
     At step  108 , circuit  14  is configured to identify the number of class changes from one class to another in the arranged subject data. Referring again to FIG. 5, circuit  14  is configured to read the class data from each subject data, proceeding from point  34  along line  32  on number ray  30 . At point  40 , a class “X” is read. The next consecutive or neighboring point along line  32 , point  42 , indicates a class “O”, i.e., a change in class from class X to class O. Circuit  14  is configured to identify this as a class change and to increment a class change counter (e.g., a memory location). Circuit  14  continues along number ray  30  until it reaches point  44 , a class “O” point, and point  46 , a class “X” point. Circuit  14  identifies this as another class change and again increments the class change counter. Circuit  14  continues in this manner until all or enough of subject data is read. The number of class changes represents the suitability of this characteristic for classification of subject  20 . If a large number of class changes exist, then the characteristic is less suitable for classification of subject  20 . If a small number of class changes exist, then the characteristic is more suitable for classification of subject  20 . 
     At step  110 , a further advantageous feature is shown. Circuit  14  is further configured to measure the interval between class changes in the arranged subject data, such as intervals  48  and  50  in FIG.  5 . The greater the intervals between class changes, the more suitable the characteristic is for classification of subject  20 . The lesser the intervals between class changes, the less suitable the characteristic is for classification of subject  20 . As can be seen, either or both of the number of class changes and the size of the intervals may be used by circuit  14  to determine the suitability of the characteristic for classification of subject  20 . 
     At step  112 , circuit  14  is configured to generate a suitability value for each characteristic based on the number of class changes and/or the size of the intervals in the arranged subject data. The suitability value can then be used to compare the several characteristics of subject  20  relative to one another to determine which one is most suitable for classification. According to one exemplary equation, the suitability value is calculated as follows:          ∑     i   =   1     n          1       d   i     +   a                              
     wherein i=an index, n=the number of class changes, d(i)=the interval between class changes, and a is a constant used to vary the relative weight of the number of class changes versus the size of the intervals. This equation accommodates for the situation wherein d(i)=O, i.e., two subject data have the same characteristic value but different classes. The smaller this suitability value, the better the characteristic is for classifying the subjects. The larger this suitability value, the worse the characteristic is for classifying the subjects. 
     At step  114 , one or more of the characteristics is selected as most suitable for classification of subject  20 . Circuit  14  may be configured to perform this step automatically by simply comparing the suitability values, or this may be done by a user via user input device  18  after viewing the suitability values, number of class changes, and/or intervals between class changes on display  16 . Once steps  102 - 114  are complete, circuit  14  is configured to automatically classify subjects, for example, during production, using the best characteristic or group of characteristics for classification purposes. 
     FIGS. 6 and 7 are number rays  120  and  130  illustrating a first characteristic being relatively good for classification and a second characteristic  130  being relatively bad for classification, respectively. Number ray  120  includes only four class changes, each separated by at least a small interval. Number ray  130  includes approximately eleven class changes, most separated by a very small interval. 
     The measured characteristics of the subjects, as mentioned, can be any type of characteristic about the subjects which can be sensed or computed. In addition to those mentioned, circuit  14  may be configured to calculate further characteristics based on sensed characteristic data. For example, from a digital image of the subject, circuit  14  may calculate the mean value, variance, diameter, standard deviation, etc. of points within the digital image, each of which is a further characteristic of the subject which may be suitable for classification purposes. As another example, a microphone may receive a sound signal from the subject. One method of testing the quality of a tile is to tap the tile with a hammer and record the sound resonating therefrom. This sound signal is a characteristic of the subject, and data derived from the sound signal, such as, maximum amplitude, frequency, decay time, square root, absolute square, etc., are further characteristics of the subject which may be suitable for classification purposes. The system and method disclosed in 
     FIGS. 3-5 above identifies which of these characteristics is best suited for classification of the tiles, for example, into “good” and “bad” parts. 
     While the exemplary embodiments have been illustrated and described, it should be understood that the embodiments disclosed herein are offered by way of example only. For example, the subject data, comprising characteristic data and class data, may be stored and arranged in various types of data structures and/or on various types of memories. Further, in addition to two-classification schemes (e.g., “good” parts and “bad” parts), subjects may be classified into three, four, or more classifications. The invention is not limited to a particular embodiment, but extends to various modifications that nevertheless fall within the scope of the appended claims.