Abstract:
Various embodiments of a method for monitoring a machine condition are provided. An embodiment of the present invention provides a method of monitoring a machine condition, comprising the following steps: modeling a normal signal model performed by detecting a signal for monitoring condition of a normal machine and converting the detected signal to a normal signal model in time domain using a hidden Markov model (HMM) algorithm; calculating a probability value data of the monitoring signal at a subject machine performed by detecting a signal for monitoring condition of the subject machine in real-time and converting the detected signal to the probability value data relative to the normal state signal model using the HMM algorithm; and determining a section having deficiency where the probability value data of the monitoring signal at the subject machinery is not maintained constantly relative to the normal signal model.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Korean Patent Application No. 10-2010-0012970 filed on Feb. 11, 2010 and Korean Patent Application No. 10-2010-0077026 filed on Aug. 10, 2010, the entire disclosures of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    Non-destructive inspections, which apply monitoring signals such as electricity, ultrasound wave, etc. to a machine or mount sensors to detect and compare normal signals to the detected signals, can be used for monitoring machine conditions. 
         [0003]    Hereinafter, a prior art method of monitoring a welding deficiency will be described as an example. 
         [0004]    Arc welding systems using robots or automatic welding devices are broadly used in the industrial field. In an arc welding operation, welding deficiencies such as lack of fusion, blow holes, etc. can be caused by pre-processing inferiority of a welding spot, non-homogeneity of a welding rod, insufficient supply of welding gas, etc. 
         [0005]    Considering the arc welding operation, it is difficult for an operator to monitor the welding operations continuously. Further, it is hard to optically find a welding deficiency such as a blow hole formed in the welding spot. Thus, even if the welding deficiency has occurred, it may not be monitored perfectly. For example, in a car manufacturing system, a door panel or a body frame of the car is press-molded and parts with various forms and sizes are welded thereto by a welding robot. Then, they will be painted and assembled. In these continuous manufacturing processes, the welding deficiencies may not be monitored substantially. Also, as another example, in welding operations of a thick metal plate of heavy machinery for construction, a welding line is welded repeatedly. If a deficiency occurs in a first welding operation, then it is hard to detect the deficiency since the deficiency is usually examined optically after the welding operations are completed. In order to monitor the deficiencies of a machine which cannot be detected optically, non-destructive inspection such as ultrasonic wave inspection, X-ray inspection, etc. can be performed. However, the non-destructive inspection may require much time and money. Further, it is hard to use the non-destructive inspection for monitoring the welding spot of the machine with a complicated shape. 
         [0006]    Commercial arc-welding monitoring method is performed by detecting the feed speed of a welding wire, welding current and voltage during the welding operation. For example, an arc-welding monitoring method of the prior art comprises the following steps: detecting signals of welding current and voltage and feed speed of the welding wire in a normal welding state; detecting signals of welding current and voltage and feed speed of the welding wire in a subject welding spot; comparing the signals detected at the subject welding spot to the signals of the normal welding state; and determining a section having deficiency where signals in the subject welding spot are different from the signals in the normal welding state. Deficiency occurrence signals can be determined by a change of minimum value or average value of the current or voltage at the welding spot, an abnormal peak point, abnormal peak points per unit time, change of feed speed of the welding wire and the like. They can be observed optically and compared to the normal state signals. 
         [0007]      FIG. 1  is a block diagram showing an example of a prior art arc-welding monitoring device. As shown in  FIG. 1 , the prior art arc-welding monitoring device includes a first controller  11  for an arc-welding power source  10 , a power converter  12 , a first voltage sensor  13  and a first current sensor  14 . The device further includes elements, which are mounted on the outside of the arc-welding power source  10 , such as a second controller  23 , a second voltage sensor  21  and a second current sensor  22 . They are electrically connected at the arc-welding spot  30 . Thus, the second voltage sensor  21  and the second current sensor  22  can detect the change of the current and the voltage during the arc-welding operation. Then, the deficiency occurrence can be determined by comparing the changes of the current and the voltage to those of the normal welding state. 
         [0008]    However, in a real welding state, irregular and significant noises are often generated. Further, the signals may fluctuate when the subject welding spot is welded normally. Thus, the prior art method may not precisely detect a deficiency. 
       SUMMARY 
       [0009]    Various embodiments of a method for monitoring a machine condition are provided. In one embodiment of the present disclosure, by way of non-limiting example, the method of monitoring a machine condition comprises the steps of: modeling a normal signal model performed by detecting a signal for monitoring condition of a normal machine and converting the detected signal to data of the normal signal model in time domain using a hidden Markov model (HMM) algorithm; calculating a probability value data of the monitoring signal at a subject machine performed by detecting a signal for monitoring condition of the subject machine in real-time and converting the detected signal to the probability value data relative to the normal state signal model using the HMM algorithm; and determining a section having deficiency where the probability value data of the monitoring signal at the subject machinery is not maintained constantly relative to the normal signal model. Further, it is preferable that the method further comprises a step of updating the normal signal model to include the probability value data of a section determined as having deficiency when the deficiency has not occurred at the section. 
         [0010]    Further, in another embodiment of the present disclosure, a method of monitoring a welding deficiency comprises the steps of: modeling a normal welding signal model performed by detecting welding signals at a welding portion in time domain during a welding operation, checking normal welding sections without deficiency, extracting a normal welding signal data from the normal welding sections, and converting the normal welding signal data to the normal welding signal model of the welding operation using a hidden Markov model (HMM); calculating probability value data of the welding signal at a subject welding portion relative to the normal welding signal model performed by detecting the welding signal at the subject welding portion in real-time and converting the detected signal at the subject welding portion to probability value data using the HMM algorithm; and determining a section having deficiency where the probability value data of the welding signal at the subject welding portion is not maintained constantly relative to the normal welding signal model. The welding signal can be voltage signal or current signal. The normal welding sections can be checked by at least one of an optical inspection, non-destructive inspection and a section inspection. 
         [0011]    It is preferable that the normal welding signal is pre-processed to a time function by filtering or time weighting, etc. Further, the pre-processed normal welding signal can be converted to characteristic vector array by transformation. The characteristic vector array can be one or combination of a mean value, a standard deviation, a power spectral coefficient, a filter bank, a cepstral coefficient, a wavelet coefficient, a coefficient of auto-regressive (AR) model, etc. 
         [0012]    It is also preferable that the welding signal detected at the subject welding portion is pre-processed to a time function by filtering or time weighting, etc. The pre-processed welding signal at the subject welding portion can be converted to characteristic vector array by the transformation. The characteristic vector array also can be one or combination of a mean value, a standard deviation, a power spectral coefficient, a filter bank, a cepstral coefficient, a wavelet coefficient, a coefficient of auto-regressive (AR) model, etc. 
         [0013]    The normal welding signal model and the probability value data of the welding signal at the subject welding portion can be one of an initial probability, a state transition probability, a symbol observation probability, a state mean vector, and a state and covariance matrix. The HMM algorithm can be one of a forward/backward algorithm, a scaled forward/backward algorithm, a k-means clustering method, a modified k-mean clustering method, an expectation-maximize (EM) algorithm and a Baum-Welch algorithm. 
         [0014]    The normal welding signal model for the welding operation time can be obtained by collecting a plurality of normal welding signal models at the normal welding sections extracted from one welding operation. Alternatively, the normal welding signal model for the welding operation time can be obtained by collecting a plurality of normal welding signal models at the normal welding sections extracted from a plurality of welding operations. 
         [0015]    If the section determined as having the deficiency occurrence does not have deficiency, then the normal welding signal model can be updated to include the data of probability value in the section to the normal welding signal model. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Arrangements and embodiments may be described in detail with reference to the following drawings in which like reference numerals refer to like elements and wherein: 
           [0017]      FIG. 1  is a block diagram illustrating an example of a prior art device for monitoring welding deficiency schematically; 
           [0018]      FIG. 2  is a conceptual diagram illustrating a welding device using a method of monitoring welding deficiency according to an embodiment of the disclosure; 
           [0019]      FIG. 3  is a flow chart for modeling a normal welding model according to the method for monitoring welding deficiency according to an embodiment of the disclosure; 
           [0020]      FIG. 4  is a flow chart for calculating a probability value of a welding signal at the subject welding portion of the method for monitoring welding deficiency according to an embodiment of the disclosure; and 
           [0021]      FIG. 5  shows a voltage graph at the subject welding portion and a graph of a probability value of a welding signal at the subject welding portion in time axis which is obtained by the process of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    A detailed description may be provided with reference to the accompanying drawings. One of ordinary skill in the art may realize that the following description is illustrative only and is not in any way limiting. Other illustrative embodiments may readily suggest themselves to such skilled persons having the benefit of this disclosure. 
         [0023]      FIG. 2  is a conceptual diagram showing an example of a welding device using a method of monitoring a welding deficiency according to an embodiment of the disclosure. As shown in  FIG. 2 , a welding robot  200  includes a plurality of arms  220 ,  230  and  240 , which are extended from a base  210 . The arms  220 ,  230 ,  240  have a plurality of rotatable joints  211 ,  212 ,  221 ,  231  and  241  to thereby move a welding torch  250  to a subject position. The welding torch  250  also has a joint  251 . A welding wire  201  is guided to the welding robot  200  by a wire guide  202  while the welding wire  201  is fed to welding torch  250  by a torch cable  260 . 
         [0024]    The welding robot  200  comprises: a controller  140  adapted to control movement of the welding robot  200  and feed speed of the welding wire  201 ; a welding machine  130  including two terminals  132  and  133  which are connected to the welding torch  250  and a subject welding portion  300  fixed to the a jig  310 ; and a computer  110  including a user interface. Current and voltage can be detected at the two terminals  132  and  133  of the welding machine  130  during the welding operation. The detected current and voltage signals are amplified by the amplifier  131  and digitalized by an A/D converter  120  to be displayed on the computer  110  in real-time. 
         [0025]    In order to measure the voltage at the subject welding portion  300 , the two terminals are connected to a voltmeter in a parallel manner. To measure current at the subject welding portion  300 , one of the terminals is connected to an amperemeter in a serial manner. Further, the computer  110  is connected to the controller  140  to thereby obtain information about welding operation (e.g., start of the welding operation, finish of the welding operation, information of the subject welding portion, information of the welding line, etc.). 
         [0026]    The voltmeter and the amperemeter can be connected to the two terminals  132  and  133  of the welding machine  130 . Thus, the welding voltage and the welding current can be measured in real-time or immediately after the welding operation. The signal feature of the welding current is different from the signal feature of the welding voltage. However, when a welding deficiency has occurred during the welding operation, the signal features are changed simultaneously. Thus, analyzing results using a hidden Markov model (HMM) algorithm are similar to each other. Accordingly, one of them can be selected to analyze the welding deficiency. 
         [0027]    A HMM algorithm is usually used for analyzing voice recognition. In voice recognition, various voice signal models are formed previously. When an arbitrary voice signal is inputted, the probability values of the inputted signal relative to the respective voice signal models are calculated by the HMM algorithm. Then, the voice signal model corresponding to the highest probability value is deemed as the inputted voice signal. In this embodiment, this HMM algorithm is applied to techniques for monitoring defects resulting from welding. 
         [0028]    To monitor the welding deficiency, only a normal welding state model is needed. After the modeling of the normal welding state model, probability value data of the welding signal at the subject welding portion relative to the normal welding state model are calculated continuously. Thus, if the welding operation at the subject welding portion is performed normally, then the calculated probability values of the welding signal relative to the normal welding state are maintained at relatively high value stably with little fluctuation in time. On the contrary, if there is a deficiency at the subject welding portion, then the probability values relative to the normal state model may drop rapidly. Thus, if there is a section where the calculated probability values are considerably dropped, then the section can be determined to have a weld deficiency. 
         [0029]    Hereinafter, referring to  FIGS. 3 and 4 , a process of modeling the normal welding signal model and monitoring the deficiency occurrence during the welding operation is discussed in detail. 
         [0030]      FIG. 3  shows a modeling process of the normal welding signal model. As shown in  FIG. 3 , a welding current signal (or welding voltage signal) is detected at the terminals of the welding machine  130  of a welding robot  200  during the welding operation in the time domain, S 1 . After the welding operation, the welded portion such as the weld line is inspected by an optical inspection, non-destructive inspection, a section inspection and the like, S 2 . Whether the welded line has a deficiency can be determined by the inspection. Further, the precise positions of the deficiency occurrence during the welding operation can be also determined by the inspection. 
         [0031]    The feature of the welding current signal (or welding voltage signal) can be magnified in the time domain by a pre-process (e.g., filtering or time weighting) for making data of the normal signal sections, S 3 . After the pre-process, the data of the normal welding signal section can be converted to a characteristic vector array (e.g., a mean value, a standard deviation, power spectral coefficient, filter bank, cepstral coefficient, wavelet coefficient, coefficient of auto-regressive (AR) model, etc.) by a transformation that can extract the signal feature involved in the data, S 4 . To obtain HMM parameter, a HMM algorithm (e.g., a forward/backward algorithm, a scaled forward/backward algorithm, a k-means clustering method, a modified k-mean clustering method, an expectation-maximize (EM) algorithm, Baum-Welch algorithm, etc.) can be applied to the characteristic vector array, S 5 . The result according to the selected algorithm can be a normal HMM model of normal welding state (i.e., normal welding state model), S 6 . 
         [0032]      FIG. 4  shows a process to obtain probability value data of the welding signal at the subject welding portion. As shown in  FIG. 4 , when the subject welding portion is welded, the current or voltage is detected during welding operation at the subject welding portion, S 11 . For example, in order to obtain 10 KHz signal data in the time domain, 2,000 sampled data are needed per 0.2 sec. These data are pre-processed to a time function by a pre-process such as filtering, time weighting, etc. S 12 . The time function is converted to the characteristic vector array (e.g., a mean value, a standard deviation, power spectral coefficient, filter bank, cepstral coefficient, wavelet coefficient, coefficient of auto-regressive (AR) model, etc.) by the transformation used in the step S 4 , S 13 . The HMM algorithm used in the step S 5  is applied to the converted characteristic vector array, S 14 . The result obtained from the step S 14  is completed as the probability value data of the welding signal at the subject welding portion relative to the normal welding signal model, S 15 . 
         [0033]    If the time range of the normal signal section is shorter than the welding time of the welding operation at the subject welding portion, then the normal signal section can be used continuously several times to obtain a normal welding signal model corresponding to the probability value data for the welding operation at the subject welding portion. 
         [0034]    Generally, a difference between the maximum value and the minimum value of the probability value data calculated by the HMM algorithm is very considerable. Thus, the logarithmic value of the probability value can be used. Logarithmic data of the probability value data can be calculated during the entire welding operation. 
         [0035]      FIG. 5  shows a graph of a voltage signal at the subject welding portion and a graph of the probability values obtained at the subject welding portion. If the welding operation at the subject welding portion is performed without deficiency, then the probability value data of the welding signal at the subject welding portion are uniform except the initial and final sections. As shown in  FIG. 5 , if there is a deficiency, then the probability value drops considerably forming a huge downward peak A. The section having a huge peak A can be determined as a section with deficiency occurrence. 
         [0036]    The welding signal involves many noises. However, the considerable peak fluctuation of the probability values calculated by the HMM algorithm is occurred at the section which has the high probability of deficiency. 
         [0037]    If the welding operation is performed normally, then the feature of the welding signal can be changed by various factors such as a condition or pre-process of the subject welding portion, non-uniform quality of the welding wire, etc. Thus, the modeling process of making the normal welding signal model can be performed by using one normal welding signal model of a normal welding section repeatedly or by using normal welding signal models collected from the data of normal signal models at the normal welding sections extracted from one welding operation or by using normal welding signal models collected from the data of normal welding sections extracted from welding operations at the same welding line. 
         [0038]    The determination of the deficiency occurrence in real welding operation can be improved gradually. For example, in earlier monitoring work for finding the welding deficiency, the graph of probability values at the subject welding portion relative to an appropriate normal signal model can fluctuate in partial or whole section of the welding portion. The fluctuated section can be determined as a section having a deficiency occurrence. It is usually caused by various reasons such as the condition of the welding surface, non-uniform material property of a welding rod, etc., which can change the feature of welding current or welding voltage. Thus, after inspection, if there is not any deficiency at the fluctuated section, then the normal welding signal model is updated to include the fluctuated probability value data at the section to thereby form a new normal welding signal model. Such fluctuation can be determined as a normal welding signal by the update of the normal welding signal model. Thus, the normal welding signal model can be strengthened gradually. 
         [0039]    The above embodiment described the method of monitoring a welding deficiency. However, the disclosure is not limited thereto and can be applied to a technical field of monitoring a machine condition or deficiency. 
         [0040]    Although embodiments have been described with reference to a number of illustrative embodiments thereof, it should be understood that various other modifications and embodiments can be devised by those skilled in the art that will fall within the spirit and scope of the principles of this disclosure. More particularly, numerous variations and modifications are possible in the component parts and/or arrangements of the subject combination arrangement within the scope of the disclosure, the drawings and the appended claims. In addition to variations and modifications in the component parts and/or arrangements, alternative uses will also be apparent to those skilled in the art.