Accuracy improvement of indenting test results by using wireless cable indenting robot

Journal of Mechanical Science and Technology 6 (9) (0) 7~70 www.springerlink.com/content/78-9x DOI 0.007/s06-0-070-9 Accuracy improvement of indenting test results by using wireless cable indenting robot Kyung-Nam Jang, Jong-Soeg Kim, Sun-Chul Jeong, Kyung-Heum Park and Sung-Yull Hong,* Korea Hydro & Nuclear Co., LTD, Central Research Institute, Daejeon, Korea School of Mechanical Engineering, Yeungnam University, Gyeongsangbuk-do, Korea (Manuscript Received March 8, 0; Revised April, 0; Accepted May, 0) ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Abstract In nuclear power plants, there are many cables that perform safety-related functions. These cables should implement condition monitoring during the operation period in the NPP, in order to assess the remaining qualified life and extend the qualified life. In this study we focused on the indenting method, which can measure the hardness of the cable jacket. This method is selected because it is nondestructive and requires short testing time and small sized equipment. In order to address the problems of the existing indenting test equipment, we developed new indenting test equipment, which could automatically move on the surface of the object cable. The newly developed equipment is designed for a small-sized and lightweight robot using wireless communication in order to implement condition monitoring in a harsh environment or locations that are inaccessible to the tester. The developed wireless cable indenting robot is composed of three parts, mechanical and electrical hardware parts and remote-control part. In order to verify accuracy improvement of indenting test data, an indenting test for cable specimens using both existing indenting test equipment and wireless cable indenting robot was performed. Analysis of the indenting test results shows that the accuracy of the results obtained using the wireless cable indenting robot is improved remarkably. Keywords: Condition monitoring; Safety-related cable; Indenting test; Wireless communication ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------. Introduction * Corresponding author. Tel.: +8 80 6, Fax.: +8 80 67 E-mail address: syhong@yu.ac.kr Recommended by Editor Jai Hak Park KSME & Springer 0 In nuclear power plants (NPP), there are lots of safetyrelated equipment, including cables. The owner of NPP should demonstrate with reasonable assurance that all the safetyrelated equipment for which a qualified life or condition has been established can perform their safety function without experiencing common-cause failures before, during, and after applicable design basis events. For this reason, all the safetyrelated equipment including cables under the harsh environment should perform the equipment qualification (EQ) and condition monitoring until the end of the qualification life according to IEEE std []. In particular, condition monitoring is a very useful method to extend the qualified life of equipment or cable installed in NPP. Among the safety-related equipment installed in NPP, we focused on condition monitoring of cables which is the one of the most important tasks in order to extend the qualified life of the cables. In this paper, we introduce a wireless cable indenting robot that can perform non-destructive condition monitoring of cables installed in NPP, and compare the performance with that provided by other indenting test equipment.. Indenting test equipment for condition monitoring. Condition monitoring of cables The condition monitoring program requires that the characteristics subject to aging deterioration be monitored at specific intervals and compared with specified acceptance criteria. The acceptance criteria shall be based on post-age-conditioning characteristics for the qualified cable []. NRC Regulatory Guide. requires that condition monitoring should be implemented for safety related power, instrumentation and control cables and recommends that condition monitoring programs may include any appropriate technique, supplemented with walkdowns to look for visible signs of anomalies attributable to aging with particular emphasis on the identification of localized adverse environments or hot spots []. Draft Regulatory Guide DG-0 published in US NRC introduces condition monitoring techniques for electric cables used in nuclear power plants []. Among the many methods available for condition monitoring, we used the indenting method, which can measure the hardness of the cable jacket; this method is non-destructive and requires short test time and

76 K.-N. Jang et al. / Journal of Mechanical Science and Technology 6 (9) (0) 7~70 Table. Indenting test results obtained using existing indenting test equipment. (Unit : g f/mm) (a) Developed by EPRI Fig.. Existing indenting test equipments. (b) Developed by KEPRI Tester No. Specimen No. Average 0 80 8 89 8 8 8 96 9 0 99 0 Average 7 7 Fig.. Irregularity of the cable surface. Fig.. Test specimens. small sized equipment. In 99, EPRI developed the first cable indenter that can measure the hardness of the cable jacket, as shown in Fig. (a) []. In 00, KEPCO Research Institute (KEPRI) developed a computer (PDA) equipped cable indenter that uses same methodology as the indenter produced by EPRI, as shown in Fig. (b) [6]. Specimen Specimen Specimen Specimen Specimen Fig.. Enlarged cable surface pictures of specimens.. Problems of existing indenting test equipment In order to identify problems of existing indenting test equipment, we performed indenting tests using the KEPRI developed indenting test equipment for different types of specimens, as shown in Fig.. Three testers were selected, and each tester performed an indenting test 0 times per specimen. The results of the tests showed that the average of variation was 7 g f /mm. The specific results are provided in Table. It was found that the existing indenting test equipment suffered problems of low reproducibility and large variation of test data. These problems are caused by human error, irregularity of cable samples and measurement error in the results of the analysis of the test data. First, with regard to human error, the angle of insertion of the indenting probe to the cable surface can influence the test results. In each test, the angle of insertion may be different because the tester should clamp and release the cable specimen whenever performing the test. Also, the clamping skill of each tester is a critical factor. Second, irregularity of cable samples can influence the test results. The enlarged cable surface picture shown in Fig. (a) Fig.. Problem of indenting test for multi-core cable. reveals that the cable surface is uneven. Hence, according to the position of the cable surface, which the indenting probe contacts, the indenting test results may be different. Observing a 00x enlarged picture of each cable specimen surface by microscope, as shown in Fig., it is found that specimens and have rather even surfaces, but specimens,, and have rather uneven surfaces. From an analysis of the test results according to the cable specimens, as shown in Table, specimens that have uneven surfaces have a two-fold greater amount of large variations. In addition, most cables are multi-core type, meaning they have more than two cores in the cable jacket, as shown in Fig.

K.-N. Jang et al. / Journal of Mechanical Science and Technology 6 (9) (0) 7~70 77 (a) Normal data Fig. 6. Normal and abnormal indenting test data. (b) Abnormal data (a) Scheme of the system Fig. 7. Wireless cable indenting robot. (b) Wireless cable indenting robot. Because the indenting test entails pressing the cable jacket with the indenting probe and measuring the pressing force and the indented depth, the existence of a core under the measuring position can influence the test results. Lastly, measurement error can lead to large variation of the test results. Measurement error can be found from the graph of pressing force and indented depth. Normally, as shown in Fig. 6(a), as the pressing force increases, the indented depth accordingly increases. However, when measurement error occurs, the indented depth is decreased during the indenting test, as shown in Fig. 6(b). If data obtained under circumstances of measurement error are included in the test results, the accuracy of the test results will be decreased. (a) Indenting module (b) Moving module. Development of wireless cable indenting robot Our aim is to develop new cable indenting test equipment that can resolve the problem of existing indenting test equipment. Furthermore, we considered that most cables in NPP are installed in harsh environments or spaces that tester cannot access. To address the first problem of human error and second problem of irregular cable surface, we designed a robot that can move on the surface of the cable and perform indenting test automatically. If the robot can clamp and release the object cable for itself, and can perform the indenting test in next position automatically, human error from repeated testing and the requirement of testing skill can be avoided. Also, if the robot can acquire abundant data quickly, representative value of indenting test results can be selected using a statistical method, regardless of irregularity the of cable surface.. System of wireless cable indenting robot We focused on designing a small-sized and lightweight robot, because the robot should travel along the surface of the object cable. Furthermore, in order to perform testing in harsh environment or space that tester cannot access, the robot should be wirelessly operated from a control system. As shown in Fig. 7(a), the robot system is composed of a cable indenting robot including mechanical hardware part and electrical hardware part, and remote-control part. The cable indenting robot communicates with the remote-control part (laptop PC or mobile device) using Bluetooth communication. The developed wireless indenting robot is shown in Fig. 7(b). Fig. 8. Mechanical hardware part. (c) Clamping module. Mechanical hardware part The mechanical hardware part is composed of an indenting module, moving module, and clamping module. The indenting module performs indenting test with an indenting probe and is located at center of the robot. The indenting probe can move down perpendicularly via use of a DC servo motor and spur gears. The pusher and spring flatten the cable surface so that the probe remains at right angles with the cable, as shown in Fig. 8(a). From the results of indenting test, the modulus value (g f /mm), which is the force per indented depth, is obtained. After the indenting test, the indenting module returns the original position, in order to perform the next test. The moving module moves on the surface of the object cable. This module consists of two DC-servo motors and wheels with a Viton-ring, in order to move on the surface of the cable smoothly, and an assist wheel to move straight, as shown in Fig. 8(b). The robot can automatically move with constant intervals for continuous indenting testing. The clamping module can clamp the cable during the indenting test and helps the robot move straight along the surface of the cable. The clamping module consists of two DC-servo motors, a V-block, and ball screw bearings, as shown in Fig. 8(c).

78 K.-N. Jang et al. / Journal of Mechanical Science and Technology 6 (9) (0) 7~70 Fix the Robot on Installed Cable Calculate Modulus Acquire Indenting Test Data No The Number of Test > 0? The Data is Normal? Yes No Delete Data Yes Determine Representative Value Evaluate Degradation Level Comparing with Unique Cable Database Fig.. Procedure for indenting test. applied, and also a mobile device with a Bluetooth module can be applied. The control program was coded by Visual C++, divided into a communication & control panel, data display panel, and state display panel, as shown in Fig. 0.. Indenting test algorithm Fig. 9. The concept of the electrical hardware part. Fig. 0. Remote control program.. Electrical hardware part The electrical hardware part is composed of a motor control module, power supply module, and wireless communication module. The concept of the electrical hardware part is illustrated in Fig. 9. The PCB is designed for triple floors in order to minimize occupied space. As the main controller, a DSP with 0 MIPS is applied. A Li-ion battery that has capacity of 7.V and 900 mah is applied for the power supply. The output voltage is divided into voltages of. and V for the controller and motor driving powers using a transforming circuit. For wireless communication, a Bluetooth module that has a communication standard of the.0 version and. GHz ISM band is applied. This module can communicate with the remote-control part within a range of 00 m.. Remote-control part The remote-control part can command the cable indenting robot to perform the indenting test and receive the indenting test results. For the remote-control part, a laptop computer is In order to minimize measurement error, we propose an algorithm that eliminates abnormal data shown in Fig. 6(b). We focused on the linear relation between the measured force and indented depth. When the indenting test is completed, the measured force and indented depth data are transferred to the remote-control program. Linear fitting for the force and depth relation using the least mean square method is performed, and the least mean square error is calculated. If the least mean square error exceeds selected criteria, it is determined that the relation between force and depth is not linear. In this algorithm, test data having large least mean square error is deleted as it is deemed erroneous. The indenting test using the wireless cable indenting test robot follows the procedure shown in Fig.. At the start of the test, the robot should be fixed on the object cable. According to the signal of the starting test, the robot performs the indenting test. Once the first indenting test is completed, the result data are transferred to the remote-control program and assessed in terms of whether they are normal or abnormal data using the abnormal data detection algorithm. If the test data are normal data, a modulus value is calculated and restored. This sequence is repeated 0 times while the robot moves on the surface of the object cable automatically. Finally, a representative modulus is selected by a statistical method, and the remaining lifetime or degradation level is evaluated through comparison with the unique database of the object cable.. Test results of indenting test using the wireless cable indenting robot An indenting test using the wireless cable indenting robot was performed in order to assess how much the accuracy of the test results was improved. Test specimens were the same those described in Section. Three testers, also corresponding with those in Section, were selected, and each tester performed indenting test 0 times per specimen, according to the procedure in Fig.. The results of the tests showed that the

K.-N. Jang et al. / Journal of Mechanical Science and Technology 6 (9) (0) 7~70 79 Table. Indenting test results obtained using wireless cable indenting robot. (Unit : g f/mm) Tester No. Specimen No. Sa mp l e M e a n Pe r c e n t Sa m p le S t D e v 00 0 0 80 0 60 00 000 00 Average. 9. 0 0.9 7..8 6. 6.8.9 7.9.9. 6. 6..8 6.7 6 6.9 Average. 0..9.9 Gage R&R (ANOVA) for Existing equipment Gage R&R Repeat Reprod S Chart by Tes t er Com pone nt s of Variat ion X bar Chart by Tes te r Part-to-Part % Contribution % Study Var UCL=7. S=0.0 LCL=8.7 UCL= X= LCL=7 S p e c i me n average of variation was.9 g f /mm, as shown in Table. In order to verify the accuracy of the developed robot system, we performed a Gage R&R analysis for both the test data using existing indenting test equipment in Table and the test data using the wireless cable indenting robot in Table with Minitab Statistical Software. The Gage R&R analysis results are presented in Fig. and Fig., respectively. In the case of the existing indenting equipment, the total Gage R&R study variation percentage was.86%. The repeatability and the reproducibility were.8% and.0%, respectively. In the case of the wireless cable indenting robot, the results of the 00 600 800 00 600 Av e r a g e 800 00 000 00 E x is t ing e quipm ent by S pe cim en E x is ting equipme nt by Tes t er T e st e r S pecim en * Tes t er Int eract ion Sp e c i m e n Fig.. Gage R&R results for existing indenting test equipment. Gage R&R (ANOVA) for Developed Equipment Sa mp l e M e a n Pe r c e n t Sa m p le S t D e v 00 0 00 000 00 Com pone nt s of Variat ion % Contribution % Study Var 0 Gage R&R Repeat Reprod Part-to-Part S Chart by Tes t er 0 UCL=.60 0 S=8. 0 LCL=. X bar Chart by Tes te r UCL=68 X=0 LCL= 00 000 00 00 000 Av e r a g e 00 00 000 00 D ev elope d E quipm ent by S pecim en S p e c i me n D ev eloped E quipm ent by Tes t er T e st e r S pecim en * Tes t er Int eract ion Sp e c i m e n Fig.. Gage R&R results for wireless cable indenting robot. Tester Tester analysis showed that the total Gage R&R study variation percentage was.%. Because the criterion for the study variation percentage is 0%, it is found that the developed robot system is stable. In particular, the repeatability and the reproducibility present the accuracy of.6% and.%, respectively. From the analysis of Gage R&R, it is found that the accuracy of the developed robot system is improved remarkably. From a quantitative point of view, the average of variation is reduced by 8%, from 7 g f /mm to.9 g f /mm. The reason that the variation average is different for each specimen is that the representative modulus value is different. Representative modulus value of specimen, is more than twice that of specimen. Thus, as the modulus value becomes higher, the variation of the test data accordingly increases.. Conclusions In order to assess the remaining lifetime of cable and extend the cable lifetime, condition monitoring using an indenting test method, which is measures the hardness of the cable jacket, will be implemented. The existing indenting test equipment suffers some problems, including large variation of test results, because of human error, irregularity of cable samples, and measurement error, as seen in the results of an analysis of the test data. This reduces the accuracy of estimation of the remaining lifetime of the cable. In this paper, in order to improve the accuracy of the indenting test results, we have introduced a new concept for cable indenting equipment that can automatically move on the surface of the object cable. If the robot can clamp and release the object cable for itself, and can perform the indenting test in next position automatically, human error from repeated testing and the requirement of testing skill can be avoided. Also, if the robot can acquire abundant data quickly, representative value of indenting test results can be selected using a statistical method, regardless of irregularity the of cable surface. Furthermore, the newly developed equipment is designed for a small-sized and lightweight robot using wireless communication in order to implement condition monitoring in harsh environment or location that the tester cannot access. In order to minimize the measurement error, we applied an abnormal detection algorithm to exclude the abnormal test data, in the procedure of the indenting test. We performed an indenting test for cable specimens using both existing indenting test equipment and wireless cable indenting robot, respectively, in order to verify the accuracy improvement of indenting test data. From the results, the average variation of test data was reduced by 8%, from 7 g f /mm to.9 g f /mm. It is found that the accuracy of the indenting test data is remarkably improved by using the wireless cable indenting robot. In the future, if a database of all safety-related cables installed in NPP is constructed, indenting test using the proposed wireless cable indenting robot will play an important role in condition monitoring of those cables.

70 K.-N. Jang et al. / Journal of Mechanical Science and Technology 6 (9) (0) 7~70 References [] IEEE std -00, IEEE standard for qualifying class E equipment for nuclear power generating stations, IEEE Power Engineering Society (00). [] IEEE std 8-00, IEEE standard for qualifying class E electric cables and field splices for nuclear power generating stations, IEEE Power Engineering Society (00). [] Regulatory Guide., Qualification of safety-related cables and field splices for nuclear power plants, U.S. Nuclear Regulatory Commission (009). [] Draft Regulatory Guide DG-0, Condition monitoring program for electric cables used in nuclear power plants, U.S. Nuclear Regulatory Commission (00). [] EPRI, Cable indenter aging monitor, EPRI NP-78, Electric Power Research Institute (99). [6] J. S. Kim, Evaluation of cable life based on condition monitoring, 0 fall meeting of Korean nuclear society (00). Kyung-Nam Jang was born in Gwangju, Korea, in 977. He graduated from Jeonnam National University, Korea, in 00, and received his M. S. degree from Korea Advanced Institute of Science and Technology, Korea, in 00. He works as a Researcher at KHNP Central Institute and is interested in equipment qualification. Jong-Seog Kim was born in Busan Korea, in 99. He received his Ph.D from Chung-Nam national university in 00. He works as a principal researcher of KHNP Central Institute and is interested in environmental qualification of NPP.