IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 21, NO. 2, APRIL 2016 851 Voice Coil Navigation Sensor for Flexible Silicone Intubation Fang Wu, Jiyun Jeon, Seung Ki Moon, Member, IEEE, Hae-Jin Choi, and Hungsun Son, Member, IEEE Abstract—This paper presents a novel navigation sensor for guidance of a flexible tube based on magnetic induction. In many applications, such as pipe inspection and medical intubation, the flexible tube has been utilized for inner sur- face monitoring and inspection. In particular, for medical intubation that inserts a stomach tube or endoscope, ac- curacy of locating the flexible tube mainly relies on expe- rienced operators. Although X-ray tomography and oxygen sensors have been applied to detect the location of the in- serted tube, the implementation is usually time consuming and costly, an equipment being bulky so that their practical applications are limited. On the contrary, magnetic sensors, offering contact free and compactness in size, have been preferred in the areas of position/orientation detection. In this study, a compact orientation sensor based on magnetic induction has been developed; both magnetic field genera- tor and detector are designed to minimize the space along the flexible tube. Magnetic mutual inductance is analyzed with the extended-distributed multiple-pole model, and de- sign parameters are optimized to determine an operating range of orientation angles. Performance of the sensor is numerically simulated and compared to experimental re- sults. The results show the accuracy in a wide operating range and the potential in many applications. Index Terms—DMP method, inductor, magnetic dipole, magnetic sensor, navigation sensor, orientation sensor. I. INTRODUCTION N AVIGATION sensors have become increasingly impor- tant in the medical instrumentation [1], [2] during the past decades. For minimally invasive surgical robotics [3], [4] and patient monitoring [5], [6], flexible catheters, injecting needles and endoscopy should be tracked [7] in real time. An optical Manuscript received November 11, 2014; revised February 27, 2015 and June 10, 2015; accepted August 12, 2015. Date of publication September 16, 2015; date of current version February 24, 2016. Rec- ommended by Technical Editor D. Stoianovici. This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) supported by the Ministry of Science, ICT, and Future Planning (NRF-2014R1A1A1038252). F. Wu and J. Jeon contributed equally to the work. (Corresponding author: Hungsun Son.) F. Wu is with the Engineering Product Development, Singapore University of Technology and Design, 487372 Singapore (e-mail: fang_wu@sutd.edu.sg). J. Jeon and H. Son are with the School of Mechanical and Nuclear Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Korea (e-mail: jyjeon@unist.ac.kr; hson@unist.ac.kr). S. K. Moon is with the School of Mechanical and Aerospace Engi- neering, Nanyang Technological University, 639798 Singapore (e-mail: skmoon@ntu.edu.sg). H.-J. Choi is with the School of Mechanical Engineering, Chung-Ang University, Seoul 156-756, Korea (e-mail: hjchoi@cau.ac.kr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMECH.2015.2476836 sensor [8] provides direct visual identification, but it requires a light source and free line-of-sight. When a medical device is inserted into the human body, an intubation path is usually complex, and the light source is difficult to maintain. In addi- tion, the size of devices is as important as tracking accuracy since available space in an organ is very limited. A bulky light source and camera could cause perforation and fatal damage [9]. An oxygen sensor has also been used to monitor the oxy- gen level and improve the intubation accuracy. However, the sensor diameter is usually a few centimeters, and it may not be suitable for narrow medical tubes, such as a nasogastric tube. An X-ray imaging system is one of the possible solutions and provides good imaging quality. However, the extra cost of X-ray scanning could increase the medical expenses and exposure to a radiation hazard could be harmful to both patients and clinic staffs. To meet such specific demands in medical instrumenta- tion, an electromagnetic tracking research [10]–[12] has been widely studied to detect the position and orientation by attaching a small sensor to the medical device. It utilizes compact devices without light source and radiation risk. Two different magnetic sources: 1) static magnetic field [13], [14] and 2) time-varying electromagnetic field [15], [16], have been used for orientation and position measurement. By compar- ison, magnetic sensors with permanent magnets utilize the static magnetic field and have the advantage of excitation free, since no power supply or signal generator is needed. However, due to limitation of controlling the magnetic field, the permanent- magnet-based sensor is difficult to be designed and customized for medical devices. On the contrary, both the excitation field and sensor utilizing electromagnets offer more flexibility as the field can be controlled as desired. Magnetic induction has been exploited to measure the orientation and position of medi- cal tracking devices. Existing navigation systems, such as NDI Aurora [17], CORPAK [18], and ScopeGuide system [19], etc., utilize a field generator that provide a known and controllable ex- citation field. NDI Aurora utilizes an external excitation source. When the magnetic sensor is inserted into the human body, the magnetic field is recorded and analyzed for its position and orientation. Both CORPAK and ScopeGuide systems insert the electromagnetic transmitters inside the human body with the medical device. Then, the positions of the transmitters are measured by a large sensor array outside the human body. All systems have been commercialized and proved to be useful. However, either the excitation device or sensing device should be inside the body, while the other one placed outside. Therefore, the electromagnetic interference with the surrounding medical instruments and ferromagnetic objects could cause a large error, and a bulky device makes the medical procedure more complex. 1083-4435 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.