2492 IEEE TRANSACTIONS ONINSTRUMENTATION AND MEASUREMENT, VOL. 56, NO. 6, DECEMBER 2007 Saxophone Reed Inspection Employing Planar Electromagnetic Sensors Subhas Chandra Mukhopadhyay, Senior Member, IEEE, Gourab Sen Gupta, Senior Member, IEEE, Jonathan D. Woolley, and Serge N. Demidenko, Fellow, IEEE Abstract—This paper deals with the research and development of a prototype of a planar electromagnetic sensor for use in nondestructive evaluation (NDE) of saxophone reeds. Analytical modeling and experimental research have been employed, leading to very promising results that could serve as a starting point for development of a commercial-grade sensor. In addition to the reed testing, the sensor could have wide applications in NDE and testing of several other products such as meat, dairy products, etc. Index Terms—Analytical model, interdigital configuration, meander, mesh, nondestructive evaluation (NDE), planar electro- magnetic sensors, saxophone reeds. I. I NTRODUCTION A SAXOPHONE reed, as shown in Fig. 1, is a small piece of bamboo that is attached to the mouthpiece of a saxophone. When the player blows into the saxophone, the reed vibrates, creating sound. The reed is therefore partly responsible for the tone and ease of use of a saxophone. There is nothing more frustrating for a saxophone player than playing on a bad reed. Reeds wear out after a few weeks of use and must be replaced. The problem with reeds is that the quality is highly variable—in a box of ten, three or four reeds are usually found to be “bad” when played and are thus discarded. Since the reeds are expensive, and, more importantly, the qual- ity of the music depends on them, the authors have decided to address the problem of reed evaluation and to develop the theo- retical foundation, design solutions, and working prototype of a sensor to evaluate and grade saxophone reeds. The authors also believe that such a sensor would find application in numerous other areas such as process industry, agriculture, dairy, services, meat, etc. A planar electromagnetic sensor has been chosen for this study. This is because research using a planar magnetic sensor has demonstrated the detection of defect in printed circuit boards and the estimation of near-surface material properties of conducting and magnetic materials. The outcome of the re- Manuscript received May 29, 2006; revised March 12, 2007. S. C. Mukhopadhyay is with the Institute of Information Sciences and Technology, Massey University, Palmerston North 4442, New Zealand (e-mail: S.C.Mukhopadhyay@massey.ac.nz). G. Sen Gupta was with the Institute of Information Sciences and Technology, Massey University, Palmerston North 4442, New Zealand. He is now with the School of Electrical and Electronic Engineering, Singapore Polytechnic, Singapore 139651. J. D. Woolley is with Alcatel NZ, Wellington 6015, New Zealand. S. N. Demidenko is with Institute of Information Sciences and Technology, Massey University, Wellington 6015, New Zealand. 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/TIM.2007.908253 Fig. 1. Sensor and different sections of a tenor and an alto saxophone reeds. search is very encouraging, and the successful results have been published in international journals [1]–[6]. The planar electro- magnetic sensing technique employing a radio frequency (RF) signal is based on the interaction between the measurement signal and the material under investigation. The configuration of the electromagnetic sensor used to inspect the quality of the saxophone reeds is of the planar type. The sensor consists of two coils: One coil, known as the exciting coil, carrying an RF signal, generates an electromagnetic field. The generated elec- tromagnetic field interacts with the material being investigated. The resultant electromagnetic field is measured by another coil, known as the pickup coil, which is placed above the exciting coil. The ratio of the voltage of the pickup coil to the current of the exciting coil is defined as the transfer impedance. The trans- fer impedance is a function of many parameters, such as the per- mittivity, conductivity, and permeability of the material being measured, the liftoff, operating frequency, and thickness of the material under test (MUT), etc. There is no direct method avail- able to separately determine the material properties, but it is possible to evaluate them indirectly by measuring the variation of impedance or from the absolute value of the impedance. It is possible to monitor the quality of the system under test by the measurement of the transfer impedance of the sensor system. The advantage of this method over many other testing methods is that the test material is not physically touched or altered. The method is therefore a form of nondestructive evaluation (NDE) and is useful in situations where it is impractical or infeasible to destroy or damage a test sample using chemical or physical testing means. This may be because a test sample is very expensive or because every item produced must undergo the test, and therefore, every item must survive the test unaltered. Another advantage of NDE is that it may be applied as a safety check to items that are already in use. As NDE does not damage 0018-9456/$25.00 © 2007 IEEE