Studies of the factors influencing the imaging performance of the capacitive imaging technique Xiaokang Yin a,n , David A Hutchins b , Guoming Chen a , Wei Li a , Zhiqian Xu c a Center for Offshore Engineering and Safety Technology, China University of Petroleum (East China), Qingdao 266580, PR China b Advanced Imaging and Measurement Laboratory, School of Engineering, University of Warwick, Coventry CV4 7AL, UK c College of Mechanical and Electronic Engineering, China University of Petroleum (East China), Qingdao 266580, PR China article info Article history: Received 21 March 2013 Received in revised form 3 June 2013 Accepted 4 July 2013 Available online 11 July 2013 Keywords: Capacitive Imaging (CI) Planar probe Imaging performance Hidden depth Lift-off abstract The capacitive imaging (CI) technique has been successfully used in many NDE applications. Previous work of the CI technique has been focused on exploring the application range of this technique, and the factors that may affect the imaging performance have never been studied systematically. In this work, the factors influencing the imaging performance of the CI technique on both non-conducting and conducting specimens are discussed and studied experimentally. The studies of the factors are helpful for under- standing the relationship between the experiment conditions and the CI performance and may provide indications on how to improve the CI performance. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Planar capacitive sensors have been widely used for a wide range of applications, such as proximity sensing [1], displacement measurement [2], thickness gauging [3], material characterisation [4] and NDE applications [5–9]. In the past, capacitive techniques have not been considered routinely in mainstream NDE, in con- trast to say eddy current methods. This is partially due to the historic use of conducting materials (which are suitable for eddy current inspections) in critical components. In addition, for some capacitive methods, the specimen is often sandwiched between two electrodes so that the specimen is exposed to a uniform electric field, which is only practical for specimens in a relatively thin plate shape and requires access to both sides of the specimen. However, in recent years with the ever increasing use of non- conducting materials (i.e. polymers and glass fibre composites) in manufacturing and the development of co-planar capacitive sensors, the number of NDE applications for which the capacitive methods may be appropriate has been expanding. In most of the NDE applications, the capacitive sensor systems are for non-imaging purposes, and the property changes in the specimens (e.g. water intrusion [10], buried objects [11] or the presence of defects [5]) are estimated based on the relation between the capacitance change and the physical property change. While for the capacitive imaging (CI) technique discussed in this work, the actual image of the possible defects is directly sought via a direct mapping from a set of measurements. There are indeed many other capacitive sensors for imaging applications reported in literatures, most of which are model based imaging and tomo- graphic imaging [12], rather than direct imaging. In the model based imaging method, a model of the image and the imaging system is constructed (usually in the form of measurement grids or look-up tables), and this framework is then used to infer information that is not directly available [11,13]. In the tomo- graphic imaging methods, the sensitivity distribution of a sensor is obtained by solving the forward problem and then the image is reconstructed by solving the inverse problem. Other direct ima- ging methods have also been reported sporadically, but they were either designed as array (and sometimes works in a differential mode for edge detection [14]) or with multi sensing elements. The CI technique uses the capacitive coupling between the active electrodes and the material under test. Previous work indicated that, the capacitive nature of the CI technique makes it possible to work on various types of specimens which do not need to be conducting. With a careful probe design, the CI technique can be used to detect defects on and beneath the surface of non- conducting specimens and describe surface features on conducting specimens even through a relatively thick insulation layer. At first sight, the CI technique may seem similar to the well-known eddy current methods, with the coils simply replaced by flat electrodes. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ndteint NDT&E International 0963-8695/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ndteint.2013.07.001 n Corresponding author. Tel.: +86 15969832916. E-mail address: xiaokang.yin@hotmail.com (X. Yin). NDT&E International 60 (2013) 1–10