Using Parametric Design to Reduce the EMI of Electronics Products - Example of Medical-Grade Touch Panel Computer

Abstract

With technical advancement and development, the amount of electronic equipment is increasing, while the functions of products are enhanced, and the routing density of Printed Circuit Boards (PCBs) becomes larger. In the electronic industry, medical instruments are used to diagnose, treat, mitigate or prevent human diseases, and maintain and promote health. Industrial PCs for medical use and their accessories should be immune to interference from external electromagnetic noise, and should not become interference sources of electromagnetic noise radiation, so they have become issues of interest with respect to ensuring safety of medical equipments in medical operation environments in recent years. This research relates to parametric design using the Taguchi Method in the early stage of product development for medical-grade touch panel computers. In considering the use of Radiated Emission (RE) in Electromagnetic Compatibility (EMC) as a response value, the experiment covers control factors such as PCB and mechanism design related parameters. In addition, peripheral devices used in conjunction with a product are considered as noise factors when the product is in use, while interaction between the control factors is studied. The Taguchi Method is used to select an appropriate inner/outer orthogonal array, and a response diagram and a variance method are used for analysis to provide an optimal set of design parameters, in which the number of routing layers of a riser card is 6; the EMI filter on the isolated card is 600 Ω; the shunt capacity for the clock on main board is 33p; and the isolated card is grounded. Moreover, it is found that an interaction exists between the number of routing layers of the riser card and the EMI filter of the isolated card. From the result of the experiment, with such a set of parameters, the SN (Signal to Noise Ratio) lies in the confidence interval, indicating good reproducibility of the experiment. Such a parametric design effectively improves the electromagnetic interference (EMI) characteristics of a product to meet design specifications required by customers, accelerate the R&D process of electronic products, and pass EMI test regulations required by various countries in order to improve industrial competitiveness. 1. BACKGROUND AND MOTIVATION OF THE RESEARCH The Industrial PC (IPC) was first used in factory automation to act as a CNC (Computer Numerical Control) and as the control core of machines, such as a CNC lathe, capable of monitoring, controlling or testing machines or instruments in manufacturing process. With advanced techniques and more convenience required by human beings in life, its new application has gradually been oriented to industrial automation and life automation. In additional to the manufacturing industry, computer automation control is currently used in the financial sector, telecommunication network industry, and even medical care and monitoring. Received 18 September 2018, Accepted 28 November 2018, Scheduled 12 December 2018 * Corresponding author: Chien-Yi Huang ([email protected]). 1 National Taipei University of Technology, Taiwan. 2 A Test Lab Techno Corp., Taiwan. 3 State University of New York at Binghamton, USA. 14 Huang, Chen, and Greene From the medical industry’s position in people’s welfare, the scope of the medical service market is extending, including various medical care related industries, such as the traditional hospital industry, pharmaceutical industry, and emerging biotech industry. Generally, the medical industry refers to industries/institutions regarding prevention, examination, treatment, rehabilitation, nursing and care for physical and mental health and disease, including manufacturers and suppliers of medical devices, manufacturers and dealers of various medicines, as well as various people working in various medical institutions, such as hospitals/clinics, inspection stations, and nursing homes. In the electronic industry, medical instruments are used to diagnose, treat, mitigate or prevent human diseases, and maintain and promote health. Medical devices feature a diverse set of products, high technical complexity and integration, high margins, high uncertainty of R&D and profit, as well as a high closure of market, etc. However, Small Medium Enterprises (SMEs) play a main role in the medical device industry in Taiwan, and traders are mainly import oriented. Production, manufacturing and channels have been increased in recent years. Since the domestic market is small, and finished products are mainly export oriented, raw materials are mainly sourced overseas. Most manufacturers are assembly Original Equipment Manufacturers (OEMs) based on production technology, and most products are social welfare devices. The amount of electronic equipment is increasing, while the functions of products are enhanced, and the routing density of Printed Circuit Boards (PCBs) becomes larger [1, 12, 18]. Further, the clock rate of the Central Process Unit (CPU) is becoming faster, so that the operating frequency tends to high frequency. Crosstalk issues will appear accordingly to cause Electromagnetic Interference (EMI), coupling signals and time delays, such that normal functions of the product are influenced in operation. Medical equipment with reversible interference issues can result in faulty diagnosis or treatment. The accuracy of EMI/EMC measurement depends on many factors, including whether the measurement environment is qualified, whether the measurement instrument is standard, whether the electromagnetic coupling between the measurement instrument and the Equipment Under Test (EUT) is eliminated, whether artificial operation procedure is correct, etc. There are standard regulations or international certifications for many existing EMI/EMC measurement procedures, environments, instruments, etc. In recent years, advanced countries, such as USA, Japan and the European Union (EU), have proposed regulations for EMI levels generated by electronic products, together with control, successively in order to protect radio communication and computer information control. For example, regulations such as the Federal Communications Commission (FCC) of USA, Voluntary Control Council for Interference (VCCI) of Japan, Communate Europpene (CE) mark of European Union (EU) and Ctick of Australia require that electronic information products which are to be imported shall apply and be tested to be qualified prior to legal transportation and sale. The Bureau of Standards, Metrology and Inspection (BSMI) has also implemented control measures since 1996 in our country. As for hand phone, notebook, as well as medical devices emit electromagnetic fields (EMF), these emissions are a risk exposure for the human beings in the proximities due to the energy that is absorbed by tissues. In these conditions, the specific absorption rate (SAR) is defined as the amount of energy absorbed by tissues. Factors affecting the amount of absorbed energy are features of the wave, features of the body, and also environmental features. About the features of the wave, the SAR depends on the features of the signal emitted by the radiation source, such as frequency [14] and polarization [11]. About the dependence with the body, the SAR depends on the type of tissue, the posture of the body [22], frontal or back incidence [10], etc. Accordingly, before selling products to domestic or overseas markets, in most cases domestic electronic companies cannot pass the EMI test standards required by the regulations of client countries, so that the time to market is delayed, and business opportunities are lost significantly. With respect to the causes and improvement measures of EMI, scholars have already had discussions, in which Kchikach et al. [15] indicated that EMI comes mainly from the high switching frequency of DC voltages of power converters. That research is limited to influence Common Mode (CM) current noise on switching power supply without mentioning how to improve the EMI of PCB [16]. Adams [2] indicated that the direction of cm current causes cables or wires on products to generate electromagnetic radiation, radiation immunity and conduction radiation. Also, a low cost EMI troubleshooting technique is designed for engineers to evaluate products quickly at an early stage in order to comply with EMC standard. Bait-Suwailam et al. [4] studied EMI generation mechanisms Progress In Electromagnetics Research C, Vol. 89, 2019 15 and improvement approaches in the aspects of theory research and design in order to improve EMI by blocking the interference path to suppress interference, and by using grounding, shielding, filtering and PCB layout. Colotti [8] indicated that EMI is formed due to three keys, including an interference source, coupling path and interfered susceptor, and EMI issues are mitigated by reducing the noise of the interference source, or developing and maintaining relevant control plans. However, the research above is only limited to explanation of theory without describing countermeasures and introductory methods which improve EMC effectively in detail for specific products or regulations. In references, scholars also used masking approaches to reduce EMI noise. Lin et al. [19] set forth the EMI masking characteristic of methyl vinyl silicone composite materials with bamboo-like carbon nanotubes of different concentrations. However, the concentration of liquid cannot be controlled easily, so the conductivity is influenced, thereby reducing the effectiveness of masking. Zhao et al. [26] proposed independent cables and motors for evaluating the EMC design of one converter and simplified different model constructions/configurations to execute this verified hypothesis. Moreover, some scholars studied grounding. Researchers used grounding approaches to reduce EMI noise [6]. Liu [20] studied and controlled EMI due to a grounding loop, which is increased with the comple