J. Cent. South Univ. (2013) 20: 385–392 DOI: 10.1007/s11771-013-1499-6 Retraction control of motorized seat belt system with linear state observer LEE Kang-seok, CHOI Chin-chul, LEE Woo-taik Department of Control and Instrumentation Engineering, Changwon National University, Changwon 641-773, Korea © Central South University Press and Springer-Verlag Berlin Heidelberg 2013 Abstract: A design and verification of linear state observers which estimate state information such as angular velocity and load torque for retraction control of the motorized seat belt (MSB) system were described. The motorized seat belt system provides functions to protect passengers and improve passenger’s convenience. Each MSB function has its own required belt tension which is determined by the function’s purpose. To realize the MSB functions, state information, such as seat belt winding velocity and seat belt tension are required. Using a linear state observer, the state information for MSB operations can be estimated without sensors. To design the linear state observer, the motorized seat belt system is analyzed and represented as a state space model which contains load torque as an augmented state. Based on the state space model, a linear state observer was designed and verified by experiments. Also, the retraction control of the MSB algorithm using linear state observer was designed and verified on the test bench. With the designed retraction control algorithm using the linear state observer, it is possible to realize various types of MSB functions. Key words: motorized seat belt system; motorized seat belt (MSB) system; linear state observer; state space model; augmented state; retraction control Foundation item: Project supported by the Second Stage of Brain Korea 21 Projects and Changwon National University in 2011–2012 Received date: 2012–05–31; Accepted date: 2012−12−01 Corresponding author: LEE Woo-taik, Professor, PhD; Tel: +82−55−213−3668; E-mail: wootaik@changwon.ac.kr 1 Introduction 1.1 Motorized seat belt system The seat belt system is the most representative car safety system. Car safety systems can be classified under two headings, namely, passive and active safety systems. A passive safety system can protect passengers after accidents, while an active safety system can protect passenger before accidents [1]. The motorized seat belt (MSB) system is representative of an active safety system which can protect passengers before car accidents. Recently, due to increasing interest in car safety, research to improve the traditional seat belt system to an MSB system has progressed [1–3]. The MSB system provides functions to protect passengers and improve passenger convenience, as shown in Table 1. The pretension, dynamic support and haptic alarm are functions to prevent additional injury by car accident and to warn passengers about dangerous driving conditions. Slack removal and belt parking are functions to prevent a reduction in MSB system performance in a dangerous situation caused by slack and poor seat belt return. The MSB functions have their own required belt tension. The required belt tension is determined by the function’s purpose. The MSB system checks the driving conditions and decides a function which is appropriate for the situation [3]. The MSB system contains an MSB body and MSB electronic control unit (ECU). The MSB body is a form of the traditional seat belt body with a DC motor. Also, the MSB body contains a clutch to hold the generated seat belt tension [1–2]. The MSB ECU contains a micro-controller, H-bridge inverter and signal acquisition circuit for measuring the applied current and voltage of the DC motor [3]. The MSB system generates belt tension and provides various functions through the operation of the MSB ECU and DC motor. 1.2 Limitation and possibility of improvement To realize the MSB functions in Table 1, information about the state of the MSB system is required. This information includes the seat belt winding velocity and tension of the seat belt. Measurement of this state information with sensors increases the system cost and complexity. Furthermore, it is difficult to measure the generated tension of the seat belt, though exact belt tension is a requisite bit of state information needed to realize MSB operations. Figure 1 shows the change of state information in DC motor operation. The DC motor operation is classified by 3 sections: Section (1) is the operating start