Journal of Mechanical Science and Technology 27 (3) (2013) 875~884 www.springerlink.com/content/1738-494x DOI 10.1007/s12206-012-1237-2 Velocity control of a secondary controlled closed-loop hydrostatic transmission system using an adaptive fuzzy sliding mode controller † Hoang Thinh Do 1 and Kyoung Kwan Ahn 2,* 1 Graduate School of Mechanical and Automotive Engineering, University of Ulsan, Korea 2 School of Mechanical Engineering, University of Ulsan, Korea (Manuscript Received November 5, 2011; Revised August 9, 2012; Accepted January 19, 2013) ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Abstract A secondary-controlled hydrostatic transmission system (SC-HST), which considered being an energy-saving system, can recuperate most of the lost vehicle kinetic energy in decelerating and braking time and it shows advantage in fuel economy improvement of vehicle. Almost secondary control units (SCU) in SC-HST inherently contain nonlinear characteristics such as dead-zone input. Therefore, it is difficult to obtain precise position or velocity control by conventional linear controllers. This problem limits the application of SC-HST in industry and mobile vehicle. This paper gives a description of SC-HST and proposes an adaptive fuzzy sliding mode controller (AFSMC) for velocity control of SCU. Experiments were carried out in the condition of disturbance load by using both the proposed controller and PID controller for the comparison and evaluation of the effectiveness of the proposed controller. The experimental results showed that the proposed controller was excellent from the standpoints of performance and stability for the velocity control of SC-HST. Keywords: Secondary-controlled closed-loop hydrostatic transmissions; Secondary control unit; Hydraulic hybrid vehicle; Hybrid electric vehicle ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction The demand of fuel is incessantly increasing in recent years while the natural resources have been exhausted. Besides, the air pollution caused by vehicles has been being a serious prob- lem. To satisfy the desire of fuel economy improvement of mobile vehicles, some energy saving systems have been pro- posed, in this research, a hydraulic system similar to a serial hybrid hydraulic vehicle (HHV) was proposed for experiment. It is called the secondary-controlled hydrostatic transmission system (SC-HST). SC-HST has been widely used in both mobile and industrial applications since 1980 [1]. In compari- son with conventional hydrostatic transmission (HST), an accumulator was added to change the relation between pump and motors from flow coupling to pressure coupling. The high pressure accumulator allows the HST to recuperate most of the lost kinetic energy of vehicle during decelerating and brak- ing time. In such systems, the velocity of SCU is controlled by adjusting its displacement [2]. The displacement is regulated by changing the angle of swash plate due to an electro- hydraulic mechanism to maintain the require torque and ve- locity as the system pressure changes. This advantage of SC- HST improves fuel economy of vehicle. However, one of the most obstacles is the difficulty in velocity or position control of SCU, such as variable displacement hydraulic pump/motors (PM). The displacement control mechanism (DCM) is a high- order system with voltage input and displacement output. In addition, nonlinear characteristics such as dead-zone input exist in SCU due to the overlap of the DCM spoon and the “Self blocking” of the swash plate [3]. Besides, the mechani- cal and volumetric losses of SCU are based on load conditions, and they are often nonlinear. In Refs. [4, 5], optimal-tuning PID controllers for velocity and position control in hydraulic systems were applied. The performance of the systems was satisfied for such controllers, but they neither showed a systematic design nor a guarantee of the stability of the system. A robust PID controller [6] and a robust static output feed- back controller [7] were designed for network control systems such as DC motor. The effectiveness of these approaches was demonstrated in simulation. A state output feedback controller was designed for non- fragile dynamic vibration absorber system [8]. Significant improvement in suppression of vibration was shown via ex- ample. Robust filter [9, 10] and robust energy-to-peak filter [11-13] are employed for network control systems with time-varying delays, randomly missing data and uncertain linear system. Simulations illustrated the usefulness of the usefulness of the * Corresponding author. Tel.: +82 52 259 2282, Fax.: +82 52 259 1680 E-mail address: kkahn@ulsan.ac.kr † Recommended by Associate Editor Kyongsu Yi © KSME & Springer 2013