Safety Operation of n-DOF Serial Hydraulic Manipulator in Constrained Motion with Consideration of Contact-Loss Fault  
[PERSON]  
1 Graduate School of Mechanical and Automotive Engineering, University of Ulsan, Daehakro 93, Nam-gu, Ulsan 44610, Korea; [EMAIL_ADDRESS] (H.V.A.T. ); [EMAIL_ADDRESS] (H.A.T.) [PERSON]  
1 Graduate School of Mechanical and Automotive Engineering, University of Ulsan, Daehakro 93, Nam-gu, Ulsan 44610, Korea; [EMAIL_ADDRESS] (H.V.A.T. ); [EMAIL_ADDRESS] (H.A.T.) [PERSON]  
2 School of Mechanical and Automotive Engineering, University of Ulsan, Ulsan 44610, Korea; [EMAIL_ADDRESS]; Tel. : +82-52-259-2282  
The currently growing attractiveness in fields of industrial applications such as polishing, welding, wiping, assembling, etc. reasonably results in requirements of workability in constrained motions. Following the literature of robotic manipulators in the relevant fields, many scholars proposed methods to improve system performance and reliability such as using force estimation [22; 23; 24], adaptive laws or intelligent techniques [25; 26; 27] in terms of tracking performance, or estimation of environment characteristics [28], observers [29; 30; 31], and impedance behaviors [32; 33] for enhancing system reliability. Generally, hybrid position/force control, impedance, and/or admittance controls which are directly involved in force regulation are frequently applied to deal with problems of physical interactions. Despite achieving good outcomes, safety operation of the manipulator in cases of sudden contact-loss due to moving out the constrained framework, step-change of environment, accidentally unloading or payload change is rarely mentioned. Stability, robustness, and especially safety are mandatory requirements when the robot is interacting with the environment. To cope with and achieve human-like behaviors in those possible aforementioned issues, an energy-tank concept-based passivity control was proposed [34; 35; 36]. This concept is based on the fact that system potential energy is stored in springs or stiffness components (treated as virtual tanks) during the working processes. When the contact-loss or sudden change occurs, the end-effector is still being regulated such that force tracking is satisfied and the energy is released simultaneously; thereby, resulting in unexpected behaviors. Therefore, auxiliary variables are designed to passively detect and switch their states for deactivating force command signals and dissipating the energy when the contact-loss happens. Furthermore, a new trajectory reshaped for smooth and safe operation is also mentioned. This technique was expanded to several fields such as bilateral telemanipulation [37; 38; 39], surgical robotics [40; 41; 42], and industrial manipulator applications with variable stiffness [35; 36; 43]. In [35], [PERSON] et al. utilized a unified passivity-based force/impedance control based-energy tank to not only deal with the contact-loss but also guarantee the stability and robustness of the KUKA/DLR LWR-III robot. In [36], [PERSON] et al. successfully investigated behaviors of a pneumatic actuator robotic system with unified torque/impedance control-based energy tank. Based on the literature overview, this paper suggests using a virtual energy tank concept as a fault detection method to deal with a problem of accidental contact-loss on an n-degree-of-freedom (n-DOF) serial hydraulic manipulator in constrained frameworks. The structure of the n-DOF serial hydraulic manipulator is derived for designing the proposed methodology in which the problem of contact-loss is taken into consideration as prior criterion for safe operation. To evaluate the effectiveness of the proposed method, simulations are given on a 6-DOF serial hydraulic KIRO robot. According to the aforementioned problems, contributions of the paper can be listed as (1) a fault detection based on a virtual energy-tank variable and passivity control methodology is exploited to deal with the sudden contact-loss due to step-change in environments.