“…Optimal control method has been proposed nearly 70 years ago (Bellman and Dreyfus, 2013; Pontryagin, 1959), and plenty of literatures are recorded (An et al, 2021; An et al, 2021; Wei et al, 2021). Adaptive dynamic programming (ADP) theory is a key direction to undertake approximate optimal control system issues of discrete-time (Hu et al, 2021; Luo et al, 2020; Zhu et al, 2020a), continuous-time (An et al, 2021; Shan et al, 2020; Wei et al, 2020), data driven (Gao et al, 2018; Li et al, 2020; Su et al, 2020), and furthermore robot systems with input/output constraints (An et al, 2020; Lu et al, 2020; Ren et al, 2019), external disturbance (An et al, 2019; Song and Lewis, 2020; Xia et al, 2020), actuator failures (Dai et al, 2018; Jiao et al, 2018; Ma et al, 2020), and so on. Kong et al (2021b) developed n -link manipulator approximate optimal law via saturation nonlinearity.…”