# State of the Art: Bipedal Robots for Lower Limb Rehabilitation
## 1 Introduction  
Bipedal robots are robots with two legs; the main difference between them and other robots is their bipedal locomotion. The research on bipedal robots began in the 1960s. In 1968, [PERSON], who worked at General Motors in the U.S., developed a controlled bipedal walking robot named Rig [1]. This started the research on bipedal robots. From 1968 to 1969, Yugoslavia's famous scientist [PERSON] proposed an important theory on bipedal robots, the Zero Moment Point (ZMP) stability criteria [2]. Since then, scholars and research institutions in various countries have started research work on bipedal robots. The main development course of the bipedal robot is shown in Table 1 and Figure 1. \begin{table}
\begin{tabular}{c c c} \hline
**Time** & **Scientist/Institution** & **Achievement** \\ \hline
1968 & [PERSON], United States & Rig \\
1969 & [PERSON], Yugoslavia & ZMP stability criteria \\
1969 & [PERSON], Japan & WAP-1 \\
1970 & [PERSON], United Kingdom & “Witt” type robot \\
1984 & [PERSON], Japan & WL-10 RD \\
1986 & Honda, Japan & E0 \\
1988 & National University of Defense Technology, China & KDW-1 \\
1989 & Mogeer, United States & Passive Dynamic Waking \\
1990 & [PERSON] et al., United States & Neural networks, SD-1 \\ \hline \end{tabular}
\end{table} 
Table 1: The main development of the bipedal robot [3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35]. \begin{table}
\begin{tabular}{c c c} \hline
**Time** & **Scientist/Institution** & **Achievement** \\ \hline
1993 & Honda, Japan & P-1 \\
1997 & Honda, Japan & P-3 \\
1997 & [PERSON] and [PERSON].[PERSON]. Pratt, United States & Virtual model control \\
1999 & MIT, United States & COG \\
2000 & Honda, Japan & ASIMO \\
2000 & Sony, Japan & SDR-3X \\
2002 & Beijing Institute of Technology, China & BHR \\
2004 & South Korea & HUBO \\
2004 & RobotCub Consortium, Italy & iCUB \\
2005 & University of Florida, United States & Rabbit \\
2005 & MIT, America & Domo \\
2007 & Aldebaran Robotics, France & NAO \\
2008 & University of Tehran, Iran & Surena I \\
2009 & Technical University of Munich, German & LOLA \\
2009 & Aldebaran Robotics, France & Romeo \\
2010 & AIST, Japan & HRP-4C \\
2012 & NRL, United States & SAFFiR \\
2013 & Institute of Robotics and Mechatronics, German & TORO \\
2013 & Boston Dynamics, United States & PETMAN \\
2015 & University of Tehran, Iran & Surena III \\
2016 & Boston Dynamics, United States & Atlas \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont_. Figure 1: The historical development of bipedal robots. (**a**) WAP-1; (**b**) WAP-3; (**c**) WL-5; (**d**) WL-9 DR; (**e**) WL-10 RD; (**f**) WL-12; (**g**) Honda P-1; (**h**) BIP2000; (**i**) Honda P-2; (**j**) Honda P-3; (**k**) ASIMO; (**l**) KHR-2; (**m**) HUBO; (**n**) NAO; (**o**) LOLA. ((**a**-e**,**g**,**i**-m**) were reproduced with permission from [36]; (**f**) was reproduced with permission from [37], Copyright Springer, 1997; (**h**) was reproduced with permission from [38], Copyright Elsevier, 2004; (**n**) was reproduced with permission from [39], Copyright Springer, 2014; (**o**) was reproduced with permission from [40], Copyright Springer, 2013). Japan has a prominent position in the field of bipedal robot research and has made many milestone achievements. In 1968, Professor [PERSON] of Waseda University first started the development of bipedal robots in Japan. From 1969 to 1984, [PERSON]'s laboratory launched more than 10 bipedal robots. The walking speed of the robot has developed from WL-5 with 45 s per step, to WL-10 RD with 1.3 s per step [36]. As a pioneer in bipedal robot research, [PERSON] has made a significant contribution. Since 1986, Honda has launched seven E series robots, three P series robots, and an intelligent robot called ASIMO. Honda's research work, especially the P-3 and ASIMO, pushed the development of bipedal robots to a new level, making the development, production, engineering, and marketing of biped robots practical.