Humanoid Robots

To appear like human, robots must walk on two legs; i.e. they must have biped locomotion. Several control methods and algorithms have been developed in this field.

When analyzing biped locomotion, we separate it into four categories- Planning, Stabilization Control, Gait Generation, and Trajectory Generation, as shown in Figure 1-2. This thesis covers the areas shown in blue.

Figure 1-2 Classification of researches for biped locomotion.

High-level planning is mainly concerned with collecting and fixing the pattern of walking. That enables us to plan the path and goal appropriately, so we can take the planned information into the environment. The planning result combines several low-level planning motions, including walking in straight line, in curves, up stairs, with

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different stride lengths, and at different speeds [32]. The information from high-level planning enables the accomplishment of stable gait generation, thus ensuring that the robot walks with asymptotic stability. The stabilization controller uses several algorithms such as the ankle stabilizer and ZMP limiter. Trajectories such as COG and ZMP for each joint are optimized or fixed, thus enabling the robot to walk more smoothly and stably—more humanly.

1.2.1 Development of Humanoid Robots

In 2010, HONDA corporation celebrated birthday of 10 years old humanoid robot

“ASIMO” [12]. ASIMO is one of the successful humanoid robots. Its height is 130 cm and weight is 54 kg, and it has 34 DOFs. This remarkable robot can do lots of human motions including walk, run, waving hands, nod, etc. ASIMO really impresses on most of researchers in robotic territory. HONDA humanoid robot triggered the world’s research on humanoid robots. Therefore, new humanoid robots has been fabricated, such as HRP-2, HRP-3,HRP-4, HUBO [54], WABIAN, and so on.

The National Institute of Advanced Industrial Science and Technology (AIST) develops the humanoid robots of HRP series. Different from the ASIMO, HRP series adopt the feminine style, and the appearances are similar to the human beings. HRP-2 [24, 26] was a remarkable humanoid robot whose height is 154 cm and weight is 58 kg.

This robot not only has the cooling system for longer motion but also utilize FEM

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(finite element method) to reduce mechanical resonance. HRP-2 is a prototype for subsequent HRP series robots. HRP-3 [27] was improved some problems of HRP-2, and it had been designed to work outdoors. The latest HRP series was HRP-4C [28] which was designed in 2009. This robot has a fashionable body shape, and its human-like face does impress robotic researchers.

Korea Advanced Institute of Science and Technology (KAIST) developed a humanoid robot, HUBOⅡ [54]. Its appearance is similar to predecessor, HUBO (KHR3), and it has 40 DOFs for various motion. HUBOⅡ only weights 45 kg which is lighter than his brother HUBO (55kg), and capable of walking two times faster (1.4km/h). It also can run up to 3.3 km/h.

WABIAN-2 [49], was designed by Waseda University, and it can walk much like the human beings. Since WABIAN-2 has toe joints and heel joints, it can walk with toe-off and heel-contact. Moreover, WABIAN-2 has additional joint in waist, and make its knee joints more straight during walking period. This robot adopted the genetic algorithm to generate the joint trajectory, and the walking motions are similar to the human beings.

Boston Dynamics designed an anthropomorphic robot, PENTMAN [82], for testing chemical protection clothing. It was used by the US Army. This robot had to be supported mechanically and had a limited repertoire of motion. The predecessor, BIG

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DOG, was also fabricated for delivering goods and materials during warfare. Although PENTMAN’s robust balance is not as good as BIG DOG, it has a walking speed of up to 7.2 km/h (4.4 mph).

Unlike most of the biped robots, DLR developed a biped robot, DLR-Biped [51], which walked with torque joints. This modular structure, DLR-KUKA Lightweight-Robot (LWR), can keep the development time and cost low. It has 6 degrees-of-freedom each legs, and has a walking speed of up to 0.15m/s.

LOLA is an anthropomorphic autonomous humanoid robot [40] which was developed by Technical University of Munich. This robot has 25 DOFs for performing various tasks. In order to generate high-speed walking, further design goals can be improved, such as high center of mass and low moments of inertia of the leg links. By investment casting, this robot largely reduces the weight, but it still has the humanoid appearance (height :180cm). Although LOLA is fabricated with aluminum, the strength of mechanism is very solid.

1.2.2 Stable Walking for Humanoid Robots

Stability is critical for a humanoid robot to walk. Locomotion is governed by a planning process to determine stride length, positions of end-effectors, and stability in motion. Most current researchers believe that a combination of high-level dynamic algorithms and appropriate mechanisms will help achieve stability.

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The use of a simple model is advantageous as it allows rapid calculation and real-time implementation. Kajita et al. adopted the Linear Inverted Pendulum Model [21, 22, 23] and Minakata et al. proposed the Virtual Inverted Pendulum Method [42]. Both methods can simplify models for easy calculation, however, such simplification may allow modeling errors to render biped robots unstable in certain circumstances. It is therefore necessary to consider the limitations of these models.

Some researchers, lacking the needed stability algorithm, generated locomotion through trial and error. Lee et al. [36] adopted an evolution algorithm (EA) to determine walking parameters, while Kuffner et al. [33] proposed a heuristics search to generate a sequence of footstep locations. Although they are very time-consuming, the contribution made by these heuristics search methods for biped robots is still significant. By constructing a huge database and developing feasible experiences for robots, researchers have enabled robots to perform more actions of different types.

1.2.3 Human-Like Walking

In recent years, an ever increasing number of humanoid robots can walk stably, prompting the belief that, just because they are labeled humanoid, they can walk or run just like human beings. They, however, walk with bent knees because of the limitations imposed by modeling or hardware [12, 22, 25, 48]. Sano et al. [59] used the inverted pendulum’s angular momentum to generate ankle torque. This was a start to recognize

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that human feet are not permanently parallel with the ground during locomotion.

Wikipedia [88] stated that human gait consists of (1) forefoot strike, (2) midfoot strike, and (3) heel strike. This definition separates the foot into toe, midfoot, and heel, each with its own functions Some researchers have therefore begun to study the influence of toe and heel [66, 71], and, unlike earlier researchers, have introduced the viewpoint of using the straight knee for part of the robotic stride [38, 41, 50]. Of the four postures used in human locomotion, only one (midfoot strike) occurs in conventional humanoid robots, whereas forefoot strike (toe-off motion), heel strike (heel-contact motion), and straight knee are all missing.

There are many advantages to implementing natural walking. Stride length, limited in conventional humanoid robots because of their constantly parallel-to-the-ground feet, can be lengthened by adding toe and heel functions. By straightening the knee for part of the stride cycle, power consumption can be reduced.

The disadvantages of adding these functions are potential loss of stability resulting from the small contact area when heel or toe is the only part of the foot on the ground.

This presents a great challenge, in particular maintaining the stability of a walking

robot.