When you hear the term humanoid robot, you may think of C-3PO, the human-cyborg-relations android from Star Wars. C-3PO was designed to assist humans in communicating with robots and alien species. The droid, which first appeared on screen in 1977, joined the characters on their adventures, walking, talking, and interacting with the environment like a human. It was ahead of its time.

Before the release of Star Wars, a few androids did exist and could move and interact with their environment, but none could do so without losing its balance.

It wasn’t until 1996 that the first autonomous robot capable of walking without falling was developed in Japan. Honda’s Prototype 2 (P2) was nearly 183 centimeters tall and weighed 210 kilograms. It could control its posture to maintain balance, and it could move multiple joints simultaneously.

In recognition of that decades-old feat, P2 has been honored as an IEEE Milestone. The dedication ceremony is scheduled for 28 April at the Honda Collection Hall, located on the grounds of the Mobility Resort Motegi, in Japan. The machine is on display in the hall’s robotics exhibit, which showcases the evolution of Honda’s humanoid technology.

In support of the Milestone nomination, members of the IEEE Nagoya (Japan) Section wrote: “This milestone demonstrated the feasibility of humanlike locomotion in machines, setting a new standard in robotics.” The Milestone proposal is available on the Engineering Technology and History Wiki.

Developing a domestic android

In 1986 Honda researchers Kazuo Hirai, Masato Hirose, Yuji Haikawa, and Toru Takenaka set out to develop what they called a “domestic robot” to collaborate with humans. It would be able to climb stairs, remove impediments in its path, and tighten a nut with a wrench, according to their research paper on the project.

“We believe that a robot working within a household is the type of robot that consumers may find useful,” the authors wrote.

But to create a machine that would do household chores, it had to be able to move around obstacles such as furniture, stairs, and doorways. It needed to autonomously walk and read its environment like a human, according to the researchers.

But no robot could do that at the time. The closest technologists got was the WABOT-1. Built in 1973 at Waseda University, in Tokyo, the WABOT had eyes and ears, could speak Japanese, and used tactile sensors embedded on its hands as it gripped and moved objects. Although the WABOT could walk, albeit unsteadily, it couldn’t maneuver around obstacles or maintain its balance. It was powered by an external battery and computer.

To build an android, the Honda team began by analyzing how people move, using themselves as models.

That led to specifications for the robot that gave it humanlike dimensions, including the location of the leg joints and how far the legs could rotate.

Once they began building the machine, though, the engineers found it difficult to satisfy every specification. Adjustments were made to the number of joints in the robot’s hips, knees, and ankles, according to the research paper. Humans have four hip, two knee, and three ankle joints; P2’s predecessor had three hip, one knee, and two ankle joints. The arms were treated similarly. A human’s four shoulder and three elbow joints became three shoulder joints and one elbow joint in the robot.

The researchers installed existing Honda motors and hydraulics in the hips, knees, and ankles to enable the robot to walk. Each joint was operated by a DC motor with a harmonic-drive reduction gear system, which is compact and offered high torque capacity.

To test their ideas, the engineers built what they called E0. The robot, which was just a pair of connected legs, successfully walked. It took about 15 seconds to take each step, however, and it moved using static walking in a straight line, according to a post about the project on Honda’s website. (Static walking is when the body’s center of mass is always within the foot’s sole. Humans walk with their center of mass below their navel.)

The researchers created several algorithms to enable the robot to walk like a human, according to the Honda website. The codes allowed the robot to use a locomotion mechanism, dynamic walking, whereby the robot stays upright by constantly moving and adjusting its balance, rather than keeping its center of mass over its feet, according to a video on the YouTube channel Everything About Robotics Explained.

“P2 was not just a technical achievement; it was a catalyst that propelled the field of humanoid robotics forward, demonstrating the potential for robots to interact with and assist humans in meaningful ways.” —IEEE Nagoya Section

The Honda team installed rubber brushes on the bottom of the machine’s feet to reduce vibrations from the landing impacts (the force experienced when its feet touch the ground)—which had made the robot lose its balance.

Between 1987 and 1991, three more prototypes (E1, E2, and E3) were built, each testing a new algorithm. E3 was a success.

With the dynamic walking mechanism complete, the researchers continued their quest to make the robot stable. The team added 6-axis sensors to detect the force at which the ground pushed back against the robot’s feet and the movements of each foot and ankle, allowing the robot to adjust its gait in real time for stability.

The team also developed a posture-stabilizing control system to help the robot stay upright. A local controller directed how the electric motor actuators needed to move so the robot could follow the leg joint angles when walking, according to the research paper.

During the next three years, the team tested the systems and built three more prototypes (E4, E5, and E6), which had boxlike torsos atop the legs.

In 1993 the team was finally ready to build an android with arms and a head that looked more like C-3PO, dubbed Prototype 1 (P1). Because the machine was meant to help people at home, the researchers determined its height and limb proportions based on the typical measurements of doorways and stairs. The arm length was based on the ability of the robot to pick up an object when squatting.

When they finished building P1, it was 191.5 cm tall, weighed 175 kg, and used an external power source and computer. It could turn a switch on and off, grab a doorknob, and carry a 70 kg object.

P1 was not launched publicly but instead used to conduct research on how to further improve the design. The engineers looked at how to install an internal power source and computer, for example, as well as how to coordinate the movement of the arms and legs, according to Honda.

For P2, four video cameras were installed in its head—two for vision processing and the other two for remote operation. The head was 60 cm wide and connected to the torso, which was 75.6 cm deep.

A computer with four microSparc II processors running a real-time operating system was added into the robot’s torso. The processors were used to control the arms, legs, joints, and vision-processing cameras.

Also within the body were DC servo amplifiers, a 20-kg nickel-zinc battery, and a wireless Ethernet modem, according to the research paper. The battery lasted for about 15 minutes; the machine also could be charged by an external power supply.

The hardware was enclosed in white-and-gray casing.

P2, which was launched publicly in 1996, could walk freely, climb up and down stairs, push carts, and perform some actions wirelessly.

P2, which was launched publicly in 1996, could walk freely, climb up and down stairs, push carts, and perform some actions wirelessly.King Rose Archives

The following year, Honda’s engineers released the smaller and lighter P3. It was 160 cm tall and weighed 130 kg.

In 2000 the popular ASIMO robot was introduced. Although shorter than its predecessors at 130 cm, it could walk, run, climb stairs, and recognize voices and faces. The most recent version was released in 2011. Honda has retired the robot.

Honda P2’s influence

Thanks to P2, today’s androids are not just ideas in a laboratory. Robots have been deployed to work in factories and, increasingly, at home.

The machines are even being used for entertainment. During this year’s Spring Festival gala in Beijing, machines developed by Chinese startups Unitree Robotics, Galbot, Noetix, and MagicLab performed synchronized dances, martial arts, and backflips alongside human performers.

“P2’s development shifted the focus of robotics from industrial applications to human-centric designs,” the Milestone sponsors explained in the wiki entry. “It inspired subsequent advancements in humanoid robots and influenced research in fields like biomechanics and artificial intelligence.

“It was not just a technical achievement; it was a catalyst that propelled the field of humanoid robotics forward, demonstrating the potential for robots to interact with and assist humans in meaningful ways.”

To learn more about robots, check out IEEE Spectrum’s guide.

Recognition as an IEEE Milestone

A plaque recognizing Honda’s P2 robot as an IEEE Milestone is to be installed at the Honda Collection Hall. The plaque is to read:

In 1996 Prototype 2 (P2), a self-contained autonomous bipedal humanoid robot capable of stable dynamic walking and stair-climbing, was introduced by Honda. Its legged robotics incorporated real-time posture control, dynamic balance, gait generation, and multijoint coordination. Honda’s mechatronics and control algorithms set technical benchmarks in mobility, autonomy, and human-robot interaction. P2 inspired new research in humanoid robot development, leading to increasingly sophisticated successors.

Administered by the IEEE History Center and supported by donors, the Milestone program recognizes outstanding technical developments around the world.

30 Years Ago, Robots Learned to Walk Without Falling