Chapter 4

How ASIMO Walks Like a Human

“The true legacy of ASIMO is not measured by the miles it walked, but by the generations of engineers and innovators it inspired.”

Walking is one of the most natural activities humans perform. Most people learn to walk as infants and eventually do so without conscious thought. Every day, we take thousands of steps while talking, carrying objects, climbing stairs, or avoiding obstacles. Behind this seemingly simple action lies an extraordinarily complex process involving the brain, muscles, bones, joints, and sensory organs.

For engineers attempting to build a humanoid robot, walking represents one of the greatest challenges in robotics. A robot must constantly calculate how to shift its weight, where to place its feet, how much force to apply to each joint, and how to respond if the ground changes unexpectedly. A single miscalculation can cause the robot to lose balance and fall.

One of ASIMO’s most remarkable achievements was its ability to walk, run, turn, climb stairs, and maintain balance in ways that closely resembled human movement. These abilities were the result of decades of research in biomechanics, control engineering, computer science, and artificial intelligence.

This chapter explains how ASIMO learned to move like a human and why its walking technology became one of the most significant milestones in the history of humanoid robotics.

Understanding Human Walking

To understand ASIMO’s movement, it is helpful to first examine how humans walk.

When a person walks, the body is never completely still. The center of gravity continuously shifts from one foot to the other. As one foot lifts from the ground, the entire body briefly balances on a single leg before the opposite foot touches the floor.

During every step, the brain performs countless calculations. It receives information from the eyes, inner ear, muscles, joints, and skin, then sends commands to dozens of muscles to maintain balance and coordinate movement.

Humans perform these calculations almost instantly and without conscious effort.

Replicating this process inside a machine proved to be one of the greatest engineering challenges Honda had ever undertaken.

The Problem of Balance

Imagine standing on one leg with your eyes closed. Even though you remain standing, your body continuously makes tiny adjustments to prevent you from falling. Your ankle moves slightly, your hips shift, and your muscles tighten or relax in response to changes in balance.

A robot must perform similar adjustments.

Unlike humans, however, robots cannot rely on instinct. Every movement must be measured, calculated, and controlled by computers.

If the robot leans too far forward, it may fall.

If it shifts its weight too slowly, it may lose stability.

If one foot lands only a few centimeters away from the intended position, balance may be compromised.

ASIMO solved these problems through an advanced balance control system that constantly monitored the robot’s posture and adjusted its movements in real time.

Static Walking Versus Dynamic Walking

Early robots used a method known as static walking. In this approach, the robot ensured that its center of gravity remained directly above its supporting foot before taking the next step.

Although this method was safe, it resulted in slow and unnatural movement. The robot paused after every step, making its walking resemble a cautious shuffle rather than a smooth stride.

Honda engineers wanted ASIMO to move more like a person.

To achieve this, they developed dynamic walking, in which the robot continuously shifts its weight forward while moving. Instead of stopping between steps, ASIMO maintains momentum, allowing its movements to appear fluid and natural.

Dynamic walking is much more difficult because the robot spends part of each step in a temporarily unstable position. The control system must predict future body movements and adjust them before instability becomes dangerous.

This predictive capability became one of ASIMO’s defining features.

Intelligent Real-Time Control

ASIMO’s movements were coordinated by sophisticated onboard computers that processed information hundreds of times each second.

These computers continuously answered questions such as:

• Where is the robot’s center of gravity?
• Which foot is supporting the body?
• How fast is each joint moving?
• Is the robot leaning too far?
• Is the floor level or sloped?
• Has someone stepped into the robot’s path?

By combining data from multiple sensors, the computer generated new motor commands almost instantly.

This rapid feedback loop allowed ASIMO to adapt to changing conditions while maintaining stable movement.

The Role of Gyroscopes

One of the most important sensors used in ASIMO was the gyroscope.

A gyroscope measures rotational movement and helps determine the robot’s orientation.

If ASIMO began leaning unexpectedly to one side, the gyroscope detected the change immediately.

The control system then adjusted the position of the legs, hips, or ankles to restore balance before the lean became large enough to cause a fall.

Modern smartphones also contain miniature gyroscopes that allow screen orientation to change automatically when the phone is rotated.

In ASIMO, however, the gyroscopes played a far more critical role by helping the robot remain upright while walking.

Accelerometers and Motion Detection

In addition to gyroscopes, ASIMO used accelerometers.

Accelerometers measure changes in speed and direction.

Together with gyroscopes, they allowed the robot to determine whether it was accelerating, slowing down, climbing stairs, or changing direction.

This information helped ASIMO coordinate smooth transitions between different movements.

For example, when beginning to walk, the robot gradually increased its speed instead of making sudden jerky motions.

Likewise, when stopping, it slowed down progressively to maintain stability.

Foot Pressure Sensors

Every step ASIMO took generated valuable information.

Pressure sensors embedded in the robot’s feet measured how body weight was distributed across the soles.

If excessive pressure appeared on one side of a foot, the control system recognized that the robot’s posture required adjustment.

The computers then modified joint positions to redistribute weight more evenly.

This constant monitoring helped ASIMO maintain stable footing even while turning or climbing stairs.

Pressure sensors also confirmed whether a foot had made proper contact with the ground before the next step began.

Predicting the Next Step

One of Honda’s greatest innovations was enabling ASIMO to predict future movements rather than merely reacting to current conditions.

The robot continuously estimated where its body would be a fraction of a second later.

This predictive capability allowed ASIMO to:

• Prepare for the next step.
• Adjust stride length.
• Compensate for changes in speed.
• Navigate curves smoothly.
• Avoid losing balance during turns.

Instead of waiting until instability occurred, ASIMO attempted to prevent instability before it developed.

This proactive strategy greatly improved walking efficiency and safety.

Walking Forward

Walking forward required precise coordination between both legs.

Each walking cycle involved several stages:

1. Shifting body weight onto one leg.
2. Lifting the opposite foot.
3. Swinging the leg forward.
4. Positioning the foot accurately.
5. Placing the foot gently on the ground.
6. Transferring weight to the new supporting leg.
7. Repeating the process continuously.

Although these steps appear simple, hundreds of individual motor commands were required for every stride.

Each joint had to move at precisely the right time and speed.

Even a small error could affect the robot’s stability.

Walking Backward

Walking backward proved even more challenging than walking forward.

Humans naturally rely on visual information to guide movement.

When walking backward, visibility becomes limited.

ASIMO addressed this challenge by using stored environmental information together with its sensor systems.

The robot carefully adjusted its foot placement and maintained slower, more controlled movements while reversing direction.

Side-Stepping

Unlike many early robots, ASIMO could move sideways without rotating its body.

Side-stepping allowed the robot to navigate narrow spaces and avoid obstacles while maintaining its orientation toward a person or object.

This capability required independent coordination of both legs and continuous balance adjustments.

Side-stepping proved especially useful during demonstrations involving interaction with people.

Turning Smoothly

Changing direction while walking introduces additional complexity.

When humans turn, they naturally lean into the turn while adjusting stride length.

ASIMO used a similar strategy.

The robot modified hip rotation, foot placement, and upper-body posture simultaneously.

Rather than stopping before changing direction, ASIMO could perform smooth curves during continuous walking.

This made its movements appear much more lifelike.

Climbing Stairs

One of ASIMO’s most famous abilities was climbing stairs.

Most wheeled robots require elevators or ramps because stairs interrupt continuous rolling motion.

ASIMO approached each stair individually.

The robot first identified the height and position of the step using its vision system.

It then:

• Lifted one foot.
• Positioned it securely on the next step.
• Shifted body weight upward.
• Lifted the second foot.
• Repeated the process until reaching the top.

Descending stairs required even greater precision because the robot had to control downward movement while maintaining balance. These demonstrations became iconic examples of Honda’s engineering achievements.

Running on Two Legs

In 2004, Honda introduced a new version of ASIMO capable of running. Running differs fundamentally from walking. During walking, at least one foot remains in contact with the ground. During running, there is a brief moment when both feet leave the ground simultaneously. This airborne phase creates additional instability.

To run safely, ASIMO had to:

• Generate greater leg power.
• Predict landing positions.
• Absorb impact upon landing.
• Restore balance immediately.

Initially, ASIMO reached running speeds of approximately 3 kilometers per hour. Later versions increased this speed to about 9 kilometers per hour (5.6 miles per hour). Although slower than an average human runner, ASIMO became one of the first humanoid robots capable of stable running.

Recovering from Disturbances

Real-world environments are unpredictable. Someone might accidentally bump into the robot. The floor may be uneven. A carried object could shift unexpectedly. ASIMO’s balance control system continuously monitored these disturbances. If a gentle push occurred, the robot adjusted its posture by:

• Moving its arms.
• Repositioning its feet.
• Shifting its hips.
• Altering ankle angles.

These corrective actions often prevented falls before they happened. This ability significantly improved the robot’s reliability during public demonstrations. Energy-Efficient Walking

Walking consumes considerable energy. Every movement requires electricity to power motors throughout the body. Honda engineers optimized ASIMO’s walking algorithms to reduce unnecessary motion. The robot avoided excessive joint movement and selected efficient step lengths whenever possible. Smooth acceleration and deceleration also reduced energy consumption. These improvements allowed ASIMO to operate longer between battery charges. Inspiration from Human Biomechanics

Honda did not simply attempt to copy human walking. Instead, engineers studied biomechanics to understand the principles behind efficient movement. Researchers analyzed:

• Joint angles during walking.
• Muscle coordination.
• Center-of-gravity shifts.
• Arm swinging.
• Foot placement.
• Stride length.
• Running mechanics.

They then translated these biological principles into mechanical systems and mathematical algorithms. The result was a robot that moved naturally while remaining mechanically reliable.

A Milestone in Robotics

ASIMO’s walking technology represented a turning point in humanoid robotics. Before ASIMO, most bipedal robots moved cautiously and slowly. After ASIMO, engineers worldwide recognized that smooth, dynamic walking was achievable. Its innovations influenced countless research projects involving humanoid robots, autonomous mobility, rehabilitation devices, powered exoskeletons, and prosthetic limbs. Many of today’s advanced humanoid robots continue to build upon concepts first demonstrated by ASIMO. The robot proved that graceful movement requires far more than powerful motors. It depends on the seamless integration of sensors, computers, control algorithms, and mechanical engineering—all working together in perfect harmony. In the next chapter, we will examine the sophisticated sensor systems and artificial intelligence that enabled ASIMO to see, hear, recognize people, interpret its surroundings, and interact naturally with the world around it.