From the Tesla Optimus working in the California factory to the Unitree H1 performing the Yangko dance on the stage of the Chinese New Year’s Gala.

The revolution of intelligence and automation driven by motors is turning science fiction into reality. Just like the scenes in the Terminator movies, humanoid robots have now stepped out of the movie screen and entered the real world. It seems that the tipping point for the explosion of the robotics industry has arrived.

On April 16,2023, the robotics world waved goodbye to an icon as Boston Dynamics, the legendary creators behind those jaw – dropping backflipping machines, shared a bittersweet farewell video for their hydraulic – powered humanoid robot Atlas.

In the video, Atlas gave its final performance with its iconic smooth movements and amazing flexibility, including those complex walking, jumping and backflip movements that once amazed the world. Every movement demonstrated the possibilities that hydraulic technology has brought to humanoid robots over the past decade. As Atlas slowly bows at the end, the era of hydraulic robots drew to a close.

Merely one day later, Boston Dynamics unveiled its next-generation Atlas powered by electric motors. This not only brought significant changes in appearance but also greatly improved the overall performance of the robot. The redesigned Atlas has more flexible electric joints, and multiple joints are capable of performing a full 360-degree rotation in all directions. This is a direct benefit of its advanced electric drive system, which enables unprecedented range of motion.

The Industry Trend of Humanoid Robots

At the World Robotics Conference in 2024, Marc Raibert, the founder of Boston Dynamics, personally explained the reason for switching from hydraulic drive to motor drive.

“I think there are two key elements. A robot that… a hydraulic robot—I love hydraulics, and we built many robots with hydraulics. I think there’s still room for construction robots that are maybe humanoid or humanoid-ish that will continue to use hydraulics because of the strength-to-weight and those kinds of things.”

“But two things: one is that they are messy. With endless effort, we could never get Atlas to stop oil from coming out. When you put oil robots into places like factories and homes, they just can’t be dripping oil. It’s very hard to find people skilled in hydraulics because people think of it as an old-world thing, even though Atlas has lots of innovation in how its hydraulics work—right to the servo valve, the HPU (hydraulic power units)—all along there’s really innovative stuff going on.”

“And then lastly, I think electric motors have gotten strong enough so that at that scale they can compete.”

Marc Raibert’s response basically sums up the reason for Boston Dynamics’ transformation. This transformation aligns perfectly with the global trends in the robotics industry. Most of the robots showcased at NVIDIA’s GTC conference last year adopted electric drive solutions. Even the industry pioneer Boston Dynamics must strike a balance between idealism and commercial realities.

Cost determines fate

The old version of Atlas is equipped with 28 hydraulic cylinders throughout its body, along with supporting hydraulic pumps, valves and motors. The bionic-designed leg skeleton needs to be manufactured using metal 3D printing technology. The manufacturing cost of each robot is as high as $2 million, and the maintenance cost is also extremely high. It not only requires a professional team to deal with frequent failures such as oil leakage and cylinder bursting at any time, but also requires daily maintenance and regular in-depth servicing.  

Compared with the complex structural design and maintenance issues of the hydraulic system, the motor drive solution, relying on supply chain integration and modular design, has not only reduced the manufacturing cost to tens of thousands of dollars, but also significantly reduced potential failure points due to its simplified structure. While improving reliability, it has significantly reduced the maintenance frequency, which is of decisive significance for scenarios that require long-term continuous operation or remote operation.

Such comprehensive advantages, ranging from the design architecture to manufacturing and maintenance, have ultimately created an insurmountable cost gap between the two technical routes, directly determining their different fates in the commercialization process.

Hydraulic drive or electric drive?

Since the hydraulic system is so expensive, why did it become the early research direction for Boston Dynamics?

Let’s go back to the 1990s. With the support of DARPA (Defense Advanced Research Projects Agency of the United States), Boston Dynamics began to develop military robots. At that time, the battlefield requirements were extremely stringent. The robots were required to transport supplies in the mountains of Afghanistan and the deserts of Iraq, and they even had to be able to get up on their own after being knocked over by bomb shockwaves.

In such extreme scenarios, robots need to have powerful driving force to cope with them. However, at that time, battery and motor technologies were not mature. Therefore, Boston Dynamics needed to use solutions with lower energy consumption and higher power density. Among servo systems of the same mass, the hydraulic servo system can provide higher torque and power density.

The working principle of hydraulic systems resembles muscle contraction, generating immense force through high-pressure oil pumps driving piston movement in hydraulic cylinders. The early-developed BigDog could carry 150kg payloads through snow-covered terrain, with hydraulic joints demonstrating spring-like impact absorption capabilities. This formidable power originated from hydraulic pressures reaching 20-30 MPa (equivalent to 300 atmospheres), enabling instantaneous power output several times greater than conventional electric motors.

Even today, hydraulic systems retain advantages in power output. Unitree H1 achieves a maximum joint torque of 360 N·m, while the hydraulic Atlas boasts knee torque of 890 N·m and hip torque of 840 N·m.

However, modern motor technology has achieved significant breakthroughs. Permanent magnet motors now achieve over 80% efficiency, with high-end models exceeding 95%, compared to hydraulic systems 30-50% efficiency limited by frictional and thermal losses.

Permanent magnet motors also have advantages in terms of response speed and precise control, enabling them to perform better in high-precision tasks. The high-difficulty movements such as 180-degree waist rotation and standing up without support demonstrated by the electric-driven version of Atlas prove that the motor system already has the same level of motion capabilities as the hydraulic solution. And the low enough cost is sufficient for people to ignore some of the performance gaps of the motors.

Pressing commercialization pressure

Commercialization pressures are the ultimate reason for Boston Dynamics’ technological transformation. Boston Dynamics has changed hands three times among Google, SoftBank, and Hyundai Motor, with investors’ urgent profit demands compelling the company to reassess its technological roadmap.

Behind this transformation also lies a reflection of the deep-seated competition within the global manufacturing ecosystem. The United States has technical advantages in high-end and sophisticated fields such as hydraulic systems, but it has faced difficulties in implementing these technologies due to the hollowing out of its domestic supply chain. In contrast, through the synergistic effects of industries such as electric vehicles and consumer electronics, China has established a complete ecosystem encompassing everything from electric motors and sensors to control systems.

A more direct comparison comes from China’s Unitree Technology: its electrically driven Go2 robotic dog costs only $900 and sells for less than $1,600. In contrast, Boston Dynamics’ electrically driven Spot robot dog has a unit cost of $48,000 and a selling price of up to $75,000, with a cost difference exceeding 50 fold.

Boston Dynamics’ ultimate decision to collaborate with South Korea’s Hyundai Motor is also an attempt to utilize the manufacturing resources in Asia to break through the bottleneck of commercialization.

“survival of the fittest”

At the dawn of the large-scale application of humanoid robots, we may be witnessing the opening of a new era for humanity.

Hydraulic systems will continue to leverage their unique strengths in specialized engineering sectors, while electric drives are emerging as the passport to the era of general-purpose robotics.

As Atlas transitions from laboratory prototypes to production lines, and military-grade performance yields to consumer-grade experience, technological evolution will ultimately obey the ultimate law of survival of the fittest.

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