"Before the s, the power density of hydraulic motors was approximately a 100 times greater than that of electric motors. Today, that gap is just a factor of 10 with advances in the strength of permanent magnets like Neodymium (Nd-Fe-B).” (1)
In April , Boston Dynamics announced a significant transition in their Atlas robots—moving from hydraulic to electric actuators in their next generation of humanoid robots. Here’s the demonstration if you’re curious (along with clips from their previous platform).
Over the last few years, there’s been a huge surge in capital investment for humanoid robots. Tesla's Optimus, Figure AI ($675 million raised at a $2.6 billion valuation), Agility Robotics, and Astribot are prime examples of emerging companies in this sector.
As robotics trends shift towards greater efficiency and commercial viability, our team began exploring a few crucial questions.
Upon its debut in , Atlas was originally designed as a disaster response robot in partnership with DARPA. Seven Atlas robots were built for the DARPA Robotics challenge, a prize competition where teams worked on software/hardware to accelerate the development of advanced robotics (software, control logic) specifically for requirements related to responding to natural and man-made disasters.
With the original scope established as an R&D initiative, Atlas achieved insane feats of mobile manipulation - multi-terrain movement, forward rolls, and even backflips (parkour!). Hydraulic actuators were the natural choice for the explosive and dynamic movements required. For a long time, and still today, hydraulic actuators were the only types of actuator that could generate the necessary forces for the required movements.
Because of the energy storage in a small location and the dynamics, hydraulics can push more loads than electrical.
Hydraulics operate based on Pascal's Law, which states that pressure remains constant throughout a fluid. This principle allows a small force applied over a small area to generate a much larger force over a larger area. The operating pressures for these systems typically range from - psi. To put into perspective, a 3-inch bore at psi generates around 15,000 lbf (imagine moving a kg weight with a bore diameter that’s just slightly larger than the width of your !)
In a system like Atlas with 28 degrees of freedom, joint movement is achieved through a network of hydraulic pistons and fluids to move its limbs. These hydraulic pistons, arranged either axially or radially, leverage their own rotation to adjust the pressure of each piston. For even greater complexity, configurations like the Stewart Platform come into play that allow a single system to manage multiple degrees of freedom.
Each hydraulic cylinder can be produced compact and separate from other components, perfect for generating force in tight spaces like joints. The larger hydraulic power unit, which controls flow and pressure, is stored elsewhere. Perhaps it helps explain the body-centered design of the hydraulic Atlas!
These systems are complicated. We’re talking about assemblies that require multiple components like valves, tubes, and filters operating at extremely high forces. Some parts, such as relief valves, are crucial for enhancing the system's resilience to shock loads. As the number of parts increased along with mechanical complexity, systems like the hydraulic Atlas require frequent maintenance to achieve the desired performance over life. Common maintenance includes keeping the system free of air, which can slow down response times due to air's compressive nature. Other challenges include repeatability (position, speed and force) of a hydraulic cylinder, worn seals, leaks, and pressure drops are major reliability considerations for a hydraulic-based architecture.
“It is not a matter of IF hydraulics will leak, but a matter of when and by how much.”
Despite these challenges, the team managed to get Atlas to perform impressive movements with the aid of sophisticated software controls.* By precisely controlling speeds and torques at specific moments, they could execute complex motions, such as this backflip, all through the use of hydraulics/servo-hydraulics.
*The mechanics of software control for robotic walking merits more than another post. This Reddit thread goes into the history of different types of walks (stable vs. dynamic), approaches from different companies, and more.
When researching more into the mechanical architecture of the Atlas platform, our first question was: how do you even select actuators?
At its core, actuators are the part of the robotic system that convert stored energy into movement. These movements can be powered by mediums like air, electricity, or liquids—the corresponding actuators for each medium include pneumatic, electric, and hydraulic powered actuators. In a full system, actuators are integrated with sensors and control systems to allow for intelligent control in dynamic environments.
To select an actuator for a particular application, the core requirements boil down to answering several questions:
For example, considerations for industrial systems typically use high-rated power actuators and operate in fixed working environments, while legged robots often operate in complex environments that require fast-motion responses.
In more recent years, the power of electric motors has become increasingly competitive with that of hydraulics. Up until the s, hydraulic motors had a power density about 100 times greater than electric motors. Today, that gap has narrowed to just 10 times (1). This surge in electric motor performance is largely due to the development of stronger magnets, like Neodymium (Nd-Fe-B).
With a shrinking gap in power output, electrical motors became a more compelling option due to several inherent advantages.
Efficiency
Electric actuators have an efficiency range of 75-80%, compared to hydraulics' 40-55%. This efficiency comes from electric motors drawing current only when needed, whereas hydraulic systems must constantly maintain pressure.
Number of Components
Electric actuators also require far fewer components than hydraulic systems. The diagram below from the same paper illustrates typical setups for each actuator type. While electric motors need more space at the force generation point, the overall system occupies much less space. Maintenance is also significantly reduced since electric motors don't require filters, oils, or other long-term consumables.
Operating Temperature
A major consideration for hydraulic systems is temperature management. Hot temperatures can degrade the hydraulic fluid’s quality, and cold temperatures can increase its viscosity which impacts repeatability of the system. Hydraulic fluid temperatures above 180°F (82°C) can damage most seal compounds and speed up oil degradation. While electric actuators have performance differences across temperatures, the operating window is much wider and can be deployed with extreme-temperature greases.
Accuracy & Precision
For all considerations listed above, the primary advantage is the flexibility of motion control capabilities in an electric actuator system—precise control over position, velocity, acceleration/deceleration profiles, output force, and complex control of all motion variables are all within reach. Electric actuator motion control is far beyond the capabilities of a hydraulic system.
When introducing the newest version of Atlas, Boston Dynamics was very intentional about calling the humanoid robot a product. Specifically, a product designed with an architecture in mind for use in factory settings (as well as lofty promises for use in our own lives). It’s clear the product requirements have shifted from the original DARPA challenge to one centered around commercialization efforts, where the benefits of a hydraulic based system don’t translate well.
Removing hydraulic actuators in their system de-risks system reliability and service maintenance. The use of electric actuators helps with increased power efficiency, reduces weight, and assembly complexity while sacrificing peak force & torque output. You don’t need robots performing parkour on a factory floor, as cool as that would be!
This wouldn’t be Boston Dynamics first time pivoting architectures as they have experience transitioning Spot from a hydraulic to electric actuator based system for commercialization. In an interview with CEO Robert Playter, he shares that launching Spot as a product required the whole company to take on new talent in manufacturing, service, and quality roles.
"A decade ago, we were one of the only companies putting real R&D effort into humanoid robots. Now the landscape in the robotics industry is very different." -Robert Playter
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Actuators are crucial for executing human-like movements. However, their use in humanoid robots poses specific system requirements. Five factors are decisive for motor selection.
Humanoid robots, with their imitation of humans, represent the next evolutionary stage in robotics and exert a special fascination. The race for the first series-production-ready humanoid robot is one of the most exciting trending topics in the tech world. A market analysis by the management consultancy Horváth from March predicts that the first human-like robots could be mass-produced for industrial use as early as .
Gallery Gallery with 5 imagesWhen it comes to hardware, so-called actuators are crucial for executing human-like movements. They function as the equivalent of human joints and muscles: drive technology components that enable movements in a system—whether rotational or linear.
Actuators consist of a combination of gears, motors, sensors, ball bearings, and encoders. The more degrees of freedom required, the more actuators are needed. Currently, humanoid robots under development are capable of realizing between 16 and 60 degrees of freedom.
Humanoid movements can be controlled by electric, hydraulic, or pneumatic drive systems. The prevailing practice is to use a specific actuator in each joint, consisting of a gearbox, torque motor, encoder, and motor control. Torque motors are highly pole, electric servo motors with high torque and relatively low speeds. Below are the system requirements that are crucial for motor selection in a humanoid robot.
The torque density of an electric motor is a measure of how much torque the motor can generate per unit volume or weight. It is a crucial factor for the performance and compactness of a motor, particularly important for applications where weight is critical—such as in robotics. The total weight of a humanoid robot is significantly determined by the weight of its joints. The lighter the motors in the joints, the lower the total weight. This, in turn, results in a longer battery life, which allows for a higher payload and thus an overall higher dynamics of the robot.
TQ motors are particularly torque-dense compared to other motors, meaning they can achieve double the torque at the same size or the same torque at half the size. TQ achieves this through a unique winding technology that maximizes the copper fill factor compared to conventionally wound electric motors. TQ is currently the only motor developer on the market that fully exploits the physical possibilities thanks to certain manufacturing steps: it is not possible to fit more copper in the respective motor size. Thus, TQ motors have the highest copper fill factor on the market.
The compact design of the drive enables efficient space utilization and weight reduction, lowers the center of gravity of the robot, and improves stability. Robots with a compact design navigate tight spaces and maintain balance when maneuvering through crowds. An additional advantage of the motors is their large hollow shaft, which is very important for routing cable harnesses internally and achieving a clean robot design.
For the robot to perform controlled, smooth, and versatile movements, the precision of the motor is extremely important. The more precise the drive, the more direct the connection between the robot's movement and the "visual process," consisting of sensor and camera technology. Each joint point is defined by three-dimensional vectors. For the highest possible precision, it is very important that the motors, especially in the sum of several joint points, always assume the "correct" position. Otherwise, even minimal deviations in individual joints, such as the hip, knee, and ankle, may accumulate into significant misalignments. The precision of an electric motor increases with the so-called number of pole pairs, which is a crucial factor for the motor's behavior, as it has a direct impact on control, positioning, and regulation of the motor.
In designing their frameless servo motors, the TQ Group places special emphasis on achieving a high number of pole pairs. TQ electric motors offer very precise position and torque control over a wide range of conditions, from low speed and high torque to high speed and low torque. This is crucial for the natural, fluid movements expected from humanoid robots.
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The motor must be able to react or be controlled accordingly to any change in the environment (e.g., a robot steps into an unforeseen hole or the ground gives way). For a humanoid robot to respond flexibly and quickly to changes in the environment like a human, excellent dynamic control and drivability as well as a fast response time are crucial.
How does an electric actuator achieve this high reaction time and dynamics? To react in real-time, very high torques are needed for a short period to, for example, quickly anticipate and adjust the foot movement to the hole in the ground. This torque is then multiplied for a brief moment. This is referred to as the overload capacity of a servo motor and as peak torque, meaning the maximum torque a motor can generate in the short term. The peak torque of TQ motors is approximately three times higher than their nominal torque, or the constant value that can be reached in the long term. Specifically, TQ motors achieve peak torque in the double-digit Newton meter range (Nm) and thus have a market-leading overload capability.
Efficiency, that is, how much power is lost over the battery life, determines how long a battery-operated humanoid robot can work. The principle is: the highest efficiency, due to low copper losses, extends battery life. Copper losses describe the losses caused by the electrical resistance of the windings in a motor. They are released in the form of heat and are one of the main sources of loss in electrical machines. Motors with high power loss consume a lot of electricity, which shortens battery life and thus limits working time.
This means that the efficiency factor plays a much more important role in a mobile, humanoid robot than in a collaborative robot (Cobot) that is connected to the power grid. For the practical use of humanoid robots, high efficiency is crucial for their continuous operation. The TQ torque motors have an efficiency of 90% or higher and achieve particularly low copper losses in watts.
The robustness and reliability of the motors are also important factors for applications in humanoid robotics. Especially during the testing phase, falls occur frequently. A robust, maintenance-free design ensures that the joints remain intact and functional despite the learning curve. Drives face the greatest challenges in space. Even with temperature fluctuations from -40°F to 257°F, they must function reliably here. On the ISS (International Space Station), an ILM motor from TQ was used, among other things, in the arm of the robot Rokviss and performed precise tasks in zero gravity—over five years and hundreds of tests, always reliably and with the highest performance.
In industrial application scenarios as well, such as in a production or warehouse hall, it can happen that a humanoid robot, which has to lift heavy loads, drops objects or is subjected to sudden impacts. The robustness of the robot's joints protects it from damage and ensures continuous and reliable operation even under demanding conditions.
Robot manufacturers also face the question of whether to choose a motor manufacturer that supplies standard components or to opt for a custom solution. At TQ, customers receive a selection of standard sizes that can be tailored to the specific application if desired. The advantage of TQ's frameless servo kits is that they are modifiable in diameter and their so-called stack length, allowing their performance and dimensions to be tailored to the specific application. Especially for quantities of more than a few hundred units per year, a custom solution can be the best option. It allows for complete control over design, materials, and the manufacturing process.
The saying "Hardware is hard" aptly applies to the development and production of robot joints and the necessary components. This is because the core competence of some humanoid robotics manufacturers is more likely to be found in the areas of artificial intelligence and software, while experience in the industrialization of robots and development of mechatronics is sometimes lacking. Additionally, the development and production of such a complex system, like a robot joint, are often underestimated. TQ is one of the few providers in Germany that can industrialize complete robots and has already proven it.
Against this background, it can make sense to rely on the technology leaders in their respective fields. TQ often develops entire motor-gear units for mobile applications. This is a decisive advantage that helps save valuable time in the current race for the first mass-produced humanoid robot.
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