STRANGER Robot alu metallic

Proposal Muscle Design
Natural rubber (NR) with high energy storage density. Tensile strength 0.5–1.2 N/mm²–comparable to that of human muscles. Natural rubber is inexpensive, lightweight, durable, and biodegradable.
Muscles made of natural rubber (NR) can withstand 100 to 200 million load cycles
Rubber muscles can be designed in various ways: with directly tensioned muscle fibers, or with twisted rubber strings. The second option is particularly suitable when one wants to achieve isometric muscle work, i.e., when one wants to regulate the tensile stress—and thus the force—while keeping the length constant.

Implanted Automatic Force Sensitivity
Tensioned rubber cords are excellent reversible energy storage devices. They function not only as spring and damping elements, but also as force sensors. Any change in the applied forces, whether internal or external, is sensed by the material and triggers reflexive responses. This essentially eliminates the need for expensive force sensors. The stretch of the rubber cord, which corresponds to a specific force, could then be measured using low-cost sensors. The entire robot body essentially functions like a multi-channel force sensor—even without any electronics.

Advantages: Active (motor-driven) joints are now technically mature and applicable. As developments since 2023 show, motor joints with harmonic drive or planetary gears, rotary and linear actuators, sensors, software and AI are capable of mastering the complex dynamics of a multi-joint kinetic system. 
Among the most fascinating aspects are the high power-to-weight ratio, the fully integrated modular design, and the ability to incorporate spring-like properties—that is, the elasticity and compliance of the robot joints—into the motor-gear combinations using only software and sensors. In this way, the stiffness of the joints can be continuously adjusted within milliseconds from soft and compliant to hard and rigid.

Disadvantages: Active motor-driven joints are the main cost factor and worsen the power-to-weight ratio, primarily due to unfavorable lever ratios. They require custom made motors with high torque density, reduction gears with high gear ratios combined with low reflected inertia (a nearly impossible trade-off), zero backlash, and sophisticated torque and position sensor systems. And they force us to develop complex software control systems. The computational demands are enormous—without onboard supercomputers and AI, it would be impossible to control the complex dynamics of a bipedal robot composed of active, motor-driven joints.

Advantages: Biological systems have passive joints that are moved by muscle-tendon actuators. They are not controlled by software, but apparently function according to much simpler principles: an elastically pretensioned skeleton enables passive dynamics and elastic behavior that embodies a morphological intelligence. This results in natural, reflexive behavior coordinated by muscles whose activities are amplified or inhibited by neural networks. The entire system is based solely on forces. Such design principles would largely eliminate the need for expensive force sensors—as well as most of the computational effort required by robots with motor-driven joints.

Passive joints are simple and non-expensive. They can be combined with muscle-tendon actuators made of rubber cords, whose tension is regulated by standard electric motors. The motors can be positioned at the most optimal locations. If useful, muscle-tendon actuators could also be supplemented by direct-drive torque motors (this design option can be implemented in the pelvic module of the STRANGER robot, which has space for two torque motors).

Disadvantages
Real artificial muscles, such as dielectric elastomer actuators (DEAs) are still in the early stages of development. Muscle-tendon systems made of natural rubber or PA-strings (fishing line) are used in some experimental projects and robotic arms, but not yet in fully functional life-size robots, as far as is known.

Furthermore, their elastic behavior is subject to complex nonlinear dynamics that are essentially impossible to simulate and control algorithmically.

Physical Limits
Despite all the extraordinary progress that has been made made in recent years (there are now at least 50 major companies worldwide that aim to manufacture humanoid robots), motor joints are not actually the optimal solution for bipedal robots. So far, this has been the main approach, though it is sometimes combined with muscle-tendon actuators, particularly for the hands. However, the design of motor-driven joint systems is already reaching its physical limits.

As a result, two main categories have emerged among humanoid robots: lightweight, fast, and more affordable robots, and heavier, slower models that, while capable of handling heavier loads, are physically unable to walk in a natural and graceful manner due to their sheer strength. The higher gear ratios in their transmissions make them practically “blind” to reaction forces below a certain threshold (keyword: reflected inertia), which is why they are only allowed to take small steps. Even a pebble could cause them to stumble.

Service Life of Motor-Gear Combinations
Another problem is the durability and cycle life of the motor-gear combinations. Just imagine the reaction forces acting on the robot with every step. Depending on the state of motion, these forces can reach several times the robot’s body weight. These forces must be able to act passively and reverse the joint motors; otherwise, they will immediately destroy the gears. Or the robot loses feedback with its environment because it doesn’t sense the reaction forces acting on it at all. This is also a security issue.

Force Sensitivity: Actio = Reactio
To walk effectively and perform physical work, a bionic robot requires morphological intelligence: a body that senses all forces (acceleration and gravity) and reacts reflexively to changes in real time. All of this is based on the fundamental principle of mechanics: Actio = Reactio. This principle can and must be integrated into the robot’s mechanical structure from the very beginning—with the highest possible force sensitivity in both directions, based exclusively on elastic pretension.

Toward an Optimal Biomechanical Design
The goal is to understand the holistic nature of such an elastic biomechanical system and to optimize its physical behavior through structural measures in such a way that both energy consumption and control effort are minimized.

Cleverly designed muscle-tendon actuators would be lighter, more robust (100 to 200 million load cycles), more energy-efficient, and fully elastic. From a purely physical standpoint, they would be superior to motor-driven joints and at least ten times more cost-effective.

Even though many of the newly emerging humanoid robot start-ups have high hopes, the functionality, cost-effectiveness, safety, and autonomy of humanoid robots have yet to be tested—a process that, as with the full autonomy of self-driving electric cars, could take 5 to 10 years. So we still have some time; nothing is certain yet.

We may well find that it is far more cost-effective to pay a worker—who can easily withstand several billion load cycles over the course of his working life—60,000 dollars a year than to employ a robot, which is cumbersome compared to a human, costs 60,000 dollars or more per year in maintenance and depreciation, and is capable of doing much less. And who certainly won’t be buying the products they manufacture. This is where macroeconomics comes into play. So we still need to figure out which applications are truly useful and cost-effective.

With the STRANGER ROBOT project, we are essentially working on technologies that foreshadow artificial muscles. That is why we should strive to a better undestand the biomechanics of elastically pretensioned robots and, above all, to implement them experimentally, since physical simulations alone are not sufficient for this purpose–and, due to the nonlinear behavior of rubber muscles, are also hardly practical.

This includes the effects of muscle loops spanning multiple joints, the optimal configuration and placement of the muscle-tendon actuators, and an anatomical or structural design of the knee and hip joints that ensures the robot consumes virtually no energy while standing, without losing its passive responsiveness.

Artificial muscles and applications for humanoid robots are still in the development stage, in part because, until now, there has been no cost-effective skeletal structure (i.e., standard platform) available for experiments that could be equipped relatively quickly with artificial muscles and thus with morphological intelligence–a need that can now be met by the STRANGER ROBOT R&D CHASSIS.

An assessment of wether the required specifications can be achieved in the most extreme load case–standing up from a squatting position against the body weight of STRANGER (assuming 450 N = 45 kg)–using a rubber-muscle-tendon system driven by ordinary e-motors:

– What pretensions are generated in the rubber cables passively by lowering into a squat?
– How work the muscle groups in the lower leg and thigh together with the hip and back muscles?
– What contribution makes the rotation of the pelvis around the hip joints, caused by activated spline muscles and hip drives, the torso's inclination, and the position of the arms to shorten the the load-lever arms?
– How much tension force must be generated in the thigh and calf muscles, the hip joint drives, and the spline muscles, in addition to the pre-tension, for a brief moment to initiate the rise from the squat?
– What is the required torque of the electric motor, and what is its mass?
– What is the optimal muscle-tendon-motor design configuration?

In bio-inspired humanoid robot design, there is still plenty to explore, invent, and improve.

The humanoid robot STRANGER is characterized by a unique plate-skeleton-design with fractal complexity that follows simple geometric rules. This design is particularly well-suited for rapid laser-cutting prototyping during development and testing phases, as well as for manufacturer kits, and perhaps even affordable DIY kits.

The design strategy is also unique: While the size, ergonomics, kinematics, and range of motion of a humanoid robot are, in principle, invariant design targets (these parameters are ultimately intended to correspond to those of the average adult), the drive and control principles are subject to rapid scientific and technological progress. For this reason, the design strategy was divided into two main components: The chassis, with at least 57 degrees of freedom (DoF), represents the ergonomic and kinematic invariants and serves as a universal biomechanical platform in the R&D and prototyping process. The nature of the drive and control concepts represents the variables that can be integrated according to the state of the art.

The kinematic chassis with passive joints serves as an R&D design template and, once built as a prototype, as an assembly platform into which a variety of actuators and components can be integrated, particularly artificial muscles. It is also possible to implement active motor joints using proprietary actuator technology and to adapt the chassis design accordingly.

The STRANGER ROBOT chassis saves time and money in product development and testing processes. The savings start right from the 3D CAD model: Designing a feasible, life-size robot chassis measuring 1.8 meters typically requires several thousand hours of design work. On top of that, there are the real-world tests of the prototypes (the STRANGER chassis in its current form—with a closed chassis—has already been built and sold more than 100 times). The 3D model of the STRANGER ROBOT chassis, modeled in Fusion with masses, inertias, and joints, can be adapted by the licensee to proprietary drive concepts and internal components and used with physics engines such as MuJoCo (open source) or Isaac Sim (NVIDIA, free). The conversion of XML structures to MuJoCo is done via the URDF format, 99% of which can be handled by AI. Isaac Sim offers native URDF/USD support.
The STRANGER ROBOT chassis can be manufactured with high precision on high-speed laser cutting machines in just a few hours. This avoids costly tooling and prototyping expenses from the outset and enables rapid modifications. Rapid laser prototyping is 100 times faster & cheaper than 3D printing. Birch plywood offers excellent lightweight material properties and cost-effectiveness. The modulus of elasticity of the 3-ply birch plywood used is remarkable (10,000 N/mm² along the grain, aluminum 70,000 N/mm²), while its volume-weight is seven times lower than that of aluminum (birch plywood 0.38 / alu 2.7).
The STRANGER ROBOT chassis forms the supporting structure. The plate-skeleton-design provides ample installation space and protection for drives and control modules. Frames, reinforcements, feed-through openings, mounting points, and the assembly process are customized to the customer’s specific components, while maintaining the external geometry and design.
The STRANGER ROBOT chassis serves as a universal R&D template and biomechanical standard platform for humanoid robots, robot arms and 5-finger-hands. Once the custom licensee design has been tested in the physics engine and all CAD modifications have been completed, production of the prototype can begin. Even after the research, development, and prototyping phases, the plywood chassis can be quickly and flexibly adapted to various current and future drive systems as well as to any necessary geometric changes. Once completed and tested, the STRANGER ROBOT chassis can serve as a cost-effective manufacturer’s kit, particularly for on-demand production and small-batch manufacturing.
The design of the STRANGER ROBOT is timeless, not subject to fashion trends, and is open—in the truest sense of the word—to any technology. The type of actuators represents a variable in this design concept. Possible options include motor joints and linear roller actuators with elastic properties (active joints), or muscle-tendon drives designed according to bionic principles (passive joints), or a combination of both. The artificial muscles could, for example, consist of electroactive polymers, or simply of rubber strings tensioned by electric motors.
The STRANGER ROBOT chassis can be equipped with clothing and protective padding. For mass production, the chassis could be manufactured using light-alloy castings or other suitable materials and technologies, e.g., laser- or waterjet-cut aluminum frames, spot-welded. And, of course, the skeleton design can also be covered with paintable, impact- and shatter-resistant body parts (e.g., made of PUR-RIM, such as automotive bumpers)
The STRANGER ROBOT chassis design can be used both as a universal humanoid robot template, biomechanical R&D platform, and as a manufacturer’s kit. This could be of particular interest to robotics companies and suppliers, as well as startups—and, with AI support, even to talented makers. With the STRANGER ROBOT design platform, you can
- accelerate the development and testing of your drive technology in the physics engine
- significantly accelerate the implementation of proprietary joint actuators and/or artificial muscles and control principles
- produce a low-cost DIY robot
- develop affordable personal robots to tap into the consumer market
- develop high-performance sports robots, and much more

STRANGER Robot alu metallic

Ergonomics and Range of Motion

Design Options

Passive Joints & Artificial Muscles

Active Joints vs Passive Joints

Problems: Cost, Durability, Safety

Conclusion

Coming Next: Proof of Concept

The grace of walking is an expression of the underlying physics

Seven Reasons to get a STRANGER ROBOT Design License