Hypocycloid Torsional Spring

Tech ID: 30374 / UC Case 2018-677-0


UCLA Researchers in the Department of Mechanical and Aerospace Engineering have developed a spring device capable of outperforming the current gold standard of actuators seen in humanoid robots.


The need for automation of physical tasks has led to robot designs that resemble humans, referred to as humanoids. These humanoid machines are still unable to interact with their surrounding environments as humans do. An underlying limitation to many humanoid designs is the inability to vary stiffness in limbs. Humans possess this ability, which allows for swift torque transfer when muscle fibers stiffen, while retaining mechanical compliance when muscle fibers flex. To solve this fundamental shortcoming in humanoids, many research groups have surveyed the construction of new, compliant actuators that can adjust their stiffness based on the task that robots are given. However, since such actuators introduce a significant amount of weight and complexity due to an additional actuator that is designed to change their stiffness, practical implementation in humanoid robots remains difficult. Instead, researchers have investigated a way to optimize the single stiffness setting to maximize the compliance while minimizing the bandwidth reduction. However, the single stiffness setting faces numerous problems related to a tradeoff between compliance and bandwidth performance. Therefore, there exists a current need to develop novel actuators capable of resolving the aforementioned issue and the ability to maintain the weight of the hardware materials used.


UCLA researchers in the Department of Mechanical and Aerospace Engineering have developed a novel torsional spring for the next generation of compliant actuators for humanoids. Their design is based on the conventional hypocycloid mechanism with notable modifications. These modifications center around a series of gears that act in an antagonistic effect, mirroring the mechanical motions of flexors and extensors in human muscles. In such configuration, the torsional spring exhibits a seemingly paradoxical relationship (i.e. stiff and compliant). In other words, the newly developed torsional spring can transmit torque instantly through rigid bodies while yielding a significantly large angular displacement thanks to internally moving parts. Further,the use of compression springs with a moment arm mechanism allows for the minimization of size and weight, making the hypocycloid torsional spring the most compact torsional spring to date.


  • Applications where high performance bandwidth and large angular displacement are desired simultaneously.  
  • Applications where a more compact form torsional spring is required.
  • The use of this actuator design in antagonistic configuration may be used in humanoid robots to simulate muscle contraction and allow for interaction with rapidly changing environments.


  • Exhibits a high performance bandwidth (as seen in a stiff spring) while maintaining large deflection angles (as seen in a compliant spring). 
  • Can be configured for a wide range of torque and angular displacement ranges.
  • Compact in size compared to conventional torsional springs, allowing for ease of integration into machinery including humanoids.  
  • The actuator with the hypocycloid torsional spring design overcomes the tradeoff between compliance and bandwidth performance that has been considered unavoidable in conventional series elastic actuators.

State Of Development

The actuator model has been simulated for instant torque transfer, shock absorption and energy storage capabilities. A prototype actuator with the hypocyloid torsional spring has been built and is currently in the testing phase.

Related Materials

  • ASME 2018 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference. “Design of a Novel Torsional Spring for the Next Generation Compliant Actuators.” Volume 5B: 42nd Mechanisms and Robotics Conference. Quebec City, Quebec, Canada, August 26–29, 2018


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  • Hong, Dennis W.
  • Yoon, Jeong H.

Other Information


Hypocycloid Actuator, Humanoid Design, Muscle Stress Simulator, Robotics

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