In order to perform human-like movement, an actuator is placed at each degree of freedom (DOF) in a humanoid robot. Additionally, these actuators must be packaged in an arrangement that approximates human structure and appearance. In this innovation, a rotary actuator assembly incorporates a brushless DC motor, a gear reduction, a variety of sensors, and a custom planar torsion spring to provide motive force, passive compliance, and torque sensing within an anthropomorphic package. The actuator, in various size scales, was designed for the humanoid robot described in “Dexterous Humanoid Robot,” (MSC-24739), NASA Tech Briefs, Vol. 38, No. 6 (June 2014), p. 52.

The frameless, brushless DC motor and harmonic drive gear train components used in each actuator are custom housed within a support structure, and a planar torsion spring is connected between the gear train output and the joint output. Two high-resolution absolute position sensors measure the spring input and output positions. Thus, a joint torque measurement can be taken using this spring deflection measurement and knowledge of the joint’s spring rate. Measuring torque in this way can be more robust than traditional strain- gauge-based methods and it allows the robot’s control system to take advantage of the naturally compliant, low-impedance behavior of the passive springs.

An upper arm assembly for the humanoid robot includes a chain of series elastic actuators to mimic the joints in the human arm. With the addition of a forearm assembly, the humanoid robot has full 7-degree-of-freedom arms with which to perform work. The series elastic actuator design and the embedded control system that takes advantage of each joint’s sensing capabilities enables the robot’s arms to perform position-, force-, or impedance-based tasks. This gives the robot the ability to perform human-like work in human environments while being sensitive to the interaction forces it experiences in its environment. Thus, the robot is comfortable to work with and around, and it has great potential as a human assistant in a variety of situations.

This work was done by Joshua S. Mehling, Brian Kristian Griffith, Nicolaus A. Radford, Robert O. Ambrose, and Lucien Junkin of Johnson Space Center; Chris A. Ihrke and Donald R. Davis of General Motors Corp.; and Adam H. Parsons and Frank Nobel Permenter of Oceaneering Space Systems. For further information, contact the JSC Technology Transfer Office at (281) 483-3809.

Title to this invention has been waived under the provisions of the National Aeronautics and Space Act {42 U.S.C. 2457(f)} to General Motors Corporation. Inquiries concerning licenses for its commercial development should be addressed to:

General Motors Research & Development Center
30500 Mound Road
Warren, MI 48090-9055
Phone No.: (313) 556-5000

Refer to MSC-24736-1.


Motion Control & Automation Technology Magazine

This article first appeared in the August, 2014 issue of Motion Control & Automation Technology Magazine.

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