Future robotic exploration of near- Earth asteroids and the vertical and inverted rock walls of lava caves and cliff faces on Mars and other planetary bodies would require a method of gripping their rocky surfaces to allow mobility without gravitational assistance. In order to successfully navigate this terrain and drill for samples, the grippers must be able to produce anchoring forces in excess of 100 N. Additionally, the grippers must be able to support the inertial forces of a moving robot, as well gravitational forces for demonstrations on Earth. One possible solution would be to use microspine arrays to anchor to rock surfaces and provide the necessary load-bearing abilities for robotic exploration of asteroids.

The Ankle Mechanism (left), and the interior of the ankle (right), showing the gimbal systems, springs, and actuation device for the engagement wires.

Microspine arrays comprise dozens of small steel hooks supported on individual suspensions. When these arrays are dragged along a rock surface, the steel hooks engage with asperities and holes on the surface. The suspensions allow for individual hooks to engage with asperities while the remaining hooks continue to drag along the surface. This ensures that the maximum possible number of hooks engage with the surface, thereby increasing the load-bearing abilities of the gripper. Using the microspine array grippers described above as the end-effectors of a robot would allow it to traverse terrain previously unreachable by traditional wheeled robots. Furthermore, microspine-gripping robots that can perch on cliffs or rocky walls could enable a new class of persistent surveillance devices for military applications.

In order to interface these microspine grippers with a legged robot, an ankle is needed that can robotically actuate the gripper, as well as allow it to conform to the large-scale irregularities in the rock. The anchor serves three main purposes: deploy and release the anchor, conform to roughness or misalignment with the surface, and cancel out any moments about the anchor that could cause unintentional detachment.

The ankle design contains a rotary DC motor that can drag the microspine arrays across the surface to engage them with asperities, as well as a linear actuator to disengage the hooks from the surface. Additionally, the ankle allows the gripper to rotate freely about all three axes so that when the robot takes a step, the gripper may optimally orient itself with respect to the wall or ground. Finally, the ankle contains some minimal elasticity, so that between steps, the gripper returns to a default position that is roughly parallel to the wall.

In order to give the ankle freedom to rotate about all three degrees of freedom, the gripper is mounted on a series of gimbals similar to those found on a gyroscope. The rotation of the gimbals about radial directions is limited by springs, which bring the gripper back to a default position in between steps of the robot. These springs have a relatively low spring-constant so as not to induce large torques that may upset the gripper’s hold on the rock. Additionally, microspine engagement is achieved through a motor that turns a spool and pulls on cables connected to the spine arrays. A linear actuator that pulls the microspines up and away from the rock face provides disengagement. Previous microspine robots have been limited by their feet, which only sustain forces in one direction and only work on globally smooth surfaces like brick walls and concrete. The omnidirectional anchors extend the potential of legged robots using microspines to natural rock, and would allow gripping at any orientation including inverted or in zero gravity.

This work was done by Aaron Parness, Matthew A. Frost, and Nitish Thatte of Caltech for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management
Mail Stop 321-123
4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.


This Brief includes a Technical Support Package (TSP).
Robotic Ankle for Omnidirectional Rock Anchors

(reference NPO-48315) is currently available for download from the TSP library.

Don't have an account? Sign up here.