June 14, 2024

Compliant Grappling, using a RAFTI

by Jaclyn Ghareeb

PickNik has developed new capabilities to successfully grapple difficult-to-capture resident space objects (RSOs) such as satellites. This has been achieved by implementing admittance control in MoveIt Pro. With admittance control our robots are able to complete a successful grasp of an object that is in motion or when the initial approach is misaligned. This greatly increases user confidence in performing remote and autonomous tasks.

Admittance control is a force control technique for position and velocity-controlled robots. The aim of admittance control is to modify the system dynamics to act as a stable spring mass damper system about a specified frame such as the end effector. Second order stable dynamics are desirable since they are robust to unmodeled perturbation, such as in complex contact. Admittance computations require force and torque readings at the specified frame which can be achieved with a force torque sensor to comply with external forces. Admittance control provides a greater tolerance in accuracy needed to perform contact tasks.

In the video above, we demonstrate admittance control first with the robot grasping a mock launch adapter ring on a mock satellite. Using MoveIt Pro the system is able to identify the launch adapter ring and plan an approach to grasp the ring at an allowable location. While attempting the grasp, the mock satellite is moving and admittance control enables the robot to comply with the external dynamics and complete the task. Without this capability, the robot would not be able to adjust to the changing location and the grapple would not be a success.

In the second half of the video, the robot is attempting to grasp a RAFTI, a fueling and docking interface from Orbit Fab, which allows spacecraft to be refueled both on earth and in space. The robot’s initial approach is slightly off, but with MoveIt Pro’s admittance control, it is able to adapt to the force feedback and adjust to fully connect with the RAFTI, and complete the grapple.

In Space, the need to be able to rapidly adjust to changing situations is critical. It is important to complete the grapple on the first attempt and eliminate the waste of fuel on a second approach. MoveIt Pro allows robotic arms to adjust to force feedback and complete the grapple even if something in the environment changes. MoveIt Pro increases the reliability of robotic systems and gives robot owners the confidence they need.