Xavier In this image, RoboCrane is the structure made up of three inverted triangles, joined at the upper vertices to create a fourth triangle. Six winches, two at each foot, control cables that determine the position of the platform. Two of the winches can be seen as the yellow cilinders in the lower center of the image, at the base of one triangle. The platform is the triangular structure "hovering" within the larger structure of the RoboCrane. The blue cabinet, marked NIST, houses the control electronics for the system. On the far left of the image, one one leg of the RoboCrane, is a red box which contains the emergency stop, or "kill" switch. Two more of these are not easily visible in the image. The red robot at the bottom of the image is Bullwinkle.

RoboCrane is a 20-foot high inverted Stewart platform built by the National Institute of Standards and Technology (NIST). Six cables stretched between a large triangular platform and six winch motors enable the platform to move freely with six degrees of freedom in a roughly ten foot cubed workspace. The winches are equipped with rotational encoders, which supply data to the control cabinet. The encoders are the crane's only sensors. The crane can handle payloads in excess of 855 kilograms, moving at speeds up to 3 centimeters per second.

Low level control loops for the crane winches are handled by an array of PIC-SERVO boards and one PIC-IO board, built by J. R. Kerr. Each winch is controlled by a PIC-SERVO, while the PIC-IO manages the RS-232 communications with an outside computer. The boards receive commands from that computer, as well as an encoder feedback signal. The boards provide full PID control of position or velocity, with built-in trapezoidal velocity profiling when in position mode. Their maximum servo rate is 1953.12 Hz, while their maximum command rate to the joint actuators is 1000 Hz. The actual command rate from the software controller to the PICs is in the neighborhood of 20 Hz.

Currently, RoboCrane is working at CMU's Field Robotics Center with the robots Xavier and Bullwinkle on a project for NASA's Space Solar Power Technology Advanced Research and Technology program on the TRESTLE project.

Greg Armstrong
Last modified: Thu Feb 17 14:22:04 EST 2005