Page 26 - Rapid Learning in Robotics

12 The Robotics Laboratory

carry the required payload of about 3 kg and which can be turned into an
open, real-time robot, was found with a Puma 560 Mark II robot. It is prob-
ably “the” classical industrial robots with six revolute joints. Its geome-
1
try and kinematics is subject of standard robotics textbooks (Paul 1981;
Fu, Gonzalez, and Lee 1987). It can be characterized as a medium fast
(0.5 m/s straight line), very reliable, robust “work horse” for medium pay
loads. The action radius is comparable to the human arm, but the arm is
stronger and heavier (radius 0.9 m; 63 kg arm weight). The Puma MarkII
controller comprises the power supply and the servo electronics for the
six DC motors. They are controlled by six parallel microprocessors and
coordinated by a DEC LSI-11 as central controller. Each joint micropro-
cessor (Rockwell 6503) implements a digital PD controller, correcting the
commanded joint position periodically. The decoupled joint position control
operates with 1 kHz and originally receives command updates (setpoints)
every 28 ms by the LSI-11.
In the standard application the Puma is programmed in the interpreted
language VAL II, which is considered a flexible programming language by
industrial standards. But running on the main controller (LSI-11 proces-
sor), it is not capable of handling high bandwidth sensory input itself (e.g.,
from a video camera) and furthermore, it does not support flexible control
by an auxiliary computer. To achieve a tight real-time control directly by
a Unix workstation, we installed the software package RCI/RCCL (Hay-
ward and Paul 1986; Lloyd 1988; Lloyd and Parker 1990; Lloyd and Hay-
ward 1992).
The acronym RCI/RCCL stands for Real-time Control Interface and Robot
Control C Library. The package provides besides the reprogramming of the
robot controller a library of commands for issuing high-level motion com-
mands in the C programming language. Furthermore, we patched the Sun
operating system OS 4.1 to sufficient real-time capabilities for serving a re-
liable control process up to about 200 Hz. Unix is a multitasking operating
system, sequencing several processes in short time slices. Initially, Unix
was not designed for real-time control, therefore it provides a regular pro-
cess only with timing control on a coarse time scale. But real-time process-
ing requires, that the system reliably responds within a certain time frame.
RCI succeeded here by anchoring the synchronous trajectory control task

1 Designed by Joe Engelberger, the founder of Unimation, sometimes called the father
of robotics. Unimation was later sold to Westinghouse Inc., AEG and last to Stäubli.