Technology in Europe (August 2005)

Good business in Europe
Radstone Embedded Computing (UK) has reported preliminary results for the year ended March 31, 2005. Turnover was up 14 percent, group profit before tax increased 15 percent. The order book indicated a nine percent higher value than a year ago. Radstone had invested heavily into new facilities last year. A number of new products have been announced recently. For more information, visit www.radstone.com.

VMEbus technology space application
DLR (Germany), the German Aerospace Research Center e.V. has an Institute of Robotics and Mechatronics among its many institutes. The Center does robotic research, and the first robot built for the D2 Mission in 1993 was hosted onboard an American spacecraft but was very heavy. It was the first fully functional robot arm in space and was controllable from a ground station. It successfully completed some extremely complex tasks which have not been repeated since then. Approximately one hundred experiments were completed during the one-week flight of the spacecraft. VMEbus technology was used then, and is being used again in the latest and most advanced robot, called LWR III.

In one experiment, the initial ROTEX robot arm could catch a free flying golf ball in a zero gravity environment inside the spacecraft. Apart from this already complex task, it was further complicated by the fact that the required computer equipment in those days (1993) was too heavy to be carried onboard the spacecraft together with the astronauts. Therefore, several computers from DEC, Silicon Graphics, and a collection of Personal Computers (PCs) were located in the German Control Center in Germany to facilitate calculations in a special type of “real time.” The robot arm was equipped with different types of laser sensors in the wrist and in each of the four fingers of the robot arm. Additional pressure and LED sensors were located in the gripper for more precise touch/feel and position detection. Two cameras provided position information to the control computers.

All the data from the sensors and cameras were downlinked to the Control Center in Houston, Texas and forwarded via satellite across the Atlantic Ocean to Germany. When the data about the position of the ball, robot arm, and fingers arrived at the Control Center in Germany, it was several seconds old. The computers calculated the new position (a few seconds ahead) and data was sent to control the necessary movements of arm, hand, and fingers to hopefully catch the free-flying ball. Sometimes it took several iterations of these actions to close the loop.

The robot hand did, in fact, catch the ball. If it would have missed even slightly, the ball would have bounced off a finger or wrist, flying out of reach of the robot arm. The robot arm was constructed with then-available technology and was so heavy it could not move its arm with its own built-in motors when operated on the ground. It could only perform during zero gravity space travel.

There was a VMEbus system (consisting mainly of semiconductor memory) literally in the middle of a pool of computers, which were all performing different parts of the trajectory calculations, position predictions, and navigation. All computers had VMEbus adapter cards from their different native computer bus systems to VMEbus. These VMEbus adapter sets came from Bit 3 Computer Corporation (now part of SBS Technologies, Inc.) and were supplied by Meilhaus Electronic in Germany. Ethernet or any other available switching products could not be used because of the real-time requirements and high-data rates. Essentially this was a huge shared memory pool connecting more than a dozen heterogeneous computers with almost zero overhead. It should also be noted that the VMEbus system in the middle was the only major element that was not duplicated. Everybody trusted in the reliability of the VMEbus technology, which performed as required.

From the description above it is clear why DLR puts so much emphasis on “light weight” in robot arm construction. The current generation LWR III with seven degrees of freedom has all the electronics, motors, gears, torque sensors, and so on inside the arm. Figure 1 depicts an exploded view of the elements of one of the seven robot arm joints. There are no huge motors outside tucked away in a sturdy pedestal; they’re all integrated. This new version can still catch a ball. It even has algorithms to figure in air drag which is just one of many parameters when used here on earth in a “normal” environment.

Many of the control and sensor feedback loops are processed inside the arm. The main control is done using a standard VMEbus system with three 6U cards. Two of these are CPUs, each working on its own or in cooperation, taking advantage of VME’s standard multiprocessing feature. A real-time SErial Real-time COmmunications System (SERCOS) optical fieldbus on a PMC mezzanine controls the arm, and a real-time SpaceWire fieldbus controls the fingers. SpaceWire (IEEE 1355.2 or ECSS-E-50-12) is a low-latency variant of IEEE 1355 for the specific need of space operations. ANSI/VITA 13 Heterogeneous InterConnect (HIC) is the variant for use on a VMEbus backplane with appropriate pin assignments. Different fieldbuses were used because of certain features that were required for different operating conditions.

During this long-term evolution of generations of LWR robots, VMEbus technology was always part of the robotic arm development; there was never a reason to replace it with different technology. The LWR III has an extremely lightweight arm made from structured carbon fiber, giving it a totally smooth surface. The joints are galvanically decoupled. Since it has torque feedback sensors, operators can hold the robotic arm and gently move it into a desired position or along a desired trajectory for training purposes. This saves lots of programming hours in traditional trial-and-error programming. All joints are such that there is no danger of operators’ fingers or hands getting squeezed between two arm sections.

Algorithms have been developed to hold an egg with a gentle grip and hard objects with a strong grip as shown in Figure 2. The fingers can also loosen the grip to a point just before the object would fall from its own weight. Figure 3 shows the robot hand serving a cup showing the Bavarian flag white-and-blue design. These algorithms and control sequences could be done on a single VMEbus CPU, but in an experimental environment are spread over two CPUs for easy testing. Additionally, one CPU can be removed from service while other maintains the arm’s operation. For more information, visit www.robotic.dlr.de/.

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Hermann Strass is an analyst and consultant for new technologies, including industrial automation, computer bus architectures, mass storage technologies, and industrial networking. He is an active member of several national and international standardization committees, and is the VITA Europe Technical Coordinator for European activities.

For further information, e-mail Hermann at: [email protected].