Schaeffer Magnetics Designs Actuators for Space Robot

SCHAEFFER MAGNETICS DESIGNS ACTUATORS FOR SPACE ROBOT WITH ALGOR

Companies that design and manufacture components for use in space must be on the cutting edge of technology and supply parts which meet the highest possible quality standards. Schaeffer Magnetics, Inc. is accustomed to meeting these requirements. After 24 years, the Chatsworth, California based company is widely regarded as the world's most experienced source for spaceflight electric motors, actuation components and systems.

Currently, the company is using Algor Finite Element Analysis (FEA) software in the development of joint actuators for the Flight Telerobotic Servicer (FTS). The FTS is NASA's first operational robotic system and will be used in the assembly, maintenance, service and inspection of spacecraft. Current plans call for a developmental test flight on the space shuttle in late 1993 and a demonstration flight soon after. The FTS will be operational in the late 1990s and may be utilized in the construction of Space Station Freedom.

The FTS has two six-foot long manipulator "arms" and a single stabilizing "leg" which are driven by 19 Schaeffer Magnetics actuators. According to Stefan B. Delin, Ph.D., Senior Analytical Engineer, "The performance of the FTS during its twenty-year operational life is primarily dependent upon the structural characteristics of the manipulator arms. In addition, the required high levels of accuracy and positional repeatability can only be achieved through minimization of joint and link compliances. For that reason, we have developed more than 40 Algor finite element models to study the elastic behavior of the joint actuators."

Drawing shows location of the Shoulder Roll and Yaw Actuator on the Flight Telerobotic Servicer (FTS) Manipulator Arm subassembly.

Modeling the Actuator

To construct the Shoulder Roll Manipulator Actuator model, Dr. Delin glued together individual finite element models of the actuator's three primary structural components, the sleeve, housing and frame. The basic geometry for the models was imported directly from 2-D AutoCad drawings, eliminating the need to recreate them and, therefore, the chance of errors.

Adding Loads and Forces

The completed model was grounded to simulate the interface between the housing and the FTS body. Beam elements were used to represent two bearings that are located where the housing and the actuator's structure connect. All loading forces and moments were applied to the model through a single node at the mass center of the actuator. The finished model contains 958 brick, 368 plate and 42 beam elements. It has 2287 nodes and required 7857 equations.

The model of the Flight Telerobotic Servicer Actuator contains 1368 elements, 2287 nodes and required 7857 equations. Both Al Alloy and Ti Alloy materials were used.

Analysis

"Our design analysis process focused on the extensive requirements for the actuator's stiffness, specified stress levels, and fracture mechanics safe life," says Dr. Delin. "Once the design met these requirements, the analyses were repeated with the goal of weight reduction. FEA was the only design analysis method that could provide sufficient accuracy and a thorough understanding of the actuator's structural behavior while we were still in the design stage of the project."

A large number of analyses were performed on the actuator model. According to Dr. Delin, "First, a static analysis was performed. The model was loaded with nine unit loads and the deflected shapes were used for stiffness analysis. The stress levels were thoroughly analyzed with 72 combinations of operational and nonoperational loads. This was accomplished with Algor's COMBLC postprocessor program. These loads reflect all major events during the life of the system from launch, to in-orbit operations, to landing."

"The most critical part of the process is the final stage of fracture mechanics safe life analysis," says Dr. Delin. "This analysis uses the part's geometry and stress values as inputs, therefore highly accurate results are crucial. The results of the Algor stress analysis were used for a consecutive, comprehensive fracture mechanics analysis using the NASA FLAGRO program."

Two computer systems were used for the analysis. The model was created and analyzed with the Hyper version of Algor's FEA System on an IBM compatible, 486 25MHz microcomputer with 12 Mb of RAM and a 120 Mb hard drive. Additional analyses were performed on a DEC MicroVAX 3100. Results were transferred with the Kermit protocol.

Ruben Nalbandian, M. Sc., Schaeffer Magnetics Director of Engineering (behind desk at right) with Dr. Stefan Delin, Senior Analytical Engineer.

Results

"Based on the stress analysis results, the design was changed and optimized with a significant weight reduction," says Dr. Delin. "Two additional runs were performed after which the final design configuration was determined."

About Algor

About the Algor FEA System, Dr. Delin says, "Algor has excellent pre- and postprocessors, powerful graphics and accuracy at a price that has no match on the market. The system offers extensive capabilities for sophisticated analyses that only a few years ago were done with mainframes at much higher cost.

"The professional customer support is a major reason for the system's success and the upgrade policy guarantees that our software will stay on the cutting edge of technology." Dr. Delin is now involved in thermal and dynamic analyses and the construction of additional models.