Real-Time Workshop 7.4

MathWorks Tools Help Control Spacecraft Vibration

Challenge

Contrary to popular belief, space is not a calm, inert, and empty environment but a highly volatile one. "In space, things are always changing," explains Sharp. "You are in and out of the sun. Spacecraft are subjected to extreme thermal gradients [the temperature of the front and back of the spacecraft can vary by hundreds of degrees Celsius] and are "prone to 'jitter' as a result of on-board disturbances." The effects of these disturbances will be appreciated by "anyone who has shaken a camera while taking a photograph—and ended up with a blurred image." Projects like MACE and DOT have virtually zero tolerance for vibration. As Sharp explains, "to achieve precision optics [on the DOT] we're working with tolerances of 10 nanometers, which is about how far a manhole cover is displaced if you place a dime on it."

The vibration controls on the MACE II and DOT projects needed to adapt to changes in the structural dynamics of the spacecraft during operation. Neglecting these dynamics can lead to instabilities in the control system. It is difficult and time-consuming to model a system like this because it must include so many structural vibration modes. SDL sought an approach to adaptive structural control that would not require such an extensive system model.

"The MathWorks was able to save us a lot of money by saving us time. If we had to write algorithms in C, rather than automatically produce C code using Simulink and Real-Time Workshop, the development of STF would have taken 10 times longer or more."

—Thomas Sharp, Ph.D.
Vice President
Sheet Dynamics Ltd.

Solution

Using MathWorks tools, SDL created a control system based on Spatio-Temporal Filtering (STF) technology. STF is the software brain of the control system. It uses adaptive algorithms and structure-mounted sensors such as strain gauges and accelerometers to track or control individual modes of vibration. The STF-based system controller is robust enough to compensate for sensor or actuator failure and to "learn" about sources of vibration and take corrective action.

SDL used MathWorks tools at every stage in the process: to analyze spacecraft vibration, simulate STF-based control systems, and create adaptive algorithms. They began by using MATLAB to create states-space models directly from NASTRAN finite element models. They used MATLAB for dynamic structural analysis and, before producing a single piece of hardware, used Simulink to simulate the behavior of their vibration control systems.

They found the MathWorks tools flexible and easy to work with: "We ran Simulink on our laptop computer to try out different approaches," recalls Sharp. SDL used data from that "laptop simulation" to design, simulate, and build a mechanical control system for active vibration control on spacecraft.

SDL then used Real-Time Workshop and Simulink to automatically generate the C-language "intelligent" algorithms used in the STF-based system. These algorithms will be used during an experiment on the International Space Station scheduled for December 2000.

Sharp, who used MathWorks tools throughout his undergraduate and doctoral work, notes that they brought several benefits to his team: "We can write software, but our real expertise is in the control and analysis of structural vibration. The MathWorks tools are easy to work with. We saved a lot of time and money using Simulink block diagrams and Real-Time Workshop instead of writing C code. The MathWorks tools are true enabling technologies."

Results

Products Used