Actuation Precision Control of SMA Actuators Used for Shape Control of Inflatable SAR Antenna
- Paper number
IAC-05-C2.4.03
- Author
Dr. Fujun Peng, Canadian Space Agency, Canada
- Coauthor
Dr. Yan-Ru Hu, Canadian Space Agency, Canada
- Coauthor
Dr. Alfred Ng, Canadian Space Agency, Canada
- Year
2005
- Abstract
There has been an increasing interest in the application of inflatable structures in space programs. This kind of structures has unique advantages in achieving low mass and high packaging efficiency. We are currently working on an in-house R\&D project to develop a large surface area to mass ratio inflatable space structure with possible applications as a Synthetic Aperture Radar (SAR) antenna. The key components of this inflatable structure are inflatable tubes, membrane and the links installed in-between stretching the membrane. It can be rolled into a small volume and fixed on a small satellite bus for launching. When it gets into orbit, the inflatable tubes are filled with gas and roll out, and the Kapton membrane will be deployed accordingly. It is expected that the membrane will be subjected to flatness problems during its lifetime in orbit due to the thermal variations in space. A pure passive control method may not be sufficient to maintain the membrane flatness since the inflatable structures will experience different space environment. Therefore, an active control system , which is used to adjust the tensions according to the thermal variations, is currently being studied. The genetic algorithm is applied in this active control system to search for the optimal tensions that minimize the membrane wrinkles. Shape memory alloy (SMA) actuators are installed in series with the links, which realize the tensions according to the instructions from the control system. SMA actuators are attractive due to their unique properties such as high force, long stroke, small size, light weight, and silent operation, etc. Their poor stability and controllability, however, are obsticales for us to use them to exert tensions to the real structure. There is not a stable reliable relationship between its displacement output and the electrical input current, i.e., the actual displacement output is not only affected by the input current, but also by some other factors such as cooling conditions, load conditions. It is not possible, therefore, to achieve the tensions we need by simply input a fixed electrical current to SMA actuators. Some attempts have been made to solve this type of problems by adjusting the heating electrical current flowing in the SMA wire. Several control algorithms, such as PID (Proportional Integral Derivative) control, PWM (Pulse Width Modulation) control, optimal control, have been proposed, and Preisach model and neural networks are developed to describe hysteresis property of SMA actuators. However, it is hard to model the SMA hysteresis precisely, and consequently the stability of the control system is not guaranteed. This may become an obstacle to use SMA actuators in space missions. In this paper, a simple control strategy is proposed, which is based on the idea of adjusting the SMA wire temperature as fast as possible. The control system monitors the actual strain output in real time in a high sampling rate. When the actual strain is less than the desired value, the input current is set a relatively large value to make the SMA wire temperature increases quickly above the transition temperature (but not high enough to burn SMA). When the actual strain output becomes higher than the desired value, the input current is set to zero so that SMA actuator cools down at the fastest speed. Such a control strategy is simple, stable, and requires no hysteresis model or thermal model. Tests are conducted on controlling a SMA wire actuator of 170 mm long and 0.015 mm in diameter. The results indicate that the SMA wire strain output tracks sinusoidal response and ramp response very well. When tracking step response, the overshoot and steady errors are very small. The results also show that higher sampling rate and larger input current are helpful to get better tracking accuracy. Tests are also conducted in thermal vacuum chamber. The results indicate that the required current for effectively activating SMA actuator is smaller, but relatively overshoot is larger. Another phenomenon observed in this case is that SMA cooling speed is too slow due to the fact that radiation is the only way for cooling and air-cooling is not available. This implies that active thermal control is needed for space applications in case multi-cycle actuations are required. This control strategy is then used for active shape control of an inflatable structure, and great improvement is achieved in the membrane shape.
- Abstract document
- Manuscript document
IAC-05-C2.4.03.pdf (🔒 authorized access only).
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