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The Forefront of Space Science

Measuring Plane Profile of Space Structures by Grating-Projection Method
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We can qualitatively observe structure profiles after deployment in orbit using images shot with onboard cameras or isolated cameras (Fig. 2 and 3), but we need to understand the phenomena occurring in orbit to attain high-functionality or high-reliability. Quantitative measurement is indispensable and thus an in-situ measurement technique of plane profiles is required. This requirement is critical for structures requiring high-accuracy plane shape, such as large parabola antennas and reflection planes of radio telescopes and converging mirrors. Further, in-orbit plane profile measurement is imperative when controlling surface shape in orbit.

For surface measurement on the ground, widely used methods include stereo observation by multi-camera imaging and/or laser-displacement meter. Using targets on the observed surface, we obtain distance or coordinate information as a point. Dynamic tracking of the position-attaching target is also conducted. One profile-measurement option is scanning by laser-displacement meter, but it takes much more time as the area size increases. To address these issues, we focus on the lattice-projection method, which allows us to conduct measurement in a short time as a single object.

The grating-projection method projects a sine waveform light and dark stripe onto the measured object from a projector, photographs it with a digital camera, and obtains the object profile per pixel using brightness value per object pixel. Our laboratory uses a commercial liquid crystal projector. This method's characteristics are: enables acquisition of plane profile as an aggregate of coordinate per pixel of the shot image; requires few photos; short analysis time from shooting image to obtaining measurement result; simple compact instrumentation allows installation of the measurement system on future satellites; etc. It's a new but existing technique in principle (Fig. 4). It is believed that we can measure the plane profile up to an accuracy of several micrometers in ideal measurement conditions in laboratory.

Figure 4
Figure 4. Measurement example of a plaster figure by the grating-projection method (courtesy of Motoharu Fujigaki, Associate Prof., Wakayama University)

We applied the technique to shape measurement of a ground-test model of a parabola antenna reflector made of metal mesh as shown in Fig. 1. Metal mesh frequently used for surface construction elements on space structures has a large porosity ratio because it is made of a fabric of metal fibers. This also means that it has high transmissivity of visible light. Since mesh generates diffuse reflection by its metal luster, however, it is a poor measurement object for the method of visible light. Nevertheless, it was demonstrated that it is possible to measure the whole surface as a single unit with almost same accuracy as a scanning laser-displacement meter (Fig. 5).

Figure 5
Figure 5. Measurement example of a φ1500mm size-reduction model for a ground test of a mesh antenna to be equipped on satellite

First, we project a stripe pattern to the two reference planes and produce virtual coordinates between the plane panels. It is also acceptable to project the grating to a single moving reference plane instead of two. The object to be measured is placed between the panels and the grating is projected onto it in the same manner. We can obtain an image where the projected sine waveform gratings are distorted according to the object's uneven surface, i.e., phase change image per pixel. On the principle of interpolating the phase change at the object surface by referencing the phase of the sine waveform light and dark stripe projected onto the two panels, the coordinate values are calculated per object pixel to obtain the plane profile. In the traditional method, we cannot measure objects larger than the reference plane since the object must be positioned between the two panels. In addition, when measuring distant objects, we need to place the reference planes in the distance. Accordingly, when measuring large-size space structures in orbit, we need the same size reference planes.

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