The present invention relates generally to shape determination and, more specifically, to methods and systems for determining the shape of surfaces and/or structures associated with large quasi-planar systems.
Several methods and systems have been proposed to determine the shape of surfaces and/or structures associated with large quasi-planar systems, such as, radar antennas. Such systems include, for example, range-based metrology systems that utilize either optics or radio frequency (RF) and scanning devices or multiple sources/targets, image-based systems (e.g. videogrammetry), and local sensors (e.g., strain-gages and/or accelerometers) that are combined with metrology and/or image-based systems. These systems have their respective limitations, as will be further described below.
Range-based metrology systems (e.g. optical, RF) use measurement of distance to estimate the shape of the surface. In such systems, the sensitivity of the measurements decreases as the deformation component normal to the direction of distance measurement increases. This illustrates some of the inherent limitations of range-based metrology systems, i.e., the need to have the source at a distance from the plane of the surface for which the shape is to be determined, and a potential low sensitivity of the measurement to the quantity of interest.
An alternative is to use various metrology sources distributed along the length of the surface to avoid the low sensitivity problem. However, this introduces additional complexity due to a number of factors including, for example, the need to determine the relative motion between the various sources so as to allow their measurements to be reconciled (non-collocated sources mounted along the structure will deflect/move with respect to each other), and the larger number of components in the system.
A range-based metrology system (e.g. laser metrology) can use either a scanning source, which revisits various target points on the surface of interest at regular intervals or various source/target pairs to avoid the need to scan. Scanning systems introduce performance limitations due to the additional complexity associated with such systems and the time it takes to generate a complete scan (i.e., time to cover entire surface). The use of various source/target pairs removes those limitations at the cost of increasing the number of components. In summary, high cost and overall complexity are severe limitations of existing range-based metrology systems.
Image-based systems have the advantage of covering a large area with high resolution without the need to scan. Such systems that have been proposed for shape determination of large quasi-planar systems, such as radar antenna, use videogrammetry technology and are based on stereo imaging. These systems reconstruct a three-dimensional image and/or displacement from two or more camera images taken from different locations and require depth of view when taking the images. More specifically, the cameras need to be mounted on a back (or front) structure at a distance from the surface to be measured. These systems suffer from similar inherent limitations as those discussed for range-based metrology systems, e.g., the low sensitivity for points at the far field and the need for mounting at a distance from the surface. In addition, for increased accuracy, camera lines-of-sight must be registered with respect to each other. This could be a complex proposition because the cameras need to be separated, i.e., mounted on different portions of the structure, and metrology is necessary to achieve the required registration. Calibration would be complex and time varying due to the relative motion between the cameras. Furthermore, even without the registration problem, computations are complex and slow resulting in low update rate and the resolution and accuracy are limited. Although in terms of complexity and cost, these systems compare favorably to range-based metrology systems, it is unlikely these systems can achieve the performance required for a large radar antenna.
Sensors that provide measurements of local deformation (such as strain-gauges) or motion (such as accelerometers) can be used to augment range-based or image-based technology. These sensors do not provide a direct measurement of surface shape. Their signals need to be integrated to generate the desired information and, therefore, they can only be used in conjunction with a system that provides direct shape information for removal of errors that accumulate over time due to drift and bias on the sensors. For extremely large structures (e.g. space-based radar), the accelerometers have limited value due to the low frequency of the structural modes of interest and of thermal induced deformations.
Hence, it would be desirable to provide a system that is able to provide shape determination for large quasi-planar systems in a more efficient manner.