Portable device to measure the displacement maps on surfaces in the three spatial directions

New and innovative aspects

The new device allows an alignment, calibration and correction of displacements needed to obtain quantitative and accurate results.

Main advantages of its use

More economical and computationally less expensive.

Specifications

The present invention is a combination of two techniques the Projection Bands (PF) and the 2D Digital Image Correlation (DIC-2D), which we call briefly PF + CDI. The proposed system allows, using a single camera, to obtain displacement maps in the three spatial directions on the surface of the displayed object.

The information contained in these maps is the displacement values ​​that each element of the surface has experienced, what allows for example to locate the greatest deformation areas of the object and therefore the areas more likely to suffer breakage. As a non interferometric technique is relatively robust against external agents and therefore is susceptible to being implemented in an industrial environment (depending on the required resolution level).

With the device, large deformations can be measured, which makes it especially interesting for highly deformable materials (rubber, natural rubber, textiles, ...) Moreover it only requires the acquisition of an image in each deformation state of the object, allowing real time measurements (limited by the speed camera capture).

Currently the 3D Digital Image Correlation (CDI-3D) is probably the technique commercially available that it is being used more to measure how the surface of an object is deformed. In this 3D CDI-track of the elements of the surface is done by means of a stereoscopic vision, It is required a minimum of two cameras with which you get the two images, from two points of view, of each state of the object’s surface deformation. Each state of deformation requires a 3D reconstruction of the object and between every two reconstructions a digital correlation is applied for the displacements of each surface element. The cameras should be perfectly calibrated and synchronized, therefore the CDI-3D complexity and computational cost is high, what explains its high cost especially if you want to measure the deformations at high speed where expensive high-speed cameras are required.

However, this new device that combines the 2D Digital Images Correlation with the Stripes Projection. It uses a single camera and a projector which projects laterally one stripes pattern on the surface of study. This area of ​​study must also have a random spotted (eg. previously painted). Any displacement suffered by the spotted is measured by the CDI-2D technique obtaining the displacement in plane (x and y direction) of the variation in the projected stripes, displacement out of plane (z-direction of the camera optical axis) using the PF technique. The PF + DIC object of this patent enables an alignment device and calibration necessary to apply the two techniques.

The PF + CDI device consists of the following parts:

An image acquisition subsystem which consists of a digital camera with its corresponding lens, a lighting subsystem which consists in a projector (2) that projects structured light (in the case of Figure 1, equispaced and parallel stripes) on the surface of the object studied, and an alignment and calibration subsystem consisting of a laser (3), a sliding platform along an axis (4), a tiltable platform on two axes (5) and a tilting head (6) that supports the entire device with two controls (7) and (8) which allows to rotate around two perpendicular axes and perpendicular to each other and also to the optical axis of the digital camera. Finally, the device includes an image processing subsystem and a control device (9).

As it is shown in the Figure 1, the surface displacements under study (10) are measured respect to a reference flat surface (11). The surface of the object under study may be the reference surface itself (if it is flat before deformation) may be used either as such other flat surface mottled. To measure displacements, the optical axis of the camera (12) should be perpendicular to the reference surface and the projector must emit a structured light beam on the surface of study, so that the angle defined between the digital camera optical axis and the projector the optical axis (13) is non equal to zero. This implies, for the case where output are stripes, that the stripes arising from the intersection of the light beam emitted by the projector to the reference flat surface, is deformed or displaced in accordance with the deformation or the displacement in the direction out of plane(that is the direction of the camera optical axis).

Applications

• Mechanical behavior of objects such as aerospace and automotive, or other facing the development of new materials.
• Medical and biomechanics to allow, for example, evaluate the distortions suffered by tissues or prostheses to external efforts. Its applicability is emphasized in the study of materials that exhibit large deformation capacity (plastics, silicones, rubber, stretch fabrics...)