Study of distance range visual measurement in the new Depth From Defocus method

Abstract

Abstract-The article presents the results achieved while researching a distance measuring method belonging to Depth From Defocus techniques. The method has been developed to determine the shape of a flaccid diaphragm used in the Ventricular Assist Device (VAD). The shape is determined on the basis of distance measured between the CCD sensor plate of a camera and the markers located on the flaccid diaphragm. The goal of this paper is to present the influence of marker size on distance range measured between the camera and the diaphragm used in a pneumatic prosthetic heart.The article presents the impact of marker size on the result of a distance measurement method [1,2]. The method was developed for a video sensor to determine the instantaneous stroke volume of blood SV from a pneumatic pump heart assist device (VAD), Fig. 1a. When measuring SV, the flaccid diaphragm is equipped with markers which position and orientation in 3D space is subject to change in relation to the camera position. What is important in the method is that when measuring the position of the camera and all settings: the lens and the camera (focus, aperture, focal length) are invariable. Method [1, 2] also defines in 3D space the position of all the markers on the basis of analysis of only one image frame. In this respect, it has no equivalent in the literature. The vision systems mentioned in the literature typically create a stereoscopic system [3]. The distance in such a system is calculated knowing the optical parameters of the cameras and their mutual position. Measuring systems equipped with one camera are also used [4]. The distance to the object in these systems is determined using, for example, a perspective inverse transform [5]. A single camera configuration with an autofocus function is also possible. The distance to the object is then determined with a lens equation [6]. Other methods used to measure the distance are photogrammetry [7][8] and a fringe projection technique [9][10][11]. Their use, however, depends on: the scope of measured distances, required measurement speed, required image sensor resolution, light source type as well as target size and sensor weight.When determining the shape of a flaccid diaphragm, real time operation is particularly important (the shape of the diaphragm may change with a frequency of up to 3Hz) as * E-mail: Krzysztof.Murawski@wat.edu.pl well as simultaneous multipoint measurement of the position in 3D space in respect to all of the markers, Fig. 1b. The development of a soft-sensor for measuring the SV of a pulsating heart assist pump results from the needs and problems mentioned in [12]. In [12][13] it was demonstrated that SV can be determined by using the acoustic Helmholtz resonator theory. This method, however, has substantial limitations. For this reason the search for techniques of determining SV is still valid. The knowledge of changes dynamics in SV will enhance the safety of VAD usage, which currently depends on visual inspection: "One of the main advantages of the extracorporeal, po...

Optical distortion compensation in the visual measurement with the new Depth From Defocus method

“…The membrane is controlled by the gas forced into the pneumatic chamber. This is a continuation of the earlier research on the use of image processing and analytical techniques to determine the membrane shape and stroke volume of an artificial ventricle [3][4][5][6][7].…”