Research On The Technologies Of Large Scale Coordinate Mesurement Based On Active Vision

In view of the problem of low accuracy in the measurement of large sacle for staticsolo-vision coordinate measurement system, a new concept of active visionmeasurement has been proposed. The subject “research on the technologies of largescale coordinate measurement based on active vision” is to study a new visioncoordinate measurement system which introduces active vision to solo-visioncoordinate measurement system base on optical probe to improve its large scalemeasurement accuracy. For this novel measurement system, some key problems such asmodeling of active vision coordinate measurement system, acquisition of movingoptical probe, tracking of moving optical probe, calibration of position and attitudebetween camera and rotator have been researched and some solutions have beenproposed.In this paper, we researched the structure of active vision coordinate measurementsystem and the measurement model has been established. First, as optical probe usuallyadapts its position and attitude to the measured point during the whole measurementprocess, acquisition and tracking of the moving optical probe through zooming,rotating and pitching of camera are applied to obtain appreciate size images of opticalprobe in CCD center area; Second, we use solo-vision measurement module to calculatethe optical probe tip coordinate under the current camera coordinate system whichequals the measured point coordinate; At last, coordinate transformation is applied totransform the coordinates gained under every coordinate system to the world coordinatesystem to implement the point coordinate measurement of large scale workpieces.Acquisition of moving optical probe is to capture appropriate size and sharplyfocused images of optical probe at CCD through lens’ zooming and focusing whileoptical probe moving with measured points. Focal length of variable lens is classifiedinto several levels to obtain appropriate size images, that is, the focal length is firstlydivided according to work distance and percentage of image plane, then a fast method isused to estimate the focal length exportation curve, at last locate the specific focallength on this cure. A combined auto focusing approach with depth from focus methodand depth from defocus method is proposed to capture sharply focused images, that is, aDFD method based on autocorrelation of derivative of defocus blured image is used firstly to give a fast but rough estimate of the right focus position, then a DFF methodchosen Variance function, most sensitive to point light source, as criterion function isused to find accurate focus position in the rough field. Experimental results showsystem can acquire moving optical probe and obtain sharply focused images withappropriate size in measurement range.Tracking of moving optical probe is to capture images of optical probe in CCDcentral area through camera’ rotating and pitching while optical probe moving withmeasured points. A new tracking method based on logical partition of image plane isproposed to meet the fast and accurate tracking requirement in active visionmeasurement, that is, image plane is parted into ideal region, tracking region andsearching region. When centroidal of feature image points distributes in searchingregion, a fast tracking method base on the errors of centroidal of feature points is usedto keep optical probe in field of view of camera; When centroidal of feature pointsdistributes in tracking region, an accurate tracking method base on PBVS is used tokeep image of the optical probe in the center of CCD plane. Experimental results showthe effectiveness of this tracking method.Cause the calibration of position and attitude between camera and rotator is the keyto accuracy of measured data combination, this paper presents a valid calibrationmethod consists of linear parameter estimation base on theory of matrix direct productand nonlinear refinement through repeated measurement of fix point. To reduce thenoise corruption, quaternion is applied to orthogonalization the rotation matrix and newtranslation vector is calculated from this orthogonal rotation matrix. Experimentalresults confirm the calibration accuracy.Finally, the collectivity test was done for the large scale coordinate measurementsystem based on active vision, the experimental results show the same repeatabilityfor measurement of single point, but2-3times higher accuracy for measurement ofstandard gauge compared with static solo-vision coordinate system，and approve theeffectiveness of extending measurement range and improving measurement accuracy.