Development Of The Control System Of A Laser Mark Automatic Tracking Extensometer

It is required to easily and accurately obtain mechanical parameters of engineering materialsduring the modern social production and scientific research. So the properties of test equipmentsbring forth higher requirements for the material parameters acquisition. The extensometer is oneof the main tools to measure stress-strain properties of engineering materials. In this thesis, thehistory and development of extensometer and visual tracking control method in domestic andoverseas have been discussed generally. The control system of the laser mark automatic trackingextensometer has been designed. And it is realized that the line mark are precisely tracked by thelaser mark during the whole tensile test.In this thesis, the control system of the laser mark automatic tracking extensometer has beendesigned on the basis of the research of non-contact measurement extensometers. Firstly, thelaser mark automatic tracking algorithm based on PID has been put forward. Its principle is thatthe pixel distance is obtained using image process method based on the results of CCDcalibration and the sampling time of this control system. Then the laser mark’s trackingdisplacement in real space and the servo motor’s speed have been calculated according to thepixel distance of center points between laser mark and the line mark on the specimen. Thereforeit has been realized that the line mark are precisely tracked by the laser mark. Secondly, Due tothe cumbersome of the calibration algorithm, the error is introduced in the PID algorithm. Thus,a method based on on-line identification of an image Jacobian matrix has been adopted in thethesis. The mapping relationship of laser mark tracking distance in real word space and theimage plane has been developed. As a result, a laser mark automatic tracking algorithm has beendesigned based on uncalibrated visual servoing. Its principle is that the image Jacobian matrixelements as the filter status input, the coordinates variation of the laser mark in adjacentsampling time as the filter observed variables and the laser mark’s tracking displacement in realspace and the servo motor’s speed have been calculated according to the identified image Jacobian matrix, the pixel distance between two marks and the system sampling time. As a result,it is realized that the line mark are precisely tracked by the laser mark during the whole tensiletest.Finally, the experimental device of the control system of the laser mark automatic trackingextensometer has been constructed. The specimens of low-carbon steel, cast iron, brass andaluminum were adopted for the tensile test respectively. The tracking accuracies of PIDalgorithm for four materials are0.0329mm/4mm，0.1948mm/27mm，0.0514mm/12mm，0.0901mm/10mm respectively. The tracking accuracies of uncalibrated visual servo algorithmfor four materials are:0.0238mm/4mm，0.0064mm/25mm，0.0153mm/12mm，0.0072mm/10mm respectively. The above methods can realize that the laser mark tracks the line mark duringthe whole tensile test precisely. Besides the tracking accuracy of the latter is better than that ofthe former.