The paper presents a technique making it possible to evaluate amplitude and phase errors of the magnetoelastic support sensor used in the plastics ultrasonic welding. Measurement of the support sensor vibration amplitude allows monitoring alterations in the basic physical properties of the polymer being welded, i.e. its temperature and thickness. The support sensor operation dynamic mode is described. It is established that evaluating measurement errors with the proposed method in manufacture of any support sensor of different design makes it possible to accurately determine its performance with parameters of the polymer material ultrasonic welding process. The problem of evaluating accuracy of the magnetoelastic support sensor operation used in the welding process was considered. The support sensor amplitude and phase errors during ultrasonic welding were studied. A magnetoelastic sensor was designed and developed ensuring improved accuracy in measuring the support vibrations amplitude during the polymer material ultrasonic welding. The experiments carried out confirmed clear relationship between the support vibrations amplitude and heating of the welded samples, their thickness and properties during the ultrasonic welding.