The cathode ray tube (CRT) for computer monitors or TV sets is composed of a glass envelope and its inner parts. Among these parts, the shadow mask plays an important role in the CRT; it filters RGB (red, green and blue) electron beams emitted from electron gun to fit the desired RGB phosphors on the screen. Excessive vibrations of the shadow mask subject to external disturbances may thus lead to landing shifts of the electron beams from the desired phosphors, deteriorating the color purity of the picture on the screen. This is called the microphonic phenomenon, which is an undesirable characteristic of the CRT.
Shadow masks are classified into two groups depending upon the manufacturing method: the press-formed and tension types. Among them, the tension type shadow mask has been developed mainly for large CRTs because of its durable stiffness characteristics against deformation. However, it exhibits a very light damping effect in the vacuum environment. Once a tension shadow mask is disturbed by an impulsive shock, the resulting vibration usually lasts for one or two minutes. To effectively attenuate such prolonged vibration, adjustment of tension distribution of the shadow mask and use of additional damping equipment are widely adopted in practice.
The V-shaped tension distribution, with large tension at both ends and small tension at the shadow mask center, and the M-shaped tension distribution have been attempted to enhance the vibration reduction efficiency of the tension shadow mask. However, it has been found that the microphonic phenomenon is not sufficiently prevented by adjustment of the tension distribution alone, requiring additional damping mechanism for the tension shadow mask.
Among others, damping wire is known to be a good candidate for such damping mechanism, although there are some drawbacks such as the difficulty in assembling and the presence of its shadow on the screen. This study is mainly concerned with the vibration analysis of the tension shadow mask with damping wires.
In this work, the vibration of the curved shadow mask with V-shaped tension distribution is analyzed in consideration of the effect of wire impact damping. First of all, author develops a reduced order dynamic model of the shadow mask with V-shaped tension distribution using a commercial FEM code and the dynamic condensation method, and a lumped-parameter model of the damping wire. In the wire modeling process, the nodes of wire models are chosen so that they match with the nodes of the mask model and thus the contact interaction between the wires and mask surface can be conveniently described. The contact mechanism between the damping wire and shadow mask plate is modeled as a nonlinear contact-impact model composed of spring and damper elements, of which parameters are determined from the Hertzian contact theory and the restitution coefficient, respectively. For the sake of the computational efficiency as well as accuracy, the deduced impact stiffness coefficient is evaluated and further reduced in the simulations.
The analysis model of the shadow mask with damping wires is experimentally verified through impact tests of shadow masks performed in a vacuum chamber. Using the validated analysis model of the shadow mask with damping wires, the ‘design of experiments’ technique is applied to search for the optimal damping wire configuration so that the vibration attenuation of the shadow mask is maximized.
