Vision-Based Measurement System for Instantaneous Rotational Speed Monitoring Using Linearly Varying-Density Fringe Pattern

The instantaneous rotational speed (IRS) of a rotating machine is key information to understand the machine operation and diagnose the potential faults. A novel nonprojection fringe vision-based system was proposed to realize the measurement of IRS, which was simply composed of an artificial linearly varying-density fringe pattern (LVD-FP) as a sensor and a high-speed camera as a detector. The designed LVD-FP, printed by a normal printer, was pasted around the surface of the rotary shaft completely along the fringe density change direction. Part of the LVD-FP was captured by the high-speed camera. The fringe period density of each imaged LVD-FP intensity changed due to the rotation of the shaft, from which the rotational angle could be obtained. Subsequently, both the instantaneous angular speed (IAS) between two adjacent frames and the IRS could be accurately and efficiently obtained. The influences of the key parameters of the proposed system on the measurement accuracy were investigated by simulations and experiments. Both the simulation and experimental results demonstrated that the proposed system can accurately measure the IAS and correctly track the wide range IRS with the advantage of noncontact and cost effective. In addition, the rotation direction of the shaft could be directly obtained from the sign of the IRS curves. Therefore, the proposed system could provide a novel vision-based approach for IRS measurement, especially when the conventional speed sensor cannot be installed on the rotary shaft. The good performance of the system in IRS measurement makes it attractive for fault diagnosis of rotating machinery.