Portable and long-range displacement and vibration sensor that chases moving moiré fringes using the three-point intensity detection method

Interferometry with single-point detection is widely used for the precise measurement of short-range displacements, when its range is less than a quarter of the wavelength. The moire technique is also used for the same purpose when the displacement is smaller than half the period of the superimposed gratings. In both interferometry and the moire technique, the response function of the system takes a periodic form by increasing the displacement value. For larger values of the displacements, the moment the detected signal experiences one of the extremum values, the signal trend does not reflect the direction of the motion, and it leads to an ambiguity in the motion reconstruction. Since an interference pattern has a sinusoidal intensity profile, by using three-point detection and the aid of conventional spatial phase shifting, we have recently proposed a new method for chasing moving interference fringes and were able to remove the disability of the interferometry in discriminating of the direction of motion for long-range displacements (see Opt. Laser Technol.103, 387 (2018)). But the transmission function of a moire pattern is a triangular or trapezoidal function. Therefore, the conventional phase shifting algorithms are not applicable for moire fringe chasing. In this work, first we introduce a new method for data acquisition in moire-based displacement and vibration sensors. We use a three-point intensity detection method for chasing moving moire fringes and we introduce a new algorithm based on the data of three point detectors to remove the disability of the moire technique to discriminate the direction of motion for long-range displacements. With the aid of three-point intensity detection, a high speed, high accuracy, and long-range displacement sensor based on the moire technique is built. This sensor can be used in the calibration of mechanical positioning sensors. Also, using a data acquisition procedure we have introduced, in a moire-based vibration sensor, the vibration parameters can be determined in a simple and reliable manner.