Optical triangulation displacement sensors detect linear displacements of objects without mechanical contact. They have simple structure, good resolution, and long operating range. However, there are several errors generated from speckle effects, environmental effects, and electronic noises, etc. To reduce errors from the electronic noises, the easiest way is to average the measurement outputs. Because the electronic noises are random in nature, their variance can be reduced with the averaging operation. However, this method is inherently time consuming process. To decrease the averaging time, several sensors or better signal processing hardwares are needed. So it increases the size of the measurement system and is not costeffective. In this paper, we propose a simple and cost-effective system structure for optical triangulation displacement sensors, which simplifies the averaging by inserting a transmission-type diffraction grating. When an incident ray enters to the diffraction grating, the grating separates the incident ray into several rays by the diffraction effect. The diffraction grating helps us to attain several signals simultaneously. Theoretical analysis is given and the feasibility of the proposed system is verified through experiments.
Optical triangulation displacement sensors are widely used for their non-contact measurement characteristics, sub-micron order resolution, simple structure, and long operation range. However, errors originating from surface inclination, speckle effect, light source fluctuation, and detector noise limit the wider use. In order to minimize these errors, the structure for optical triangulation displacement sensors, which is composed of an incoherent source and a linear CCD, has been proposed. But using a linear CCD causes several problems in signal processing. In this paper, we propose an adequate signal processing system for the proposed structure. With the help of the proposed algorithm, the limited resolution problem of CCD can be solved.
Point triangulation probes (PTBs) fall into a general category of noncontact height or displacement measurement devices. PTBs are widely used for their simple structure, high resolution, and long operating range. However, there are several factors that must be taken into account in order to obtain high accuracy and reliability; measurement errors from inclinations of an object surface, probe signal fluctuations generated by speckle effects, power variation of a light source, electronic noises, and so on. In this paper, we propose a novel signal processing algorithm, named as EASDF (expanded average square difference function), for a newly designed PTB which is composed of an incoherent source (LED), a line scan array detector, a specially selected diffuse reflecting surface, and several optical components. The EASDF, which is a modified correlation function, is able to calculate displacement between the probe and the object surface effectively even if there are inclinations, power fluctuations, and noises.
Multi-degree-of-freedom (MDOF) displacement measurement systems are needed in many application fields; precision machine control, precision assembly, vibration analysis, and so on. This paper presents a new MDOF displacement measurement system that is composed of a laser diode (LD), two position- sensitive detectors (PSDs), and a conventional diffraction grating. It utilizes typical features of a diffraction grating to obtain the information of MDOF displacement. MDOF displacement is calculated from the independent coordinate values of the diffracted ray spots on the PSDs. Forward and inverse kinematic problems were solved to compute the MDOF displacement of an object. Experimental results show maximum absolute errors of less than plus or minus 10 micrometers in translation and plus or minus 30 arcsecs in rotation.
A new hardware compensation method reducing displacement measurement errors, caused by tilt of index scale in moire type linear encoder, has been developed. In the conventional moire type linear encoders, the detectors are aligned perpendicular to the line of moire fringes and this structure is very sensitive to an unwanted tilt of the gratings. In this paper, newly designed grating, named as phase shifted grating, is developed to compensate non-orthogonal error. By adopting the phase shifted grating instead of conventional index one, it is possible to reduce non-orthogonal error of the moire type linear encoder.