A High Revolution Absolute Optical Encoder Using Tapred Grating

Optical encoder is a measuring device that assemble optic, mechanical, electricity as one. The main component is the encoder which made using a certain encoding, modulated the incident light, and then transformed the physical measurements, such as angle, displacement, into electrical signals with photoelectrical technology. The signal is received and processed to abtain the measurement information and achieve position detection and real-time dynamic control as well as speed, acceleration and other measurements. As the optical encoder having advantages of the high resolution, wide measurement range, using of reliable, easy to maintain, it is widely applied in military, national defense, automation and other areas, such as radar, theodolite, numerical control machine tools, robots, etc.Different to the incremental optical encoder, absolute encoder can get absolute position to achieve real-time measurement, have no accumulated error, and many other features. Therefore it plays an important role on many occasions. However, there is a contradiction of resolution and size in the absolute encoder, the resolution improvement depends on the number of laps increased. For this problem, we designed a tapered grating using diffraction method to get further subdivided on the basis of the traditional absolute encoder, so that the encoder can be both small and high-resolution.First, giving a analysis and discussion to the available coding methods and principles of absolute encoder, mainly Gray code, matrix code, single-ring absolute codes, summerizing the advantages and disadvantages to select the appropriate rough division method.On the basis of the traditional encoder, the theory of tapered grating is innovative uses to achieve further subdivided. Tapered grating have cyclical changing pitch and in the context of a smaller pass-ray can be approximated as a uniform grating. The diffraction fringes reflect the current location information. Set reasonable grating parameters can be achieved very high resolution, the high subdivid efficiency than traditional method can effectively reduce the encoder size. In this paper, the tapered grating subdivid principle, tapered grating parameter settings, grating diffraction phenomena simulation and experimental verification have be elaborated.According to the characteristics of tapered grating, suitable encoder system is designed, including the encoder lighting, image data receiving and processing. Then the factors having an effct on the encoder accuracy and error are anaylzed and given, some advice is presented to beter the encoder.