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978-3-8439-0980-8, Reihe Ingenieurwissenschaften
Untersuchungen zur Industrialisierung von miniaturisierten optischen Drehwinkelsensoren mit diffraktiver Kodierscheibe aus Kunststoff
146 Seiten, Dissertation Universität Stuttgart (2013), Softcover, A5
An advanced optical rotary encoder concept is presented within this work. The basic idea is to use a micro structured plastic disk as code wheel which is manufactured by injection moulding. The solid measure consists of micro structures which act like a diffraction grating. To industrialize this concept, a compact incremental and also an absolute encoder were designed and manufactured.
The incremental encoder has a diameter of 36,5 mm and a height of 29 mm with a resolution of 2048 increments per revolution. The code wheel with a diameter of 30 mm is made of polycarbonate. The design concept is to use an optical MID (Moulded-Interconnect-Device) module which contains a laser diode and photodiodes as well as a lens and an aperture. Due to the precise MID module it is possible to assemble the encoder without any steps of adjustment.
The absolute encoder requires small dimensions of only 16 x 16 x 18 mm³ with a resolution of 6 bit. The code wheel with a diameter of 9 mm is made of PEEK. PEEK was necessary due to high temperature requirements of the whole encoder during an optional reflow soldering process at 260°C. The optical components like laser diode, photodiode array and aperture were directly integrated in an optical module with dimensions of 5 x 5 x 1,6 mm³ similar to an QFN package. In case of larger quantities it is also possible to develop an Opto-ASIC with integrated photodiodes and signal processing. In this way, a small miniaturized low cost encoder module can be manufactured.
The experimental characterization of the encoders was performed on a test bench for rotary encoders. The influence under temperature during operation of the encoder was investigated from -40° to +85°C in an environmental test chamber. It could be shown that the influence is insignificant if the power of the laser diode is regulated by a monitor diode. Without power regulation the signals decrease about 50% at high temperatures.
Furthermore, comprehensive investigations regarding the tolerances of the encoder assembling have been done. For example the distance of the encoder disc has to be within a tolerance range of ±100 µm.
For both encoder types several prototypes have been assembled without any adjustment. All correctly assembled encoders were working without failures. This shows that the encoding method with diffraction gratings on a plastic code wheel can be used to build up miniaturized optical low cost encoders with high resolution.