High-accuracy position feedback system Kimura, Kyle; Mah, Brian; Chu, Yin-Chi
Zaber Technologies Inc. is a Vancouver based company that manufactures linear actuators and linear slides that are used for precision motor control applications. Some applications include camera and microscope positioning, scanning, semiconductor manufacturing and laser cutting, just to name a few. Currently, their linear slides have positioning accuracies of around +/- 10 um with a 0.1 um resolution or better with open-loop control. The purpose of this project was to develop an interferometer based encoder system that could be added to a linear slide to provide position feedback information and increase the resolution. The initial objectives of the project were to develop a working system that could fit in a 3 cm³ volume, have a resolution of 50nm or better and be able to measure distances up to 2 m at speeds up to 2 m/s. Another longer term objective was to design the device so that it could be manufacturable for under $50. The plan for the project was to first develop a large scale system that could meet the performance requirements before focusing on miniaturization to meet the size and manufacturability requirements. A conventional Michelson interferometer was constructed, with one of the mirrors attached to the stage of a Zaber linear slide. The movement of the stage was measured by measuring the movement of the interference fringe patterns using a 16-element photodiode array and a dsPIC33F digital signal controller. By the end of the project, movement of the stage could be measured with a resolution of 53 nm. However, this could only be done at speeds up to 5.56 um/s over a range of 3.2 cm with the final, large scale apparatus. Several technical limitations and challenges faced throughout the design and development stages of the project prevented many of the original project objectives from being achieved. The overall project goal was gradually reduced from designing the manufacturable, miniaturized system to completing the position measurement implementation on the large scale system. The main technical limitation that prevented the system from being miniaturized was the size of available lasers with the required coherence length. Small infrared DFB lasers are available that could potentially be used to miniaturize the system in future development. However, these were not used for the project and instead a large, HeNe laser was used because one was readily available and it was easier to work with a visible light source. One of the main challenges that was faced was that the signal from the interference pattern was easily disrupted from vibrations and other inherent noise. A smoothing algorithm had to be implemented to help reduce noise. This algorithm, along with sequential data sampling of each photodiode element (multi-channel sampling was not implemented due to time constraints) greatly reduced the maximum speed that measurements could be made. Interferometer alignment also proved to be a major challenge for the project and would require a lot of consideration for a manufacturable design. The interference fringe patterns could be lost or disoriented very easily from misalignment. The interferometer had to be adjusted several times throughout the project. The alignment also changes as the linear slide moves back and forth which became the limiting factor for the operating range of the system. This project concluded with several opportunities for further development towards a final design. One improvement would be changing to faster data sampling hardware such as an ADC with simultaneous sampling and an FPGA. Different signal processing techniques could also be explored to help reduce noise and improve performance. Other provisions, such as vibration isolation or compensation techniques would also need to be considered in the future.
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