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UBC Theses and Dissertations

The study of field programmable gate array based servos in atomic, molecular and optical physics experiments Yu, Shi Jing


The use of Field Programmable Gate Array (FPGA) is becoming increasingly popular in new designs for instrumentation tools. Among them, the FPGA-based servo is emerging as a replacement for the traditional analog servo as a more versatile, automated and remotely controllable alternative. Despite the demonstration of FPGA servos for the control of lasers in the literature, the practical constraints of an FPGA servo have not yet been fully investigated. This work presents an open-source FPGA servo design that is capable of reaching a total signal latency of 200 ns including both conversion delay and computation delay. This work also investigates various limitations inherent in a digital implementation of a servo arising from the computation precision of an Infinite Impulse Response (IIR) filter and the effect that signal quantization has on the transfer function that a digital servo can implement. These technical details are not widely discussed, but are important both for the design and the operation of the FPGA servo. Applying the FPGA servo in an intensity stabilization application allows direct tests of these limitations. In particular, this work compares the performance of the FPGA servo and a high-performance commercial analog servo with a focus on key specifications including the closed-loop bandwidth, noise floor and the resolution of the transfer functions. For closed-loop control scenario with a bandwidth below 1 MHz, we achieve better performance with the FPGA servo than the analog servo through the use of more complex transfer functions including a Proportional and Integral Cubed (PI3) and a Proportional Integral and Integral (PII) with lag-lead.

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