Building Active Feedback Control: Methods At The Nanoscale
By James MacKay, Mad City Labs
An active feedback control system can be an invaluable technique in experimentation or testing, but its creation and operation also can be perilous. Minor mistakes can undermine results, while more significant errors can damage instrumentation. Understanding the factors that affect an active feedback control system — including tolerances, noise floor, and the interactions between setup components — leads to safer, more successful utilization.
Active feedback control can be accomplished in either a closed-loop system or an open-loop system, depending on the demands of the experiment. In a closed-loop system, a portion of the output is provided back to the input, informing the controller of changes necessary to meet the desired output. In an open-loop system, the desired output does not depend on the control action.
All Mad City Labs nanopositioning products achieve sub-nanometer resolution because they utilize a position sensor accurate to less-than-a-nanometer precision, providing picometer performance in most cases. This sensor also can be applied to other applications that need sub-nanometer resolution — often the impetus for creating a closed-loop feedback system.
Additionally, one can create a software feedback loop just as easily as a hardware-based feedback loop. Mad City Labs provides a USB interface to help accomplish just that. While it is true that a software feedback loop operates more slowly, it is faster to implement than a hardware one. Software may, indeed, be too slow for some tasks, but it is not so slow that its use does not warrant an attempt, because the approach can be quickly vetted.
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