Saturday, November 10, 2012

Improving Single-Molecule Experiments With Feedback Control of Optical Traps

D. G. Cole
This article explores various types of feedback control—position feedback, which was shown to be equivalent to force feedback, rate feedback, andintegral feedback—for the purpose of improving instrument performance for single-molecule experiments. The ability of each of each types of feedbackto lower the measurement signal-to-noise ratio (SNR) is evaluated andcompared to the open-loop case. While position feedback does not result in any improvement in the SNR, the cases of rate feedback and integral feedback both resulted in improvements in the measurement's SNR. Rate feedback is shown to effectively “cool” the beads held in the optical trap, thereby limiting the effect that Brownian disturbances have on the beads' motion. Integral feedback is shown to improve the SNR of the measuredsignal of interest and is robust and easy to implement. It is also shown that integral feedback acts as an exogenous force estimator. Ultimately, feedback does not provide better resolution as measured by SNR than an open-loop filtering approach can but does provide other advantages, including the ability to control other variables and to make a more robust instrument that can be easily adapted to changes in experimental conditions or the environment.
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