title: Implementation and Tuning of an Optical Tweezers Force-Clamp Feedback System.
abstract: Feedback systems can be used to control the value of a system variable. In optical tweezers, active feedback is often implemented to either keep the position or tension applied to a single biomolecule constant. Here, we describe the implementation of the latter: an optical force-clamp setup that can be used to study the motion of processive molecular motors under a constant load. We describe the basics of a software-implemented proportional-integral-derivative (PID) controller, how to tune it, and how to determine its optimal feedback rate. Limitations, possible feed-forward applications, and extensions into two- and three-dimensional optical force clamps are discussed. The feedback is ultimately limited by thermal fluctuations and the compliance of the involved molecules. To investigate a particular mechanical process, understanding the basics and limitations of the feedback system will be helpful for choosing the proper feedback hardware, for optimizing the system parameters, and for the design of the experiment.
authors.affiliation: Center for Plant Molecular Biology, Universität Tübingen, Auf der Morgenstelle 32, D-72076, Tübingen, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Bugiel
authors.affiliation: Center for Plant Molecular Biology, Universität Tübingen, Auf der Morgenstelle 32, D-72076, Tübingen, Germany.
authors.forename: Anita
authors.initials: A
authors.lastname: Jannasch
authors.affiliation: Center for Plant Molecular Biology, Universität Tübingen, Auf der Morgenstelle 32, D-72076, Tübingen, Germany. erik.schaeffer@uni-tuebingen.de.
authors.forename: Erik
authors.initials: E
authors.lastname: Schäffer
mesh_terms: D000465:Algorithms; D008962:Models, Theoretical; D052898:Optical Tweezers; D055095:Optics and Photonics / Q000295:instrumentation / Q000379:methods; D000072760:Single Molecule Imaging / Q000295:instrumentation / Q000379:methods
keywords: Force clamp; Force spectroscopy; Kinesin; Optical tweezers; PID controller; PID tuning; Single-molecule biophysics