Digital Control of Interferometric Metrology Lines

A recent paper presented principles and digital control of Fabry-Pérot interferometers, showing some ongoing and potential applications in the field of precision engineering. Among these, metrology lines were mentioned, the aim being to reveal sub-nanometric displacements in view of dimensional stabilization/monitoring of demanding space telescopes. The core is a confocal optical cavity, in vacuum, whose light path is sensitive to supporting structure length variations. The latter are tracked and revealed by a frequency actuator which acting on the incident light keeps the interferometer signal (the tracking error) centered to zero although the optical path varies. Concept and digital control of metrology lines are outlined and some experimental results from an on-ground breadboard are discussed. Due to on-ground and test contingencies, a displacement actuator (piezo-electric ceramics) was also active on the cavity, which required actuator coordination. Digital control was designed around a stylized model of plant and disturbance, called the Embedded Model, to be included in the control unit. Control algorithms and code are derived from the model; control gains either derive from the model or from closed-loop eigenvalues. No performance optimization is sought, but requirements are met and stability/performance is guaranteed in the presence of neglected dynamics and actuator/sensor constraints.