Long Range High Accuracy Optical Delay Line

Benefits

The use of magnetic bearings as guiding elements and of a one-stage magnetic actuator allows for long range, high precision and high bandwidth without wear.

Performances

Axial Positioning

The stroke is 5mm and the positioning precision is 12nm_rms over the full closed-loop bandwidth of 100 Hz. The latter is mainly limited by the control structure interaction (CSI). Due to the saturation of the actuator, the settling time of the step response (Figure 1) depends on the amplitude of the command. In this configuration, the Optical Delay Line works in a perturbation rejection mode and the achieved sensitivity to perturbation is -60dB at 10 Hz (Figure 2).

Radial Guiding

The guiding system has an infinite static stiffness, only limited by the force produced by the magnets. The guiding error mainly depends on the sensor resolution (122 nm quantification steps) and on the bandwidth of the control system (1500 Hz), the resulting guiding error is about 100nm_rms (Figure 3). The four pairs of magnetic bearings dissipate a total of 3.5 W. This comes both from the large bias current (1.5A) required to have the necessary control force, and from the static control current that compensates for the gravity. The use of permanent magnets to generate the bias magnetic flux can drastically reduces the first, while the second is not relevant for space application. Neglecting these static currents (i.e. horizontal bearings), the power dissipation can be estimated at 60 mW (Figure 4).