Unified Optical Scanning Technology

Chapter 4.5.3 - Scanner Devices and Techniques: Suspension Systems and Position Control

4.5.3 Suspension Systems and Position Control

In the galvanometer and resonant scanners, the bearings and suspension systems are the principal determinants of detailed scan uniformity. A given radial run-out (of a ball bearing, for example) will cause more shaft wobble the shorter the suspended distance between bearings. Thus the shaft length and mass are critical design factors. Sufficient stiffness and length of the armature are required to restrain cross-scan wobble. However, to maximize the upper oscillating frequency, the armature is restricted in size and mass. Fortunately, the reciprocating armature suspended within such bearings tends to repeat its path (and its perturbations) faithfully over many oscillations, making adjacent scans of these devices more uniform than if the same shaft rotated continuously within the bearings, as in a motor.

Some bearings are elastic flexure, torsional, or taut-band devices that are mounted rigidly at their ends and contribute almost no scan perturbations. Because of the high mechanical Q of these suspensions, they are most often applied to the resonant scanner [Tuc]. When properly damped, they can also serve for galvanometer scanners, suffering a small reduction in peak-to-peak excursion but gaining in more uniform scan with very low noise and extended life. Some considerations are their low radial stiffness and possible cross-coupling of perturbation from the torsion, shift of the rotational axis with scan, and pos-sib le appearance of spurious elastic modes when lightly damped. Most of these factors can be limited or circumvented by design trade-off and application of appropriate installation procedures [Mon].

Many galvanometer and resonant scanners are equipped with elegant position-sensing devices and control systems, which provide high accuracy and repeatability and render a smoothed and extended bandpass. Several magnetic encoders and capacitive and optical position detection schemes are utilized [Mon,Ayl] for integration into servo control systems that approach accuracies to 1 μrad of positional finesse.

 

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