Unified Optical Scanning Technology

Chapter 4.5.1 - Scanner Devices and Techniques: The Galvanometric Scanner

4.5.1 The Galvanometric Scanner

Historically, the galvanometer scanner evolved from the famous D'Arsonval meter movement having a 'moving coil' armature suspended within a strong permanent magnet field [Bei2, Fig. 27]. It has appeared in three forms [Mon1]. A modified multiturn moving coil armature and solid cores of either iron or magnetic material to provide increased torque. The moving magnet transducers generally render the highest resonant frequencies and the highest scan speeds.

As shown in Figure 4.19a, the typical contemporary moving magnet galvanometer transducer is similar to a torque motor. Permanent magnets in the stator provide a fixed field that is augmented (±) by the variable field developed by the control current through the stator coils. Seeking a new balanced field, the rotor (and mirror) executes a limited angular excursion (±Φ/2) that is restrained by an elastic suspension system. The galvanometer is a broadband device, damped sufficiently to operate over a wide range of frequencies, from zero to an upper value related to its mechanical resonance. Thus it can provide the basic sawtooth waveform for raster formation, having a long active linearized portion and a short retrace interval of time τ. Figure 4.20 (solid line) shows a typical galvanometer sawtooth scan. Of the total cycle period T, a useful linear ramp portion occupies 0.7 T (70% duty cycle). At moderate frequencies (~100 Hz), duty cycles >85% have been achieved [Gad]. Also, as a broadband device, it can provide random access, positioning the beam rapidly to an arbitrary location within the time τ. For this important feature of waveform shaping, the galvanometer was early categorized [Bei2] as a low-inertia scanner (Fig. 4.1). Ex-

Fig. 4.19 Examples of galvanometer and resonant scanner transducers. Permanentmagnets provide fixed field, augmented by variable field from controlled current through stator coils. [a] Galvanometer: Torque rotates iron or magnetic core ±Φ/2. Mirror surface (not shown) on shaft axis. [b] Resonant scanner: Torque induced into single-turn armature coil (looped in plane perpendicular to paper) rotates the mirror ±Φ/2 suspended typically within torsion bars. One stator coil may be nondriven and supply induced current for velocity pick-off. From [Bei7] Wiley-VCH (1995) Encyclopedia of Applied Physics Vol. 12. Fig. 18 on p. 352.

Fig. 4.20 Waveforms (Φ vs. time) of two vibrational scanners having same periods and zero crossings. Solid line, galvanometer with linearized scan at 70% duty cycle. Dashed line, resonant scanner providing 33.3% duty cycle (unidirectional) with 2:1 slope change, or 40% duty cycle with 3.24: 1 slope change. See text for further data. From L. Beiser, Laser Scanning Notebook, SPIE Press (1992). Reproduced by permission of the publisher.

cellent reviews of galvanometer design have been published recently [Mon2,Ayl].

 

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