Unified Optical Scanning Technology

Chapter 4.5.2 - Scanner Devices and Techniques: The Resonant Scanner

4.5.2 The Resonant Scanner

A resonant scanner transducer is illustrated in Fig. 4.19b, a single-turn coil (within its controlling magnetic field) coupled directly to the mirror [Mon]. Other forms utilize armatures of iron or magnetic materials [Rei‚Tuc]. With damping removed almost completely, large angular oscillations can be sustained only very near the resonant frequency of the armature and its suspension system. The resonant scanner is thus identified with near-perfect sinusoidal oscillations at a fixed and typically high frequency. Figure 4.20 (dashed lines) illustrates such a sinusoid having the same zero crossings as those of the (solid line) galvanometer sawtooth function. Contrary to its prevalent designation as 'low inertia,' the resonant scanner acts as a pendulum, providing rigid time increments, as though it exhibited high inertia. In some designs, a small amount of inertial tuning can be provided to trim the resonant frequency slightly. Although the rotary inertia of the armature system is low, allowing high cyclic rates, it permits no random access and no waveform shaping, as do the galvanometer, acoustooptic, electrooptic, and other wideband scanners designated as low-inertia devices. Thus, in the Figure 4.1 categorization of optical scanning techniques, the 'oscillatory resonant' scanner is classified as 'high inertia.'

With its high mechanical Q (very low damping), the resonant scanner provides only harmonic scans. Because we typically seek a linearized scan segment, significant adaptation of the sinusoid is often required to render scan utility. As illustrated in Figure 4.20 (dashed lines), we need to utilize a central portion of the sine function that is sufficiently linear to linearize the image further by timing the pixels, as by extracting them out of memory at a complementary rate [Twe]. Some numeric values are useful indicators of the degree of correction required. To limit the variation in pixel rate to 2:1 (velocity at crossover to be twice that at the selected scan limit), one may restrict the angular scan to 60°/90° = 66.7% of its peak excursion. When scanning with only one slope of the sinusoid (as for raster formation on a uniform medium transport), this represents a duty cycle of only 33.3%. To raise the duty cycle, one must accommodate a greater variation in data rate. If, for example, we limit the useful scan to 80% of its excursion (40% duty cycle; one slope), the velocity variation rises to 3.24x.That is, the data rate at crossover is 3.24 times that at the scan limit. Accompanying this scan velocity variation is the corresponding variation in dwell time of the pixels, resulting in loss of pixel exposure or detectivity: 2:1 for 33.3% duty cycle and 3.24:1 for 40% duty cycle. This, too, may require compensation over the full scan interval, utilizing position-dependent sensitivity controls [Mon,Rei,Twe,M&G] to provide complementary pixel modulation. In contrast, the broadband galvanometer can render a linearized scan at much higher duty cycles. At a duty cycle of 70%, its data rate need be only 1.43x that for an ideal duty cycle of 100% (Section 4.3.3).

Also, its data rate at crossover is increased significantly over that of the galvanometer. For equal peak-to-peak excursions (Fig. 4.20), the bandwidth is approximately 2½ times that of the galvanometer (with 70% duty cycle), as represented by their relative slopes at crossover. For equal angular excursions of both, with the resonant scanner operating at, for example, 40% and 33.3% duty cycle, its bandwidth at crossover rises by a factor of approximately 3.1 and 3.7 times that of the galvanometer, respectively. With thoughtful provision for the above comparative factors (in electronic and system control), the resonant scanner renders application in compact linearized scanning tasks, including accommodation for operation in the forward and return scan directions, for increased duty cycle.

 

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