Comparing Beam-Deflecting Systems: Part II

Last month in this space, we explored the most commonly used beam-deflection system available to system designers. In this month's installment, we will consider the characteristics of those systems and examine their cost/performance attributes.

All beam-deflector design alternatives exhibit behavioral characteristics that have significant impact on the optical systems employed for either generating or collecting images with precision. The task of selecting the superior cost/performance alternative begins with understanding those characteristics, and usually ends with selecting a deflection scheme that meets a given performance requirement for the lowest overall system cost (please note the emphasis on system cost).

In an attempt to cover this broad topic with a degree of conciseness, my discussion will focus on comparing rotating mirrors, hologons and galvo-driven mirrors as they apply primarily to printing machines and vision systems. AO (acousto-optic) deflectors have applications peripheral to these two substantial market segments and are therefore not considered.

Choosing among rotating mirrors, hologons and galvo-driven mirrors offers designers alternatives that have significant effects on a system. The conclusion of a two-part consideration begun last month.

Generating or collecting images
The beam deflector's job is to generate a continuous repetitive series of scans that superimpose themselves in time and space. Figure 1 schematically represents the deflector in an oversimplified optical system. In order for a scanning spot to be accurately positioned repetitively both in time and space on the scan line, it is necessary to control the velocity stability of the beam deflector as well as its tracking accuracy.

Scanner velocity stability
The velocity stability of deflectors affects the accuracy with which scene elements (pixels) can be located. A typical scanner optical system employs a “start-of-scan detector” to synchronize the system with the electronics that manipulate pixels. During the time intervals between start-of-scan pulses, the system relies on the velocity stability of the deflector to know with precision the location of the scanning spot by “clocking” the scan.

Polygonal mirrors, hologons and galvo mirrors are all driven by motors powered by electronic controllers. The quality of these motors and controls, plus the quality of the suspension systems that couple the rotor to the stator in the motor, directly affect velocity stability.

Polygonal mirrors and hologons employ conventional motors that rotate continuously in one direction, generally have high inertia, and exhibit velocity stability in the range of 0.1% to 0.01% (V/V mean). This range exists because of variations in rotor inertia (increased inertia = improved velocity stability) and variations in speed requirements (higher speeds = improved velocity stability). Additional elements that influence velocity stability in conventional motors are shown in Table 1.

To some degree these elements affect galvo-driven mirror deflectors velocity stability as well; however, galvos differ in some important ways from conventional motors. To begin with they are oscillatory devices. At low speeds they can be driven with a variety of current (amperage) waveforms and, provided a high torque/inertia ratio exists, the galvo will respond to generate a variety of deflection “waveforms.” Up to a few hundred hertz of deflection frequency, the current waveforms are faithfully reproduced optically in the deflected rays.

At higher speeds, galvos are used in the “resonant mode” of operation. Both galvo/deflector and electronic controller are tuned to provide resonant oscillation. The scan waveform in this case is a sine wave. The device is a bi-directional deflector, and usually the most linear portion of the sine wave is used for the active scan, and the more nonlinear portion and “turnaround time” become dead time. In contrast to conventional motors, galvos require low-inertia rotors in order to provide high-frequency response.

Reducing scan jitter
Scan jitter (or wobble) is defined as errors in scan spot position at right angles to the scan direction. This defect results in “banding” in recorded images and in pixel-location errors in image-collector systems.

Scan jitter results from multiple causes. In the case of polygonal mirrors, jitter contributions come from the manufacturing tolerances of the polygonal mirror, its mounting to the motor driver and the orbital accuracy of the motor itself. Several active and passive correction schemes are employed to reduce scan jitter to acceptable levels in polygonal mirror optical systems with tight specifications on jitter. High-resolution laser printers, automatic optical inspection systems and high-quality facsimile transmit/receive machines are examples of applications where correction is usually required.

A “wash” in terms of cost
In the case of volume-produced laser printers, the polygonal mirror component usually represents a cost of $20 or less. The “motor,” be it galvo, hologon driver, or polygon driver, is a wash in cost terms, so the cost tradeoff is a consideration of deflector-element cost plus corrector element cost.

Many scanner applications are relatively insensitive to scan jitter. Examples are bar-code readers, UPC symbol readers, laser gauges, etc. In these cases the cost/performance tradeoff on the deflection optics is simpler, since it involves the deflector element only.

About scanner speed
One figure of merit for a deflector system is speed. Speed here is defined as the number of resolved picture elements per unit of time that can be addressed by the deflector and scan lens combination. If one assumes diffraction limited performance for the lens (which is frequently achieved in laser optical systems), the lens may be ignored in meriting the deflector system. For comparison purposes, a figure of merit for scan speed can be expressed simply by the formula shown in the box.

Of course the units of measure (for comparison purposes) are only important in that they must be applied consistently. Table 2 (from Part 1 of this article in March) provides approximate practical pixel-rate limits for the various deflector alternatives. In the case of polygonal mirrors and hologons, limiting speeds are primarily determined by centrifugal stresses in the rotating deflector element. In galvo deflectors the primary limits are the torque/inertia ratio of the motor-deflector combination and, in some cases, the torsionally induced stresses introduced in lightweight, low inertia mirror structures.

Deriving scan efficiency
Scan efficiency (as defined earlier) is less than 100% for all the deflector alternatives except in the cases of polygonal mirrors and hologons where facet overfilling is acceptable (trading against light loss). Figure 2 shows an optical system that employs a polygonal mirror with two facets simultaneously filled with incident light. In this example, Beam A is shown completing a scan as Beam B simultaneously begins a new one with no loss of time.

The most common application, however, utilizes underfilling of facets, in which case a first approximation of scan efficiency is derived by examining the ratio of beam diameter to facet length in the scan direction. That ratio in percent subtracted from 100% yields scan efficiency. The same general rules apply for hologons. The scan efficiency for resonant galvos is simply that percent of the sine wave trace that contains sufficient linearity to meet the system requirement.

The technical task of selecting from deflector design alternatives to achieve high performance and low cost is critical and requires a detailed analysis of the complete scanning optical system and its costs. The intent here has been to provide some insight into important e

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