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Lasers & Sources

Scanning the Scene

Choosing the right galvo for your application requires understanding system requirements.

From oemagazine May 2004
30 May 2004, SPIE Newsroom. DOI: 10.1117/2.5200405.0008

Although my first exposure to galvos was many years ago, I still marvel at their capacity to perform tasks with great accuracy at lightning speeds. A decade ago, I thought the market expansion for galvos had peaked. Having witnessed double-digit growth since that time, I am now convinced that galvo applications are endless. Galvos increase productivity and, along with their laser counterparts, have enabled new technologies to emerge and whole industries to flourish.

Galvanometers are used across a broad spectrum of industries including transportation, biotechnology, entertainment, medical diagnostics and therapeutics, and defense (see figure 1). They are used to collect images in confocal microscopy, DNA biochip imaging, and lidar applications. They can produce images by scanning information onto film or plastics. Galvanometers are the workhorses behind many laser-based, materials-processing applications such as ablating, cutting, drilling, marking, and welding. Galvanometers project laser beams that are used in the construction of buildings and jets. In entertainment, they dazzle us with laser motion displayed in light shows. When you think you have seen it all, someone will come up with a new application. One of my favorites was the cattle rancher who wanted to brand his cattle from a helicopter.

Figure 1. Galvo scanners can produce laser-processed objects such as parts made using laser-based rapid prototyping; images marked on fabric, wood, metal, and plastic; and products with laser-scribed codes.
Getting Started

To select the appropriate galvanometer system, start by listing the basic parameters, which are derived from your specific application. These parameters, which include optical wavelength, field size, working distance, focused spot diameter, accuracy, process time, and lifetime, will help guide you to some basic choices for an optical design and the appropriate galvanometer. As with many engineering decisions, there are tradeoffs. The goal is to select a galvanometer that provides the appropriate platform to handle the mirror size, speed, accuracy, and scanning lifetime requirements for your application.

Figure 2. This moving magnet galvanometer features a 30-mm beryllium scan mirror.

A laser scanning galvanometer consists of a limited-rotation torque motor with a sensitive position detector and a mirror attached to one end (see figure 2). The limits of rotation are typically bounded by about 40° mechanical (±40° optical). The rotor may be supported by flexures or ball bearings. The hallmark of a good galvanometer is its ability to produce a high-bandwidth response to a given command signal, while maintaining accuracy and repeatability over a wide range of operating conditions.

Galvanometers are controlled with servo drive amplifiers. These devices close the loop around the galvo position detector based on an input command from the host controller. Host controllers are typically PC-based boards that convert a set of vectors into microstep commands for the servo drives. Most servo drivers can accept a range of input command voltages and some a 16-bit digital command. Instrument-quality servo drives use low-noise and low-thermal-drift components.

We evaluate galvo speed in different ways depending on the application. Small-step response is relevant in point-and-shoot applications such as laser drilling. In text marking, the ability of a scanner to maintain accuracy and speed is emphasized, often measured in characters per second. In raster imaging applications, it is the galvo's ability to continuously perform large moves at high frequency that determines speed performance.

Galvo response is dependent on the inertia of the rotor and mirror load, the available torque, and the system bandwidth. The bandwidth is limited by resonances found in the entire rotor structure including the mirror. A galvo with a high torque-to-inertia ratio and stiff rotor/mirror structure will provide optimal speed, which is why advanced galvo designs use materials with high stiffness-to-weight ratios, such as ceramics.

Users often misunderstand scan speed. The benefit of a fast scanner can only be realized if accompanied by accuracy and enough laser power to process materials at high speed. Frequently, applications cannot take advantage of the full capability of a given high-performance galvo, in which case a more modest galvo is appropriate. You wouldn't, for example, buy a racecar for city driving or connect a fire hose to a sink faucet.

We measure angular sensitivity of the position detector in microamps per degree of rotation. Maintaining a high signal-to-noise ratio requires high-sensitivity position detectors and low-noise electronics. Accuracy and repeatability of galvos can be compared in terms of signal-to-noise ratio. High-signal, low-noise scan systems typically achieve the highest accuracy.

Repeatability is the ability of the system to scan to a position and come back to that position accurately. We can class repeatability as short term or long term. Long-term repeatability can be affected by temperature-induced drift, caused by thermal effects on the galvo position detector and servo drive. We can divide drift into offset drift and gain drift. Offset drift causes the center position of the field to move. Gain drift causes the field size to change.

Although the repeatability of scan systems generally ranges from 1 part in 10,000 to 1 part in 40,000 of the scan field, short-term accuracy depends on other factors such as optical (lens) distortion and system alignment. System-level compensation is often required to achieve the desired accuracy.

Long-term accuracy can be enhanced by controlling the temperature of the scanner, by selecting low-drift electronic components, and through the use of fiducial targets located on the work piece or in the target area. The latter requires the system to periodically observe the targets and incorporate compensating data into the command signals.

Linearity of the position detector is a measure of its ability to produce a constant change in signal per unit of rotation. Most galvos can produce linearity of 99.9% or better, even at large scan angles. If necessary, residual non-linearity can be characterized and mapped out by the host control system.

Mirror and Galvo Choices

Mirror size and shape is based on the diameter of the beam to be reflected, the incident angle, and the required scan angle. Scanning a 10-mm beam requires a 10-mm clear aperture at all scan angles. Trimming extraneous material from the mirror to reduce inertia results in the typical elliptical or polygon-shaped galvo mirror.

The choice of mirror thickness represents a compromise between stiffness and low inertia. Optical flatness is typically λ/4 at 633 nm or better. Excessively thin mirrors cannot achieve desired flatness and can have structural resonances that limit servo bandwidth. Manufacturers base material selection on several factors, including cost, stiffness, and density. Low-weight, high-stiffness materials like beryllium or silicon carbide can reduce mirror inertia, enabling higher scan speeds. These materials cost more than the more commonly used fused silica or BK-7. Once the mirror is selected and its inertia is known, you can choose the appropriate galvo size. Typically galvos can drive loads six to 10 times higher than their inertia.

Many galvos use ball bearings to support the rotor, a design that is successful for many applications. In more demanding applications, however, ball bearings can wear excessively and fail prematurely. Ceramic bearings offer improved lifetime. All ball bearings produce some amount of mechanical noise, and skidding of the balls reduces overall repeatability of mirror position.

Figure 3. Steel flexure designs can enhance scanning lifetime (top left). Ceramic rotors (bottom left) and silicon carbide mirrors (right) increase speed in modern galvanometers.

Flexure suspensions are free from wear issues and offer the highest repeatability. Applications requiring small scan angle, fast raster performance, or high duty cycles may be candidates for flexure-suspension galvos (see figure 3). These galvos support the rotor with spring steel flexures, which can last indefinitely. They also offer the added benefit of smoother scanning and no requirement for lubricants.

Jitter is the deviation of instantaneous mirror velocity from the average scan velocity when the mirror is scanning through a given angle. This can be an issue in imaging applications such as microscopy or film imaging. Flexure galvanometers and low-noise servo drives offer improved performance in applications requiring smooth scanning with low jitter.

Galvo improvements have come in the form of higher-strength magnets, stiffer rotor materials, improved rotor support, and reduced-inertia mirror materials. Design modifications such as ceramic rotors, spring steel flexures, and silicon carbide mirrors are increasing scan rates and scan life.

Digital encoders as galvo position detectors have demonstrated improved accuracy for high-end applications. Servo drivers with digital signal processors provide alternatives to analog servo drivers. Digital servos have some advantages, but noise and accurate modeling of the galvos have been problematic. Laser and scanner control software, as well as PC interfaces, have been improving, though. PC interface boards and software for 3-D applications are now available that can run outside the PC with a USB interface.

As sure as the sun rises, innovative uses of galvos and lasers will continue to emerge. With the basics listed above, you can choose the right galvo scanner for your application. oe

Rolland Zeleny

Rolland Zeleny is a founder and the vice president of sales and marketing for Nutfield Technology Inc., Windham, NH.