Amit Shahar
General Scanning
Billerica, Mass.

Optical scanning can rightfully be termed the Ferrari of motion control applications because the required speed and accuracy is typically an order of magnitude or two faster than conventional motion control systems. Optical scanners control the movement of a mirror, which in turn steers a laser beam for applications such as marking characters on everything from medical products to packaged food.

The fastest industrial controllers can typically execute a command in 400 µsec, while optical scanners react in just 10 to 20 µsec. The positional resolution of optical scanners is typically measured in microradians, which equates to millionths of a rotation. This article offers an overview of these super-high performance, servo controlled motion systems and focuses on substantial gains achieved through the recent innovation of tuning servo performance in the "application domain."

Optical scanners were originally developed for quick and accurate laser beam positioning. Typical applications include engraving, cutting, welding, perforation, and today's leading application area, laser marking. The automotive industry uses laser marking to identify parts with vehicle identification numbers and part numbers, and to engrave components such as switches and instrument panels. Lasers are also used to mark medical products so their origins and lot numbers can be tracked. For example, pacemakers, artificial joints, mechanical heart valves, and surgical instruments all carry these codes.

The MPower motor strategy used in the Lightning Digital Scanner platform is a three-step process that inertially matches the motor and load (mirror and mirror mount). Treated as a system, the final configuration optimizes operating efficiency as well as power transfer.

Lightning scanners incorporate digital servo drivers employing advanced DSP technology. The drivers maintain a high signal to noise ratio, which greatly improves performance. They also provide dual-axis control in a form factor barely larger than many single axis boards.

Elements of an optical scanner

A typical optical scanner is composed of two elements — a galvo motor and a one- or two-axis servo driver. Beginning with the galvo motor, this is not simply a motorized mirror, but a complex system that can only achieve high performance when all the inertial elements — mirror, mount, and motor — are designed and tested as an integrated working unit, just like a Ferrari.

Galvo motor optimization usually begins with the mirror, which must have the correct flatness, reflectivity characteristics, and appropriate size for the application. Size determines how large a laser beam can be used, which in turn dictates the power that can be delivered. The mirror needs to be stiff to provide adequate servo bandwidth and positioning accuracy. However, increasing stiffness raises inertia. So, a balance must be struck when optimizing mirror performance for its contribution to galvo motor inertia.

The mirror mount is the next item to be considered. Again, minimum inertia, maximum mechanical stiffness, and in many cases, the ability to support multiple mirror orientations are critical parameters. The combined inertial performance of the mount and mirror must closely match to the rotor to achieve optimal motor performance. Typically, an inertia ratio of no greater than 3:1 should be observed to arrive at the highest servo bandwidth and greatest positional accuracy.

Other considerations regarding galvo motor design affect performance and long-term durability. Because these motors are limited rotation devices with poor lubricant circulation, special hybrid bearings with ceramic balls and stainless steel races are used to maximize motor life.

In addition to wear, their extreme performance means intense heat buildup within the motor. This heat must be conducted to the motor case so it can be dissipated before damage occurs. Optimized coil winding and forming techniques improve power transfer to the rotor and coil-to-case thermal conductivity. This has the three-fold benefit of improving heat transfer, increasing motor efficiency, and minimizing thermal drift. Most importantly, it enables the best balance of torque and response time.