Optical scanning: the Ferrari of motion control

May 1, 2007
Optical scanning can rightfully be termed the Ferrari of motion control applications because the required speed and accuracy is typically an order of

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.

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.

When treated as an optimized system, the final mirror, mount, and motor configuration is capable of performance in the kHz region. This compares to hundreds of Hz for the typical motion control application.

Smart servo driver design

The next element of the system is the digital servo driver that operates the galvo motor. Typical digital servo drivers offer up to 5 kHz bandwidth and clock rates of about 100 kHz.

As an example of technological advancement, the Lightning Digital Scanner has a second-generation, 16-bit driver that achieves 50 percent higher bandwidth than analog drivers, resulting in dramatically improved speed and accuracy. Digital signal processor (DSP) technology is far faster and enables features such as improved compensation for ambient temperature changes, which affect position accuracy, error compensation, and programmable digital filters to deal with system resonance. In addition, the DSP servo driver provides an interface to automated servo loop tuning software, a new technology from General Scanning that promises rapid application development and low cost of ownership.

Software supercharges servo loop tuning

Tuning the servo loop to the requirements of optical scanning applications is where the most interesting advancements in servo control have occurred. Consider this example: One laser marking application achieves 700 characters per second at very high quality, while another produces legible characters at a blazing speed of 1,700 characters per second. In each case, the servo parameters are adjusted differently.

Individual rough tunes were created to generate each character set. And, since the application required an x and y axis, each axis had to be optimized separately by trial and error and then matched to each other through additional manual tuning procedures. This is a time-consuming, iterative process that demands expensive equipment and considerable technical expertise. But all of that is about to change.

Servo tuning made simple

With “application domain” tuning, software enables users to observe a servo tune recipe's results while operating the system in its intended application. In contrast, the traditional manual tuning approach is to observe an intermediate indicator such as step-time, which is a hit and miss approach to perfecting the servo tune.

Using a more advanced method, such as the Lightning Digital Scanner's TuneMaster automated servo loop tuning toolset, the time required to optimize scanner system performance for the application can be reduced by up to 90 percent. This has the benefit of faster time to market as well as the ability to reduce production and support costs.

The process begins by using a tune template to develop a servo tune recipe. The user optimizes the recipe by running a normal application, changing various servo parameters, and viewing the results. For example, in a marking application, if the corners are too round, you tweak a parameter to make them more square. If the character-writing rate is below target, you back off on precision to increase speed. The ability to automatically tune to the application with software — rather than manually adjusting a set of abstract variables — is a dramatic advancement as it virtually eliminates manual servo tuning procedures.

Automated servo tuning software also enables improved unit-to-unit consistency and lower production costs. Here's how it works. As a system moves through production and is ready for final configuration, a technician simply downloads the appropriate tune recipe from the database, clicks a button, and the software matches both scanner axes automatically, optimizing their performance for the selected application. The manual tuning that was required due to the inherent variability of galvo motor performance has been rendered unnecessary. This automatic servo loop optimization software thus improves unit-to-unit consistency, while also improving production throughput and reducing costs.

This new software approach can also reduce field support and inventory costs. System performance can be maintained at “like new” levels at the click of a button. The automated tuning process compares current servo loop performance with performance data that was recorded on the day the system was commissioned. The software then re-optimizes servo loop performance with a single click of the button. Thus, system performance and user satisfaction can be improved with little or no additional cost.

If repairs are required in the field, lower-cost, off-the-shelf units can be used, rather than stocking pre-tuned scanners at service depots around the world. This automated software lets technicians optimize new components at the end user's site. Thus, the cost associated with stocking pre-tuned spares in the field is eliminated. The driver can also be configured to record periodic snapshots of system performance, which can then be used to determine when preventive maintenance is needed.

Summing up the software

Software that enables application domain tuning is a new concept that can affect much more than system performance. It can substantially reduce both time to market and the need for highly skilled production staff. It can also cut production time while improving unit-to-unit consistency, as well as enable “like new” performance over a system's service life.

For more information, e-mail the editor at [email protected] or visit gs-scanners.com.

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