Digital Servos, Software Chart New Directions in Optical Scanning
- Monday, 01 January 2007
Optical scanners, or servo-controlled, limited-rotation motors with laser-beam steering mirrors, were first introduced 40 years ago by General Scanning. Since then, they have become the enabling technology behind many innovative products across many different industries, including medical imaging, industrial machining, product identification, biomedical research, automotive manufacturing, and many more.
While optical scanner performance has improved dramatically, little has changed in the way systems are developed, manufactured, and supported since the introduction of this technology. Today, new systems are emerging that consist of three elements: innovative scanner motors, advanced digital electronics, and a comprehensive software toolset.
To put this advancement in perspective, it is useful to review past solutions and their strengths and weaknesses. The original optical scanning technology was based on bulky motors controlled by analog servo drivers. Performance tuning for a single- or multi-axis system was a labor-intensive and time-consuming manual procedure. But it was the only game in town.
The original limited-rotation motors were fairly large and prone to wear and drift from thermal changes. Advances in materials and manufacturing techniques resulted in smaller, faster, more accurate motors. While these newer motor designs perform dramatically better and are far more reliable than those of just a few years ago, performance improvements are reaching a practical limit. So how does one achieve any meaningful improvement in motor speed and accuracy? It is done through inertia matching.
Even though scanner motors are now smaller and more efficient, and mirrors have become lighter and stiffer, system performance often falls below its potential. The reason is because engineers don’t often address component synergy. When configuring a scanner motor, little attention was paid to the optimal inertial match between the total load (mirror and mount) and the motor’s rotor assembly. In fact, some manufacturers exacerbate the problem by specifying that the load inertia can be up to ten times larger than that of the rotor. It turns out that the inertial match between the load and the rotor is one of the most important criteria in achieving optimal motor performance.
Figure 1 illustrates this new understanding of optical scanner motors. Rather than viewing the scanner motor as a collection of discreet components, General Scanning’s new scanner motors are configured as integrated assemblies, and designed for loads that do not exceed three times the inertia of the rotor.
Digital Servo Driver
Optimized scanner motor design is one step toward reaching the performance limit of an optical scanner-based system, but actual system-level performance, as compared to scanner motor performance, is largely dependant on how well a servo driver controls the motor.
Figure 2 illustrates a typical system design. Until recently, optical scanning servo drivers were analog devices. Engineers needed extensive servo knowledge and considerable time to optimize system performance for a given application. “Tuning the servo” was a trial-and-error process aimed at achieving the best balance of speed and accuracy, while eliminating the system’s resonance-frequency. In a dual-axis system, keeping a pair of optical scanners in lockstep became increasingly difficult as speeds increased.
The advent of digital servo drivers improved servo tuning significantly, but even today’s digital drivers are still rather limited. Some will let engineers load pre-defined tune files or allow rudimentary “self-tuning” to compensate for the inherent variations in all scanner motors. Yet, no digital driver offered real control over the tuning process. And with dramatically different tuning requirements for applications such as marking, welding, cutting, and drilling, servo tuning is still a time-consuming manual process.