Blog 3 Reasons Why Digital Servos Help Systems Operate Smarter

3 Reasons Why Digital Servos Help Systems Operate Smarter

Eric Ulmer

Eric Ulmer | August 14, 2019
Product Manager working with customers to provide flexible solutions to new challenges

Contributing Author: Johanna Brito, Marketing Communications Manager

Manufacturing systems are constantly evolving. From requiring higher performance, changing system processes, and increasing technology changes, it all trickles down to components that can keep up with shifting demands. As laser processing for medical, imaging, marking and manufacturing applications continue to grow exponentially, so has the demand for optimized performance delivered through digital technology. Traditionally, the analog servo driver has been the benchmark for performance thanks to its flexible tune configurations, but as technology demand has changed, users have started to adopt digital technology to their systems. Digital servos steer laser beams with higher precision, flexibility and finesse than analog servos. Let’s break down all the key benefits of why switching from analog to digital technology helps manufacturing systems operate smarter.

Work Smarter with State Space Technology
One of the biggest benefits of digital servos is State Space technology because it uses real-time system feedback communication, syncing the system and component seamlessly. This feedback is brought into the State Space Model using an Observer (FIGURE 1). Every time the actual drive voltage is applied to the real galvanometer (also refer to as galvo), a simulated voltage is applied to the model of the galvo to generate its current state. The Observer combines the measured system feedback with calculated feedback, and outputs a correction feedback to the controller.

FIGURE 1. Shown is the typical control loop used within an Observer-based digital servo.

Since the State Space Model provides insight to the future state of the system, it allows for predictive positioning control. For example, if the model knows the required acceleration needed to complete a job, it can auto adjust based on its capacity. If the drive capability is insufficient, the command input can be modified to stay within physical limits of the system, allowing the servo driver to work smart and deliver at peak performance throughout the course of the job. In the following sections, we explore how this example of command optimization can improve marking throughput from 30% to 100% over analog servo technology without sacrificing mark quality.

Maintain Quality with Zero Tracking Delay
A common pain in traditional marking is the velocity-limited commands used in vector patterns. Because of large acceleration requirements at the vector end points, the analog servo driver must include filtering to add a delay in the galvo’s response to a given command, limiting the amount of drive voltage required to change speed and direction. This delay is referred to as the system’s tracking delay and is directly related to the system’s step response time and maximum linear speed. Typical galvo systems have tracking delays between 100 and 1000 µsec. The marking job file must add compensation time for these delays to maintain pattern quality, which limits throughput.

The digital servo reduces this tracking delay by 50–100% without giving up stability. With knowledge of the galvo and mirror dynamics accurately captured in its model, the control algorithm within the digital servo optimizes the input command profile to limit the acceleration to a value that will keep the servo within drive voltage limits. As a result, the additional filtering can be removed to greatly reduce the tracking delay and delivering increased throughput. In some cases, the digital servo control algorithm can eliminate the tracking error completely. For example, Cambridge Technology’s DC3000 Plus digital servo driver introduces a small delay in the command stream. It uses this delay to analyze the command stream and smooth areas of high acceleration. It then dynamically adapts the bandwidth, when necessary, to apply all of the available drive voltage to lock onto the post-processed command, eliminating the tracking delays (FIGURE 2).

FIGURE 2. Tracking delays introduced by typical analog servos (a) are eliminated by the digital DC3000 Plus servo (b).

Using zero-tracking-delay algorithms, micro-precision features in the pattern that would normally be limited by the servo bandwidth are now limited only by the drive voltage, allowing up to 2X faster speeds for the smallest features in marking, micromachining, and trepanning. Applications like converting and additive manufacturing also benefit by the fact that consistent laser energy density is applied at corners and other precision features in the pattern, enabling crisp pattern quality.

Take a look at the example below (FIGURE 3) to see how reduced tracking delays are highlighted by comparing the character quality produced by laser marking using an analog servo with that of the DC3000 Plus digital servo driver running the same 6220H galvo and 10 mm mirror configuration at equal mark/jump speeds and delay settings.

FIGURE 3. Characters marked by a laser on marking paper have significantly higher quality when using a digital servo (a) compared to an analog servo (b).

The relatively small delay settings have been optimized for the digital servo driver and achieve marking speeds of 1275 CPS. To obtain similar character quality using the analog servo driver, the delay settings would need to be increased by nearly 3X and result in marking speeds of 900 CPS. This means that the optimized command from the digital servo still maintains all the critical features of the original pattern, so the mark quality remains high while increasing throughput by 40%.

Simplified Production and Field Service Thanks to Auto-Tuning
The galvo system must be closely aligned to the model’s default state for the auto-tune process to complete successfully. For this reason, auto-tuning can help identify marginal components during the assembly process. For example, large shifts in the resonant frequency could indicate an improperly installed mirror. High noise levels during the fine-tuning process could point to a marginal cable connection. In short, auto-tuning is a comprehensive verification step to ensure high-quality assemblies.

In contrast to the digital servo, the analog servo tuning process is largely isolated from component-level variations. The tuning process is essentially monitoring the position feedback of a black box system while adjusting the servo gain terms to achieve the desired response to a pre-determined command input. As long as the tuning terms can be adjusted to achieve the desired response, the system is considered verified. Even experienced tuners may not see a slight shift in resonant frequency from a marginally attached mirror.

With digital servos, auto-tuning is a simplified process, making it far more accessible to new users. It enables flexible inventory control: since the digital servo can be tuned at any time, it can be delivered separately from the galvo and mirror in efficient quantities. Additionally, field service is simplified with auto-tuning in that the process requires very little technical input for successful calibration. Once the field service person has identified the faulty component, they only need to replace the defective part and initiate the re-tuning process.

Making the switch from analog to servo drivers is driven by new application requirements that only digital control servos can meet. This is largely because of the State Space model and its key advantages over analog servos. Furthermore, next-generation digital servos are expected to improve and support evolving application requirements. This could include better modeling, faster update rates, higher resolution command and position feedback, predictive maintenance algorithms, integral system safety checks, and enhanced synchronization to system controllers and lasers.

This post has been adapted and originally posted in Industrial Laser Solutions.