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Challenges in Motion Control for Precision Laser Processing

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Laser manufacturing, combining optics, electro-mechanics and computer sciences, is widely applied in many technology manufacturing applications. According to industry reviews and forecasts from Laser Focus World and Industrial Laser Solutions magazines in early 2013, global laser sales have returned to the pre-Recession levels of 2008 with an upward tendency, with metal processing taking the majority of global laser material processing output in recent years. In specific applications, surface treatment (such as laser marking and engraving) represents 42% of all material processing and takes the first place, leading laser cutting and laser welding—which take second and third places respectively, comprising 34% of overall material processing applications. Laser material processing is applied in manufacturing areas including automotive, aviation, electronics, machining, and steelworks. Additionally, "Global and China Laser Equipment and Processing Industry Report, 2012-2014" released by Global Information (GI), points out that global laser sales in 2011 increased by 14% from USD7.46 billion in 2010, and were expected to grow moderately by around 2% in 2012.

Figure1: Global laser material processing distribution by application, 2009
(Source: Indus. Laser Solution, Y09)
Figure2: China's laser equipment market distribution, 2011
(Source: Global Information, Y11)

China's laser equipment market growth in 2011 exceeded the global rate. As a result of the macroeconomy, as demand from China's industrial sectors for high-power laser equipment declined, small and medium-power segments maintained growth. China's potential demand for laser equipment is considerable, with emphasis on automotive, semiconductor, and electronics sectors. China's laser processing services are heavily concentrated on precision metal machinery and laser drilling applications, with the two accounting for more than 60% of the nation's laser processing market.

Precision laser processing and cutting are popularly used in silicon wafer slicing for solar cells, cell phone screen cutting, semiconductor wafer slicing, and CNC machines, among others. High-end motion control products for laser equipment must accommodate micro-scale precision in contour cutting and adjusting released energy to cope with different materials while yielding the best possible results.

These issues and current solutions are discussed here with presentations of demonstration testing.

Low Laser Cutting Precision

Laser power is usually controlled by pulse frequency and mark/space ratio adjustments, requiring instant and precise positioning control as well as rapid shifts between velocities and power required. However, if, when cutting a non-linear signature, velocity is destabilized, especially during changes in direction and stops, and if laser power output fails to adjust synchronously, scorching occurs, as shown.


Figure 1: Scorching resulting from inferior laser power adjustment

Laser power is generally adjusted by Pulse Width Modulation (PWM), controlled through changes in mark/space ratio and usually performs better at stable speeds. If cutting motion accelerates and laser frequency is not adjusted consistently, the laser beam fails to deliver in time, resulting in non-uniform burning and scorching, as shown.


Figure 2: Non-uniform burning

Difficulties in Precision Trajectory Acquisition

Laser cutting requires superior motor control to maintain trajectory accuracy and prevent contour distortion, as shown. As an open-loop controller is unable to correct errors and perform compensation during operation, a closed-loop system can ensure more precise and accurate outcomes. The closed-loop controller, however, requires considerably more precise PID tuning for tracing effect, which can be time-consuming.


Figure 3: Curved lines are distorted in the absence of adequate PID compensation


Figure 4: Non-precise path-tracing (left) compared with precise tracing (right)

Difficulties in Laser Power Adjustment

Frequently, workpieces to be cut, such as solar power panels or cell phone touch screens, comprise multiple layers of different materials, each of which requires a different laser power level for cutting. Most laser controllers currently available, however, provide only a single lookup table by which correlation can be set between velocity and laser power, the table requiring a reset when switching materials, meaning the same trajectory must be processed several times. This negatively affects throughput.

Velocity Planning is Time-Consuming

When using lasers to cut more complex shapes, simple speed planning is insufficient to produce desired outcomes. For example, when cutting touch screens for cell phones, splines or longer geometric or curved lines are frequently encountered, and non-precise velocity control can lead to mechanical vibrations in acceleration and deceleration or serious distorted contours, such as overcuts or dithering, as shown. Because product designers normally provide only positions for the required contours without supplementary speed planning data, material processing manufacturers must plan speed by themselves, with repeated revisions impeding both accuracy and throughput.


Figure 5: Dithering laser trajectories resulting from inaccurate velocity planning


New Generation Motion Control Cards Overcome the Challenges of Laser Processing

Improving Transient Response of PWM Control

Current motion control cards employ software-enabled duty-cycle-PWM controllers which may fail to instantly and stably control PWM time sequencing. To differentiate between velocities and contours, the new generation of motion control cards uses hardware-based PWM, which combines multiple control methods including frequency modulation, duty modulation, and blend modulation, to allow laser machines to release different levels of energy corresponding to changes in cutting velocity. A lookup table dictating different velocities and corresponding duty-cycle percentages is used to prevent over-melting. ADLINK refers to this energy correlation as VAO (Velocity Amplitude Output), as shown.


Figure 6: Multi-PWM control modes


Figure 7: VAO table

Using multi-VAO for dynamic modulations

PWM using multiple VAO lookup tables can manage multi-layered materials without repeatedly cutting the same trajectories, helping to reduce production time and improve production efficiency and capacity, as shown.


Figure 8: Multi-VAO

Precise trajectory tracing and easy PID tuning

To increase contouring precision, new generation motion control cards use full closed-loop control to minimize errors and improve legacy control card efficiency, as indicated in Figure 9.

For high-precision trajectory-tracing, a PID controller is needed, with some new generation motion control cards providing easy tuning programs to facilitate quick PID parameter setting, as indicated in Figure 10:


Figure 9: Optimal trajectory tracing; the purple waves indicate errors


Figure 10: Easy tuning tools

Automatic velocity and trajectory planning

New generation motion control cards use algorithms generated by the SoftMotion module algorithm to compute out and automatically generate optimal contouring capacity.

The LookAhead function provided by SoftMotion can compute out and predict an upcoming angle in the trajectory, allowing the machine axes to slow and smoothly complete the required trajectory.

SoftMotion requires only three simple parameters to carry out operations, "max. velocity", "max. acceleration" and "error tolerance"( as shown in Figure 12). With these, the SoftMotion algorithm can compute and map out optimal kinematic trajectories for complex contours.


Figure 11: LookAhead function


Figure 12: MotionCreatorPro 2 velocity planning and setting

Demonstration

New generation motion control cards developed by ADLINK incorporate the new functions and technologies described to improve laser cutting quality and efficiency with optimal path tracing and minimized processing errors.

Table 1 shows specifications of the test machine, which uses servo motor and ball screw with maximum velocity at 800 mm/s. Optimal closed-loop PID parameters are acquired with ADLINK Easy-Tuning programs, which reduce errors to ±2 measuring units (±5µm in this case).

In this demonstration, a contour is processed (as Figure 13 shows) comprising 4,500 short lines, with errors at four round angles and four long straight lines presented in Table 2. The overall laser processing errors at round angles are less than 2.2µm and errors at long straight sides are less than 0.5µm.

From the enlarged images shown, it can be seen that laser energy was released within a range and distributed uniformly whereby the processed trajectory is smooth and without dithers. This demonstrates that ADLINK's new generation motion control card can realize not only ordinary multi-axis interpolation motion but also complex contouring processing with laser cutting. The test demonstrates the real time laser delivery strength and feedback tracing speed that can conserve CPU resources and guarantee processing efficiency.


Table 1: Test Equipment Specification


Figure 13: Easy tuning tools


Table 2: Trajectory Tolerance

ADLINK's advanced motion control card PCI-8254/8258 provides highly efficient motion control performance, using state-of-the-art DSP and FPGA technologies to offer high-speed and high-efficient hybrid analog and pulse command types. It also provides servo update rate up to 20 kHz through hardware-based PID-FF closed-loop control. With downloaded programs the ADLINK motion control cards can execute up to eight individual tasks simultaneously. ADLINK also provides a variety of easy-to-use tools including rich motion control functions and diagnosis and operational interfaces for users to realize faster and more precise motion control. SoftMotion technology enables significant reduction of development periods and provides superior synchronous motion control performance, saving up to 25% to 50% in costs for system integrators.

Conclusion

The laser processing industry will become increasingly prevalent in the future, with application in fields ranging from automobile body, smart phone, TV display panel, and device housing to medical areas such as dental fabrication and laser therapy. Highly efficient laser processing is also a positive response to carbon-reduction agendas, with many countries having invested heavily in laser processing technology to lead technological accomplishment. In the Greater China region, over 200 laser equipment manufacturers are competing for market share. However, faced with high-end products imported from Europe and the USA, these local manufacturers have to enhance their software capacities and product quality to gain a foothold in the market and increase revenue.

ADLINK Technology, Inc., with over 10 years of experience in developing and providing motion control technologies and long-term partnership with system integrators, has successfully incorporated complex velocity planning and laser power calculation into a single motion control card to allow users to focus on CAM trajectory planning without worrying about complex mathematic calculations. This creates market add-value and makes ADLINK products different. Looking into the future, the advance in use of 3D processing equipment will play the role of currently popular 2D CNC machine tools but with the added demand for more complex surface treatment capabilities.

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DSP-based 4/8-axis Advanced Motion Controllers
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