PTB >> Distributed Intelligence Drives High-Energy Laser Manufacturing

Historically, high-energy (Jouleclass)
nanosecond YAG lasers have been confined primarily to laboratory research facilities and government labs. System designs were unique for each facility and the low number of systems worldwide made the learning curve for system improvement slow. It is only in recent years that the maturity of the technology has reached a state that makes it possible to consider these lasers for industrial applications.
In order to be successful in an industrial environment, these systems need to be user friendly and reliable. Subassemblies must be modular and easy to service. Finally, they must be capable of monitoring their performance parameters and maintaining them consistently with a high duty cycle. One of the emerging applications for this class of laser system is laser shock peening. While first demonstrated in the mid- 1970s, it was only in the late 1990s that the technology was advanced to the point of commercial acceptance.

Shock Peening Lasers

Laser shock peening is a technique that generates compressive stress on the surface of a metal part, greatly enhancing its resistance to metal fatigue and micro-cracking. A sacrificial layer is applied to the surface of the target and is covered by a transparent layer - usually water. The laser energy is absorbed by the sacrificial layer and the plasma is contained by the water layer. The shockwave is transmitted into the target material, creating compressive stress in the surface of the part.

Continuum (Santa Clara, CA) has been supplying laser systems for laser shock peening applications since the late 1990s. With an installed base of over 30 systems, the company has worked closely with the early adopters of the technology in developing the right tools for the job. While early systems used many of the same components as traditional laser sources, more recent systems have advanced to include features that make their introduction into true industrial environments possible, including distributed intelligence and monitoring systems.

High-energy YAG lasers consist of an oscillator and a series of amplifiers working together to create the necessary energy for the application. Repetition rates can range from 10 Hz to 50 Hz and more. Figure 1 shows a laser that delivers in excess of 25 J at 1064 nm, and in the configuration shown, outputs 4 J at 532 nm in a 4-ns pulse.
The system architecture is modular, with distributed microprocessing throughout. Each oscillator or amplifier set has a dedicated microprocessor that monitors temperature and is available to provide controls for the system’s Q-switch, as well as accessories such as an intra-cavity shutter, automated attenuator, external interlocks, pyro detectors, and management of harmonic generators.

The microprocessor in each power supply controls the recirculating water supply, monitoring deionization (DI) resistance, flow, and temperature. It also manages the high voltage delivered to the lamps. Improvements in design of the core supply technology have led to longer lamp lifetimes, with up to a fivefold increase in some systems.

GUI Delivers ‘User Friendly’

The power of the system is evident when it is all brought together under a sophisticated graphical user interface (GUI), as shown in Figure 2. The main feature of the GUI is the layout of the laser head and its various pump chambers. Each pump chamber is individually addressable by the user, with a graphic
representation of its current state — green for normal operation, blue if offline, and red if an error is present.

Below the layout are the individual controls for setting the laser’s output parameters and a series of monitors that displays pump chamber temperatures and output energies as measured at different points in the beamline. The GUI monitors temperatures to stay within preset limits. If the temperatures are too low, the laser is not yet warmed up and cannot be advanced into a processing mode until warm-up is complete. If a temperature is too high, the laser is brought into a standby condition for operator intervention.

All setup commands are under password protection and cannot be altered by the system user without adequate permission. Three levels of security are present: one for the operator, one for the setup technician, and one for service and maintenance.

The system stores setups in a “recipe” file. Each recipe has all of the voltage and delay settings for a specific output power. A series of recipes can be combined into a “cookbook” that can be linked to a processed part, an individual user, or a specific development program. For example, a set of pulse energies can be called by an external system such as a robot that is holding and processing a part. The robot can then call pulse energies on the fly, changing the energy of each pulse from shot to shot.

The GUI stores all operating parameters and sensor feedback in a log file. The user can choose which functions to store and how often data is taken. The file can then be used for quality control, comparing completed parts to their processing parameters. The help file contains system information from the manual, and there is also a user-editable help file knowledge base that makes it possible to keep solutions for past problems local to the user for immediate reference.

Each section of the GUI code is modular and interchangeable. Using this approach, it is possible for Continuum to prepare a unique custom interface for each customer.

A wireless Bluetooth-enabled remote control is available. It does not directly control the laser system, but synchronizes with the GUI and sends a subset of the system commands from the operator to the system control computer. Operating on a simple, commercial Windows CE device, the remote control makes alignment and service straight forward without the need for connecting wire. Since the remote is a Bluetooth device, it is encrypted for security and will only operate within 30 meters of the GUI. At any time, the GUI can disconnect the remote for safety.
Inexpensive computing power, Bluetooth technology, and intelligent user interfaces are bringing high-energy YAG lasers into mainstream industrial applications.
This article was written by Mike LaHa, program manager, and Curt Frederickson, director of marketing, at Continuum Inc. For more information, contact Mr. Frederickson at C.Frederickson@continuumlasers.com or visit http://info.ims.ca/5785-200.