New Laser Control Techniques from Polaris Motion
The two broad classes of lasers are Continuous Wave (CW) and pulsed. CW lasers are typically lower cost and have the advantage of high average power. Pulsed lasers have lower average power, but the short pulse photon energy breaks the material’s chemical bonds instead of burning. Lasers are used for material altering processes such as annealing and quenching for metals, and modifying impedance in semiconductors.
Many manufacturing processes such as cutting, drilling, welding, etching, engraving and ablation require laser control. For each of these processes, there is a preferred technique for controlling the delivery of laser energy. Laser control minimizes the size of the Heat Affected Zone (HAZ) for the process, ensures that cuts are clean, and often obviates the need for material post processing. Fine laser control is needed for powder bed fusion (PBF) and for directed metal deposition (DMD). Polaris Motion’s suite of laser control techniques supports both CW and pulsed lasers.
Continuous Wave (CW) Lasers
A Continuous Wave (CW) laser is a laser that emits a continuous laser beam with controlled heat output. For CW lasers Polaris Motion provides both Pulse Width Modulated (PWM) control and analog control. With PWM duty cycle control, the percentage of time that the PWM signal is high controls the intensity of the laser. As an example, if maximum power is 200 W, corresponding to 100% duty cycle, then a duty cycle of 25% results in 50 W of continuous laser power. Duty cycle can be changed instantly.
Fig. 1. PWM Duty Cycle Control of Continuous Wave (CW) Lasers
Many lasers have an analog control input. These inputs are typically 0-10 V or ±10 V, and can take any value in that range. The laser power output is directly proportional to the control signal input. In the 0-10 V case, if the maximum input signal of 10 V corresponds to 200 W of laser power, then 3 V would result in 60 W. The analog input voltage can be changed instantly, resulting in instant changes in the laser output power.
Fig 2. Analog Control of Continuous Wave (CW) Laser
PWM Control for CW Lasers
Constant power CW lasers have been popular for a long time due to their wide availability and low cost. However, to avoid hot spots and over-burning, laser processing must occur at a constant speed to match the constant output power of the laser. By using this technique, a constant amount of laser energy is deposited per unit length along the processing path, see Fig. 3b. The problem with constant power lasers is that maximum processing speed is limited by the cornering speed. The cornering speed, which is limited by the machine’s ability to accelerate and decelerate, becomes the maximum overall speed of the process. If an operator tries to go fast along the straightaways and slow around the corners to maximize part per hour throughput, over-burning at the corners will unfortunately result.
Fig. 3. Obsolete Methods for CW Laser Control
Fortunately, Polaris Motion offers the ability to change laser power automatically using duty cycle Pulse Width Modulated (PWM) control. Laser power can be changed instantly to match the laser processing speed along the work path, resulting in even deposition of laser energy (fluence) as shown in Fig. 4a. Furthermore, if it is necessary to have variable laser energy deposition for a special process, then the ratio of laser power to speed can be changed automatically, as indicated in Fig. 4b.
Fig 4. Polaris Motion Laser Control Methods
Analog Control for CW Lasers
Polaris Motion also supports analog control of CW lasers. The simple method is proportional control. The laser output power is simply proportional to the control signal input, which can be made proportional to the speed along the path as one example. However, Polaris Motion also offers Analog Laser Pulse Shaping (ALPS™), see Fig. 5. The operator has a number of control levers, including: the pulse shape itself which is created in a table, the amplitude (A), the on-time of the pulse (T1), and the pulse period (T2). ALPS™ is useful for controlling the thermal energy density of a melt pool for both laser welding and for metal 3D printing applications.
Fig.5. Analog Laser Pulse Shaping (ALPS™)
A pulsed laser is a laser that emits a succession of high energy pulses. Pulsed lasers are classified as nanosecond (ns), picosecond (ps) or femtosecond (fs). The faster the laser, the smaller the heat affected zone and the greater number of material types that can be processed. Pulsed lasers are typically used for precision material processing applications in the semiconductor, military, medical, scientific and hand-held device markets. At Polaris Motion the two ways of controlling pulsed lasers are (1) fixed pitch laser control (FPLC) and (2) pulse-on-demand control (PODC).
Fig. 6. Digital Control of Pulsed Lasers
Fixed Pitch Laser Control
With Fixed Pitch Laser Control (FPLC), the laser is triggered to fire an optical pulse at fixed increments along the laser path even under varying speeds of motion. In the geometry example (Fig. 7), laser spots are placed along the path at constant increments even as the motion speeds up along the straightaways and slows down at the corners. The density and rate at which laser pulses are triggered are settable parameters, which can be changed on the fly. In fixed pitch mode, the pulse placements are independent of the path points.
Fig. 7. Fixed Pitch Laser Control
With Pulse-On-Demand Control (PODC), laser pulses may be placed at arbitrary, exact locations on any geometry. In PODC mode, the location of the pulse placement needs to align with path points. If exact placement of laser pulses is required, then Position-Velocity-Time (PVT) path planning is recommended. If placement of pulses can be specified within a small tolerance, then linear and circular interpolation path planning can be used.
Fig. 8. Pulse-On-Demand Laser Control
About Polaris Motion
Polaris Motion specializes in motion control solutions for Computer Numerical Controlled (CNC) machines in laser-based manufacturing, diamond tool cutting, grinding of optical surfaces and other specialized high performance areas.
To request more information or a quotation for Polaris Motion products, contact IL Photonics.