13-11-2012, 05:20 PM
A mechatronics approach to laser powder deposition process
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Abstract
This paper introduces a mechatronics approach for the development of a closed-loop control system utilized in laser powder deposition.
The laser powder deposition process, as a manufacturing technique, is combined with a feedback control system to increase the
quality of the final formed parts. The interconnections between the technologies involved in this complex mechatronics system along with
the development of a CCD-based optical detector are explained. The optical CCD-based detector monitors the process zone to provide a
series of single pass band images of the near-locus region. A pattern recognition algorithm is incorporated into the feedback device to
obtain the clad’s height and angle of solid/liquid interface in real-time. This feedback device is blended in a PID-based controller, which
is designed using a knowledge-based model, to adjust the laser pulse energy for enhancing the output of the process. The experimental
assessments of the developed system are also presented at different process conditions and disturbances when Fe–20%Al was deposited
on mild steel. It is shown that the PID-based controller can effectively improve the geometrical characteristics of the clad around the
operating point by overcoming the effects of various disturbances.
Introduction
Laser powder deposition (laser cladding by powder
injection) has received significant attention in recent years
due to its unique features and capabilities in various industries
involved in metallic coating, high-value components
repair, rapid prototyping, and low-volume manufacturing.
This emerging laser material processing technique is an
interdisciplinary technology utilizing laser, computer-aided
design and manufacturing (CAD/CAM), robotics, sensors
and control, and powder metallurgy.
In this process, a laser beam melts powder particles and
a thin layer of a moving substrate together to create a bulk
layer on the substrate as shown in Fig. 1. A great variety of
materials can be deposited on substrates to form a layer
(which is known as ‘‘clad’’) with a thickness of 0.1–2 mm.
Automated laser powder deposition apparatus
An automated laser powder deposition technique, as a
complex mechatronics system, synergies five common technologies:
laser, computer-aided design (CAD), robotics,
sensors/control, and powder metallurgy. The central intelligent
decision making unit connects all these technologies
together to provide a fully functional system. Such a system
can be applied in a variety of ways to overcome the limitations
of existing open-loop laser powder deposition
technology, particularly in having control over the dimensions
of the layer and control over the maximum temperature
of the process zone to minimize the distortion, residual
stress, cracks and pores in the produced parts.
Closed-loop control system
The first step for the design of a controller is to develop
a proper model for the process. It was experimentally
shown that the clad’s height is a linear function of the laser
pulse energy around an operating point [17]. Several second
order discrete models and one Hammerstein–Winner
nonlinear model were developed around different operating
points to predict the clad’s height based on the laser pulse
energy changes [17].
Closed-loop system assessment in the process startup
In the layered manufacturing process, it is important to
fabricate the desired slice with high precision in the thickness.
Initiation of a layer using an open-loop laser powder
fabrication system with a minimum possible energy, which
eventually results in a desired clad height, causes a gradual
growth in the clad height. The developed closed-loop control
system can overcome this problem by applying higher
energy in the start up of the cladding process and reducing
the energy afterward. Fig. 10 compares the experimental
results when a closed-loop and open-loop systems were
used. As seen, the closed-loop system improved the clad
height significantly especially in the start of the process.
As also seen, the amount of energy in the process startup
increases to compensate for the lack of consolidation
energy. Thereafter, the level of energy decreases to the level
of steady state due to the accumulative heat in the substrate.
The figure shows that small overshoot occurred
while a closed-loop control system was used. This increases
the desired clad height by 15% at the start-point of the
clad.
Conclusion
Based on the earlier discussed results, the following conclusions
can be drawn on the development of closed-loop
PID-based control system for laser powder deposition:
1. The CCD-based optical detector located normal to the
process zone can effectively measure the clad height. It
can also be used for measuring the angle of liquid/solid
interface in real-time. However, the application of this
type of detector is limited to simple straight tracks of
the process, not to a complex and curvature tracks.
2. The PID-based controller designed using a knowledgebased
model can be utilized in the laser powder deposition
process and can effectively improve the geometrical
characteristics of the clad around the operating point by
overcoming the effects of disturbances.