Applying Lasers to the Worlds of Scanning and Stereolithography
A few years ago the world was forever
changed, with the buzzwords "point clouds," "surface contours" and
"scan density" becoming permanent definitions for the CAD world. What
was the catalyst for this change? Lasers.
Until lasers were developed, the only way to depict 3-D objects was through one-point-at-a-time data capture methods and CAD hardware capable of modeling. Anyone in the industry will tell you that lasers can be pretty boring - just sitting there outputting photons. What makes them interesting is what you do with those photons and points once you have them. But how far have lasers moved us towards the perfect CAD model or SLA part?
Solid State or Gas
To the layman, lasers are complicated technical designs and though
several types exist, the two most recognized lasers on the market today
are solid state and gas. Solid state lasers typically consist of a
dozen or two separate optical components. Each optic needs to be
manipulated in the alignment process of the laser, sometimes in two
axes, and others up to five axes of movement. Each optic has to have a
very good mechanical positioning mount that can be locked into position
so that it will not move for the lifetime of the laser, allowing it to
survive shocks. As it operates, the laser generates heat. This heat
needs to be managed so it will not misalign the optical train. The
ultraviolet solid state laser has an electrical-to-optical conversion
efficiency of a few tenths of a percent that, according to industry
scribes, is very good. To get one watt of UV power out, these lasers
have to pull a few hundred watts of electrical power from the wall,
while the rest of the energy goes into heat that must be managed.
But argon ion lasers - the gas laser often used in stereolithography systems - while delivering one watt of UV output, can consume 20,000 watts or more of energy to perform. These systems also need to be water cooled at rates of five gallons a minute. Due to the laser's design complexity and production rates, it's been hard for manufacturers using it to bring down their costs. But whether using gas or solid state, it is important for companies to have good power sources and the lasers to have good pointing stability and lifetimes in order to get the most from their use.
"When I first saw an SLA system using an argon gas laser, I saw this giant power cord attached to this machine and thought that it must be cranking out power like mad," says Diane Kalisz, VP of development for 3D Systems, Inc. (Valencia CA). "I asked how much power was being output and the answer was less than half a watt! That more than anything else shows the difference between solid state and argon gas lasers. The power conversion for gas lasers is pretty poor, which made recurring cost for high power gas laser systems quite high."
Laser Stabilizers and Scanners
One critical aspect of lasers for stereolithography application is
their pointing stability - or how much jitter occurs in the beam. For
example, a person using a laser pointer during a lecture can't hold
their arm steady enough to avoid the laser dot moving all over the
screen. Well, the same thing can occur with a beam when mapping point
clouds or tracing a part. In SLA that can be very important because for
a part to be accurately traced the beam can't be moving all over the
place. It can be just as important for scanners. Capturing the real
world and turning it into a digitized one using lasers can be extremely
difficult.
Older methods of measuring and modeling the surface geometry of complex structures (like one-point-at-a-time capture) are often incomplete and expensive, which lead to inaccurate models and costly delays. But lasers have allowed the capture of complex surfaces and items that would otherwise be deformed if touched. When used with surface modeling software, laser-based systems can determine the surface of highly contoured parts. Various schemes are used to scan and map objects. The main three methods are:
- Triangulation: Sends out a laser beam from one end of a known length oblong scanner and captures the return beam at the other end. By triangulating the angle, the scanner calculates the distance to the point. One drawback with this technology is that when objects are farther out the angle becomes very small and difficult to calculate precisely. Accuracy falls off rapidly over longer distances.
- Pulsed laser: Produces up to 1,000 distinct, individual dots in each sweep then measures the time of flight for each point. To do so with millimeter accuracy, the scanner is equipped with a timing device that measures Pico seconds. Usually a laser beam spreads out over distances. Unfocused, the beam spreads to as much as 120 mm at 100 meters, which makes the accurate capture of fine details such as pipes and stairs impractical.
- Amplitude modulated laser beam: These types of scanners sweep a continuously varying beam by way of rapidly rotating mirrors and capture the return with a receiver that detects the reflected energy. The receiver matches the return waveform to the output modulation and calculates the distance to the object.
Lasers in Use
But the key to finding a solution that fits your problem is finding the
laser that matches your application. In laser SL, for example, a liquid
photopolymer resin is selectively cured using an ultraviolet laser
beam. The laser is focused onto the surface of the resin scanning the
surface. Moving the beam's focal point, the resin is hardened almost
instantaneously. In this way, each layer is of a shape corresponding to
a horizontal slice of the model. The machine's software provides each
slice of the CAD model to the system. After each layer is solidified,
the part is lowered in the system, and the subsequent layers are built
upon one another. The entire process is repeated, building layer after
layer until the complete part is finished. At completion time, the part
is fully submerged in liquid resin and is then lifted out of the resin
and can be removed for final curing and cleaning.
"The most important use of this technology is helping manufacturing companies improve the quality of their products," says Alex Laymon, president of DPSS Lasers, Inc. (San Jose, CA) - a laser developer/manufacturer. "There is a nice tie-in between the two practices of scanning and SLA because the output of one can be built onto the next one. For example, laser-scanning companies might output IGES files so that they can go directly into CAD, or they might output files that go directly into stereolithography files or one of the other RP machines - manufacturers will tend to hang their hats on whatever works for them."
For more information contact Alex Laymon of DPSS (San Jose, CA) at (408) 298-7755; John Farr of Polhemus, Inc. (Colchester, VT) at (800) 357-4777; or Diane Kalisz of 3D Systems, Inc. (Valencia, CA) at (661) 295-5600.
