Calibrating a focused beam of energy in a solid freeform...

Optics: measuring and testing – Lamp beam direction or pattern – With lamp focusing

Reexamination Certificate

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Details

C356S121000, C356S123000, C425S174400

Reexamination Certificate

active

06646728

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates in general to calibrating a focused beam of energy in a solid freeform fabrication apparatus, and, in particular, to a method of measuring the propagation characteristics of the beam to produce beam propagation data. The beam propagation data can be used to verify that the beam is operating within tolerance, an and/or produce a response that can be used to further calibrate the beam. The invention is particularly useful in determining asymmetric conditions in the beam.
2. Description of the Prior Art
Recently, several new technologies have been developed for the rapid creation of models, prototypes, and parts for limited run manufacturing. These new technologies can generally be described as Solid Freeform Fabrication, herein referred to as “SFF.” Some SFF techniques include, for example, stereolithography, laminated object manufacturing, selective phase area deposition, multi-phase jet solidification, ballistic particle manufacturing, fused deposition modeling, particle deposition, laser sintering, and the like. In SFF, complex parts are-produced from a build material in an additive fashion as opposed to traditional fabrication techniques, which are generally subtractive in nature. For example, in traditional fabrication techniques material is removed by machining operations or shaped in a die or mold to near net shape and then trimmed. In contrast, additive fabrication techniques incrementally add portions of a build material to targeted locations, layer by layer, in order to build a complex part. Generally, SFF technologies such as stereolithography, selective laser sintering, and the like, utilize a computer graphic representation of a part and a supply of a build material to fabricate a part in successive layers. The build material is typically a powder, liquid, or paste that is solidified, cured, or sintered when stimulated by a focused beam of energy. Normally, the focused beam of energy is selectively scanned across successive layers of the build material to produce a three-dimensional object. Often, the focused beam of energy used is a high powered laser, such as, for example, an Ultra-Violet generating laser used to cure liquid photopolymer materials.
There are many parameters that must be controlled when utilizing a focused beam of energy in an SFF apparatus. For example, the width of the beam and the intensity of the beam are important characteristics that typically must be precisely controlled in order to produce three-dimensional objects of high quality and consistency. In addition, there must be some process or procedure to track the location of the focused beam and or monitor the condition of the beam. Previous expedients in monitoring a beam can be found in, for example, U.S. Pat. No. 5,267,013 to Spence, which discloses an apparatus and method for obtaining the profile intensity of the beam in a stereolithography machine. The apparatus utilizes a sensor comprising a photodetector located behind a pinhole. The photodetector takes measurements of a laser beam as the beam is moved over the sensor, and a beam intensity profile is produced. The profile provides useful information that is indicative of how the beam will cure a photopolymer material, and the information can be used to optimally select various solidification parameters such as cure width, or the like. Undesirably, however, the information is only two-dimensional and does not clearly indicate the true condition of the laser beam. For example, if the beam has an asymmetric condition such as an astigmatism, the two-dimensional data of the profile is insufficient, by itself, to detect the condition, let alone compensate for it.
Until recently there was no agreed upon standard to characterize a beam. However, the “M
2
” standard for characterizing a beam has recently been adopted by the passing of ISO 11146. As used herein, “to characterize a beam” means to obtain sufficient measurements from the beam to be able to map the three-dimensional propagation characteristics of the beam and/or calculate the values of the beam according to the “M
2
” standard. The M
2
standard, wherein the M
2
value is herein referred to as “the times-diffraction-limit number,” takes into account the threedimensional nature of a focused beam to quantify the propagation characteristics of the beam. Generally the value of M
2
is indicative of how close a beam is to an ideal beam. For example an M
2
value of 1.0 indicates an ideal beam. M
2
values can be calculated from the following equation:
M
2
=(&pgr;×2×
W
0
×&THgr;(4×&lgr;)
where W
0
is minimum waist radius of the beam, &THgr; is the divergence angle of the beam, and &lgr; is the wavelength of the beam. However, to obtain these values for a real beam, three-dimensional data must be extracted from the beam. Generally, this requires taking three-dimensional measurements of the beam, not just unlinked two-dimensional profiles. In addition, when a focused beam has an asymmetric condition such as astigmatism, M
2
values or calculations must be taken or made in two different directions in order to characterize the condition. One instrument capable of making such measurements and calculations is disclosed in, for example, U.S. Pat. No. 5,267,012 to Sasnett et al. The instrument in Sasnett et al. optically transforms the propagation characteristics of the focused beam prior to taking measurements in the transformed state. The measurements are then processed via extrapolation techniques to finally determine the original propagation characteristics of the beam. Thus, the instrument in Sasnett et al. substantially alters the propagation characteristics of the focused beam prior to taking measurements. Such a device could be permanently mounted in an SFF apparatus to measure beam propagation characteristics; however, to do so is undesirable, as it is a relatively complex and expensive component that would not frequently be used. For instance, the need to completely characterize the focused beam in accord with the M
2
standard may only arise once or twice, such as during the assembly of the system in order to assure that it will operate within specification. In addition, it would be desirable to be able to perform such measurements on existing SFF equipment that may not be suited to physically receive the diagnostic device disclosed in Sasnett et al. to make the measurements.
Thus, there is a need to develop a method to characterize a focused beam in an SFF apparatus with existing equipment and without adding additional components. There is also a need to completely characterize a focused beam in an SFF apparatus to produce a response indicative of the condition of the beam. There is also a need to provide a simple and effective method to determine whether a focused beam needs to be replaced, or to change the focus point of the beam in order to compensate for an asymmetric condition found in the beam. In addition, there is a need to completely i characterize an adjustable focused beam in an SFF machine in order to eliminate any asymmetric condition detected in the beam. These and other difficulties of the prior art have been overcome according to the present invention.
BRIEF SUMMARY OF THE INVENTION
The present invention provides its benefits across a broad spectrum of SFF technologies. While the description which follows hereinafter is meant to be representative of a number of such applications, it is not exhaustive. As will be understood, the basic apparatus and methods taught herein can be readily adapted to many uses. It is intended that this specification and the claims appended hereto be accorded a breadth in keeping with the scope and spirit of the invention being disclosed despite what might appear to be limiting language imposed by the requirements of referring to the specific examples disclosed.
It is one aspect of the present invention to provide a simple and effective method to determine the propagation characteristics of a beam of energy in an SFF system.
It is an

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