A comparison study of digital sinusoidal fringe generation technique: defocusing binary patterns VS focusing sinusoidal patterns

With the recent advancements in digital technology, three-dimensional (3-D) shape measurement has played an increasingly important role in fields including manufacturing, homeland security, medical sciences, and entertainment. Over the past decades, numerous 3-D shape measurement techniques have been developed. Among these existing techniques, fringe analysis based on phase-shifting sinusoidal structured patterns stands out because of its numerous advantages. However, there are still some major challenges of the existing digital fringe projection system for accurate 3-D shape measurement and for future speed improvement. They are: (1) projector nonlinearity problem, (2) synchronization problem, and (3) exposure time limitation problem. There are currently two approaches to generate sinusoidal fringe patterns with a digital-light-processing (DLP) projector: defocusing binary patterns (DBP) and focusing sinusoidal patterns (FSP). The focus of this dissertation research is to compare these methods for high-quality 3-D shape measurement. We developed a system based on a digital fringe projection and phase-shifting technique to perform various comparison tests. The system utilizes a DLP projector to project computer generated fringe patterns onto the object and a charged-coupled-device (CCD) camera to acquire the fringe images. Conventionally, sinusoidal fringe patterns are usually supplied to a focused projector, and the DBP method is used to properly defocus the projector to generate sinusoidal patterns from binary structured patterns. We compare the performance of the new DBP approach against the traditional FSP method by analyzing the phase errors introduced by following factors: (1) defocusing degree, (2) exposure time, (3) synchronization, and (4) projector nonlinear gamma. The traditional FSP involves some practical issues for high-quality measurement. Our experiment found it is possible to generate ideal sinusoidal fringe patterns by the DBP method, and when the projector is defocused to a certain degree, the phase error induced by the DBP method is very close to that produced by the FSP approach. With the DBP method, 3-D reconstruction was shown to be feasible. Short exposure time is especially needed when measuring fast motion. For the FSP method, the minimum exposure time of the camera is limited by the projector\u27s fringe projection rate, and the phase error is very large when a very short exposure time is needed. The experimental results show that the phase error does not change very much when the exposure time alters, and if a very short exposure time is needed, the DBP method clearly outperforms the FSP method for 3-D shape measurement. It also provides a potential way to develop fast 3-D shape measurement technique. For the DLP projector, if it is supplied with sinusoidal fringe patterns, the synchronization between the projector and the camera is critical. When the projector is not synchronized with the camera, the phase error for the DBP method is much smaller than that for the FSP method when the exposure time is not multiples of projection cycle. By implementing the DBP method in our system, we could achieve 3-D reconstruction without synchronization between the projector and the camera. Projector gamma correction, which is usually a time-consuming procedure, is mandatory for the FSP method. In this research, we found no projector gamma correction is needed for the DBP method. Our experimental results demonstrated it can achieve high-quality 3-D reconstruction by the DBP method without projector nonlinearity calibration. Compared with the FSP method, the possible shortcomings of the DBP method are: (1) seemingly sinusoidal fringe patterns are still composed of high-frequency harmonics, which results in measurement error, and (2) the depth range of high-contrast fringe patterns is small. Even with these drawbacks, this new technique still has the potential to replace the conventional fringe generation technique

Process planning for the subtractive rapid manufacturing of heterogeneous materials: Applications for automated bone implant manufacturing

This research presents a subtractive rapid manufacturing process for heterogeneous materials, in particular for custom shaped bone implants. Natural bone implants are widely used in the treatment of severe fractures or in tumor removal. In order for the human body to accept the bone implant material and heal properly, it is essential that the bone implant should be both mechanically and biologically compatible. Currently, the challenge of having correctly shaped natural bone implants created from an appropriate material is met through hand-shaping done by a surgeon. CNC-RP is a rapid machining method and software that can realize a fully automated Subtractive Rapid Prototyping (RP) process, using a 3-axis milling machine with a 4th axis indexer for multiple setup orientations. It is capable of creating accurate bone implants from different clinically relevant material including natural bone. However, there are major challenges that need to be overcome in order to implement automated shape machining of natural bones. They are summarized as follows: (1) Unlike homogeneous source materials for which a part can be machined from any arbitrary location within the original stock, for the case of donor bones, the site and orientation of implant harvest need to consider the nature of the heterogeneous internal bony architecture. (2) For the engineered materials, the source machining stock is in the convenient form of geometrically regular shapes such as cylinders or rectangular blocks and the entities of sacrificial supports can connect the part to the remaining stock material. However, irregularly-shaped bones and the heterogeneity of bone make the design of a fixture system for machining much more complicated. In this dissertation, two major areas of research are presented to overcome these challenges and enable automated process planning for a new rapid manufacturing technique for natural bone implants. Firstly, a new method for representing heterogeneous materials using nested STL shells is proposed. The nested shells model is called the Matryoshka mode, based in particular on the density distribution of human bone. The Matryoshka model is generated via an iterative process of thresholding the Hounsfield Unit (HU) data from a computed tomography (CT) scan, thereby delineating regions of progressively increasing bone density. Then a harvesting algorithm is developed to determine a suitable location to generate the bone implant from within the donor bone is presented. In this harvesting algorithm, a density score and similarity score are calculated to evaluate the overall effectiveness of that harvest site. In the second research area, an automated fixturing system is proposed for securing the bone implant during the machining process. The proposed method uses a variant of sacrificial supports (stainless surgical screws) to drill into appropriate locations and orientations through the free-form shaped donor bone, terminating at proper locations inside the solid part model of the implant. This automated fixturing system has been applied to machine several bone implants from surrogate bones to 3D printed Matryoshka models. Finally, the algorithms that are developed for setup planning are implemented in a CAD/CAM software add-on called CNC-RPbio . The results of this research could lead to a clinically relevant rapid machining process for custom shaped bone implants, which could create unique implants at the touch of a button. The implication of such high accuracy implants is that patients could benefit from more accurate reconstructions of trauma sites, with better fixation stability; leading to potentially shorter surgeries, less revisions, shorter recovery times and less likelihood of post-traumatic osteoarthritis, to name a few

A Method to Represent Heterogeneous Materials for Rapid Prototyping: The Matryoshka Approach

Purpose—The purpose of this paper is to present a new method for representing heterogeneous materials using nested STL shells, based, in particular, on the density distributions of human bones. Design/methodology/approach—Nested STL shells, called Matryoshka models, are described, based on their namesake Russian nesting dolls. In this approach, polygonal models, such as STL shells, are “stacked” inside one another to represent different material regions. The Matryoshka model addresses the challenge of representing different densities and different types of bone when reverse engineering from medical images. The Matryoshka model is generated via an iterative process of thresholding the Hounsfield Unit (HU) data using computed tomography (CT), thereby delineating regions of progressively increasing bone density. These nested shells can represent regions starting with the medullary (bone marrow) canal, up through and including the outer surface of the bone. Findings—The Matryoshka approach introduced can be used to generate accurate models of heterogeneous materials in an automated fashion, avoiding the challenge of hand-creating an assembly model for input to multi-material additive or subtractive manufacturing. Originality/Value—This paper presents a new method for describing heterogeneous materials: in this case, the density distribution in a human bone. The authors show how the Matryoshka model can be used to plan harvesting locations for creating custom rapid allograft bone implants from donor bone. An implementation of a proposed harvesting method is demonstrated, followed by a case study using subtractive rapid prototyping to harvest a bone implant from a human tibia surrogate

Preparation of formyl cellulose and its enhancement effect on the mechanical and barrier properties of polylactic acid films

Cellulose was modified by formic acid to prepare formyl cellulose (FC). The amount of formyl groups in FC was adjusted by controlling the reaction time, reaction temperature, and formic acid concentration. Then, FC was used to reinforce polylactic acid (PLA) films prepared by solution casting. Scanning electron microscopy (SEM) shows that long rod-like cellulose particles were broken into short rods after formylation and the introduction of FC made PLA surface rougher. The mechanical properties of PLA/FC films were improved by the inclusion of FC. Compared to pure PLA film, the PLA/FC composite film with 1 wt% FC (containing 15.79% formyl groups) showed a 48.59% increase in tensile strength and a 346% increase in Young's modulus. The addition of FC also resulted in better water barrier properties. The moisture absorption capacity and water vapor permeability were 40.56% and 51.43% lower than those of the pure-PLA film. The enhancement in properties for PLA/FC composites could be ascribed to the improved compatibility between PLA and cellulose with the introduction of hydrophobic formate groups. The PLA/FC composite films developed in this work could be highly potential for food packaging

A comparison study of digital sinusoidal fringe generation technique: defocusing binary patterns VS focusing sinusoidal patterns

With the recent advancements in digital technology, three-dimensional (3-D) shape measurement has played an increasingly important role in fields including manufacturing, homeland security, medical sciences, and entertainment. Over the past decades, numerous 3-D shape measurement techniques have been developed. Among these existing techniques, fringe analysis based on phase-shifting sinusoidal structured patterns stands out because of its numerous advantages. However, there are still some major challenges of the existing digital fringe projection system for accurate 3-D shape measurement and for future speed improvement. They are: (1) projector nonlinearity problem, (2) synchronization problem, and (3) exposure time limitation problem. There are currently two approaches to generate sinusoidal fringe patterns with a digital-light-processing (DLP) projector: defocusing binary patterns (DBP) and focusing sinusoidal patterns (FSP). The focus of this dissertation research is to compare these methods for high-quality 3-D shape measurement. We developed a system based on a digital fringe projection and phase-shifting technique to perform various comparison tests. The system utilizes a DLP projector to project computer generated fringe patterns onto the object and a charged-coupled-device (CCD) camera to acquire the fringe images. Conventionally, sinusoidal fringe patterns are usually supplied to a focused projector, and the DBP method is used to properly defocus the projector to generate sinusoidal patterns from binary structured patterns. We compare the performance of the new DBP approach against the traditional FSP method by analyzing the phase errors introduced by following factors: (1) defocusing degree, (2) exposure time, (3) synchronization, and (4) projector nonlinear gamma. The traditional FSP involves some practical issues for high-quality measurement. Our experiment found it is possible to generate ideal sinusoidal fringe patterns by the DBP method, and when the projector is defocused to a certain degree, the phase error induced by the DBP method is very close to that produced by the FSP approach. With the DBP method, 3-D reconstruction was shown to be feasible. Short exposure time is especially needed when measuring fast motion. For the FSP method, the minimum exposure time of the camera is limited by the projector's fringe projection rate, and the phase error is very large when a very short exposure time is needed. The experimental results show that the phase error does not change very much when the exposure time alters, and if a very short exposure time is needed, the DBP method clearly outperforms the FSP method for 3-D shape measurement. It also provides a potential way to develop fast 3-D shape measurement technique. For the DLP projector, if it is supplied with sinusoidal fringe patterns, the synchronization between the projector and the camera is critical. When the projector is not synchronized with the camera, the phase error for the DBP method is much smaller than that for the FSP method when the exposure time is not multiples of projection cycle. By implementing the DBP method in our system, we could achieve 3-D reconstruction without synchronization between the projector and the camera. Projector gamma correction, which is usually a time-consuming procedure, is mandatory for the FSP method. In this research, we found no projector gamma correction is needed for the DBP method. Our experimental results demonstrated it can achieve high-quality 3-D reconstruction by the DBP method without projector nonlinearity calibration. Compared with the FSP method, the possible shortcomings of the DBP method are: (1) seemingly sinusoidal fringe patterns are still composed of high-frequency harmonics, which results in measurement error, and (2) the depth range of high-contrast fringe patterns is small. Even with these drawbacks, this new technique still has the potential to replace the conventional fringe generation technique.</p

Process planning for the subtractive rapid manufacturing of heterogeneous materials: Applications for automated bone implant manufacturing

This research presents a subtractive rapid manufacturing process for heterogeneous materials, in particular for custom shaped bone implants. Natural bone implants are widely used in the treatment of severe fractures or in tumor removal. In order for the human body to accept the bone implant material and heal properly, it is essential that the bone implant should be both mechanically and biologically compatible. Currently, the challenge of having correctly shaped natural bone implants created from an appropriate material is met through hand-shaping done by a surgeon. CNC-RP is a rapid machining method and software that can realize a fully automated Subtractive Rapid Prototyping (RP) process, using a 3-axis milling machine with a 4th axis indexer for multiple setup orientations. It is capable of creating accurate bone implants from different clinically relevant material including natural bone. However, there are major challenges that need to be overcome in order to implement automated shape machining of natural bones. They are summarized as follows: (1) Unlike homogeneous source materials for which a part can be machined from any arbitrary location within the original stock, for the case of donor bones, the site and orientation of implant harvest need to consider the nature of the heterogeneous internal bony architecture. (2) For the engineered materials, the source machining stock is in the convenient form of geometrically regular shapes such as cylinders or rectangular blocks and the entities of sacrificial supports can connect the part to the remaining stock material. However, irregularly-shaped bones and the heterogeneity of bone make the design of a fixture system for machining much more complicated. In this dissertation, two major areas of research are presented to overcome these challenges and enable automated process planning for a new rapid manufacturing technique for natural bone implants. Firstly, a new method for representing heterogeneous materials using nested STL shells is proposed. The nested shells model is called the Matryoshka mode, based in particular on the density distribution of human bone. The Matryoshka model is generated via an iterative process of thresholding the Hounsfield Unit (HU) data from a computed tomography (CT) scan, thereby delineating regions of progressively increasing bone density. Then a harvesting algorithm is developed to determine a suitable location to generate the bone implant from within the donor bone is presented. In this harvesting algorithm, a density score and similarity score are calculated to evaluate the overall effectiveness of that harvest site. In the second research area, an automated fixturing system is proposed for securing the bone implant during the machining process. The proposed method uses a variant of sacrificial supports (stainless surgical screws) to drill into appropriate locations and orientations through the free-form shaped donor bone, terminating at proper locations inside the solid part model of the implant. This automated fixturing system has been applied to machine several bone implants from surrogate bones to 3D printed Matryoshka models. Finally, the algorithms that are developed for setup planning are implemented in a CAD/CAM software add-on called "CNC-RPbio". The results of this research could lead to a clinically relevant rapid machining process for custom shaped bone implants, which could create unique implants at the touch of a button. The implication of such high accuracy implants is that patients could benefit from more accurate reconstructions of trauma sites, with better fixation stability; leading to potentially shorter surgeries, less revisions, shorter recovery times and less likelihood of post-traumatic osteoarthritis, to name a few.</p

Flexible 3-d shape measurement using projector defocusing

We present a 3-D shape-measurement technique using a defocused projector. The ideal sinusoidal fringe patterns are generated by defocusing binary structured patterns, and the phase shift is realized by shifting the binary patterns spatially. Because this technique does not require calibration of the gamma of the projector, it is easy to implement and thus is promising for developing flexible 3-D shape measurement systems using digital video projectors. Optical Society of America OCIS codes: 120.0120, 120.2650 3-D shape measurement is very important to numerous disciplines; over the years, a number of techniques have been developed, including some with real-time capability To perform high-quality 3-D shape measurement using a digital fringe projection and phase-shifting method, projector gamma calibration is usually mandatory. This is because the commercial video projector is usually a nonlinear device that is purposely designed to compensate for human vision. A variety of techniques have been studied, including the methods to actively change the fringe to be projected This Letter will present a flexible 3-D shape measurement technique without requiring gamma calibration. The idea came from our two observations: (1) seemingly sinusoidal fringe patterns often appear on the ground when the light shines through open window blinds and (2) the sharp features of an object are blended together in a blurring image that is captured by an out-of-focus camera. The former gives the insight that an ideal sinusoidal fringe image could be produced from a binary structured pattern, and the latter provides the hint that if the projector is defocused, the binary structured pattern might become ideal sinusoidal. Because only binary patterns are needed, the nonlinear response of the projector would not be a problem, because only 0 and 255 intensity values are used. Moreover, phase shifting can be introduced by spatially moving the binary structured patterns. Therefore, if this hypothesis is true, a flexible 3-D shape measurement system based on a digital fringe-projection technique can be developed without nonlinear gamma calibration. Experiments will be presented to verify the performance of the proposed technique. Sinusoidal phase-shifting methods are widely used in optical metrology because of their measurement accuracy Solving these three equations, this phase can be obtained: ͑x,y͒ = tan This equation provides the wrapped phase with 2 discontinuities. A spatial phase-unwrapping algorithm can be applied to obtain continuous phas

Superfast phase-shifting method for 3-D shape measurement

We present a 3-D shape-measurement technique using a defocused projector. The ideal sinusoidal fringe patterns are generated by defocusing binary structured patterns, and the phase shift is realized by shifting the binary patterns spatially. Because this technique does not require calibration of the gamma of the projector, it is easy to implement and thus is promising for developing flexible 3-D shape measurement systems using digital video projectors.This article is from Optics Letters 34 (2009): 3080–3082, doi:10.1364/OL.34.003080. Posted with permission.</p

A Method to Represent Heterogeneous Materials for Rapid Prototyping: The Matryoshka Approach

Purpose—The purpose of this paper is to present a new method for representing heterogeneous materials using nested STL shells, based, in particular, on the density distributions of human bones. Design/methodology/approach—Nested STL shells, called Matryoshka models, are described, based on their namesake Russian nesting dolls. In this approach, polygonal models, such as STL shells, are “stacked” inside one another to represent different material regions. The Matryoshka model addresses the challenge of representing different densities and different types of bone when reverse engineering from medical images. The Matryoshka model is generated via an iterative process of thresholding the Hounsfield Unit (HU) data using computed tomography (CT), thereby delineating regions of progressively increasing bone density. These nested shells can represent regions starting with the medullary (bone marrow) canal, up through and including the outer surface of the bone. Findings—The Matryoshka approach introduced can be used to generate accurate models of heterogeneous materials in an automated fashion, avoiding the challenge of hand-creating an assembly model for input to multi-material additive or subtractive manufacturing. Originality/Value—This paper presents a new method for describing heterogeneous materials: in this case, the density distribution in a human bone. The authors show how the Matryoshka model can be used to plan harvesting locations for creating custom rapid allograft bone implants from donor bone. An implementation of a proposed harvesting method is demonstrated, followed by a case study using subtractive rapid prototyping to harvest a bone implant from a human tibia surrogate.This is a manuscript of an article from Rapid Prototyping Journal 20 (2014): 390, doi:10.1108/RPJ-10-2012-0095. Posted with permission.</p