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TABLE OF CONTENTS

2019,  1 (6):   622 - 634

Published Date:2019-12-20 DOI: 10.1016/j.vrih.2019.09.006

Abstract

Background
To solve the problem of visualization in augmented reality (AR), for assembly process information, we report here on our study into the composition of AR assembly process information.
Methods
Our work led us to classify the visual elements of assembly processes into six categories, and after looking further into visual element expression characteristics used in assembly process information in the AR environment, standard assembly process elements have been identified and visual element layout principles studied.
Conclusion
Typical visualization elements have been presented, using an AR-based assembly instruction system.

Content

1 Introduction
A traditional assembly instruction manual expresses process information by using text, pictures, and forms. When an operator uses the manual to carry out assembly, the information on the paper needs to be translated into understandable information in his/her mind before the work can be done. This mental process will take much time and can lead to different interpretations of the information, owing to there being varied cognitive power levels among operators—thus potentially affecting assembly quality. AR allows virtual information to be overlain onto the real environment to facilitate work, and it is used in assembly field[1,2]. AR technology can reduce the difficulty of information comprehension significantly, which is beneficial to assembly quality improvement[3]. In the process of AR guiding assembly work, visualization of assembly process information is the key factor in determining the effectiveness of the guidance[4,5,6,7].
Visualized information expression can reduce people's cognitive burden, as demonstrated by Mayer et al.[8]. Bruno et al. used AR to express numerical experimental results in an industrial engineering context in a visual way, which greatly improved the readability of the data[9]. To solve a problem of chaotic augmented information display, Julier et al. proposed a filtering algorithm to reduce display information and optimized an AR information layout[10]. Rolim et al. studied AR visual expression instruction, proposed design criteria for the instruction, and designed a series of instruction sets[11].
For visualization of assembly process information in AR, Haringer et al. studied 2-D visual elements and layout pictures to express assembly process information, while, based on 2-D visual expression[12], Zauner et al. introduced a 3-D model for assembly process expression, and used 3-D simulation animation to dynamically express assembly work[13]. Their research was based on specific assembly objects, however, and lacked general application, following which Radkowski et al. began to study the relationship between commonly used assembly process information and visual element expression[14]. They proposed that the complexity of the visual expression should match the difficultly of assembly work, noting that if complex visual elements are used to express simple assembly work, it will increase the cognitive burden on operators.
Keil et al. started to describe, classify, and organize a visual method for assembly process information, based on mediation quality—although they did not consider visual element design and layout from either the technical or implementation perspectives[15]. Gattullo et al. put forward a method for converting process information into pictures and designed some picture standardization for assembly processes—although they only considered picture design[16].
The review above suggests that it is necessary to conduct a comprehensive analysis on the design of all assembly process visual elements, based on the characteristics of AR assembly processes.
To solve the problem of visualization in AR assembly guidance, an AR-based composition model for the visual expression of assembly process information has been proposed here, and key factors that affect the visual expression of assembly process information identified. After that, the type, design, and layout methods for visual elements in assembly process have been studied. Based on this work, an AR assembly guidance system has been developed, and typical visual elements have been analyzed using this system.
The main contributions of this work are as follows: firstly, six assembly process information expression visual element categories have been proposed, while, secondly, design methods for different visual elements have been systematically analyzed—and standardized visual elements have thus been established. Thirdly, the principles of visual element layout have been studied and two 3-D visual element layout methods have been proposed.
2 Analysis of process information in augmented assembly work
In the process of AR-guided mechanical assembly[17,18,19,20], assembly process information is expressed using visual elements. There are two types of AR process information—assembly process information and operational status information, and through operators and their work, the two parts of the assembly process form an information cycle. In this cycle, assembly operations provide assembly process information for operators, who then place new requirements for assembly process information, based on feedback from operational process information.
The composition of AR assembly process information—which mainly comprises three categories: product information, tooling information, and process flow information—is shown in Figure 1. Product information mainly includes product model information, part names and appearances, and assembly location information. Tooling information mainly consists of tool model and location information, while process information includes three sub-types: process names, assembly methods, and process requirements. In these sub-types, assembly methods refer to operational methodology information, assembly path information, and assembly location information, while process requirements refer mainly to quality requirement information. Operation status information includes task feedback information, which is composed of error, warning, and progress confirmation information, while help information consists of expert help and problem-solving lists. AR assembly process information contains 16 sub-items, and for each assembly step, there is at least 1 sub-item relating to operational status and assembly process information.
Through a large number of trials, we concluded that it was difficult to express complete assembly process information in a single visual AR element clearly, and that multiple visual elements had to be combined so that assembly process information could be expressed in AR guides for assembly operations. The determining factor for the information that needed to be expressed in visual elements describing each assembly operations step was related to the type, design, and layout of the visual elements. It was necessary, therefore, to identify the type of visual elements to be used to express the reality of the assembly process, to design the visual elements for the process information that needed to be expressed, and to standardize the visual element forms used for information expression. We also needed to study layout principles for the visual elements of display equipment.
3 Classification of visual elements for augmented assembly processes
Depending on the dimensional characteristics of visualization information in virtual space, an AR visual style can be divided into 3-D and 2-D visualization. In our study, six visual element categories have been proposed for use as assembly process information expressions. These were picture, static model, simulation animation, video, symbol, and text. As shown in Table 1, pictures, and videos are 2-D visual elements, static models and simulation animations were classed as 3-D visual elements, with symbols and text included as either 2-D or 3-D. Each visual element can express one or more items of sub-item process information in an AR assembly process guide.
Visual elements of an assembly process
Number Category Attribute Function
1 Picture 2-D To express assembly operation and other information
2 Static model 3-D To express product and tooling information
3 Simulation animation 3-D To express process and direction information
4 Video 2-D To express help messages and other information
5 Symbol 2-D\3-D To express object location, identify assembly paths, and provide prompt information
6 Text 2-D\3-D To express assembly process and product operation information
3.1 Pictures
A picture is a way to express process information using assembly progress photos or designed graphics. Picture content form varies, and can cover active steps or object appearance. Pictures can be used to display static process information on an intuitive basis, and need not be restricted to the object being assembled.
3.2 Static models
A static model is a way to express process information using a 3-D model. The static 3-D model illustrates assembly parts and tools that can represent the size and shape of real parts, and can prompt the operator to find the needed assembly object rapidly. Compared with 2-D drawings, the 3-D model is more intuitive and easy for the assembly operator to understand.
3.3 Simulated animations
An animation is a way to express process information using a dynamic, 3-D model, which can use movement to present the assembly process from start to finish. When using simulation animation to express process information, it is necessary to understand information such as the assembly path, assembly location, and fitting processes. Based on the display position of the 3-D model, simulation animation can be divided into two forms: precise overlay display and imprecise overlay display. Here, precise overlay display simulation animation means the virtual 3-D model assembly is shown using the actual object, with accurate positioning, while imprecise overlay display simulation animation means the virtual 3-D model uses displays in any position and without accurately overlaying the actual assembly object.
3.4 Videos
Video is a way to express process information by using continuous, 2-D dynamic images. Compared with other expressions types, video has the largest information content, allowing it to express the complete assembly operation process, and it can also be used to provide expert help or assembly demonstrations.
3.5 Symbols
Symbols are a way to express process information with labels or symbols—as either 2-D graphics or as 3-D models. They are mainly used to provide arrow information indicators, Model-based Definition 3-D labeling, and other guidance marking.
3.6 Text
Text is a way to express process information using a written description. In visual expression, simple information—such as a part name, tool type, and process serial number—can be prompted using simple text descriptions. Text is flexible, and it can describe guidance information that may be difficult to express in other ways, such as process quality requirements and inspection requirements.
4 Design of visual elements for augmented assembly process
AR assembly process information can be expressed using the six visualization elements referred to in section 3. For standard assembly operations or process information, it is useful to standardize visual process information using these AR expression elements, as this can effectively reduce redundancy in design work during content authoring[21], and can facilitate the application of AR assembly guidance.
4.1 2-D element design visualization
2-D visualization elements include mainly text, pictures, videos, and symbols. Video is generally captured from on-site operation, with video content determined by the different assembly task requirements. In our study, we considered standardized designs for three kinds of 2-D visualization elements: text, symbols, and pictures.
4.1.1   Text visualization design
Visualization of text for AR process information is achieved by converting text information into text bitmaps. Visualization design decisions required for text include determination of its font, color, size, and rendering style. Depending on the comparative importance of the various assembly process items, text information can be divided into low-, medium-, or high-level importance, with the importance level then distinguished by differences in color and background, as required to attract the operator's attention. In our work, low-level importance text was designed as yellow font, against a transparent background style; medium-level importance text was designed as red font with a transparent background style; and high-level importance text was designed as red font, with an enhanced background style. Examples of text visualization design have been listed in Table 2.
Text visualization design
Process manual Text visual design for augmented assembly
Process name Process content description Process content Rank Visual design
Parts cleaning The surfaces of the parts are scrubbed clean with kerosene and then assembled. Scrub the parts clean with kerosene Low Scrub the parts clean with kerosene
Drum shaft assembly Press the assembled drum shaft into the central hole of the big gear to ensure center hole alignment. The drum shaft is pressed into the central hole of the big gear Medium The drum shafts is pressed into the center hole of the big gear
Bearing assembly After the inner baffle ring, transparent cover, and dustproof gasket are installed on the drum shaft, press the bearing into the drum shaft Press the bearing into the reel shaft High Press the bearing into the reel shaft
4.1.2   Picture visualization design
We designed assembly operation pictures in our study. By transforming complex processes into simple, 2-D pictures, as shown in Table 3, operators can shorten the time needed to understand the target processes, thus reducing their cognitive burden. When the different assembly operations are designed as simple, 2-D visualization graphics, the key aspect is to outline the actual action required, and to show the relevant operational object—to ensure that the pictures are universally comprehensible and readable.
Picture visualization design
Process manual Augmented assembly
Process name Process description Visual design
Screw Using a tightening tool, tighten the screw connector according To the torque and rotation directions specified in the process requirements
Lubricate Inject lubricating oil into the Assembly object to reduce friction between parts
Line Using the marking tool, draw the corresponding lines on the parts, according to certain requirements
Knock For assembly fit support, the assembly is tapped in a knock operation, to ensure the firm fit of the assembly
4.1.3   Annotation symbol visualization design
Symbol visualization for AR process information is achieved by combining text and point-line bitmaps, as shown in Table 4. The symbol design requirements include decisions on the shape, color, size, and rendering style. Traditional process manuals have often used words and pictures to describe symbols that have no uniform format, so in our study, we have standardized the arrow, annotation, label, and tab symbols used for AR process information expression. Arrow symbols have been designed using line segments and to be blue; annotations consist of leads and text; labels consist of leads, numbers, and boxes; and tabs consist of lead numbers and circles.
Symbol visual designs
Name Function Visual design
Arrow Guide operator's attention; reminder to pay attention to the object indicated by the arrow; indicate direction
Annotation Annotate the assembly object and guide operators to locate the object
Label Mark the size of the assembly object and remind assembly operators to pay attention to the size
Tab Label the assembly object with a serial number, and guide operators in the sequential use of objects labeled with the corresponding sequence number
4.2 3-D visualization element design
4.2.1   3-D tool visualization design
3-D tool visualization design means using a 3-D model of a lightweight tool to express the process information on how the tool is used during the assembly process. Many kinds of tools are used in assembly operations, and for tools that meet national standards, we can design a standard visualization tool, building a tool model library for AR assembly. This means that, when such a tool in then used in different assembly applications, the appropriate tool model can be selected, without the need for a new design. This is helpful in reducing the complexity of assembly content authoring. For visualization design using standard tools, our strategy was to use CAD 3-D tool models in our study to simplify content authoring, using only information relating to model facets and spatial position. Modeling of various tool types can build a 3-D tool library for AR, and examples of four types of tool model design are shown in Table 5.
Visual design of tool model
Name Function Visual design
Open-ended spanner Provide a reminder of tool type to be used; indicate installation position; guide the nut tightening operation
Screwdriver Provide a reminder of tool type to be used; indicate mounting position; guide screw tightening operation
Socket spanner Provide a reminder of tool type to be used; indicate mounting position; guide nut tightening operation
Socket head wrench Provide a reminder of tool type to be used; indicate mounting position; guide screw tightening operation
4.2.2   3-D arrow visualization design
In AR assembly guidance, 3-D arrows can be used to direct operator attention. To build a unified arrow style for AR assembly guidance from different arrow types, we chose the same design strategy as that used for 3-D tools. 3-D CAD models of arrows were used to simplify the process and build a 3-D arrow model library. After authoring 3-D arrows for use in AR guidance, it would then be a simple matter of selecting an arrow corresponding to assembly process requirements from the model library. In our study, we designed four arrow model types for assembly guidance, namely, linear arrows, rotating arrows, broken arrows, and arrow extension lines, and these can be used to express different assembly process information, as shown in Table 6.
Visual design of arrow models
Type Function Visual design
Linear arrow Indicates installation position; guides linear movement; directs attention to an object
Rotating arrow Rotating motion guide; directs attention to the direction of rotation
Broken arrow Polyline movement guide; directs attention to polyline direction.
Arrow extension As an extension of the arrow, increases arrow range
4.2.3   3-D parts visualization design
Many standard parts are used in processes described in AR-assisted assemblies, and for different assembly subjects, there are some universal standard parts, making it appropriate to design 3-D visualization models for these, for AR. The structures of many standard part types are complex, so to reduce the memory consumption required for standard parts, simplified models have been adopted. In these models, only surface and spatial position information have been retained, while internal structure, model properties, assembly constraints, materials, and other model information are deleted. As shown in Table 7, the standard parts listed in the table are, respectively, a bearing, valve, flange, and bolt, and these four standard part types have been simplified from CAD models. Note that for non-standard parts, this method can also be used to simplify the models.
Visual designs for standard parts
Name Function Visual design
Bearing Represents mounting position, appearance, and model number of a bearing
Valve Represents mounting position, appearance, and model number of a valve
Flange Represents mounting position, appearance, and model number of a flange
Bolt Represents mounting position, appearance, and model number of a bolt
5 Layout of visual elements for augmented assembly processes
In the study of the visualization technology used to present assembly process information based on AR, it is not only necessary to design the visual elements used to express standard assembly process information, but the layout and display of visual elements also needs to be analyzed. A reasonable layout of visualization elements will benefit assembly process explanation and will increase understanding of the visual elements in display devices. The key issue in relation to the layout of visual elements for assembly processes is to achieve conversion between the real-world coordinate system of visual elements and the pixel coordinate system of display equipment—and to adjust the location coordinate values of visual elements according to the different reference coordinate system.
5.1 2-D visual element layout
2-D visualization elements mainly include text, pictures, videos, and some symbols, and the layouts of 2-D visualization elements are based on screen pixel coordinate systems. The principle applicable to 2-D visual element layout is to determine the virtual information display coordinate system of the visualization elements, and then to overlap the display coordinate system of virtual information with the pixel coordinate system. This allows the display positions of 2-D visualization elements to be obtained for the display equipment. The final step is to compose the virtual 2-D visualization elements and the real scene, obtained using a camera, and finish creation of the 2-D visualization elements display. See Figure 2 for a schematic of this process. The 2-D visualization elements are displayed in a fixed position on the screen and do not change, and both the size and shape of the 2-D visualization elements maintain their stability and good readability. If the camera position is changed, however, the 2-D visualization elements fixed display will occlude the assembly object.
To ensure good readability, the spatial proximity principle, the information cutting principle, and the attention guidance principle should be followed during development of 2-D visualization elements.
The principle of spatial proximity refers to ensuring that the visualization elements are located in the adjacent area of the annotated object. For example, when annotating information on a tool, for example, the annotation visual element should be displayed near the tool model, to reduce attention diversion, and to help operators to concentrate on observing both the models and associated annotations.
The principle of information cutting refers to the division of visual elements into blocks, to avoid overlaying the display. Human cognitive ability is limited, so visual information displayed in sequence and in blocks can reduce the cognitive burden on operators.
The attention guidance principle refers to establishing a connection between visualization elements and the object, and can help workers quickly understand the content of process information expressed by visual elements. Each visual element is used to express assembly information in relation to a specific assembly object, and the connection between the assembly object and visual elements can be established by arrows, box fingers and symbols, and when judiciously applied, this principle can help operators avoid cognitive confusion.
5.2 3-D visual element layout
3-D visualization elements are generally limited to a static model, a simulation animation, and 3-D symbols. As shown in Figure 3, the layout principle for 3-D visualization elements is based on the transformation of the assembly object’s actual coordinate system into a display pixel coordinate system. The principle can be described as follows: firstly, the coordinate values of the assembly object's relevant points in the actual coordinate system and in the pixel coordinate system are determined. Then, the corresponding points are used to calculate the parameters for the real camera, which include the matrix of external and internal parameters, and then, after obtaining the real camera parameters, the virtual camera can use these to position the 3-D virtual model in the real scene. Finally, the layout and display of the 3-D visualization elements are completed, by fusing the real camera image with the 3-D visualization elements captured by the virtual camera. Unlike a 2-D visualization element layout display, the display positions and shapes of 3-D visualization elements will change with the position of the camera, and, by taking advantage of this characteristic, using different perspectives of 3-D visualization elements can help operators understand complex assembly process information.
The layout of 3-D visualization elements can be divided into two types: precise overlay and imprecise overlay, as shown in Figure 4. Precise overlay is used to superimpose a 3-D virtual model onto its correct position in a real assembly object, in accordance with the assembly process, and this process can intuitively guide assembly operators to install parts according to the display position of the virtual model.
Imprecise overlay is used to display the 3-D virtual model in a way that can be easily seen by the operator, in a situation where it is not necessary to overlay the virtual model onto the real assembly object. Imprecise overlay location is therefore flexible, and can be applied as required by the assembly circumstances, making it conducive to expanding the application scene of 3-D visualization elements.
6 Prototyping system and case study
6.1 AR based assembly instruction system
The visualization elements of an AR assembly process should be displayed by applying an AR guidance system. In this paper, an engine assembly process is used as an example of AR assembly process visualization, and the typical visualization elements of the assembly process are visualized. The AR assembly guidance system was developed using Microsoft Visual Studio C++, and the virtual scene was managed and rendered using a virtual reality development kit, Coin3D. Image processing and video fusion for real-time guidance were developed using OpenCV, and the interface of the enhanced assembly guidance system—which mainly includes a menu window, a structure and a display window—is shown in Figure 5.
The menu window is used to manage operation of the guidance system, the structure tree window displays assembly step information, and the display window is used to display the guidance video, which integrates virtual elements with real assembly process scenes.
6.2 Typical visualization element representation
6.2.1   Picture
The purpose of this example is to guide the screw tightening process of a cylinder cover, using a picture, as shown in Figure 6. The picture is displayed on the left top of the displayer, and the idea is that during the assembly process, the operator can select the appropriate tool for tightening the bolt based on the guidance provided by the picture.
6.2.2   Simulation animation and static model
The purpose of this example is to guide an assembly sequence. In this sequence, the screw tightening assembly process for a cylinder head is shown, and then the engine cylinder head is assembled on the body, using a threaded connection. As part of the guidance, the cylinder head model is overlain on the body in real time and accurately, using simulation animation, the bolt is installed in the screw hole of the cylinder head, and the tool model (as a static model) is used to guide the nut tightening operation. The guidance visual, using the simulation animation and the static model, is shown in Figure 7.
6.2.3   Video
Visual guidance using video is shown in Figure 8, with this example referring to a cylinder cover assembly process, using video annotation. The video is captured before the assembly guidance is authored, and consists of an assembly demonstration. The demonstration video display position is at the top of view window, which can be arranged during the content authoring phase. The operators can implement assembly steps by following the video.
6.2.4   Symbols
Figure 9 shows an example of visual guidance using symbols—in this case covering annotation of a cylinder cover assembly process. The purpose and content of the annotation relate to ensuring the correct alignment of the cylinder head, and during the guidance process, the annotation box will move with movement of the parts, with the annotation lead line always staying connected to the parts. To achieve correct symbol guidance, firstly, the 3-D point coordinates of center of gravity of the part are included in every frame of animation, and then, the perspective projection calculation method is used to get the pixel coordinates onto the display plane of the center of gravity. Finally, the annotation box and the text are drawn on the display plane in every frame, using the pixel coordinates of the center of gravity as the start point.
6.2.5   Test
In this example, text is used to annotate the assembly process of a cylinder cover, as shown in Figure 10, with the text shown in the top left corner, in yellow. This facet of the assembly process involves aligning the cylinder head with screw holes and fastening the screws—and using this guidance information, an operator can easily understand and apply the correct assembly procedure.
7 Conclusions
After studying the components in AR-based assembly process information, we have proposed six AR visualization method types, for assembly process information expression, and various principles for standardizing visualization elements. To increase the readability of visualization elements, their layout and the layout principles of augmented assembly have been studied, and in due course, this allowed us to develop a prototype system for AR assembly guidance. This system was then tested, to verify the application of typical visual expression elements, in an engine assembly process.
Based on the AR assembly process visualization technology described in this paper, the combined expression of different visual element types can be studied in the future, to establish more accurate and more easily understood visualization element combinations for assembly process guidance.

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