In order to assess the relative efficiency of the user interface elements discussed in the previous chapter, as well as to investigate the merits of AR for assembly as opposed to other instructional methods, a user study was conducted. This chapter is dedicated to outlining the setup and implementation of that user study by describing the hardware utilized, the
experimental hypotheses and design of accompanying interfaces, and the logistical study structure.
APPARATUS
Hardware
The AR system supporting this user study was implemented on a single workstation, the arrangement of which can be seen in Figure 14. The primary components consisted of two LCD displays—one each for the subject and the experimenter—an overhead webcam, and a desktop computer. The LCD displays for the subject and experimenter measure 24” and 23”,
respectively. The webcam is a Logitech Pro 9000—a common consumer-grade model. The desktop computer has the following specifications:
Intel Xeon 3.47GHz CPU
NVIDIA Quadro 5000 video card 6.00GB RAM
Windows 7 Enterprise 64-bit OS
The user’s monitor was placed just above the physical assembly space, allowing him to look up slightly in order to quickly reference the augmented scene from a viewpoint closely approximating his direct view of the physical scene. The AR system performed tracking via four
fiducial markers attached to the workbench, the assembly base, and the base components of two subassemblies.
Figure 14. The workstation used to conduct the user study.
The test product utilized by this study was an axial piston motor, which the users were required to assemble—shown in its fully assembled state in Figure 15. This unit was provided by Sauer Danfoss and is a hydraulically controlled, variable (L/K series) motor. The entire unit measures roughly 7” tall, 7” wide, and 9” long including the output shaft, and weighs 34 pounds.
The tools necessary to assemble the motor were an 8mm hex key (also commonly
referred to as an Allen wrench) and a pair of snap ring pliers. Each tool was used for exactly one assembly step—namely the securement of the motor end cap with five cap screws and the installation of a snap ring around the output shaft. The remaining 14 assembly steps were all performed by hand, and are described and illustrated by the three sets of assembly instructions which comprise Appendices B, C, and D—the first of which is a paper instruction manual written based on advice from Sauer Danfoss, while the latter two consist of screenshots of the AR interfaces discussed in the next section of this chapter.
To perform complete disassembly and reassembly of the motor would require several additional specialized tools, making the full procedure impractical for the user study. As such, a small number of assembly operations were excluded. The primary exclusions involved two sets of bearings which cannot be removed or reinstalled without a bearing puller or bearing press, or other similar tools. The two bearing sets include a ball bearing mounted on the motor’s central shaft, and two journal bearings mounted on the inside of the motor housing. For the study, all of these bearings were treated as if they were permanent features of the parts to which they were attached. From an information presentation point of view no opportunities were lost due to these exclusions since bearing installations are insertion tasks, of which this assembly included many. The second exclusion involved a rubber seal which is oriented around the motor’s output shaft. Sauer Danfoss advised that this seal only be removed using a slide hammer—a process which effectively destroyed the seal. Therefore the installation of the seal over the output shaft was ignored for the study, along with the snap ring and circular plate which hold the seal in place. Again, no unique information presentation opportunities were lost due to this exclusion since the circular plate and seal installations comprise yet another pair of insertion tasks and the
snap ring installation is an exact duplicate of a prior task in which a snap ring secures the motor’s output shaft.
Software
The AR software employed for this user study was an application entitled ARMaker, designed by Rafael Radkowski. This application utilizes ARToolkit and OpenSceneGraph, and enables its users to very easily create multiple AR scenes containing numerous virtual elements and to integrate those scenes in any sequence. Authoring of such scenes involved writing an XML script to load models or images, specify positions and animations, and link objects to AR markers. An additional Python script was also written to control switching between scenes and other application logic. The XML and Python scripts written for this study are included in Appendices E and F, respectively.
For the ARMaker software to function properly and provide accurate registration of the augmented elements, the system must be calibrated for the webcam being used. Calibration was carried out using the GML Camera Calibration Toolbox [48]. Completion of the calibration procedure allowed for the webcam’s focal length and principal point—intrinsic values of the camera itself—to be communicated to the ARMaker application.