CHAPTER 3 ASSEMBLY MOTION REFERENCE MODELS AND RAW DEPTH
3.1 A SSEMBLY TASKS
In this thesis, we propose a method for robot teaching by observing assembly process demonstration.
This thesis aims to build a system for robot teaching by assembly demonstration. Assembly plan from observation requires temporal segmentation, object recognition, task recognition, grasp recognition and task imitation (Ikeuchi et al., 1992). Temporal segmentation, task recognition and task imitation are carried out for the inset assembly task in this thesis. By defining temporal segmentation and task recognition models for the assembly activities, we are trying to expand the scope to apply the proposed method. Object recognition and grasp recognition are assumed as given.
3.1.1 Assembly types
Products from manufacturing systems are ordinary formed of a number of sub-parts. A lot of sub-parts are produced by various processing methods in different times. The purpose of the assembly is to form complex part from the individual sub-parts to perform specific functions (Lotter et al., 2013). Assembly contains various type of activities such as inspection, adjustment and assembly methods. The activity groups employed during assembly are shown in Table 3.1 which is specified in DIN 8593.
Table 3. 1 Activity groups employed during assembly (DIN 8593 – IPA Stuttgart)
General activities needed during the assembly are illustrated in Table 3.1. The assembly processes are made up of various combinations of proposed activities and its methods. Each method has its own purpose and assembly orders in the assembly processes. For example, fitting type assembly process shown in Fig 3.1-(a) has transfer, magazining, positioning, clamping, fitting and force-fitting.
In the case of screw type shown in Fig 3.1-(b), materials are feed by transfer, clamping and screwing.
And also activities such as marking, positioning, suspending and force-fitting are operated during the fitting type assembly processes. Lastly insert type assembly process shown in Fig 3.1-(c) has magazining, transfer, positioning, clamping, fitting and force-fitting in the aspect of robot assembly process.
Assembly
Material feed by Inspection Adjustment Secondary operation Assembly task - Bunkering
- Magazining - Transfer - Separating - Converging - Orienting
> rotating - Turning over
> slewing - Allocation - Infeed - Discharge - Positioning - Clamping - Release
- Measurement - Testing
- Setting - Matching - Locking
- Cleaning - Deburring - Fitting - Printing - Marking - Heating - Cooling
- Laying together
> positioning
> suspending
> Fitting - Filling
> saturating - Pressing on
> keying
> screwing
> force-fitting - Basic forming
> form-casting
> form-pressing - Re-forming
> folding
> riveting
- Material combinations
> welding
> soldering
> bonding - Other jointing processes
> binding
> stitching
Figure 3. 1 (a) Activities during the fitting type assembly (b) Activities during the screw type assembly (c) Activities during the insert type assembly
In terms of assembly task which listed in table 3.1, it consists of six main groups. Main groups of assembly tasks are shown in Table.3.2. All the groups play important role in the assembly task.
However, assembly tasks consisting of placing together (Group 1) and pressing on and in (Group 2) will be mainly discussed in this paper.
Table 3. 2 Subdivision of assembly task (DIN 8593 – IPA Stuttgart)
Placing together (Group 1) is relatively considered as a sophisticated task. The placing together process is one of the important processes associated with assembly quality. Its methods and procedures affect the efficiency and reliability of assembly operations, and pressing on and in (Group3) work easily if the placing together task operated properly. Therefore, it is necessary to elaborately teach the robot
Assembly task
Group 1 Group 2 Group 3 Group 4 Group 5 Group 6
Placing
together Filling Pressing on and
in Assembly by
basic forming Assembly by
re-forming Material combination
work path in order to perform the assembly work, and it takes a lot of time and effort to teach assembly task using robot. Moreover, it is difficult to teach efficient assembly task using teaching pendant and direct teaching method.
3.1.2 Insert type assembly process
General insert type assembly process consists of material feeding and assembly task. There are various methods on insertion process. In this paper, however, assembly tasks according to placing together and pressing on and in are considered for the insert type assembly process. Insert type assembly process consists of five activities as shown in Fig 3.2. Pre-insert and insert are considered as assembly task, and pick, approach and place are considered as material feeding.
Figure 3. 2 Activities of insert type assembly process
Placing together consists of pre-insert and insert activities, and pressing on and in consists of insert activities. The distinction between the pre-insert and insert is determined such that overlapping parts are generated. The pre-insert is classified as a preparation process for the insertion process.
In the aspect of generating robot path for the insert type assembly process, all the activities are influenced by the activities of each other. For example, grip points of the part during the pick activities is used as a basis for generating a robot pass for other activities. In this regards, it is important to have a reliable grip and to define the exact grip points on the part (Angelidis et al., 2013).
Defining a reference factors is the first step to generate the robot path, and it is important to track and record the demonstration. Reference factors can be various elements such as grip points,
represent the position and direction of the assembly parts. Reference factors for the electric tooth- brush housing which is used for the case study in this paper are shown in Fig 3.3 and representation of the part position and direction is illustrated in Fig 3.4.
Figure 3. 3 Reference factors of electric tooth-brush
Figure 3. 4 Representation of the part’s pose with reference point and reference vector
Reference points represent the position of the part and reference vectors represent the orientation of the part. The trajectory and orientation of the part was gathered based on the reference point. The reference factors can be multiple and must be able to present the state of the part during the assembly process. In the case of electric tooth-brush, reference points and vectors are used to present the state of
the part during the insert assembly process. The center point of the two marker’s center points attached to the part was defined as the reference point, and the vector with the two marker’s center was used as the reference vector. The origin of the vector is reference point. Therefore, state of the electric tooth- brush housings can be present using reference point and vector in a three dimensional space.