2.1. Skill Primitives

By analyzing a person performing a task, we derived a sequence of primitives for various behaviors. The behavioral primitives are called “skills” in our research [9–11]. As shown in Figure 2, in the assembly sequence, three skills, i.e., move-to-touch, rotate-to-level, and rotate-to-insert, played an important role. A skill sequence of actions for a task was obtained by assembling these three skills and their derivatives as a working sequence. The primitives of the work sequences of machines in a plant can be considered in the same manner as the skill sequences obtained from the analysis of human actions.

Figure 2

Three fundamental skill. (a) Move-to-touch skill. (b) Rotate-to-level skill. (c) Rotate-to-insert skill.

2.2. Stratification of Tasks

For tasks in an automation plant, representation using stratification makes it easier to consider work sequences, as shown in Figure 3 [6–8]. Please refer to Nakamura et al. [6] for more information on this technique.

Figure 3

Hierarchy of tasks.

3.1. Error Classification

The errors are classified into the following four types according to the cause of occurrence: execution, planning, modeling, or sensing. As shown in Figure 4, the cause of the error is generally estimated at the time of executing a task [6–8].

3.2. Error Recovery based on Classification

Immediately following the occurrence of an error, the first step involves estimating the cause of the error, and the second step involves proceeding to the error recovery course based on the specified cause [6–8]. The corresponding system parameters are modified in the indicated error recovery course, as shown in Figure 4. This improves the system, thus making it less prone to the same errors.

Figure 4

Fundamental process flow with error recovery.

When the error is small, the process returns to an earlier step in the executed sequence, and the task sequence advances again, as shown in Figures 4 and 5. Conversely, when the error is large, the process returns to the steps in the upper hierarchy composing the task, and the recovery process then proceeds, as shown in Figure 5.

Figure 5

The expression of task stratification and the process flow of the error recovery.

3.3. Candidate Processes for Recovery

This subsection briefly describes the recovery process after an error occurs during task execution. It has been assumed that task T of start S and goal G comprises n subtasks {subtask₁, subtask₂, …, subtask_n}, and that an error occurs in subtask_m (m is one of 1, 2, …, n), as shown in Figure 6. Here, subtask_m is the minimum possible traceable unit, as described in Nakamura et al. [6]. This is the smallest unit of the skill primitive sequence, which starts with the first node that is required to go backward when an error occurs. That is, a minimum traceable unit represents a boundary, where recovery can be made by going back to a certain node or a step before that node; however, recovery is not possible beyond that step.

Figure 6

Various processes of error recovery considered for a failure occurred in subtask_m.

Here, it is assumed that an error occurs at subtask_m, and the process returns to subtask_j (j is one of 1, 2, …, m). Then, the recovery process begins from subtask_j, and returns to the original work sequence. However, it is not always possible to make the process identical to the original work sequence. Owing to various conditions, it may be necessary to use a sequence composed of different skill primitives, after an error occurs. This is why the substitute task for the subtask in the recovery process in Figure 6 is shown as “An equivalent task.”

5.1. Several Types of Recovery Processes

Let us consider attaching a hook to a vertical flat plate with four precision screws. It is assumed that the vertical flat plate has four tapped holes for these precision screws. The task comprised various skill primitives, as shown in Figure 7; however, these primitives can be divided into four sections. As shown in Figure 7a–7d, the first section is a tacking task that temporarily fastens four screws to a horizontally laid hook. As shown in Figure 7e and 7f, the second section is an erecting task, wherein a horizontally inverted hook is made to stand vertically. As shown in Figure 7g and 7h, the third section is a touching task for moving the hook to the mounting position on the vertical plane plate. As shown in Figure 7i–7m, the fourth section is an installation task for fixing the hook to the vertical plane plate by tightening the four screws.

Figure 7

An assembly task in which a hook is stuck to a plate by four precision screws.

Now, let us consider the recovery processes for the failure in the step shown in Figure 7h, where one screw that was temporarily fixed comes off and falls (Figure 8). Here, we have discussed three types of recovery processes.

• •

  Recovery Type I (RT-I)

  The first recovery type (RT-I) is the process of returning to starting step S of the original task and starting over (Figure 9). Concurrently, the hook with three screws and the one screw that fell off when the error occurred are abandoned, and the task is rerun with new parts, that is, a new hook and four new screws.

• •

  Recovery Type II (RT-II)

  The second recovery type (RT-II) is the process of returning to the step that is shown in Figure 7d in the tracking task and starting over from this step, as shown in Figure 10. After returning to the tracking task, a screw is temporarily fastened at the missing hole once again; consequently, two types of methods occurred depending upon the screw used at said time. One involves abandoning the fallen screw and using a new one in the parts box; conversely, the other involves finding the fallen screw and using it again. The former is called process [RT-II(N)], and the latter is called process [RT-II(F)].

• •

  Recovery Type III (RT-III)

  The third recovery type (RT-III) is the process of returning to the skill primitive in subtask_m, where the failure occurred and started over from, as shown in Figure 11. This method has a shorter return process; however, unlike (RT-II) in the vertical direction, it requires the insertion and temporary fastening of the screws in the horizontal direction. Mistakes are likely to occur in the temporary fixing process, owing to which the difficulty level is high. As in the case of (RT- II), there are two schemes depending upon the screw used in the relevant step. The method of discarding the fallen screw and using a new screw in the parts box is called process [RT-III(N)]; meanwhile, the method of finding the fallen screw and reusing it is called process (RT-III(F)]. Immediately after an error occurs, the first step is to estimate the cause of the error, and the second is to proceed to the error recovery course based on the specified cause.

Figure 8

An error in which a screw is dropped at (h) in Figure 7.

Figure 9

Recovery Type I (RT-I).

Figure 10

Recovery Type II (RT-II).

Figure 11

Recovery Type III (RT-III).

5.2. Suitable Process in Each Evaluation Standard

In the previous subsection, we considered five types of error recovery processes—(RT-I), [RT-II(N)], [RT-II(F)], [RT-III(N)], and [RT-III(F)]—for screw fallout errors in the task of fixing a hook to a vertical flat plate with four precision screws. Here, the priorities of these five recovery processes have been derived for each evaluation standard that is described in Section 4. For simplicity, priorities are explained qualitatively, rather than quantitatively.

1. (i)

   Cost

   Priority order = {(RT-II(N)), (RT-III(N)), (RT-II(F)), (RT-III(F)), and (RT-I)}

   The priorities of the five error recovery types under the cost based evaluation standard have been described in detail in our previous research [8]. Here, material charges, parts charges, electricity bills, and planning expenses are considered as costs. The (RT-I) process is costly, as it discards the entire unit in which the error occurs and starts over from the beginning. In contrast, the [RT-II(N)] process is not costly, as there is no search operation or difficult operation involved.

2. (ii)

   Time

   Priority order = {[RT-III(N)], [RT-II(N)], (RT-I), [RT-III(F)], and [RT-II(F)]}

   The (RT-II(F)) process requires a search operation, and must return to much earlier steps, which is time consuming and unsuitable. Alternatively, the [RT-III(N)] process does not involve a search operation, and returns only a few steps; hence, it is the optimum choice.

3. (iii)

   Reliability

   Priority order = {(RT-I), [RT-II(N)], [RT-III(N)], [RT-II(F)], and [RT-III(F)]}

   The (RT-I) process is the most reliable process, as it runs the original sequence from the beginning.

   The [RT-II(N)] and [RT-III(N)] processes are the second most reliable after the (RT-I) process, because they do not have search operations and only perform almost planned operations. Conversely, the [RT-II(F)] and [RT-III(F)] processes are not suitable in terms of reliability, because they require search operations. Moreover, [RT-II(N)] and [RT-II(F)] are more suitable than [RT-III(N)] and [RT-III(F)], respectively, because there are no difficult operations involved.

4. (iv)

   Safety

   Priority order = {(RT-I), [RT-II(N)], [RT-II(F)], [RT-III(N)], and [RT-III(F)]}

   The (RT-I) process is the most desirable in terms of safety, because it executes the original sequence from the beginning. The [RT-II(N)] and [RT-II(F)] processes are the second safest options, because they do not have difficult operations or become unstable postures. In contrast, the [RT-III(N)] and [RT-III(F)] processes are not suitable, because they require difficult operations. Additionally, [RT-II(N)] and [RT-III(N)] are more suitable than [RT-II(F)] and [RT- III(F)], respectively, because they do not involve a search process.

5. (v)

   Finishing

   Priority order = {Fundamentally the same for all five} There is no significant difference in the completion of the sample task for any of the processes. However, operations that may adversely affect the target item or its surroundings, such as scratching or scattering, should be avoided.

6. (vi)

   Recovery data

   Priority order = {Process with a significant amount of data is prioritized}

   The quantity of the dataset is important in deriving the error recovery process, using data science techniques. Even when the operation is considerably difficult to perform, considerable amounts of data allow the task to be accomplished successfully.

7. (vii)

   Parts

   Priority order = {Process of using dedicated parts is given priority}

   If dedicated parts are prepared for replacement, it is desirable to prioritize the process of using those parts. For example, suppose there is a special replacement unit that corresponds to a hook temporarily fastened with four screws, as shown in Figure 7a–7d. When an error occurs, the use of the replacement unit simplifies the recovery task.

8. (viii)

   Tools

   Priority order = {Preference is given to a process with a dedicated tool}

   Here, we will consider a case in which there is a dedicated tool for recovery. If a process that uses the exclusive recovery tool is incorporated, it is highly likely that the recovery will be quick and reliable. If there is a special tool that can temporarily fasten the hook with four precision screws in any posture, the [RT-III(N)] or [RT-III(F)] process is given the highest priority.

9. (ix)

   Workspace

   Priority order = {Process of using the workspace takes precedence}

   If there is a dedicated space prepared for replacement work, it is desirable that the process that utilizes this area be prioritized. In this assembly task, if there is a space for replacement work near the place where the failure occurs, the process that uses that region will be given priority. For example, in the RT-II process, it is efficient to use a dedicated workspace for the process of temporarily fixing the screws, as shown in Figure 10d, e, r.

10. (x)

    Operator skill

    Priority order = {Efficient recovery process of the operator is prioritized}

    We will consider a recovery process using a teaching operator. It is preferable that a process, which is the forte of an operator regarding recovery implementation, is prioritized. However, there may be differences in the level of efficiency of each operator. Hence, the processes suitable for skilled operators may differ from those suitable for unskilled operators.

We have examined the appropriate process by selecting only one out of the 10 evaluation standards at a time. In practice, however, it may be desirable to derive the optimal process using a combined evaluation of multiple standards.