## 3 Results and Discussion
### Verification of Correlation Usage and Health Threshold Identification
Table 4 shows the maximum and minimum fault warning periods obtained for each die for each type of correlation. The force profile correlation indicators provided--at the minimum--early warning alerts on quality adjustment and non-die fault initiation of 1 h 20 min and on die damage of 1 h 41 min for 4P. For 9P, the minimum warning period was 1 h 40 min for quality and non-die issues and was 1 h 41 min for die faults. The force SRS correlation, on the other hand, provided a minimum warning period on quality and non-die fault occurrences of 1 h 30 min and on die damage of 2 h for 4P. For 9P, the SRS minimum warning period was 1 h 41 min for quality adjustment and non-die damage and was 2 h 31 min for die damage. Furthermore, the more severe the damage level, the earlier was the detection of fault occurrence. For example, the force SRS correlation health threshold gave a warning period of 8 h 52 min before 4P, 5P, and 9P dies became damaged and quality adjustment was performed. Of the two analyses, the force SRS correlation provided more fine-tuned results and longer times-to-failure. The SRS tool allowed the measurement of the instantaneous impact against the natural frequency of each die impact compared to the voltage changes over time measured by the force profile. As a result, the force SRS was sensitive to changes in the natural frequencies and detected fault occurrences earlier than the force profile correlation. The warning periods for die faults were longer than for quality adjustment and non-die faults. Faults of both dies were detectable at about 8 h at the earliest and 2 h at the latest under both correlations. The 4P sensor, likely due to its more central location, detected non-die and quality issues twice as early--5 h compared to 2 h 30 min--as the 9P sensor. Table 5 gives an approximate average warning time for each die and its respective fault types, which may be preferred under a RTF stance. In either case, production would still be more optimal compared to existing practices. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{3}{*}{**Fault Types**} & \multirow{3}{*}{**Die**} & **Force Profile** & **Force Profile** & **Force SRS Profile** & **Force SRS Profile** \\  & & **Correlation** & **Correlation** & **Correlation** & **Correlation** \\  & & **Maximum** & **Minimum** & **Maximum** & **Minimum** \\  & & **Warning Time** & **Warning Time** & **Warning Time** & **Warning Time** \\ \hline Quality Adjustment and & 4P & 4 h 51 min & 1 h 11 min & 5 h 1 min & 1 h 30 min \\ Non-Die Damage & 9P & 2 h 31 min & 1 h 9 min & 2 h 31 min & 1 h 41 min \\ \hline Die Damage & 4P & 8 h 33 min & 1 h 41 min & 8 h 33 min & 2 h \\
9P & 8 h 42 min & 1 h 41 min & 8 h 52 min & 2 h 31 min \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Time prior to maintenance for 4P and 9P force and force SRS profiles using health thresholds over long time span. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Date**} & **Maintenance** & **Die** & **Maintenance Log** & **Force Profile Correlation** & **Force SRS Profile** \\  & **Time** & & **Earliest Predictive** & **Correlation Earliest** \\  & & & & **Warning Period** & **Predictive Warning Period** \\ \hline \multirow{3}{*}{9 March} & 7. 10:09–10:22 & 4P & C; D & 1 h 30 min & 1 h 40 min \\  & & 9P & & 1 h 30 min & 1 h 40 min \\ \cline{2-5}  & 8. 19:51–end & 4P & A; B; C; D & 8 h 23 min & 8 h 33 min \\  & & 9P & & 7 h 12 min & 7 h 33 min \\ \hline \multirow{3}{*}{11 March} & 9. 12:10–21:36 & 4P & A; B; C & 2 h 30 min & 2 h 20 min \\  & & 9P & & 2 h 30 min & 2 h 40 min \\ \cline{1-1} \cline{2-5}  & 10. 00:30 (end) & 4P & A & 1 h 50 min & 2 h 20 min \\ \cline{1-1} \cline{2-5}  & & 9P & & 2 h 20 min & 2 h 10 min \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._The results of the study showed that establishment of health thresholds and detection of the common faults within a period that can allow timely maintenance can be done using force data with the force SRS profile being the more sensitive.