Master Thesis - Hochschule Bonn-Rhein-Sieg

6. Evaluating the safety Master Thesis Björn Ostermann page 110 of 126
Frames per Seco
12
10
8
6
4
2
0
Offset: 12 cm
0 2 4 6 8 10 12 14 16
Lines drawn
Median Minimum Maximum
Figure 78: Distance calculation, intrusion at different distances to the robot, offset at 12cm
From those measurements it can be seen that the camera control thread, using the desired offset of
12 cm, works on at least 9 FPS with reachable space computation and decreases to at least 6 FPS with
the calculation of the distance.
Thus the reaction time of the camera control thread is:
- reachable space: 111 ms
- distance calculation: 167 ms
Within those reaction times, the acquisition of a single frame of the camera is already included. This
means that 100 ms of every camera control thread cycle is not used for calculation but is idle time,
waiting on the next image.
The path planning thread proved to be too fast to allow the measuring of a single cycle. Thus it is
assumed at 1 ms.
The robot control thread, communicating with the robot’s PLC, used around 63 ms to complete one
cycle most of the time. At 5 % of the measured cycles it used as much as 78 ms and the worst result
measured was 100 ms. The responses of the PLC to the separate commands were already included in
this time. The thread has to be assumed with its worst result, 100 ms.
When combining all those reaction times, it has to be taken into account that the path planning and the
robot control thread need the results of the previous threads to achieve their task. Because the threads
are not timed, threads can start their computation with old results, just before the previous thread is
finished. Thus, when computing the worst reaction time, the secondary threads have to be counted
twice.