1. Analysis of NAC brake tests
F. Angrilli, S. Debei, M. De Cecco, M. Zaccariotto, M. Pertile, P. Ramous

2. Encoder pulses from the same folder of OSIRIS web site.
NAC brake tests
Analysis of SHM shots taken on the 21th November 2009 to test NAC SHM brake and homepush
Data sources:
Log File: MessageLog.txt from the folder Lutetia Navigation of the OSIRIS web site;
Encoder pulses from the same folder of OSIRIS web site.

3. Shots registered in the log file: 28 for NAC.
For SHM NAC, there were 0 errors.
Three groups can be identified:
Group 1: 10 shots without brake of blade 2;
Group 2: 12 shots with brake on for blade 2 (2 homepushes);
Group 3: 6 shots with brake (1 homepush).

4. Log file
The frame rate is

5. Velocity profiles of blades were analyzed for NAC shutter.
Encoder data
Velocity profiles of blades were analyzed for NAC shutter.
The present analysis is based exclusively on the encoder pulses, that were downloaded from the OSIRIS web page.
Number of velocity profiles that were analyzed:
Blade 1: 25; Blade 2: 25.
Velocity and position profiles are grouped according to the three defined groups (for group 3 only 3 shots were available in the cited folder of the OSIRIS web site).

6. Encoder data
Group 1: NAC Blade positions vs. time.

7. Encoder data
Group 1: NAC Blade velcities vs. time.

8. Encoder data
Group 1: NAC Blade initial velocities vs. shots.

9. Encoder data
Group 2: NAC Blade positions vs. time.

10. Encoder data
Group 2: NAC Blade velocities vs. time.

11. Encoder data
Group 2: NAC Blade initial velocities vs. shots.

12. Encoder data
Group 3: NAC Blade positions vs. time.

13. Encoder data
Group 3: NAC Blade velocities vs. time.

14. Encoder data
Group 3: NAC Blade initial velocities vs. shots.

15. Group 2 (12 shots, with brake) Group 3 (3 shots)
Encoder data
The following table summarizes the maximal difference (max initial velocity – min initial velocity) of the initial acquired velocity for each group and for each blade:
Group 1 (10 shots)
Group 2 (12 shots, with brake)
Group 3 (3 shots)
Blade 1
m/s
m/s
m/s
Blade 2
m/s
m/s
m/s

16. Encoder data
Initial position analysis
If the initial position of blade 1 changes from shot to shot, the time Ttrig when the blade reaches the trigger position Ptrig (fixed) also changes:

17. Encoder data
Initial position analysis
A main effect of this variation of Ttrig is a variation of the initial acquired velocity:
the lower the initial acquired velocity and the larger the initial position variation.

18. Encoder data
Initial position analysis
The assumed acceleration profile is (only positive acceleration phase):

19. Encoder data
Initial position analysis
Exploiting this kinematic relationship between the initial acquired velocity and the initial position of blade 1, the difference P0 of the initial position between two different shots can be evaluated:

20. This is a simplified analysis, whose hypothesis are:
Encoder data
Initial position analysis
This is a simplified analysis, whose hypothesis are:
The trigger position is reached during the maximum acceleration A; this hypothesis is VERIFIED for blade 1 from the measured initial velocities.
The effect of the low pass filter (applied to motor current profiles) is low and is, in first analysis, neglected.
The uncertainty associated to the measured initial velocity is low and, thus, neglected.

21. Encoder data
Initial position analysis
The following table summarizes the maximum P0 among the shots of each group (described above for 21th November 2009 tests) and for a comparison group comprising the shots performed immediately before Steins Fly-by (5 september 2008, before 18:00).
Group 1 (10 shots)
Group 2 (12 shots)
Group 3 (3 shots)
Before Steins
Vtrig,max
m/s
m/s
m/s
m/s
Vtrig,min
m/s
m/s
m/s
m/s
P0max
0.022 mm
0.065 mm
0.036 mm
1.708 mm

22. Conclusions
When the brake is active, the repeatability of the initial acquired velocity remains good, but not as good as in the shots without the brake.
The maximum difference in the initial acquired velocity increases for both blades in the case of active brake.
Also the variation of the initial blade position increases, but the obtained values are very low compared with those obtained during Steins Flyby.
The number of shots taken is very low and with low frame rate, thus further shots with higher frame rate (film mode) could be useful to validate the above results.