PHEBUS is behaving well!

25 octobre 2018 par Aurelie Reberac
On October 22nd 2018, around 18:40 CEST, the PHEBUS instrument is the first instrument of the BepiColombo mission to confirm good health!

The instrument has been switched ON for about 25 minutes, where basic checks were performed (current consumption, connection ping test, telemetry packets time synchronization, secondary voltages verification, sub-systems temperatures, etc.), and then commands were sent to proceed successfully with the unlocking of the pointing mechanism (scanner) of the instrument.

Pointing mechanism (scanner) used to rotate the telescope (baffle + main mirror) of the PHEBUS instrument


Purpose of the scanner and the importance of its locking during the launch phase

The light collecting system of the instrument includes a stray light baffle and an off-axis parabolic mirror, and this assembly is acting as a telescope. The telescope is mounted on a one degree-of-freedom scanning system ("scanner") provided by the Russian Space Agency (IKI). This scanner allows the telescope to turn with an accuracy less than a degree in order to adjust the line of sight of the instrument.

In order to preserve the integrity of the pointing mechanism during the launch phase, where severe constraints in terms of vibration are applied, a locking system was installed to block the rotation of the scanner.

At the time of launch the telescope is in its “parking” position, facing into a support that is called "parking bracket” which is attached to the spacecraft, and protects the telescope aperture to prevent that the inner part of the instrument is illuminated by intense brightness during non-operating phases (Figure 1).

Figure 1. PHEBUS telescope facing into the parking bracket attached to the spacecraft, protecting the telescope entrance during non-operational periods.

Locking system description

The locking system (Figure 2) includes a locking axis, a spring and a custom Tini Aerospace  P25 pinpuller actuator.

Figure 2. PHEBUS locking system exploded view

The actuator (commonly named "TiNi") is the active device in the PHEBUS scanner locking system and it triggers the release of the locking axis.

When rearming the locking mechanism thanks to a specific tool set, the locking axis is pushed into the scanner flange such as the scanner is locked (Figure 3). When pushing the locking axis, the spring is compressed. The locking axis is kept in this position by the axis of the pinpuller (when the actuator has been reset). When the mechanism is activated, the pinpuller pulls its axis, and it releases the locking axis. The locking axis is then pulled out of the scanner flange by the spring.

Figure 3. PHEBUS locking system integrated into the instrument


Scanner lock release operation

The actuator can only be rearmed by hand. As a consequence, once the spacecraft is in flight, the Locking System can only be activated one time (one-shot mechanism). The actuator has two supply lines (nominal and redundant). It is activated by the DPU with a voltage pulse (duration: 1 second) of 12V either on nominal line or redundant line. Once the actuator has been activated, it opens its supply circuit (this occurs after 200-250ms). A current detection circuit is used to detect when the current flows through the activated line. This results in an event which indicates that the actuator was activated

If the locking system fails on both activation lines (i.e. baffle still locked), the telescope will remain in its position behind the parking bracket.

Telecommand sending to the instrument

The scanner lock release operation has been executed with ground station coverage in an interactive manner by the ESOC (European Space Operations Center in Darmstadt, Germany) operators who were supported by the PHEBUS team (Figure 4).

The instrument has been first switched ON in a “STAND-BY” software mode in order to perform basic checks (current consumption, connection ping test, telemetry packets time synchronization, voltages verification, sub-systems temperatures, etc.),

Figure 4 ESOC operators and 2 members of the PHEBUS team (Aurélie & Jean-François) in the BepiColombo DCR (Dedicated Control Room) at ESOC, tracking in real time the current detection! 2 other PHEBUS team members (Christophe & Benjamin) were in remote in LATMOS facilities in order to validate the operation.

The instrument was then configured in another software mode called "MAINTENANCE" (via another specific telecommand) before sending telecommands which activate the actuator. TiNi functions are protected by 2 Arm and Fire commands, where the first command allows the second one to be executed. The “Fire” command configures the Tini supply line to be activated and checks the current consumption detection. The current detection information is memorized in the DPU HK (HouseKeeping data) that will be then transmitted on ground.

If the previous steps are successful, the following telemetries will be sent:

  1. One telemetry event “Baffle Lock Success”, which means that the current consumption has been detected.
  2. One telemetry event “End of Baffle lock Activation” which means that the on-board software has powered off the TiNi line.
  3. One telemetry “DPU Housekeeping” with a specific bit (“TiNi current”) equals to “Detection” (Figure 5).
Figure 5. Tini Current bit value monitored by the instrument during the baffle lock release operation on October 22nd 18:40 CEST (16:40 UTC).


Next step will be the testing of the rotation of the telescope of the instrument, scheduled too take place on December 7th 2018 

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