SPIRE Design Description - Research Services

Draft SPIRE Design Description Document
Jiggle Stage: The jiggle stage is in the form of a split frame split and clamps together around the flex pivots.
To balance the jiggle stage the framework in the opposite corner to the coils of the actuator has been made
solid. This also increases the stiffness of the structure. This structure carries the chop stage, and is inevitably
heavier. Since the amplitude and frequency requirements in this axis a stiffer flexures pivot is used for
increased strength and reliability.. The static load on the jiggle axis flex-pivots at 50g is 27N, well below the
245N load capacity.
Position sensors for the chop axis are mounted on the jiggle stage, which means flexible cable connections
are required, unlike the jiggle stage position sensors, which mount directly on the non-moving housing. An
alternative to this may be to place the chop axis sensor also to the housing and compensate for the movement
in the jiggle axis in the look up table using this in conjunction with the jiggle axis position. The gimbal
frame is fabricated from Al-6061.
Magnetic Actuators: The magnetic actuators are located at the edge of each stage of the BSM. There is a
primary and a redundant motor for each axis. The estimated power consumption is 0.4 mW when chopping
at 2Hz with maximum amplitude and an average power consumption of 1.6 mW when jiggling at 1 Hz with
maximum amplitude. Depending on implementation, the motor for a single axis will either have:
(i) the prime coil on one side of the rocker beam and the redundant on the other, leading to an
unbalanced load but a more rugged coil, or
(ii) a balanced set of coils with the prime and redundant motor coils wound onto the same bobbin.
The coils may be potted (encapsulated) if required, and will certainly require extensive magnetic shielding
and strong thermal linking to the thermal straps. All the motor coils mount directly to the BSMs, i.e. the
chop stage air gaps must be slightly over-size to accommodate chopping whilst in various jiggle modes.
Position sensors: These sensors are Infineon (ex-Siemens) FP 212 L100-22 differential field plates that
sense the position of soft iron pieces in the moving parts. The sensors are dual InSb/NiSb magneto-resistive
elements, biased with a permanent magnet and forming part of a bridge circuit. As the position of the gimbal
frame moves relative to the position of the sensor, the resistance of the element changes in proportion to the
displacement. The sensor is connected in a Wheatstone bridge configuration driven by a constant current
source. Hence, the voltage measured across the bridge is proportional to the displacement of the mirror.
Figure 4-33 – Schematic drawing of the Infineon magneto-resistive position sensor.
Position sensors for the chop axis are mounted on the jiggle stage, which means flexible cable connections
are required. The jiggle stage position sensors mount directly on the non-moving housing and do not require
these flexible wires.
Stray light baffle: Parasitic losses in the actuators, electronics and wiring may cause the temperature of
some of the items mounted on the BSM to rise above the 4 K temperature of the Level-1 enclosure. Any
radiation emitted from these items must be shielded from the detector arrays. This is achieved through a
baffle made from 0.25-mm thick (TBC) Al-6160 (TBC) placed over the components and around the mirror.
The baffle fits tightly around the mirror allowing full movement without interfering with the primary beam.
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