Mirror Transport Mechanism at Cryogenic Temperatures

Performance Requirements

• Slow scan: 0.228 cm/sec
• Rapid scan: 0.342 cm/sec
• Flyback: ≥1.5 cm/sec
• Power dissipation: <5 mW
• Operating temperature: 1.8 K
• 26-g load vector design criteria
• Jitter: 40 µsec
• Long scan travel: 0.096 to 1.638 cm
• Short scan travel: 0.4096 to 0.102 cm

Selection Process

Three mechanisms were evaluated: a linear ball slide, a flexural-pivot four-bar linkage, and a magnetic suspension platform. The linear ball slide (stainless steel rods, then hard-coated aluminum with Teflon) had unacceptably high noise on each sweep. The magnetically suspended platform was smooth and frictionless but complex. The flex-pivot four-bar linkage was chosen for its very smooth operation, extremely low power dissipation, and simplicity of design.

Mechanical Design

The MTM was upgraded from a six-flex-pivot to an eight-flex-pivot four-bar linkage to increase rigidity of the dihedral mirror platform support. Each link has line-bored holes for true coaxial pivot centerlines, eliminating rotational stresses from misalignment. Flex pivots are captivated by slotted, clamped holes — at LHe temperature, aluminum's ΔL/L (0.0043) exceeds stainless steel's (0.003), so clamping force increases on cooldown. INCO 718 was selected for the flex pivots for its cryogenic impact resistance (~20 ft-lb).

Latch & Drive Systems

A latch mechanism captivates all moving components during launch and uncages in orbit. A 15° stepper motor through a 24:1 gear ratio drives V-shaped bearings against beryllium copper leaf springs, producing ~311 N (70 lb) of force reacted by hard-anodized cone-and-socket latches. The motor has dual windings for redundancy. The scan is driven by a unique linear motor whose only moving part is completely passive. An optical encoder (50 lines/mm grating pair) generates pulses every 20 µm to control scan reversal and trigger A/D sampling.