Hubble Presentations
Rate Gyro Assemblies (RGAs)
Mike Urban / NASA Goddard Space Flight Center
Summary
- No failure, degradation, or aging mechanisms attributable to the space environment.
- No significant change in design or materials in response to observed failures.
- Observed failures mitigated by process controls.
- Lube patch formation a random event (personal opinion).
- Flexlead corrosion an aging mechanism.
Description
- Six rate gyros
- Rate gyro channels functionally separated into electronics control unit (ECU) and rate sensor unit (RSU).
- Rate gyro channels packaged two per ECU and two per RSU.
No cross strapping.
- Three channels required for normal vehicle control.
- Four channels typically used for fail operational capability.
Rate Gyro Assembly (RGA) Functional Block Diagram
Background
- Three channels failed prior to first servicing mission; fourth showed anomalous behavior.
- Post servicing mission evaluation of returned hardware isolated failure sites.
Failure Summary
- Rate gyro channel 1
- Signature: Motor current to zero (failure).
- Source: Shorted capacitor in ECU.
- Rate gyro channel 4
- Signature: Gyro output to saturation (failure).
- Source: Failed hybrid circuit card in RSU.
- Rate gyro channel 5
- Signature: High mode scale factor transient (anomaly).
- Source: Foreign particle in flotation fluid in RSU.
- Signature: Motor current increase (anomaly).
- Source: "Lube patch" on gas bearing in RSU.
- Rate gyro channel 6
- Signature: Gyro output to saturation (failure).
- Source: Failed hybrid circuit card in RSU.
- Gyro motor spindown (failure).
- Source: Open flexlead in RSU.
Lube Patch
- Lubricant is applied to reduce bearing stiction at wheel startup.
- Lubricant can accumulate on bearing surface resulting in increased stiction.
- Mechanism not well understood but appears to be related to run time.
- Gyro 5 lube patch accumulated to the point where bearing dynamic friction affected; requiring increased motor torque (e.g., motor current) to maintain wheel speed.
- Previously observed problem:
- HEAO 1, 2, 3: 5 of 16 gyros fail to restart (4 of these recovered after repeated restart attempts).
- IUE: 2 of 6 gyros have failure signatures consistent with lube patch accumulation.
- Lube patch accumulation is major source of rejection during gyro screening.
- Processes already in place to mitigate lube patch problem.
- Change lubricant from sodium stearate to tri-ethyl amine stearate (TEAS).
- Controlled micro-pitting of bearing surface.
- Increased motor startup voltage.
- Screening: Measure variations in required startup voltage after wheel run-in.
- Current status
- Lube patch problem is still with us.
- Corrective actions seem to have mitigated its consequences if not its occurrence.
- Gyro 5 continued to operate on-orbit.
- Gyro 5 successfully restarted on the ground after return.
- Further corrective action (all procedural).
- Don't operate gyro for extended periods until you're ready to use it.
- Lube patch accumulation appears to be function of operating time.
- Once you've started using it, don't stop.
- Can't guarantee motor will restart.
Flexlead Failure
- Output axis rotation facilitated by encapsulating the wheel assembly plus necessary electronics (e.g., float) and immersing it in a viscous "flotation" fluid.
- Motor current and output axis rebalance torque signals delivered to the float by thin conductive ribbons (e.g., flexleads).
- Four flexleads carry motor current (two motor phases; two leads per phase).
- Three flexleads carry output axis torque commands (plus and minus and a common return).
- Flexleads are immersed in the flotation fluid.
- Flotation fluid
- Bromo tri-fluoro ethylene (BTFE); contains covalently bonded halogens (bromine, fluorine, chlorine).
- Selected to satisfy viscosity and density requirements: float must be neutrally buoyant.
- Flexleads
- 85% silver 15% copper aggregate.
- Selected for conductivity/mechanical characteristics (flexibility, tensile strength).
- Copper and (to a lesser extent) silver corrode in the presence of BTFE.
- Extensive corrosion observed on flexleads of all returned gyros.
- Corrosion/failure mechanism
- Residual halides react with copper phase of the flexlead aggregate.
- With copper (15% of material) gone, material is embrittled and tensile strength reduced 98%.
- Corrosion continues at slower rate on silver phase.
- Ultimate failure due to either fracture of embrittled silver lead or complete corrosion of silver.
- Residual halides an inevitable byproduct of fluid manufacture.
- Exacerbated by:
- Moisture (forms acids).
- Oxygen (replaces halogens in the polymer).
- Light and heat (accelerate polymer decomposition).
- Residual chlorides from solder flux and solvent.
.
- Indirect evidence that fluid lot used for all four returned gyros was a "bad" batch.
- Corrective action
- Specifications/screening for halide and moisture content of floation fluid.
- Modify fill procedures to eliminate exposure to air (oxygen) during fill process.
- Manage gyro operating time as a consumable.
- Gyro operates at increased temperature.
- Flexlead corrosion rate increases with temperature.
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