1、Lessons Learned Entry: 1581Lesson Info:a71 Lesson Number: 1581a71 Lesson Date: 2005-27a71 Submitting Organization: JPLa71 Authored by: David J. OberhettingerSubject: Thermal Sensor Installation Failures Remain a Problem Abstract: There is an extensive history of resistance thermal device (RTD) failu
2、res on NASA and DOD missions due to thermally-induced mechanical stress. Recent MER ground and in-flight failures, including some failures in mission critical MER applications, suggest that these devices remain sensitive to variations in the mounting configuration. Perform Package Qualification Veri
3、fication (PQV) for all critical RTD applications. For non-critical applications, develop standard, flight qualified installations and methods, and track the part pedigree. Description of Driving Event: During Mars Exploration Rover (MER-2) system-level thermal-vacuum (STT) test, it was discovered th
4、at one of two platinum resistance thermometers (PRTs) that provide temperature calibration for the Miniature Thermal Emission Spectrometer (MTES) had failed (Reference (1). The PRTs had been installed by the contractor using a liberal amount of a rigid adhesive that transferred the thermal strain di
5、rectly to the relatively brittle ceramic body of the PRT, overstressing it to failure. The mounting method had not been sujected to Package Qualification Verification (PQV), and subsequent coupon tests showed that the mounting method was consistent with failure within a few thermal cycles. One inter
6、nal calibration target PRT on each of the Mars rovers was reworked by JPL using a ribbon of RTV, a configuration that passed PQV. (The others were not replaced because it would have invalidated the calibration.) Three months after landing on Mars, 6 internal and external calibration target PRTs on R
7、overs MER-1 and MER-2 that were bonded using the original method failed, but the 2 reworked PRTs subjected to PQV functioned properly. The loss of a temperature sensor on the MTES could result in the loss of a major portion of the rovers science return. In an unrelated incident during an STT of MER-
8、1 (Reference (2), the azimuth actuator PRT failed on the Instrument Deployment Device (IDD, or rover arm). This failure occurred shortly after the IDD heaters, used at the conclusion of the test to return the spacecraft to ambient temperature, were Provided by IHSNot for ResaleNo reproduction or net
9、working permitted without license from IHS-,-,-powered off. In this case, bonding adhesive likely migrated from under the PRT body and onto the lead wires (Figure 1), and the different coefficients of thermal expansion caused the contracting platinum wire to break and open at cold temperature. inser
10、t figure and picture Figure 1. PRT bonded to the actuator case with excessive epoxy adhesive, covering the leads (small wire loops indicated byarrow) Also, a PRT on the MER-2 Lander Petal Actuator failed in flight (Reference (3). Thermal sensors provide vital information on spacecraft and instrument
11、 health, and are sometimes essential to subsystem function. A 1994 study (Reference (4) documented a trend of in-flight failures of resistance thermal devices (RTDs) on JPL, Goddard Space Flight Center, and U.S. Air Force missions. Although electrostatic discharge and radiation also caused failures,
12、 design and implementation of the sensor mounting configuration was the probable cause of most failures. For installations lacking adequate strain relief, thermal dwell or cycling may induce different expansion and contraction rates in the (internal) sensing wire (0.0006 inch diameter), or the lead
13、wire (0.012 inch diameter), versus the sensor body. Fracture of an RTD wire typically resulted in erratic readings (as the wire intermittently regained contact), followed by full scale temperature readings indicating an open circuit. References: (1) JPL Problem/Failure Report No. Z79528, February 21
14、, 2003 (2) JPL Problem/Failure Report No. Z79691, March 5, 1003. (3) JPL Problem/Failure Report No. Z81188, July 10, 2003 (4) D. Oberhettinger, “NASA Unmanned Flight Anomaly Report: Investigation of Thermal Sensor Failures Aboard Unmanned Spacecraft“ (JPL D-11377), April 1994. Additional Key Words:
15、Mini-TES; thermistor; workmanship error Lesson(s) Learned: When viewed against a history of RTD failures related to mechanical stress during NASA and military missions, recent JPL failures suggest that these devices remain sensitive to variations in bond joint geometry and in device and surface moun
16、t materials. Hence, package qualification verification (PQV) may be particularly suited to predicting RTD failure- arguably more suited than a collective mechanical design guideline for packaging (e.g., 155 deg C range for 200 cycles). The need for PQV may be overlooked in plans and contracts becaus
17、e hardware featuring RTDs may be mistakenly considered to be purely mechanical assemblies. Also, a problem with qualifying a single standard mounting configuration is the lack of analytical insight into what changes may invalidate the flight qualification. Recommendation(s): 1. Perform PQV to verify
18、 the peer-reviewed packaging design for all critical RTD applications used in both closed and open “control loops.“2. Develop a set of standard, flight qualified, mounting configurations, installation procedures, and workmanship standards for RTDs not intended for critical applications. Review the e
19、stablished engineering specifications and procedures on the use of bonding and potting compounds, and issue guidelines that account for adhesive properties, the need for mechanical stress relief, and the effects of environmental stress on RTDs.3. Assure that all piece part RTDs have a qualification
20、paper trail from the manufacturer.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Evidence of Recurrence Control Effectiveness: JPL opened Preventive Action Notice (PAN) No. Z87688 on October 24, 2005 to initiate and document appropriate Laboratory-w
21、ide action on the above recommendations. Documents Related to Lesson: N/AMission Directorate(s): N/AAdditional Key Phrase(s): a71 Flight Equipmenta71 Hardwarea71 Payloadsa71 Spacecrafta71 Test & Verificationa71 Test Articlea71 Parts, Materials, & ProcessesAdditional Info: Approval Info: a71 Approval Organization: JPLProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
