SAE AIR 5024-1999 Landing Gear Switch Selection Criteria《起落架开关选择标准》.pdf

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10、81g3g83g72g85g80g76g86g86g76g82g81g3g82g73g3g54g36g40g17g3TO PLACE A DOCUMENT ORDER: Tel: 877-606-7323 (inside USA and Canada) Tel: +1 724-776-4970 (outside USA) Fax: 724-776-0790 Email: CustomerServicesae.org SAE WEB ADDRESS: http:/www.sae.orgSAE values your input. To provide feedback on this Techn

11、ical Report, please visit http:/www.sae.org/technical/standards/AIR5024AEROSPACEINFORMATION REPORT g3AIR5024g44g86g86g88g72g71g3 g20g28g28g28g16g19g23g3g53g72g68g73g73g76g85g80g72g71g3 g21g19g20g22g16g20g19g3g3g47g68g81g71g76g81g74g3g42g72g68g85g3g54g90g76g87g70g75g3g54g72g79g72g70g87g76g82g81g3g38g

12、85g76g87g72g85g76g68g3g53g36g55g44g50g49g36g47g40g3g36g44g53g24g19g21g23g3g75g68g86g3g69g72g72g81g3g85g72g68g73g73g76g85g80g72g71g3g87g82g3g70g82g80g83g79g92g3g90g76g87g75g3g87g75g72g3g54g36g40g3g73g76g89g72g16g92g72g68g85g3g85g72g89g76g72g90g3g83g82g79g76g70g92g17g3FOREWORDThis document is intended

13、 to provide the landing gear designer with position sensing device guidance on appropriate technologies and means to achieve enhanced landing gear position sensing performance.1. SCOPE:The scope of this document is to discuss the differences between electromechanical and proximity position sensing d

14、evices when used on landing gears. It also contains information, which may be helpful, when applying either type of technology after the selection has been made. The purpose is tohelp the designer make better choices when selecting a position-sensing device. Once that choice has been made, this docu

15、ment includes information to improve the reliability of new or current designs. It is not intended to replace recommendations from sensor manufacturers or actual experience, but to provide a set of general guidelines.2. APPLICABLE DOCUMENTS:The following publications form a part of this document to

16、the extent specified herein. The latest issue of SAE publications shall apply. The applicable issue of other publications shall be the issue in effect on the date of the purchase order. In the event of conflict between the text of this document and references cited herein, the text of this document

17、takes precedence. Nothing in this document, however, supersedes applicable laws and regulations unless a specific exemption has been obtained.2.1 SAE Publications:Available from SAE, 400 Commonwealth Drive, Warrendale, PA 15096-0001.AIR1810A Design, Development and Test Criteria - Solid State Proxim

18、ity Switches/Systems for Landing Gear ApplicationsAIR4077 Mechanical Switch Usage for Landing Gear Applications3. TECHNICAL DATA:3.1 Environmental Factors:a. Shock/VibrationElectromechanical switches are affected by shock and/or vibration. Typically the switch is most susceptible in the plane parall

19、el to contact motion. In this plane, the switch contacts will tend to be separated causing the signal to be lost momentarily. Generally switch contact separation longer than 10 s is considered a failure. Once the shock and/or vibration has been lowered or removed, the contact will generally stabiliz

20、e. The typical design values for sealed switches used on landing gears are 100 to 200 gs sawtooth shock or 10 to 20 gs vibration.Contact separation, especially repeated separations during high levels of vibration, will tend to prematurely wear out the contacts. Permanent damage to the electromechani

21、cal switch usually does not occur until shock and/or vibration levels are reached that are higher than required to separate the contacts. For each operation, the switch actually sees the contacts make and break the circuit on each separation caused by the shock and/or vibration. These separations ca

22、n also cause false signals to be registered by electronic data processors without proper buffering or “de-bouncing”.Proximity devices by design are less susceptible to the effects of shock and vibration since they have no moving parts. However, the reliability of the device can be reduced due to sho

23、ck and vibration. This is especially true of one-piece proximity devices where the active electronics are subjected to this environment. Typical design values are 500 gs shock and 30 gs vibration for one-piece proximity sensors and 2000 gs shock and 50 gs vibration for two piece sensors. Reliability

24、 and mean time between failure calculations should reflect the environment.It should also be noted that the target and mounting of the proximity sensor and the actuator and mounting of the electromechanical switch should be rigid enough to withstand the dynamic loads caused by the shock and/or vibra

25、tion. Resonant frequencies of the sensor and bracketry must be designed such that sensor assembly functions are not impaired. Vibration mode shapes should be considered when selecting sensor mounting locations either on structural components or on the landing gear itself.g54g36g40g3g44g49g55g40g53g4

26、9g36g55g44g50g49g36g47 g36g44g53g24g19g21g23 g51g68g74g72g3g21g3g82g73g3g20g21_3.1 (Continued):b. Radio Frequency Interference/Electromagnetic Interference (RFI/EMI)1. SusceptibilityElectromechanical switches are usually not considered susceptible to RFI/EMI. However, the wires connecting these swit

27、ches to associated circuitry may require RFI/EMI shielding. One-piece proximity switches that include active electronics are susceptible to RFI/EMI especially in severe environments, such as near high power radar or near or during lightning strikes. The RFI/EMI environment must be carefully consider

28、ed when designing the proximity sensor and its installation. Typically there are specific RFI/EMI performance specifications that the proximity sensor is required to meet. Twisting leadwires, metal overbraids, leadwire routing, as well as the device design, will minimize susceptibility.2. Generation

29、Electromechanical switches can generate noise depending on the load. Non-arcing loads below approximately 0.5 amp are not considered to generate noise. Arcing currents, especially those controlling highly inductive loads, such as large solenoids or relays, can generate noise. Twisting leadwire, meta

30、l overbraids, and leadwire routing can reduce or eliminate generation problems when dealing with arcing loads. Transient protection networks are usually provided on inductive devices. Proximity devices generate a weak electromagnetic field to sense the target. This can be an interference problem if

31、the proximity devices are mounted closer than 3/4 in to each other.c. TemperatureTypically, electromechanical switches can withstand wide temperature ranges and rapid gradient shifts without damage or operating point shifts. Most aerospace switches operate at temperatures between -55 to 85 C with de

32、signs available from -185 to 260 C, or more. Higher temperatures require more exotic materials that increase cost and can limit life. It should be noted that O-ring seals and elastomer boot seals tend to stiffen in extreme cold that can increase operating forces and reduce release forces or delay sw

33、itch release.Aircraft proximity sensors are currently designed for a performance operating range of -65 to 150 C for two piece devices, and between -65 to 125 C for one-piece designs. Proximity switch operating and release points may shift during temperature excursions, shifts of up to 5% are common

34、. If shifts up to 5% can be tolerated, the operating temperature can be extended.g54g36g40g3g44g49g55g40g53g49g36g55g44g50g49g36g47 g36g44g53g24g19g21g23 g51g68g74g72g3g22g3g82g73g3g20g21_3.1 (Continued):Reliability of the one-piece proximity sensor will typically be highest at room temperature due

35、to the active electronics. Reductions in reliability and mean time between failures (MTBF) estimates should be made for devices which must see high temperatures or high thermal gradients, especially one-piece proximity devices with active electronics which will see this environment. Reliability and

36、MTBF calculations for two piece proximity sensors are largely unaffected by temperature extremes since their active electronics are located in the interior electronics bay of the aircraft. Current two piece sensor designs are all metal sealed units, intended to last the life of a 40,000 cycle aircra

37、ft.d. SealingElectromechanical switches range in sealing from completely open to true hermetic sealing. Hermetic sealing is defined as metal to metal or glass to metal seals with leak rate of less than 1 x 10-8cc/atm/sec. Resilient sealing is the most common seal in the aerospace industry. These swi

38、tches have O-ring seals at the plunger with glass to metal seal around the basic switch and have leak rates less than 1 x 10-8cc/atm/sec. The factors to determine seal level in an electromechanical switch are as follows:1. Use a sealed switch when the switch will be exposed to a dirty environment in

39、 storage, assembly, or operations.2. Use a higher level of sealing when the contacts will not have an arcing load. Switches that have arcing loads tend to self-clean the contacts. Low energy loads tend to be more susceptible to contamination.3. Use a higher level of sealing when it is an extremely i

40、mportant function or the switch may not be used for long periods.The seals, including the O-ring material, as well as any potting material, should be compatible with fluids in the intended environment.Current proximity switch and sensor designs have an all-metal case with an elastomer compression le

41、ad wire seal, or a hermetic connector. Sealing baseline is at 1 x 10-5cc/atm/sec, with 1 x 10-8cc/atm/sec available if using a connector. Proximity sensors are considered explosion proof by design. Potting materials are not currently used. Older proximity sensor designs with plastic faces meet the 1

42、 x 10-5cc/atm/sec criteria unless damaged by hard physical contact.g54g36g40g3g44g49g55g40g53g49g36g55g44g50g49g36g47 g36g44g53g24g19g21g23 g51g68g74g72g3g23g3g82g73g3g20g21_3.1 (Continued):e. IceIce buildup on the plunger or actuator of electromechanical devices can reduce the switchs ability to re

43、lease. Rarely is this a problem and typically only on installations where the switch is directly in the airstream. Testing in the laboratory for ice buildup is difficult and does not yield practical results. Release forces of 4 lb minimum for plunger switches subject to icing conditions are consider

44、ed sufficient.Likewise, ice buildup on proximity devices is rarely a problem. Swipe by installations in lieu of direct approach installations are preferred to reduce potential for malfunction due to the buildup of ice or foreign object damage (FOD).3.2 System Requirements:a. Contact ArrangementsMost

45、 aircraft electromechanical switches have multiple throws and poles and can be easily wired redundantly. Single pole, single throw is usually necessary if hermitic sealing is required. Generally, electromechanical switches drive banks of relays within the aircraft and switched power levels are of co

46、ncern in fuel areas, particularly in fault conditions.Proximity switches are usually single pole, single throw. Proximity sensors and switches have a “single on-off output” that can be configured into multiple outputs or logic equations through their mating electronics. This system is also “checkabl

47、e” by using built-in-test (BIT) logic.b. Load CapacityElectromechanical switches can handle extremely low voltages and currents up to high power loads. Typically, aerospace switches have silver or silver base contacts and are designed to handle from 0.5 to 5.0 amps or more. The number of operations

48、required determines the maximum current the switch can handle. Electrical life of most aerospace switches is 25,000 operations. Reducing the current will in most cases increase the number of operations the switch will make or increase the switchs reliability. It should be noted that this applies to arcing loads only. Reducing the load below arcing levels changes the failure mode. Clean silver contacts have no minimum current or voltage requirement. However, switch manufacturers will usually recommend gold or other precious metals below arcing levels. This is because gold and s