Aircraft Landing Gear System - Essay Example

?AIRCRAFT LANDING GEAR SYSTEM Part I a) Fig ARIC 429 Hardware (Helfrick 322) As can be seen from Fig the external components of ARINC 429 Data Bus are: the standard line receiver, and; standard line driver. The transmitter or line driver sends messages and receivers accept them. It can have 20 receivers at the most, with a 12 k resistance. b) ARINC 429 Databus Features Operational Specifications Simplex, broadcast bus connected in Star or Bus Drop configurations Transmission rate: low speed (12.5 to 14.5 kHz) and high speed (100 kHz) +/-1% Receiver is not allowed to respond in the same bus where a transmission has occurred LRU does not have an address but is programmed to listen for specific words Cabling consists of 78 ? twisted, shielded pair with shield grounded at both ends Source (transmitter) must be able to handle 400 ? at the maximum. Receivers must have minimum effective input impedance of 8 ? Normally designed for range of less than 175 ft (or 50+ meters) A data “one” is created when the rising edge goes from 0 to 10+/- 1 positive volts; a data “zero” is created when the falling edge goes down from 0 to 10+/- 1 negative volts Error Checking ARINC 429 uses the odd parity bit to detect error and ensure that data that is being received is accurate. It is the last bit in a message transmission constantly changing with label and data change always resulting in “Odd Parity” always containing the number ‘1.’ Data Word Format An ARINC message is usually a single data word that is 32-bit long and includes five fields: Label; SDI; Data; SSM, and; Parity. Label identifies the kind of data that being transmitted and has a value of 8. SDI (or Source Destination Identifier), specifies the value of the intended receiver. In a system with multiple receivers, each receiver is assigned a value. Data, the actual message of the transmission, uses two kinds of format: the BCD (short for Binary Coded Decimal), uses four bits, and; the BNR (short for Binary encoding). Both define units, resolution, range, number of bits used and frequency of the label. SSM (or Status/Sign Matrix) assists in the interpretation of numeric values in the data field with values such as ‘north,’ ‘east,’ ‘plus,’ ‘minus,’ etc. Finally, P, for parity bit, is the last bit transmitted (Cook et al 462). c) Comparison of ARINC 429, ARINC 629 and MIL 1553B Databuses 1. Encoding Method ARINC 429 uses a bipolar return to 0 type of encoding; MIL 1553B uses the Manchester II biphase where a logic one (1) is transmitted as a bipolar coded signal I/O and a logic zero (0) as a bipolar coded signal I/O (TSCM 9-6.2). ARINC 629 also uses binary encoding. 2. Bus Coupling Method The ARINC 429 has integrated line transmitter/receivers that software can program to receive (Rx) or transmit (Tx) and operate at specific transmission rate independently of other channels. On the other hand, ARINC 629 incorporates bus controllers into every unit and coupling is made using current transformers without cutting off wires. Meanwhile, MIL 1553B uses the transformer and direct method of bus coupling. 3. Data Word Format ARINC 429 uses a 32-bit data format. Fig. 2 illustrates the allocation of bits in the fields. It also shows the numbering of bits from 1, or the LSB (Least Significant Bit), to 32, or the MSB (Most Significant Bit). In the order of transmission, the Label is transmitted first, with the MSB going out before the LSB, but in all other fields, the LSB is transmitted first. Fig. 2 shows the order of transmission by field (label, SDI, Data, SSM and P) within every field. MSB LSB Fig. 2 ARINC 429 32-bit Word Format (AIM GmbH p. 15) Fig. 3 ARINC 429 Word Transfer Order (AIM GmbH p. 15) On the other hand, ARINC 629 uses a 20-bit date format, where the first three bits are allocated to word time synchronisation, the next 16 bits to data content and the last bit as parity bit. Fig. 4 illustrates the bit allocation in an ARINC 629 data bus system and shows that it has only three fields as opposed to the ARINC 429 that may have as many as 5 fields. Fig. 4 ARINC 629 Word Data Format (Isik 194) The MIL 1553B Databus has a similar data-bit configuration with ARINC 629, with its 20-bit word data format, the first three bits devoted to sync wave form, then next 16 bits to data and the last bit to parity field. However, the MIL 1553B has three kinds of words: data; command, and; status. Fig. 5 shows the data-bit allocation in a MIL 1553B word format and it is shown that bit allocation varies with the type of word (TSCM 9.6-3 – 9.6-4). 3) Operating Speed The operating speed of the ARINC 429 is 12.5 to 14.5 kHz at low speed 100 kHz) +/-1% at high speed. On the other hand, ARINC 629 has a speed of 2 MHz whilst the MIL 1553B runs at 1 Mbit/s (Noceti & Perez 10). 4) Bus Operation ARINC 429 operates uni-directionally such that a receiver does not respond in the same bus where message is received whilst ARINC 629 is a bi-directional distributed control bus that can support a maximum of 120 users. MIL 1553B, on the other hand, uses asynchronous operation, which means that an independent clock is used in each terminal in message transmission where information from that source is used in decoding the message received. **LEGEND: RMTE TERM ADD - Remote Terminal Address MGE ERR – Message Error INSTR - Instrumentation SVC RQT - Service Request BDT RCV - Broadcast Command Received SSYS FL - Sub-system Flag BLC TRL - Dynamic Blue Control Assistance TRL FL - Terminal Flag Part II a) Synchros provide information on angular position measurements such as that of shaft mechanisms that drive control surface. The types are: DC synchro; AC torque synchro, and; AC magnesyn synchro. The DC synchro has circular transmitter with three pick-offs with two contacts attached to the transmitter’s resistance winding and power supply. The receiver, on the other hand, contains a magnetic shaft located in the middle of three stator windings. When current enters one of the coils, it splits into the two other coils and creates a magnetic field to which the magnet in the receiver aligns. This changes when the transmitter’s input shaft is rotated and the contacts change position due to the alteration of the magnetic flux in the transmitter and in the receiver (Tooley 180-181). The AC torque synchro consists of transmitter, receiver, stator and rotor windings. The stator has three windings positioned 120° degrees to each other and rotors are supplied with 26 volts AC that flows through by slip-rings. A magnetic field is created when power flows through the stators and also in the stator windings of the receiver. Alignment between receiver’s rotor and the transmitter’s rotor is angular, but disconnects when the transmitter’s rotor is moved. It is restored when the receiver’s rotor aligns itself with the transmitter’s rotor. The Magnesyn synchro system is an AC system that consists of transmitter and receiver built just like the AC torque. The transmitter’s rotor is a permanent magnetic field that affects the power-induced stator, with toroidal windings made of soft iron. The magnetic field changes when the input shaft is rotated, affecting the receiver’s rotor (Tooley 182). b) Open Loop Diagram In the above diagram, the input is set at a value estimated to get the desired output. It is unidirectional and horizontal. There is no checking mechanism that interferes with the system (Dingle and Tooley 527). Closed Loop System Diagram In the closed loop system, input goes through a regulating controller that readies it for P, the final control element. Output is fed back to the system to determine its accuracy through a Sensor, which takes the error difference and forwards the information to the input and Controller. c) Open and Closed Loop Systems The open loop uses 2 kinds of input to determine output: measure of current system, and; model of internal behaviour. There is no mediation on the basis of performance. An example is a sprinkler irrigation system where input consists of water amount to maintain fixed moisture level of soil and length of time to keep the water valve open. No test is conducted as to effectiveness or if extraneous factors have interfered such as monsoon or drought (Stolzer 162). A closed loop system has three steps: the sender checks if the message was actually received; receiver confirms receipt, and; sender and receiver confirm accuracy of the message sent. The closed loop system was applied in the case of United Flight 232, which lost three of its hydraulic systems while on flight on July 19, 1989 over Chicago, with 185 passengers. The flight was able to land safely because communication followed the closed loop system (Salas & Maurino 272). d) Errors affecting closed loop systems Closed loop systems are less stable and slower than open closed systems because of the feedback system that may compel system correction. Also, the sensors may result in certain inaccuracies because of the noise generated. In addition, inherent time lags in closed loop systems generated by their feedback mechanism may cause error feedback to appear as positive ones (Sinsha 96). Words Cited Cook, Michael and Howard Curtis, Filippo de Florio, Antonio Filippone, Lloyd Jekinson, Jim Marchman, THG Megson, Mike Tooley, John Watkinson, David Wyatt. Aerospace Engineering Desk Reference. California, USA: Butterworth-Heinemann, 2009. Dingle, Lloyd and Michael Tooley. Aircraft Engineering Principles. Oxford: Butterworth-Heinemann, 2005. Frodyma, Pat. ARINC 429: Specification Tutorial. AIM GmbH 2001. Ingle, Al. Tech Time: Helpful Tips for the Avionics Technician. AVIONICS News July 2008. Isik, Yasemin. ‘ARINC 629 Data Bus Standard on Aircrafts’ Recent Researches in Circuit Systems, Electronics, Control & Signal Processing. 2010. < http://www.wseas.us/e-library/conferences/2010/Vouliagmeni/CSECS/CSECS-34.pdf > Nocetti, D. Fabian and Hector Benitez-Perez. Reconfigurable Distributed Control. Mexico: Springer, 2005. Salas, Maurino and Daniel Maurino. Human Factors in Aviation. 2nd Edition. London, UK: Academic Press, 2010. Sinsha, Naresh. Control Systems. New Delhi: New Age International, 2008. Stolzer, Alan and Carl Halford, John Joseph Goglia. Safety Management Systems in Aviation. Surrey, England: Ashgate Publishing, Ltd., 2008. Tooley, Michael and David Wyatt. Aircraft Electrical and Electronic Systems: Principles, Operation and Maintenance. 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