1、SAE AIR*LL84A 83 357340 0003647 5 M The Engineering Society AEROSPACE or Advancing Mobility ian Sea Air and Space B INFORMATION 400 COMMONWEALTH DRIVE, WARRENDALE, PA 15096 REPORT Submitted for recognition as an American National Standard AIR1 184 Issued 1.-73 Revised 3-27-89, CAPACITIVE FUEL GAUGIN
2、G SYSTEM ACCURACIES ABSTRACT: This document presents a concise overview of error sources in capacitive fuel gauging systems. The subject matter is directed at individuals who are responsible for specifying gauging system accuracies and, therefore, need insight into the predominant error sources in c
3、apacitive fuel gauging systems and means to minimize the errors. A brief introduction to capacitive fuel gauging is also Included as background reference. SAE Technical Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of t
4、his report is entirely voluntary, and its applicability and suitability for any particular use, including any patent infringement arising therefrom, is the sole responsibility of the user.” SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or ca
5、ncelled. SAE invites your written comments and suggestions. Copyright 1989 Society of Automotive Engineers, Inc. Printed All rights reserved. in U.S.A. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-5
6、 Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE AIR*LL84A 87 m 8357340 0003647 7 M Page 3 - I AIR1184 IRE“* A 1. PURPOSE: This report identifies those sources or causes of error which adversely*af
7、fect the accuracy of airborne capacitance-type fuel quantity gauging systems. 2. SCOPE: This report is intended to identify .the necessary analytical tools to enable making value judgments for minimizing the various errors typically encountered in capacitance systems. Thus, in addition to identifica
8、tion of error sources, it describes the basic factors which cause the errors. When coupled with appraisals of the relative costs of minimizing the errors, this knowledge will furnish a tool with which to optimize gauging system accuracy, and thus, to obtain the optimum overall system within the cons
9、traints imposed by both design and budgetary considerations. Since the subject of capacitance accuracy is quite complex, no attempt is made herein to present a fully-comprehensive evaluation of all factors affecting gauging system accuracy. Rather, the major contributors to gauging system inaccuracy
10、 are discussed and emphasis is given to simplicity and clarity, somewhat at the expense of completeness. An overview of Capacitive Fuel Gauging operation is provided in the Appendix. 3. GENERAL REOUIREMENTS: There are numerous ways in which the contributors to gauging system inaccuracy may be classi
11、fied. Parameters for such classification may be general (e.g., temperature-induced errors, geometrical errors, etc.) or specific (e.g., tank errors, tank unit errors, indicator errors, etc.). Indeed, the variety of possible classifications is virtually unlimited. For the purpose of this report, four
12、 broad categories for classification of error-causes are listed: ( i) Errors controlled by the airframe manufacturer. ( 11) Errors jointly controlled by the airframe manufacturer and gauging system supplier. (iii) Errors mainly controlled by the gauging system supplier. ( iv) Errors not controlled b
13、y the airframe manufacturer or the gauging system supplier. 4. DETAILED REOUIREMENTS: 4.1 1-rrors Controlled bv the Airframe Ma.nufacturer: systems which are typically installed in modern aircraft, the fuel quantity gauging equipment should be considered as a system which is irrevocably “married“ to
14、 and which becomes an integral part of the aircraft. With respect to the tank units particularly, their locations and geometries are influenced by the airframes basic design. system, its preliminary design should proceed in parallel with that of the Unlike many instruments and Thus, to optimize the
15、gauging Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-, SAE AIR*LL84A 89 M 8357340 O003650 51 - AIR1184 IRE“* A I Page 4 4.1.1 .FLLT!LCISCYJI i.e., the largest cross section, or the one where horizon
16、tal walls become oblique, etc. Determi ne the required number of probes - General ly, the top-and bottom-errors are the deciding factors. For example, if the bottom-error is too large when only one probe is used additional probes will be required to satisfy the accuracy requirements at the specified
17、 attitudes. Locate the probes - After the number of required probed is determined, their location at the top and bottom cross-sections, that is, the simplified cross sections, is also generally determined. If the tank differs appreciably from its approximate geometric shape in a certain region, a co
18、rresponding shift of the probes is indicated at that place. Evaluate the maxirnum errors - When the probes have been tentatively located, analytical (computer) determination of the necessary probe characterization and the resultant errors are determined. Accuracy over the entire flight attitude enve
19、lope is studied. attitude-weighted error al location is considered at this point. urthermore, it is possible to have the computer derive the errors resulting from incremental movement of the probes from their tentative locations. The results of such computations are then aiialyzed, and final “optimu
20、m“ probe locations are selected. Appropriate Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE AIR*lrLLi4A i9 8357340 0003654 12.w AIR1184 IRE“* A I Page 8 4.2.2 Installation Considerat ions There ma
21、y be no class of equipment whose performance can be degraded more drastically by improper instal lation than capaci tance-type fuel quantity gauging systems. Recognizing this, the gauging system manufacturer and airframe manufacturer must col laborate effectively in this area to ensure accurate, tro
22、uble-free performance of the gauging equipment. In an actual installation, the superimposed effects of humidity, fuel contamination, temperature, al titude, vibration and shock, fuel sloshing, fuel characteristic variations, external subsystem usage, etc., all act simultaneously and adversely upon t
23、he gauging system. Many of these influences can be worsened by the frequent cycl i ng of humi dl ty , temperature, and al ti tude produced by typi cal short-haul mission profiles, and by operation from bases where fuel cleanliness may be poorly controlled. In view of these potential difficulties, it
24、 is not sufficient that individual gauging components merely satisfy their own particular specificatlons;.it is also necessary that they be made to interact with one another and with the aircraft to produce an accurate and reliable gauging system during actual service use. 1, Single-Point Ground - T
25、he beneflts of the implementation of single-point grounding are known to greatly outweigh its cost. The baslc approach to proper implementation is to have a signal return collection point within the indicator associated with each fuel tank, and to have all signal circuitry connected thereto, each by
26、 an individual wire, to prevent circulating ground currents. From this point, a single wire is brought through a pin in the instruments connector and returned to airframe structure external to the uni t. The power return (usually 115 volt, 400 Hz), the case ground and, as applicable, the lighting gr
27、ound, should each have individual egress from the instrument, and may be returned to the airframe structure in any convenient acceptable fashion. By adopting this approach, all of the signal currents are confined within the gauging system, and none is permitted to take an uncontrolled path through t
28、he airframe, wherein it could be influenced by spurious external causes. In addition to the precautions already noted, care must be taken in the indicator design to minimize the capacitance between the input-signal lead and the indicator case. Otherwise, should the case potential be other than that
29、of the single-point ground, unwanted signals could be coupled into the system through this capacitance, Where electrostatic and electromagnetical ly-induced coupling of spurious signals into the gauging systems external wiring is concerned, the use of single-point grounding along with reasonable sep
30、aration (2 to 3 inches minimum) between gauging equipment leads and external current-carry! ng conductors is suffi clent to ensure trouble-free performance. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IH
31、S-,-SAE AIR*LLBLiA 89 83573LiO 0003655 4 Page 9 REV. I AIRllBIII A 4.2.2 (Continued): Extreme care must be taken in maintaining shield continuity of th? shielded conductor employed to transmit the signal from the inner tube (Hi-Z) in conventional AC fuel gauging systems. The coupling of any synchron
32、ous AC signa1 on this conductor will introduce a potentially large error in the reported fuel quantity. This problem is undoubtedly the most frequent cause for maintenance on AC gauging systems. 2. DC Gauging System Shielding - The modern DC system transmits a pulsatlng unidirectional IIDC“ current
33、as a signal from the tank to the signal conditioner. The twisted-triplex signal conductor used in DC systems is virtually immune to AC pickup in the audio-frequency range. In some applications, a simple braided shield over these conductors is needed to add high-frequency immunity to the system. Mult
34、i-point grounding of this shield is acceptable and results in a more reliable system. 3. Sensor Mounting Tolerances - The tolerances in the location of the mounting brackets (for internally mounted tank units) or the flange plates (for externally mounted tank units) should be taken into account. The
35、se errors can be of the order of tank dimensioning tolerances (see 4.1.3). 4.3 Errors Controlled by the Gauging Equipment Manufacturer: 4.3.1 Qceration of Heisht-Volume Data: The gauging system manufacturer is often called upon to generate height-volume data from the tank geometry descriptions furni
36、shed by the airframe manufacturer. Sometimes, however, the airframe manufacturer has the capability and desires to furnish the height-volume data directly to the gauging system manufacturer, by means of in-house computer studies of the tank geometry for ground and flight attitudes, including wing tw
37、ist and deflection if applicable. For the case when the tank description is given in terms of scale drawings with cross-sectional information, it is often necessary to approximate the curvilinear envelopes of these sections by straightline segments. The errors induced by such approximations can be m
38、ade negligible by increasing the number of segments; however, this is limited by the economics of both preparing the data and of computer time for their utilization. When the furnished cross-sectional information is listed in nature (i., when the sections are given at relatively largely spaced inter
39、vals), the tank geometry must be further approximated by straightline segments between the various sections. This can lead to additional errors. Frequently, due to schedule requirements, information given the gauging system manufacturer is preliminary in nature, and during the design phase the airfr
40、ame manufacturer alters either the envelope or the location of auxiliary equipment within the tank. design at that time, upon the scope of the changes, or upon economic considerations, the gauging system supplier will elect to resubmit the new Depending upon the state of the Copyright SAE Internatio
41、nal Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-_ _ _ SAE AIR*LLBLIA 89 W 83573qO 0003bSb b W AIR1184 IRE“* A 1 4,3,1 4.3,2 (Continued) : Informatkm Por computer analysis. In this case no additional errors beyond those alread
42、y described will result, or adjustment of the initial computer data to suit the new requirements. possible to introduce additional errors into the height-volume data; however, experienced gauge desi gners are we1 1 -equi pped to hand1 e situations of this kind, and can operate on the existing data w
43、ith minimum accuracy compromi se In the latter case, it is J,_t_6bawcra_t-.n: When the tank locations are establ i shed, and the “system constant“ (usually given in units OP picofarads/gal lon) is determined, another specially designed computer program can be uti 1 ized to establish optirnom charact
44、erization of each tank unit in the normal tank attitude. The computer then simulates the various specified pitch and roll attltudes, and, as applicable using twist and deflectlon, computes the measurlng eyrors under these conditions on the assumption of ideally characterized tank units. At th9s poln
45、t the gauge deslgner reviews the error data and evaluates the relatlve riiagnltcrdes sf the errors in the various attitudes and for the varbus Puel volumes considered. certaiti reglsns in order to stay withfti specification limits, tank unit Iserstiens ar adjusted or the normal atti tude characteriz
46、ation is eornpromlsd SI ightly, and the revised data are submitted for recornputatlati. For this reason; it is pointed out that too stringent accuracy requhments for extreme conditions are not desirable because of the adverse effect on accuracy achieved at normal attitudes. If it is neeessary to red
47、uce errors in In thi s manner, through sequential exchange of information between the gauge deslgner and the airframe manufacturer, optimum characterization of the tank units in their selected locations is attained. In computerized gauging systems the employment of “linear“ profile on tank units is
48、made posslble. can be contained in system softQare in the form of lookup tables. If individual tank units can be addressed at the computer interface, “common level“ techniques can be used to further reduce attitude errors in computerlzed systems. This enhancement is possible only if “in tank“ and sy
49、stem wiring is conflgured to permit access to each tank unit. The information which converts height to volume Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE AIR*LLBLIA 87 83573YO 0003b57 Page 11 I AIR1184 A I REV* 4.3.3 Component Design: It is generally recognized at this time that tank units of all-metal constructio
