[1]Uni'ted States Patetit Office 297-71@742 2,771,742 SAFETY CERCUIT FOR GEINERATOR U-,NIT OF THE JET FUEL CONTROL Charles B. Brahm, Windsor Locks, Conn., assi-@no@- 4, @3 United Aircraft Corporat!on, East Hartford, Conn., -,I corporation of Delaware Application December 6, 1952, Szr-,a] No. 324,546 18 CWms. (Cl. 60-39.09) This invention relates to fuel control mechanism and particularly to safety means for continuing operation of said control after failure of one of its component parts. An object of the invention is to provide a safety control signal upon failure of normal si,-Ual providin.- i-nechanism. A further object is to provide an underspeed signal upon failure of the speed signal source. A still further object is mechanism which will permit control by temperature responsive mechanism upon failure of the speed signal source. A still further object is mechanism which will permit an increase of fuel flow for undertemperature conditions when the speed responsive signal is disabled. StiU another object is the provision of means, in a fuel control that normally controls fuel flow in response to speed and temperature, for controlling fuel flow in response to temperature alone upon failure of 'Lhe speed signal source. Additional objects and advantages will be apparent from the attached specification, claims and drawings in which, Fig. I is a schematic and block diagram showin@ the general arrangement of the turbine, the ftiel supply and the electronic control for the fuel supply. Fig. 2 shows the speed sensing circuit crossov-.r limiter schematically in more detail than Fig. I including the wiring diagrams therefor. Referring to the drawings and particularly to Fi-. 1, the turbine jet en.-ine 10 may be of any well known type such as the type utilizing a compressor receiving air from an air inlet and deliverin- the air under pressure to a combustion chamber where fuel is burned. The products of combustion from the combustion chamber are fed to a gas turbine which drives the compressor. Gas exhausted from the turbine passes out through a tailpipe. The compressor may be either centrifugal or axial flow type and the turbine may be either a power jet type or a portion of the turbine power may be utilized to drive a propeller giving a combination known as a turboprop. Dur,'@ng operation of the turbo-jet type in particular, the fuel is selectively controlled bv a speed or a temperature sensitive control system. A speed is selected by lh-, pilot on a speed control 48 which will select a dalum for the speed sensitive control system. The signal frora the speed sensitive control system is compared Nvith a signal from the temperature sensitive control syst-@m in the crossover limiter and the resultant signal is used to position the pilot valve controlling the flow of fuel to the turbine. Thus control of the throttle is ef'iected by movement of the speed control lever 48. Th;s control is electronic in nature and in general is similar to that shown in Offner applicatio, Seri,l NE, 84,696, filed March 31, 1949, now Patent No. 2,697,908 entitled System of Engine Speed Control. As shown i,, Fig. 1, of the present application, a speed signal is taken from a generator 12, which may be an alternating current -enerator driven by the turbine, and is fed to a Patented Nov. 2",7, 1956 speed sensing circuit 14. In the speed sensing circuit, a speed error voltage is produced. This error volta.-e may be zero when the turbine is onspeed, positive wheii 'Lhe turbine is overspeed, and iiegative when the turbine is underspeed. The speed error signal is fed to cr-3ssover limiter 18. A temperature si.-nal is fed from tailpipe thermocoiil)les 20 to thermocouple input circuit 22 where it @@'s coir@pared with a fixed voltage and transformed into a teii-ip.-ratuire 10 error signal. The temperature error si.anal is, ainplified in the thermocouple output circuit 24 and f,-d throcgh lille 26 to feedback equalizer 16 and crossover limiter IG. 'Ihe amplified temperature error signal is zero or null wheii ontemperature, positive when the turbine is over15 temperature and negative when the turbine is undertemperature. In the crossover limiter 13, the mosl posit:ille or least ne.-ative signal of the speed -and temperatiire signals is selected to be transmitted to the main amplifier 28 where it is amplified and then fed to the proportional 20 solenoid 30 through the line 31. The proportional solenoid is of a type shown ir-, Patent No. 2,579,723, issued December 25, 1951@, to S. G. Best for Magnetic Device, to which reference mqy b@ made for a more detailed explanation. The proportioi-ial sole25 noid 30, which is normally centered but may be inoved i-n one direction with a negative si.-nal and in the opposite direction with a positive signal, operates a valve 32 controlliiig the flow of fuel from the fuel suppl3, 34 to the fuel jets 36. This fuel supply system is ol-' the type sliown 30 in application Serial No. 231,926, filed JLine 16, 1951, by D. R. Pearl and S. G. Best, for Fuel Control Syste@n for Turbine Fngine, to whicli reference m-ay be ir@ade for more detailed explanation and generally inel-ades a fiiel pump 38 for supplying fuel under pressure w;tli a pres35 sure actuated relief valve 40 for maintaining the pressure drop the same across the throttle valve 42. Valve 32 directs either the high or low pressure to the in@erior of bellows 44 to move valve 42 toward closed or opened position to thereby regulate the flow of fuel to the fuel jets 4o 36. The movement of the valve toward closed pos't,on may be limited by a stop such as 43 to provide a miiimulm fliel flow. The speed sensing circuit 14, which will be explained in more detail later, utilizes a bridge circuit to match 15 a constant voltage selected by the pilot's lever 43 aga@Tist the voltage of a speed signal generator 12, the resultin@ rectified signal is utilized to give a speed error si@anal which is applied to lines 50 and 52. Poi4,er for operating the various electronic is 50 normally provided by the .@enerator 12 opera@lin.- throti.-Ii a usual power supply 54, slipplying among other voltages a minus 13 volt bias in line 56 and a plus 85 voit voltageregulator-tube-controlled voltage in Iiiie 53. Dii-ii@ the starting cycle, however, 400 cycle A. C. voltal-e froiii aq 55 outside source is supplied through the line 60 to the po,@ver stipply. When a preselected speed is reached by the turbine and accordingly by the generator 12, the -eperator voltage acting througli relay rectifier 62 and re'iay co;l 64 will actuate switch arm 66 to disconnect the 400 cyci'e 60 source and connect in.the generator 12 to the power slil)- ly A second switch arm 68 is actuated by the col at the same time in order to ground otit li-iie 70 to connect the swinger 166 of chopper 71 to grou@-id and disable the starting circuit after a preselected speed ;s reached. 65 Refet-ence may be had to application Serial 14o. 282,204, file d by S. G. Best and T. P. Farkas, on April 15, l@952, for Starting Circuit for Electronic Jet Fuel Conlrol _for Lurther details of the startirg circuit. The therm-orotii)le 70 output amplifier and the m,ain arr@plifier are, in effect, D. C. chopper amplifiers of a well known type. The thermocouple input and chopper 74 bala@-ice the
[2]thermocouple signal against a datum which may be fixed or may be varied as desired. The correction volta.-c or @temperature error signal supplied by the thermocouple to the chopper contacts 76 aiid 78 is positive for in undertemperature condition aiid i'legative for an overtemperature ooildition. If a variable datum is used, a signal which is usually positive may b@fed to the swinger 79 of chopper 74 to modify the amplitude of the square wave produced by the chqpper 74. The swiiiger arm 82 of chopper 80 is mechanically syr@chronized with chopper arm 79 in such a way that positive signals at 76 and 78 will produce negative (i. e. undertemperattire) voltage in ithe line 26 while negative voltages in the contacts 76 and 78 will prod,,Tce positive (i. e. overtemperature) volt,-,ges in the line 26. The convention observed with respect to the illustration of rectifiers herein is that current flow (from plus to iiiinus) as distinct from @electron flow is in the direction of the arrow. The temperature error signal on line 26 is led through line 29 to resistors 130 and 132 to form a volta.-e divider circuit. A portion of the temperature error signal is led off throtigh line 142 to the crossover limiter for switching purposes, which will be explained later. In the crossover limiter, rectifier 144 coiinects line 142 with ground and limits any n,-.-ative temp-@rature error voltage appearing on line 142 to a sinall amount, approximately one volt. A portion of the temperature error signal is led off from a point between resistors 130 aiid 132 through line 146 to contact 1.48 of the chopper 71 and is also led throu.-h line 150 inio the crossover limiter from which it is selectively fed to the proportional solenoid in a madner to be described later. The speed error si-,nal is fed throtigh line 52 to the co.itact 162 of chopper 71 ' This speed error signal is led on linc 164 into the crossover limiter from which it is selectively fed to the proportional solenoid in a manner to be described later. The speed error signal, applied to lin@- 50 from the speed sensing circuit, is fed into the crossover limiter for switc@hing purposes which will be described later. The speed and temperattire error signals fed through lines 164 and 1:50 into the crossover limiter are led to rectifiers which will select one or the other as the signal to be transmitted to the main amplifier and theiice to theproportional solenoid. j-he speed error signal on line 164 is a square wave and is led torectif-@ers 168 and 170 and the temperature error signal @on line 150 is also a sqtiare wave and is led to rectifiers 172 and 174. The speed error signal is also fed into the crossover liniiter throu,-h line 50 and is led through resistors 183 and 200, the condelisers in the lines sei-ving as filters. The speed error voltage existi-@ig after passing through res'stor 188 is applied at 202 between resistors 176 and 17@ and at 204 between 184 and 186. This is a rectified D. C. error signal and is used to bias rectifiers 169, 170, 172 and 174. In the same manner the D. C. temperature error signal is led in on line 142 -and after passing through resistor 206 is applied at 208 between resistors 180 and 182 which are connec@ted to the rectifiers 168, 170, 172 and 174 to bias them. It will now be apparent that if the voltage in the speed error signal at 202 and 204 is niore positive than the voltage in ihe temperature error signal at 208, rectifiers 168 and 170 will be biased to conduction and rectifiers 172 and i'14 will be biased to non-conduction. Rectifiers 168 and 170 being conductive will pass the speed error square wave produced by chopper 71 and fed in line 164 to the crossover I;miter. Conversely, if the temperature error signal fed into junction 203 is more positive than the speed error signal fed into junctions 202 and 204, then rectiiiers 172 and 174 Nvill be biased to conduction and rectifiers 168 and 170 will be biased to non-conduction. The temperature error square wave fed in through line 150 will then be able to pass-througn re,.tifiers,172 abd 174 to the propor2,771,742 4 tional solenoid while the speed error signal becomes blocked. It is thus apparent that the most positive or the least n-.gative error signal of the speed or temperature will select its correspondin.- square wave as the signal to be transm-itted, amplified and fed to the proportional solenoid. The network showii generally at 212 is fed from the niinus 13 volt bias line of the power supply and is used to sl;ghtly bias rectifiers 168 and 170 to ,ivoid the dead10 band comrdo-,i to all rectifiers. In a s;milar way the network sliewn .-cicrally at 214 is fed from the plus 85 volt section of the power supply to provide a 'bias for rectifiers 172 and 174 to avoid the deadbaiid. The speed and top--p,-rature error si,@nals are arranged 15 so that vihen fed to the crossover limiter an overspeed or over'@emperature condition is indicated by a positive signal and conversely an undertemperature or underspeed signal is neg-,itive. In order to actuate the propor'LiOnal solenoid 30 to close the throttle valve 42, it will therofofe 20 be necessary to feed a positive signal into @the crossover limite.-. This wil.1 call for less fuel in order to overcome an overspeed or overtemperature con ion @and thus close the valve. On the other haiid, in order to actuate the propor,,;onal solenoid 30 so as to open the throttle valve 25 42, it will be necessary to feed two negative signals (i. ;C. both speed and temperature) into the crossover :limiter. If one of the signals, say the speed signal, is zero, i. e. neither pos,.tive nor negative, and the other, temperature, is negative, the proportional solenoid will not be @actuated &'O aiid hence the throttle valve will liot be che(n.-ed. This is a condition which would @exit if the generator should fail and the engine shoiild arrive at an tindertemperature cordition. It would l@hen be impossible to increase the fuel flow as the zero signal from the speed control would continue to govern. The invention of this application prevents such a condition from arising by providiqg ftlechanism which will automatica.Ity g'rve an underspeed indication upon failure of the generator. As long as both signals are negative ihe proportional solenoid will be actuated to increase the fuel flow iintil one of them becomes zero or actually becomes positive and starts to reduce the fuel flow. The speed sensina. circuit utilizes a bridge having theiidiietance 220 and poteiltiometer 43 as one pa r of le!@s 45 and the resistor 222 and resistor 224 as the other pair 'of legs. The react@tnee of fhe inductance will v-,iry directly with freqtiency aiid 'the bridge will therefo@re be in balance -,it oi-ily oi-ic speed. The alternating cii@-rent at junction 226 oi- tfic bridge is led tarou,-h a reefifier 213 and con50 denser 230 to grour,..d aiid the alt@-mating current at jlnction 232 of the bridge is led throti.-h rettifier 234 and co-ildenser 236 to g-otind. The two rectifiers are con@iected to provide voltages of odposite p6larity and are connected together by resistors i38 ir@d 240 which may r,5 be of abort 150K oqms each ind resistors 242 and 244 Piay bc of approxin-iately 500K olims each. The i-nidpoint between resistors 238 and 240 is connected through approximately 5 megohms resistance in the feedback equal;Zer to line 52 and contact 162 of chopper 71. The mid60 pont bctween resistors 242 a@--l 244 is co-inected @to line 50 ar,@d the crossover circuit 1)reviously described. From the above it will be apparer@t that when @thf-- bridge is iii balaTic--, the voltag-es produced Lly -lectifiers 228 and 234, are of opposite polarity, and current Nilill flow through C,5 resistors '@38 anC, 240 aiid '@42, 244 but the volta@r,,e being of equal and opposite pol-arity at the opposite ends of these resistors and the res;stors being equal, zero or no vol.tage will be orod,,,,ceCi aL the m-i-.TpoinL of the resistors. As the bridge be-.omes unbp@lanced ond one rectifier pro70 duces a greater or less vcltage than the oth-_r, theniidpo'fnt of the resistance 233, 240 a@nd 242, 2,14 will accordingly vary and give the necessary speed signal @to chocnge the amoupt of fuel delivered and tn,,js cl@i,,inge the speed so as to briiig the bridge bac'@@- into balance again. A tempera7@5 ture seiising T-,sistor 246 h,,iving 'a negaiive teirperatore
[3]@P,771,742 coefficient is placed in the turbine inlet to minimize the effect of air temperature changes on the operation of the jet engine and reduce its speed on a drop in temperature. The output of rectifier 228 is connected throug@h resistor 248 to one side of resistor 246 and the output of rectifier 234 is connected through reSiStOT 250 to the other side of resistor 246. A rectifier 252 is @connected to ground between resiscors 246 and 250. At a selected temperature and when the outi)ut of rectifiers 228, 234 is equal, this circuit has no efiect becguse the sum of the resistances f 246 and 248 is equal to the resistance 250. In this ciricuit if a negative signal predominates, it will be limited by rectifier 252 to a small value so as to have substantially no effect. If a positive si-nal predominates, then a positive bias will be slii)plied t'o lines 50 and 52 through resistors 254 and 256 so as to call for a reduction in ftiel and hence a reduction in speed. This positive signal in line 258 will be produced by a drop in temperature whi,ch will increase the resistance of resistor 246, thus unbalancing the 248, 246, 250 voltage divider circuit to give a positive voltage in line 258. In the event of generator failure, no voltage will be supplied in lines 50 and 52 to the crossover limiter. This will prevent actuation of the proportional solenoid to increase fuel flow because the zero speed voltage will not move the proport-onal solenoid if it is accurately centered. If the en.-ine is undertemperature, the negative temperature si,-nal will not be effective to increase fuel flow as the zero signal from the -speed circuit will predominate. If the engine is overtemperature, the proportional solenoid will be actuated to reduce fuel flow until a zero signal is received when no further c@hange in fuel 'low can take place. Hence upon failure of the generator, the engine would continue to run at whatever setting the engine failed. in practice the proportional solenoid is slightly biased so that under no signal conditions the throttle valve will creep toward minimum fuel position. Hence instead of continiiing to run at a fl-,ed setting the engine, in practice will gradually creep to the idle or minimum fuel position. In order to permit operation of the engine at full power upon generator failure, I have provided a safety circuit to supply a negative or underspeed si-,nal to the speed circuit upon failure of the @enerator so that control will be transferred to the thermocouple and the engine will continue to run at the maxiriium permissible temperature. With this device there are two choices upon failure of the generator, the engine can either be run at the maximiimpermissible temperature with the speed settling out at some balancing speed or the engine can be shut doivii. In order to provide the negative signal, I connect the minus 13 volts of the power supply through res;stor 260 and rectifier 262 at junction 264 with t'iie oiitput of rectifier 228. Jn normal operation, the rectifier 228 supplies a negative voltage varying between minus 15 and minus 30 at the junction 264 dependent upon the setting of rheostat 48. As long as the generator is operatin.- and as long as the voltage at ii-iiict:ion 264 is greater than the minus 13 volts b,-;n-. supplied through line 266 and resistor 260 to the rectifier 262, rectifier 262 will be biased to non-conductiol and the minus 13 volts will have no effect an the speed circuit. If, however, the .@enerator is disabled, then rectifier 228 will not provide any negative voltage and rectifier 262 will then be biased to conduction and will provide a negative bias in lines 50 and 52. The negative voltage supplied at point 264 when the generator is not operatillq is limited by resistances 248 and 246 which are connected to ground throligh rectifier 252 of the temperatlire conipensatin.- circuit which may be -in the nature of 400K ohms when the resistor 260 is I megohm. The other main path to ground from the minus 13 volts would be through resistor 242 and resistor 200 in the crossover circuit to ground which wolld .-ive a total resistance of approximately 700K ohms. These resistances are large enough so that they, together with resistance 260, act as a voltage divider to give,a substantial portion of the mint7,s 6 13 volts at junction 264 @and hence on lines 50 and 52. This safety circuit will operate with either the resistance 260 alone or the rectifier 262 alone. If the resistance 260 is omitted, then the full minus 13 volts subject only to the drop through the re-,tifier 262 will be appliect at junction 264 when the generator is not operating and, as explained above, none of the minus 13 volts will appear at the junction 264 when the generator is operating. If the rectifier 262 is omitted and only r,-sistance 260 is lo utilized, then as e-,plained above a substantial portion of t@lic minus 13 volts will appear at junction 264 whe@-1 the ge.ilerator is not operating. When the generator is operatii@, rectifier 228 is bi@ased to icondi,,ction during a portiln of each cycle and effectively acts as a low resistance 15 path for the minus 13 volts applied through line 266. Actually the voltage at 264 produced by the rectifier 228 lianing a larger negative value than the negative voltage sup.plied in line 266 will tend to be reduced but as the resistance of the potentiometer 48 is only in the nature 20 Of 8K ohms and the rectifier @s biased to co-.lduction and hence would have very little resistance, the major voltage drop will appear -across the resistor 260 and the minus 13 volts will hav.- very little e'@T--cct on the voltage at junction 264. It is thus apparent that the minus 13 volts applied 25 in line 266 has little or no effect on the generator signal when the generator is operating but supplies a substantial ne.-ative signal upon generator failure and thus prevents the negative volta.ae at junction Z64 from dropping below a predetermined amount. 30 Generator failure will cause switches 66 and 69 to be ope-iied so as to disconnect swinger arm 166 from ground and to connect the 400 cycle A. C. power source to the po-,ver supply to replace the lost generator supply. When the swinger arm 166 is disconnected from ground by 35 switch 63, it finds a ground connection tbr<)ugh resistor 267 in the startin,. circuit which is more fully described in application S,-rial No. 282,204 referred to above. Briefly a positive volta.-c of about 85 volts is supplied tlirough litic 268 through r,.sistor 270 and rectifiers 272 40 to junction 294. This voltage finds a path to ground through resistor 276, li@ne 278 and rectificr 230. This circuit will stipply a higher positive voltage at 294 than the negative vclta,-c be;ng supplied frorii line 29 through resistor 282 so as to comt)letely neutralize the negative 45 volta.ae from the therniocouple output and 'ehus prevent a ne.-ative voltage froin being supplied to the swinger arm 166. Groundin.- throu.-h resistance 267 will reduce the an, plitude of t'iie square wave produced by the chodper 71 only slightly and will not act to change its polarity. 50 From the above descriptio-ii it will be apparent that I have provided a saf-,ty circuit coniprisin.- resistors 260 and rectifier 262 connected with a source of negative volta.-e which will have little or no effect upon the speed control when the generator is operatin.@ but will provide 55 a negative (i. c. underspeed) sigl@al when the generator is P-ot operating so that c<)ntrol may be by te-@nperature alone. If potentio@rr@eter 43 shoiild fail, by opening, the volta.@e at 226 would greatly incrcase which would provide a 60 lar.@f,@ ne.-ative (i. e. underspeed) signal in lines 50 and 52. This signal would be of such a large magnitude that it would feed bac-kwards throu,-h rectifier 16S and supply a ne.@ative voltage to the proportional solenoid which would have to be evercome by an equal positive volt65 a@e out of the thermocouple amplifier in order to produce a null si.-nal at the proportional solenoid. This positive (i. e. oiertemperature) signal fror@-1 the thermocouple could b.- produced only by an overtemperature and probably dangerous coridition. In order to prevent 70 the large un-derspeed signal from reaching the crossover circuits, rectifiers 284 and 286 are provided to limit the negative voltage which can be attained in either line 50 or 52. Hence no excessive undersp@-ed signal can be supplied to the crossover limiter. 75 Althoti,-h the circtiit ineltiding the speed sepsing and
[4]7 safety circuit @has been described in detail and only a single ernbodiment has been used as an example in the 'description, it will be apparent that maiy modifications niay be i@-ade which wili come within the scope of the inventioti. Therefore, I do not desire to be limited by 5 the details of the embodiment whir-h has been selected 'for the piirpose o' explaining the invention. Fotexample, while particular polarities have been chosen and used in the e-mbodiin--nt described, it is apparent ',hat the sevdral polarities may be reversed witho,,it atectinlthe 10 result. W.'lat it is desired to secure by
