CORRECTION OF ERRORS IN AUTOPILOTS

Abstract: A system (1) responds to inputs (2) in parallel with a state estimator (4). The controller (6) provides the inputs (2) in response to ordered state inputs (14) and state estimator inputs. An external disturbance vector (f) affects the system (1) so that the observed values of the state variables are not the same as the estimated values. The estimated and measured values are compared (8) to give an error signal (9). The error values are fed to an Ordered State Corrector (11) which computes the error that the ordered state variable would have in the steady state and applies it in the opposite sense to increment the input ordered state (14). If more than one state variable is to be corrected, the correction is applied directly to the control inputs using Multi-State Correction, in which the steady state errors are used to find the control inputs which would cause the errors and then a correction is applied to the control inputs.

Description: CORRECTION OF ERRORS IN AUTOPILOTS The invention relates to the correction of errors in achieving required states as determined by a controller, such as, for example, an autopilot. In particular, the invention relates to the correction of errors due to an unknown disturbance * of non-zero mean. When a system is subjected to an unknown constant disturbance, the variableswhich describe the state of thesystem (statevariables) cannot, in general, be brought to desired values (the ordered state) by an automatic controllerwhose inputs consist solely of the di ferences between the actual and the ordered state (the state errors) . The inclusions of derivatives of the state variables does not remedy matters. Herein, the term "desired state" or "desiredvalue" isused todenote thevalues desired by the operator, or by an automatic systemcontrollingthe operation of an autopilot. The term "ordered state" is used to denote the input settings of the autopilot, bywhich thedesiredvalues are input to the autopilot. This controller problem can be illustrated by the following examples; a) Acontroller dependingon inputsof thedeptherror, pitchangle and their derivatives cannot bring a submarine to ordered depth if the submarine is out of trim (due to, for example, external loads, incorrect ballast or hydroplane offsets) . The addition of heave velocity and pitch rate terms is unable to remedy the problem; b) A similar controller whose input is course error cannot bring ships or submarines to their ordered coursewhen they are subjected to, for example, wind or steering offsets. The addition of sway velocity and yaw rate terms does not remedy the problem; c) Similarly to (b), ordered track cannot be achieved by a controller of this type whose inputs are distance off track and course error. In the same way as in (b), the addition of sway velocity and yaw rate terms cannot remedy this problem. The control outputs of these controllers are proportional to the statevariables, andtheirderivatives. HencetheyarecalledProportional Derivative (PD) controllers. PDcontrollersaresimplecontrollersandare commonly fitted to ships and submarines. In a submarine, for example, the hydroplane deflection is proportional to terms in the depth error (i.e. actualdepth- ordereddepth) andpitchangle, andtheirderivatives. Under a constant disturbing force, e.g. out of trim, the controller maintains a constant, but inaccurate, depth. The shortcomings of a PD controller become serious when accurate achievement of aparticular ordered stateis important, suchaswhenaccurate navigation or depth keeping is needed. The addition of a termwhich is the integral of ' the appropriate state error variable, as in a Proportional Integral Derivative (PID) controller, can overcome theproblem. However, though this can remove the steadystateerror, itreducesthestabilityofthe system and serious difficulties arise when a change is made in the ordered state. A change in the ordered state creates a step in the corresponding state error variable. Left to itself, the integrator will integrate the error throughout the transition and can only correct itself by a massive overshoot. This can bemitigated by f eezing or resetting the integrator, buttheproblemreappearswhen theintegratorhastobereleased. Thereis, as well, the problem of how and when the integrator is tobe switched outand reintroduced. Situations will always be possible in which highly undesirable side effects can occur, such as massive overshoot or the integrator not switching back in at all. These shortcomings of the PID controllerbecome seriouswhen, forexample, asubmarinewishestoascendtoa particular depth, eg periscope depth, with no or minimal overshoot. In any of the systems described above, as an alternative to the integrator, the steady state error could be corrected manually by observing the error and adjusting the ordered state accordingly. This technique requires the steady state to be achieved and observed before the correction can be made. It cannot be applied during a manoeuvre, and it has to be repeated whenever the disturbing force changes. For example, the correction cannot be ascertained during a submarine*s depth change. The directautomatic implementationofthistechnique (byautomaticallyapplying the steady state error in the opposite sense to the ordered value) fails, because success requires the error to be removed, and the necessary correction signal thereby vanishes also. A successful implementation would require an integrator, or somethinglike it, thedisadvantagesofwhich have been described above. Anothermanual procedure, similar to thatdescribed inthepreceding paragraph, uses the steady state errors to calculate offsets which are applied to the controls. This technique is open to the same objections and cannot be implemented automatically. Everything thathasbeen stated aboutaconstantdisturbanceapplies also to avarying disturbancewith anon-zeromean, including ameanwhich is varying slowly compared with the response of the system. The objective is to bring the mean state to the ordered state. Wavesconstituteavaryingdisturbancetoasubmarine. Theyaffect the system in three ways: 1. Pressurefluctuationsproportional towaveheightarepickedup by the depth sensors, causing futile flapping of the hydroplanes, which causes excessive wear and noise; 2. Oscillatory forces cause the submarine to surge, heave and pitch. These do not affect themean depth, and the control system must ignore them since they are too large to be opposed; 3. Asecond-order force, commonly referredto as 'suction force' , acts in an upwards direction only. It has a non-zero mean which must be opposed by the control system. All three effects increase rapidly as the surface is approached. The ascent of a submarine to periscope depth therefore presents a particularlydifficultproblem. Itmustbeachievedwithminimal overshoot by a manoeuvre during whichwave effects, which have anunknownbutnon-zero mean, are increasing. Modern autopilots minimise the effects of thehigher frequencies of the disturbances. They compute the most probable values of all the state variables (for a submarine these include the heave velocity, pitch rate, pitch angle anddepth) frommeasurements thatmaybenoisy. This isdoneby astate estimator, whichmodels thesystem. Itsoutput, whensubjecttothe same control inputs (for example, the hydroplane deflections), is compared with those actually occurring. Ideally, the estimation errors (observed state - estimated state) should be zero. (There is a distinction between theestimationerrors, thestateerrors ieactual- ordered, andtheestimated state errors ie estimated - ordered) . In practice there are discrepancies due, for example, to the wave effects. By feeding back the estimation errors (those that can be measured), to the model via a Kalman filter, the estimatescanbe improved. Thecontroller is similar tothePDcontroller. It includestermsproportional totheestimatedstateerrors. This typeof autopilot, possiblywith additional filtering, achieves better performance againstdisturbances atthehigher frequencies suchaswavefrequencies, but it offers no improvement against a constant or non-zero mean disturbance. An integrator can be included but with the same disadvantages as the PID. To summarise, theproblemis tobringthemean statetotheordered stateinawaywhichovercomesthedifficultiesofchangingtheorderedstate. It is coππnon to all autopilots and to many other controlled systems. It cannot be dealt with by presently available systems. The object of the invention is to provide a means to accurately achieve a desired state in the presence of unknown disturbances. In particular it is an object of the invention toprovide ameans tofacilitate accurate changing of the orderedstatewithout reducingthe stabilityof the system and without impairing the trajectory (ie the change in thevariable path) during the transition. The invention provides an autopilot comprisings a) means to provide control inputs to control one or more state variables; b) means to provide a model of the system; c) meanstoestimate fromthemodel of thesystemthevaluesofthe oneormorestatevariableswhichareexpectedtooccurin responseto the control inputs; d) means to set the desiredvaluesoftheor eachcontrolled state variables; and e) means to provide measurement of the or each controlled state variable; characterised in that the autopilot further includes i f) means tocomparetheestimatedvaluewith themeasuredvalue of the or each controlled state variable; g) means to derive the correction to thecontrol inpjutsnecessary . to achieve the desired values; and h) meanstoadjustthecontrol inputstoproducethedesiredvalues of the or each state variable. Preferably the means to provide a model of the system comprises a state estimator, as this is an effective way of providing a model of the system. Preferably the state estimator estimates, on line, the probable valuesof thestatevariablesf omobservations. Thisenablesthesystemto be controlled continuously during manoeuvres. As with all autopilot estimators, care must be taken in modelling the system. Preferably the state estimator includes a computer or microprocessor and the system is modelled by a computer program, as such an arranganent provides great versatility and the ability to deal with large amounts ofdataveryquickly. Preferably the means to provide control inputs to control the state variables comprises a controller. The controller is preferably a linear controller but need not necessarily be purely linear. Advantageously it can tolerate temporary non-linearities. Advantageously the controller takes the estimates from the state estimator as inputs, rather than the measured values obtained by direct observations, and provides the control inputs to the system. For example, in a submarine thecontrol inputs could be hydroplane deflections or in a ship they could be rudder deflections. The arrangement of a state estimator and an controller enables effective and flexible control of the system to be achieved. Preferably the means to compare the estimated values of the one or more statevariableswith theirmeasuredvalues comprises acomparatorwhich subtracts the estimated value of the statevariablefromitsmeasuredvalue, togiveanestimationerrorvalue. Preferablytheestimationerrorvalueis then used to derive the steady state error in the measured values. This arrangement is advantageous because the steady state error remains constant andbyderivingthevaluesofthesteadystateerrorthecorrectionsnecessary to remove it can be derived. It is not necessary with this arrangement to ever actually measure the steady state error. The estimation error (observed - estimated values) is preferably used to derive the changes necessary in the system to produce the desired values by deriving the correctiontothecontrol inputswhich isnecessarytoremovethesteadystate error value. Conveniently the calculations are carried out by means of a computer program. Preferably the means to set the desired values of the or each controlled state variable comprises means to enter an ordered state in the controller to set the value of the control input necessary to achieve the desired output statevariable. This desired statemaybe orderedmanually by an operator or automatically by an autopilot controller, in response to system demands. If only one statevariable is to be corrected, thecorrectiontothe control inputs is preferably achieved by applying an increment, positive or negative, to the ordered state. Alternatively the correction may be applied directly to the control inputs. Ifmorethanone statevariableis to be corrected, the correction ispreferablyappliedto thecontrol inputs. Incrementing the ordered statewill be referred to as Ordered State Correction (OSC). In Ordered state Correction, the steady state error is derived and the ordered state is incremented accordingly. Preferably the steady state error in the orderedstate is applied ina reversemanner to the ordered state to achieve the desired value of the state variable. The correction to the control inputs to correctmore than one state variable will be referred to as Multi-State Correction (MSC). m Multi- State Correction, the steady state errors in the statevariables areusedto findthe control inputswhichwouldcause theerrors andthena correctionis applied to the control inputs. In Ordered State Correction the change is preferably applied to the entered ordered state to provide the control inputto the systemwhichwill produce thedesiredvaluesofthe statevariableas originallyordered. In Multi-State Correction the estimation errors are preferably used to derive those values of the control inputs which would have produced the errors. T_hereverse of thesevalues arethenpreferablyappliedto thecontrolinputs to remove the errors. Autopilots according to the inventionmaybe used to control ships, submarines, aircraft, weapons and other vehicles. Preferably the autopilot of the invention is adapted for use as a submarine autopilot. Preferably, in such anautopilot, thecontrol inputs comprise hydroplane and rudder deflections and forward speed or propellor speed. Preferably the autopilot includes a state estimator to model the system. Such an autopilot can beused to control depth-keeping (forwhich it is particularly effective), course or track keeping etc. Inanysystem, noise (electrical, mechanicalorother) canaffectthe accuracy of the signals. Thus, if there is anynoise input the correction signals canbe affected. Advantageouslya filter is includedtofilter out noise from the steady state error signals. The filter is conveniently a simple lag, but may be another form of filtering if desired. The main autopilotwill normally include aKal an filter andpossiblyother filtering as well, but an additional filter of thecorrection channel isadvantageous. The invention further provides a method of correcting an autopilot subjected to errors of a non-zero mean, comprising the steps of; 1) inputting desired values of one or more state variables of a system into an autopilot controller in the form of ordered states; 2) providing control inputs to the system from the controller to achieve the states corresponding to the ordered states; 3) providing measurement of the actual values of the state variables of the system; and 4) modelling the system in a state estimator; characterised in that the method includes the further steps of: 5) comparing the measured values of the state variables with the values estimated by the state estimator; 6) deriving an error signal from the difference between the measured values and the estimated values; and 7) using the error signal to provide a correction to the control inputs to bring the actual states closer to the ordered states. Preferably, if only one statevariable is tobecorrected, the error signal is applied to the ordered state to provide an incremented ordered state, thus changing thecontrol inputs in order toproduce thedesiredstate of the system. This is called Ordered State Correction (OSC). Alternatively, if more than one state variable is to be corrected, the error signals are preferably used to determinewhatvalue of thecontrol inputs would be expected to produce the measured values of the state variables. A correction can then be applied to the control inputs to eliminate the difference between the measuredvalues of the statevariables andthedesiredvalues. This iscalledMulti-StateCorrection (MSC). MSC canbeusedtocorrectanerror inasinglestatevariablebutOSCispreferred in that case as it is simpler. Preferablythe increment orcorrections arederivedusingacomputer program. Preferably the computer program compares the measured values of the state variables with the estimated values. The estimation error can then be derived. The invention also further comprises a computer programmed to correcterrorsofnon-zeromeaninanautopilotinaccordancewiththemethods of the inventions. The invention will now be described, by way of example only, with reference to the drawings of which: Figure 1 is a flow diagram representing a general control system according to the invention. Figure2 is adiagramto indicatethenotationused forunderwater vehicle dynamics. Figure 3 is a flow diagram representing the operation of an autopilot with Ordered State Correction. Figure 4 is a matrix flow diagram for the system represented in Figure 3. Figure 1 shows a simplifiedflowdiagramofageneral control system according to the invention. The systemis representedby its dynamics for those state variables of interest for control of the system. This is representedbyblock 1. The systemat 1 is affectedby inputs 2 toproduce valuesofthestatevariables3. Thevaluesoftheinputs2arealsofedinto a model of the system, represented atblock 4, whichproduces the estimated values 5 of the statevariableswhichwouldbe expected fromthe system1 in response to inputs 2. The inputs 2 are controlled by a controller represented by block 6 which is setto control the inputs 2 to the systemto provide desired outputs 3 as set by ordered state inputs 7. The ordered state inputs 7 canbe input atvariouspositions inthecontrol loop (eg into the state estimator 4) butthepositionwherethe resultingcalculations are simplest is a direct input into the controller 6. The actual values 3 and the estimated values 5 are compared by a comparator 8 toproduce estimation errorvalues9. If there isnoiseaffectingtheerrorsignalsafilter10 is provided, which removes noise fromthe error signals 9 toprovidesignals11 which are thenfedback intothemodeltoprovide improvedestimatedvalues5 which are used to bring the actual values 3 of the systan 1 closer to the desired values for the system. The inventionwillbedescribed in relationtoa submarineautopilot used for depth keeping. Figure2 isused to indicatethenotationused. Thesymbolsusedare as follows: Oxyz Body axes X A * -K etc Co-ordinates of the point A, etc -- y -z Moments of inertia referred to Oxyz m Mass Mathematical Model ~ e Modified Euler angles of roll, pitch and yaw ϋ Vw Velocity components P Q R Angular Velocity components X Y Z Force components K M N Moment components H Depth of 0 below mean level of free surface S B ^S ^R Bow plane, stern plane and rudder deflection W Muw' Nur' etc Coefficients in force and moment expressions. Disturbed Motion P θ Ψ Boll, pitch and yaw increments u V w Velocity component increments p q r Angular Velocity component increments Δ X Δ Y Δ Z Force component increments Δ K ΔH ΔN Moment component increments h Depth increment § b S s $ Control deflection increments x u Mn N r etc Derivatives. The 'dot' denotes differentiation with respect to time eg H. The 'bar' — denotes the value of a variable in steady state motion eg ϋ. The 'prime' ' denotes a non-dimensional quantity based on the units of ϊ length -t , mass ^ <» I , and time ■ / , eg P 1 = ' ~- t / . * n Notation for Control Control - State-space notation (x = A.x + B.u + D.f) where X = Vector of absolute state variables Y = Vector of absolute state observations x = Vector of state error variables y ***** Vector of state error observations u ~ Control input vector f - External disturbance vector X Y x y denote estimated values E = Estimation error vector = X - X ΪE = Observed estimation error vector ****• Y - Y K = Kalman gain matrix L = Controller gain matrix ϋ is the vehicle forward speed. "Addedmass" - when the speed of a body ina fluid is increasedwork must be done, notonlyto accelerate thebodybut alsoto increasethekineticenergy of theparticles of thefluid. Theeffect is the same as ifthemass ofthe body were increased and this component is commonly referredtoas the "added ■ mass"« Itsvaluedepends onthedirectionoftheacceleration. (Nofluid is actually entrained and the effect does not requireviscosity). In the sanewaythereisanaddedmomentofinertia. Addedmass isofthesameorder as the mass of fluid displaced and is important for ships, submarines, airships and torpedoes. It isunimportant in relationtoaircraft. Thus the effective mass in the downward direction for a submarine is πu wherem-. = m _ -z\. The effective moment of inertia inthe sameplane is I 2 where I 2 = i - j . TheautopilotofFigure3 operatesoncorrespondingprinciplestothe general control system of Figure1. The systemdynamics atblock1 respond to inputs 2 inparallel with a model of the systematblock 4, which isherea state estimator. The controller 6 provides the inputs2 as acontrol input vector u in responseto ordered state inputs 14 and to inputs fromthe state estimator4 tothecontroller6. Thesystemdynamicsat1 are, however, also subjected to an external disturbance vector f. Thevalues of thestatevariables3 of thesystem1formthevectorof absolute state variables X and are measured by appropriate means 12 to give the vector of absolute state observations Y. In the sameway, thestateestimator4 provides theestimatedvector X and measurement means 13 provide thevector Ϋ of the estimated valuesof the observations 5. The presence of the external disturbance vector f affects the dynamics of the system1 so that thevalues of the statevariablesX, and the observed values Y are not the same as the estimated values and Ϋ, respectively. Thevalues YandΫarecomparedbymeansofacomparator8 to give an error signal output 9 of the observed estimation error vector YE=Y-Ϋ. The signal 9 ofYE is fedbacktothestateestimator4 toenable improved estimated values X to be produced. The error values Y- are also fed to an Ordered State Corrector 11 which computes the error (actual - ordered) that the ordered state variable would have in the mean or steady stateandapplies itintheoppositesensesoasto incrementtheinputordered state 14, such that the error in themean or steadystate is reducedto zero. The incremented ordered state signal 15 thuscausesthecontrollertoprovide amodifiedcontrol inputvector uwhichwillproducethedesiredoutputXfrom system1 inthepresenceofthedisturbancevectorf. OSCcanbeimplemented without the need to establish a steady state prior to the adjustment. The implementation of the techniquewillbedescribedwithreference to Figure 4 as well. The illustrative example being used is that of submarine depth-keeping and the equations of submarinemotionare therefore first written in state space form: I 2 .Q = *tø* + Q + M^θ + β S B + M δs S s + M θ - GL H = W - ϋ.θ The state space notation gives: X = A.X + B.u + D.f where: X = [ W, Q, θ, H JT, u = [ S ' S S IT, f = [ Z, M, 0, 0 ]T A = Z n j , ZQ ΠVJ, 0, 0 B = z S B /m 3' ~ -s /" - D = 1/m j , 0 *2' * ^' ^' ° V-S- -2' M ys 7l 2 0, 1/I 2 0 , 1 , 0 , 0 0 , 0 0, 0 1 , 0 , -ϋ , 0 0, 0 For observations of depth and pitch angle Y - [ H, θ ]T and the measurement matrix C = 0, 0, 0, 1 0, Or 1, 0 Figure 4 shows the matrix flow diagram corresponding to Figure 3, For the system dynamics X = A.X + B.u + D.f For the state estimator = A.X + BoU + K(Y - t) = hΛ + B.u + K.CCX - ) Subtracting - = A(X - X) - K.C(X - ) + D.f giving E = (A - K.C) X E + D.f Thus Xj- depends solely on f, whatever the manoeuvre the systemmay make. Provided the determinant A - K.C is not singular, X-g will attain steadyvalues givenbyX E = -(A- K.C)-1 D.f. It follows that Y-.will also achieve steady values. In the steady state of the whole system under auto control, and inserting the controller relation u = Lx, the autopilot equation is and for a system which is not sensitive to the ordered variable Α»A ~~ Α-A Hence (A + B.Dx = -K.Y„ E giving x = -(A + B.L) "1 K.Y £ Hence the estimated value of the ordered variable x(4) (the steady state estimated depth error) can be derived. Hence x 4 = (~ A _ * 4 ) + J 4 = g^ + 4 since the observed value of 4 is Y^ (i.e. H). Thusthedeptherrorinthesteadystateisfoundfromquantitieswhich can be observed during amanoeuvre. Thisvalue ( S H or( -j) is applied in the opposite sense to correct the ordered state (Hord), withthe result that the error in the steady state is reducedto zero (though it shouldbenoted that, in general, neither Yg nor x will become zero as well). Since Y E achieves a steady value independently of the system as a whole, the addition of the OSC channel has no effect on the stability of the system. OSC automaticallycomputes theerrorwhichwould occur in the steady state and adjusts the ordered state accordingly. There is no need to establish a steady state prior tothe adjustment. It isvalidprovidedthe disturbing force and the other state variables are independent of, or insignificantly changed by, the correction. The presence of a constant disturbing force is revealedby non-zero mean values of the estimation errors. These mean values can be shown to depend only on the disturbance and the vehicle forward speed. They are unaffectedby manoeuvring and depth changing. Thus the steady state depth keeping error can be found f om themean estimation errors (though the depth keeping error is not the same as the estimation error of depth) and can be applied in reverse to the ordered depth, thus correcting the depth keeping. This techniqueovercomesall theproblems of the integrator and isstableand robust. Once the steady state error and correction have been found for a particulardepth, itwillneedonlyminor, ifany, adjustmentas itismovedto another ordered depth. When one ordered statevariable is changed, changes in another state variablemay occur e.g. change ofdepthcauses changes inpitch. OSCcanbe operated evenwhen alimitto the secondarychanges is included e.g. apitch limiter. With other systems the introduction of apitch limit causes such complicatedeffects thatordereddepthmaynotbeattained. However, pitch limitinghasnoadverseaffectonOSCasOSCwillchangethedepthatwhichthe limit ceases to operate so that achievement of the corrected state can be achieved cleanly. External forcescanposeevengreaterproblemssuchastheeffectofa "seaway" on depth keeping of a submarine. In this situation there is an upward forcewhich constantlypulls the submarineoutoftrim. Ina strong seaway, present autopilots without integrators would be unable to keep the submarine below the surface. OSC can keep it within normal depth tolerances. Thosethat include integratorswouldbeunabletochangedepth in autocontrol, whereas OSC can. Thewaveforceshaveanon-zeromean (suctionforce) whichisopposed by the OSC. Even if the submarinehas no knowledge of the sea state and is notballasted against it, OSC can achievecleanflareouttoanewdepthwith the overshoot being within the motion occurring normally. OSC is comparableto (or slightlybetterthan) the integrator systan at simple depth keeping but is vastly superior when the ordered depth is changed as itcanchangetothenewdepthveryaccuratelywithno undesirable overshoot or other complications. OSC will remove one steady state error, and this maybe all that is required, but itcannot removemorethanone. Forexampleitcannotcorrect thedepthkeepingandsimultaneouslyremoveasteadystatepitchangle. (If OSC is appliedtobothdepthkeepingandpitchtheywill interactagainsteach other). Multi-state correction (MSC) is an alternative technique which is not limited in this way. Once again the key is the constancy of the mean values of the estimation error. If the hydroplane deflections needed to produce this errorarefound, they canbeapplied in reverseasoffsetstothe hydroplanes, thus correctingbothdepthkeepingandzeroingthepitchangle. Theprincipleofoperation issimilartothatdescribedforOSCwithreference toFigures3 and4. Ifthecontrols canbeoffset, byusay, soastobalance the disturbance, ie B.u + D.f = 0, then MSC enables the vector u to be calculated on line during any manoeuvre. Haying neutralised the disturbance, thecontroller brings the system exactly to the ordered state. (There are some limitations on controllability, eg two or more variables cannotbecontrolled independentlyby a singlecontrol, nor can itbeassumed thatB.u+ D.f = 0 has a solution, but thesedonot arisewith the submarine) . Providedu is included inthefeedbacktotheautopilotBmatrix (Fig. 4) then X = A.X + B(u + u) + K(Y - Y) and as before so that X E and y £ tend to constant values whatever the manoeuvre. For the steady state of the whole system in auto control As before A.X = A.x and Y E = y £ giving If (A + B.L) = Λ, the equation may be partitioned to read 0 = + I *l I K 2 ] y E where x, 2 = h- x 34 = x 3 ~ 2 x I Eliminating x , _ by multiplying the upper and lower equations'by Λ and Λ respectively and subtracting gives ( Λ ~ Λ t - . χ 3 + ( A ' - Λ 3 'B 2 )JI + ( Λ ~ λ - Λ * 5 ' κ 2 )y E = 0 Also x 34 = - (χ 34 - X3 4 ) + χ 34 = y E + 34 and in the required state 34 = o giving ( Λ " ,' - Λ " ,' B 2 ) f - Λ" χ ♦ Λ ~ ' K 2 >y E r = ( Λ.' Bl - Λ" B 2) - i ( fi - Λ - Λ.'K, + Λ " 5 'κ 2 )y E As with OSC it is necessary to smooth the elements ofu ory„. j > or the submarine, the computation simplifies considerably, and MSC gives more precise pitch limiting in a depth change than OSC. However, the offsets required to balance large disturbances, eg suction force, may . cause saturation. The A and Bmatrices areparticularly speed sensitive and should be adjusted on line, if possible. OSC andMSCareapplicablenotonlytodepth andpitchkeepingbutto course keeping , track keeping and any other type of autopilot controlled function * They are suitable for ship, submarine andweapon autopilotsand could also be used for aircraft and are suitable for use with any system including a state estimator.

Claims: CLAIMS. 1. An autopilot comprising: a) means (6) to provide control inputs (2) to control one or more state variables (3) of a system (1); b) means (4) to provide a model of the system (1); c) means (13) toestimatefromthemodel ofthesystemthevaluesof the one or more state variables which are expected to occur in response to the control inputs; d) means (14) to set the desired values of the or each,controlled state variables; and e) means (12) to provide measurement of the or each controlled state variable; characterised in that the autopilot further includes: f) means (8) to compare the estimated value (5) with themeasured value (3) of the or each controlled state variable; g) means (11) to derive the correction to the control inputs necessary to achieve the desired values; and h) means (15) to adjust the control inputs to produce the desired values of the or each state variable. 2. Anautopilot according toclaim1 characterised in thatthemeans (4) to provide a model of the system comprises a state estimator. 3. An autopilot according to claim 2 characterised in that the state estimator includes acomputer ormicroprocessorandthesystanismodelledby a computer programme. 4. An autopilotaccording toany onepreceding claimcharacterised inthat the means (6) to provide control inputs to control the state variables comprises a controller. 5. An autopilot according to claim 4 characterised in that the controller takes the estimates from the state estimator as inputs rather than the measured values obtained by direct observations, and provides the control inputs to the system. 6. Anautopilotaccordingtoanyonepreceding claimcharacterised inthat the means (8) to compare the estimated values of the one or more state variables with their measured values comprises acomparatorwhichsubtracts the estimated value of the state variable from its measuredvalue, and from this the steady state error is derived. 7. An autopilotaccording to claim 6 characterised inthat the estimation error (observed-estimatedvalues) isusedtoderivethechangesnecessaryin the system to produce the desired values. 8. Anautopilot according to anyoneprecedingclaimcharacterised in that the means (14) to set the desired values of the or each controlled state variable comprises means to enter an ordered state in the controller to set thevalue of the control inputnecessary to achievethedesiredoutput state variable. 9. An autopilotaccording to claim8 characterised in that thecorrection to the control inputs is achieved by applying an increment, positive or negative, to the ordered state. 10. Anautopilotaccording toclaim9 characterisedinthatthesteadystate error in theorderedstate is applied in a reversemannertotheorderedstate to achieve the desired value of the state variable. 11. An autopilot accordingto anyoneofclaims 1 to8 characterised inthat the correction is applied directly to the control inputs. 12. An autopilot according to claim 11 characterised in that the steady state errors inthestatevariables areusedto findthecontrol inputswhich wouldcausethe errors andthenacorrectionisappliedtothecontrol inputs. 13. Anautopilotaccording toanyoneprecedingclaimcharacterised inthat the autopilot is adapted for use as a submarine autopilot. - 14. An autopilotaccordingto anyonepreceding claimcharacterised inthat the calculations are carried out by means of a computer program. 15. A method of correcting an autopilot subjected to errors of a non-zero mean, comprising the steps of: 1) inputting desiredvalues (14) of one ormore statevariables of a system into an autopilot controller (6) in the form of ordered states; 2) providing control inputs (2) to the systemfromthecontroller to achieve the states corresponding to the ordered states; 3) providing measurement (12) of the actual values of the state variables of the system; and 4) modelling the system in a state estimator (4); characterised in that the method includes the steps of: 5) comparing (8) themeasuredvaluesYof the statevariableswith the values Ϋ estimated by the state estimator; 6) deriving an error signal (9) from the difference between the measured values and the estimated values; and 7) using the error signal (9) to provide a correction to the control inputs to bring the actual states closer to the ordered states. 16. Amethod of correctingan autopilotaccordingtoclaim15 characterised in that the error signal (9) is applied to the ordered state to provide an incremented ordered state (15). 17. Amethod of correcting an autopilotaccording toclaim15 characterised in that the error signals are used to determine what value of the control inputswouldbeexpectedtoproducethemeasuredvaluesofthestatevariables and a correction is appliedto thecontrol inputs to eliminatethedifference between the measured values of the state variables and the desired values. 18. Amethod ofcorrecting anautopilotaccording toanyoneofclaims15 to 17 characterised in that the corrections are derived using a computer programme. 19. Amethod of correcting an autopilot according toclaim18 characterised in that the computer programme compares the measured values of the state variables with the their estimated values and then derives the estimation error therefrom. 20. A computer programmed to carry out amethod of correcting an autopilot according to any one of claims 15 to 19.

Inventors: BOOTH THOMAS BRINDLEY, BOOTH, THOMAS, BRINDLEY
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1988
Main classification: G05D1
IPC codes: G05B13, G05D1