GB2217477A

GB2217477A - An engine control unit for a turbomachine

Info

Publication number: GB2217477A
Application number: GB8807918A
Authority: GB
Inventors: Andrew John Anning; David Christopher Wellings
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1988-04-05
Filing date: 1988-04-05
Publication date: 1989-10-25
Anticipated expiration: 2008-04-05
Also published as: DE3910869C2; GB8807918D0; DE3910869A1; GB2217477B

Abstract

An engine speed control unit for a gas turbine engine, e.g. an aircraft jet engine, is operative to adjust the level of fuel flow and incorporates in the control loop means for smoothing the rate of acceleration or deceleration to a newly demanded speed to avoid overshooting or undershooting the new speed. This is particularly effective when decelerating to a new low speed by avoiding engine speed undershoot and thus the subsequent re-acceleration which can cause a surge. The deceleration curve is smoothed controlling the rate of deceleration by substituting a false demand level NHd for the newly demanded speed PLd, and by progressively reducing the substituted level in an iterative control process so that actual engine speed NH is substantially assymptotic to the demanded speed after roughly four iterations. <IMAGE>

Description

An Engine Control Unit for a Turbomachine V 1 24 17477 The invention-

relates to an engine control unit for a turbomachine.

_r In particular, but not exclusively, the invention relates to the control of jet engines for aircraft but it may also find application in the control systems of gas turbine engines in general.

The scope of and restrictions on the operating envelope of gas turbine engines is well known. In particular, the area of unstable engine operation known as the surge region is well charted. in order to avoid problems with surge the running line of the engine is designed to maintain an adequate safety margin over the surge region. Unfortunately, towards the lower end of the engine speed range when intake air flow and fuel flow are low, the stable operating limits and the surge boundary tend to converge. it can be very easy, therefore, to induce a surge problem, for example, if an engine is re-accelerated from a low engine speed, say idle speed, at a relatively low airspeed. One when this can happen is when a pilot throttles back to idle speed, the engine speed undershoots and the automatic governor tries to accelerate the engine up to the selected speed.

The object of the present invention is to modify the acceleration or deceleration curve of an engine to avoid the hunting effect J n the engine speed controller by adjusting the speed control loop so that actual engine speedl approaches a new selected speed substantially asymptotically.

According to the present invention an engine control, unit for a turbomachine operative to adjust the level of fuel flow to achieve an engine speed change required by an input demand includes means for solving an engine speed control function in accordance with an iterative algorithm wherein following an input demand change when the control function is evaluated initially the true input demand is substituted by a C false demand in which the demand change is less than the true demand change.

Subsequently the algorithm restores the value of the input demand utilised in evaluating the control function to the value of the true input demand. Preferably the false demand is restored progressively to the true input value, in progressively smaller increments and each change is triggered by measured engine speed reaching a predetermined proportion of the engine speed represented by the true input demand.

The invention and how it may be carried out in practice will now be described by way of example with reference to the accompanying drawings, in which:

Fig 1 is a graph illustrating how engine speed responds to a step change in the setting of the pilot's demand lever, Fig 2 illustrates the false demands set by the invention and the corresponding change in engine speed response, Fig 3 shows n block diagram f orm the -I o f ar, reLationship between and interconnection engine, its fuel system and its digital engine controller, k 1 Fig 4 shows a f low chart for the iterative algorithm responsive to a charge in. engine speed input demand, Fig 5 shows in logic circuit form how the algorithm of Fig 4 might be applied in the digital electronic controller of Fig 3, and Figs 6 and 7 graphically illustrate the offset bias and rate of change functions referred to in Fig 5.

Fig 1 shows graphically an engine response phenomena known as idleundershoot. However, it is fairly typical for a generalised gas turbine engine when being decelerated or accelerated rapidly with a fairly large speed change to exhibit a similar response. The engine speed drops below the demanded speed level, in this example idle speed, at the completion of a deceleration and then recovers back to the idle speed. During this recovery phase when the engine is overfuelled to provide the necessary acceleration the engine J q prone to surge. The dangers and consequences of engine surge are well known.

The basic principle of the invention, as %,-,ill be appreciated from a consideration of Fig 2, is a modi-fication of the original input demand by substitution of a false demand in order to smooth engine response as engine speed approaches close to the final demand value. The dynamic response of the control system remains unaffected although the trajectory of the response is modified by effectively introducing a lag-free damping effect. Operation of the invention w411 be described with particular reference to a deceleration from an initial relatively high engine speed to idle speed, however, it will be understood that it may be applied to control the dynamic response of an engine to both positive and negative speed changes to any speed level.

Fig 2 shows the effect of the in-7ertion on engine response to a step charge in the engine speed demand to idle, requiring a relatively large reduction in engine speed. The original demand would normally produce the response shown in Fig 1. The approach control sets a false demand above the new or true demand level and the fuel flow control circuit calculates a reducing flow level with reference to the speed error between actual engine speed and this false aiming level. When actual engine speed reaches a predetermined level above the false demand the false demand is further reduced towards the true level.

In the simplest form of the invention there is only one intermediate false demand level employed and at: the true the first transition level the full value o4C demand level is reinstated. Again, in the simplest form the transition from false to true demand may be in the form of a step change. Rowever, it is preferred that the transition should be in the form of a ramp function and that the false demand level shall approach the true demand level according to an iterative algorithm in which the slope of the false demand is decreased as the final demand is approached. After the fourth ' or fifth false demand change the actual engine speed and the true demand level have substantially converged.

1 Fig 3 illustrates the basic elements of an engine speed control system of the present type. The speed of the engine 2 is controlled by the level of. fuel flow 4 supplied by a fuel system 6. The level of fuel required is determined b.,,, engine speed and also by the power required, represented by the feedback loop labelled shaft power. The output from the fuel system 6 is demanded by an electrical current 8 supplied by a Digital Electronic Control Unit or DECU 10 in response to a number of inputs and internally programmed function laws. These inputs include the pilot's lever speed demand NHd, feedback signals representing actual engine speed NH and a critical temperature parameter usually turbine temperature, and limit datum signals for temperature and engine speed.

As illustrated the DECU 10 shown in greater detail in Fig 5 comprises a plurality of interconnected function blocks. It could be constructed using discrete logic elements and dedicated electronic circuits to perform the indicated functions, but, in practice, and in the described example the DECU consists of a microprocessor based controller which is operated by specially designed software programs to perform the functional tasks shown. The blocks shoX.,M. in Fig 5 are, therefore, more akin to elements of the sofl:t-,,Tare programs than to the unit hardware itself. It follows that the engine speed control unit being described operates cyclically, sampling various inputs in a predetermined sequence, performing mathematical and logical operations according to appropriate program instructions and providing in each cycle a controlling output signal 8. This output is sustained by a buffer circuit (not shown) to control operation of the fuel 1 computation. The system 6 during the next cycle OIL cycle period of the controller in suit is roughly 40 mS so that inputs are sampled and the output is refreshed with that periodicity.

The algorithm evaluated by the digital controller 10 in Fig 3 is illustrated by the flow chart of Fig 4. In the instance illustrated, and as previousl-,;, referred in connection with Figs 1 and 2, a deceleration to engine idle speed is considered. NH represents an actual engine speed feedback signal from a tachometer mounted on the engine, PLD is a signal from. the pilot's demand lever representing desired engine speed, IDLE is a datum input signal representing engine idle speed, and NHD is an output signal connected to control operation of the Fuel System to produce a required engine speed.

Following the flow path of Fig 4, generally downwards from the top of the page, at the beginning of each processor cycle the speed control input PLD is tested b- the alogorithm, as indicated by the uppermost decision symbol, to ensure that a deceleration has been demanded by comparing the pilot's demand signal PLD with actual engine speed NH. If PLD is greater than NH then an acceleration is demanded and an appropriate algorithm is implemented. If PLD and NP are equal no change in output is required. If PLD is less than NH a deceleration is demanded the decision passes to the next step which allows the remainder of the approach control program to be effective if the demanded final speed is less than 85% of NH.

11 At the third level in the flow chart of Fig 4 a false speed demand NHD is set at an engine speed of IDLE + 8% NH until actual measured engine speed NF falls to Idle + 9% or less. The fourth level then sets the output level and further reduces the false demand level towards Idle speed at a rate of 3% per second to a minimum of IDLE + 4% NH. When the actual speed NH reduces to IDLE + 4.5% control passes to the next and fifth level where the false demand level is reduced still further at a rate of 1.3% per second to a minimum of IDLE + 1_% NH. Finally, when actual speed reaches IDLE + 2.5% a sixth level of control ramps the output signal towards its final level at a rate of 0.5% per second.

To summarise, for a substantial change in the controlled parameter a false demand is generated above the required final demand level. As the transient level of the controlled parameter approaches the false demand level the latter is moved to a new level closer to the required level. This iterative approximation is repeated through a series of intermediate le-ve 1 s until the output finally reaches the demanded level.

J:

Although described above with reference to a deceleration to idle engine speed it wi 1. 1 be appreciated that the IDLE input could be replaced by any other final required engine speed in which case the offset percentage level may be selected to suit. As described the bias and rate functions are piecewise functions but, as will also be readily appreciated from the following further description of the invention with reference to Fig 5, these ma,y be realised alternat4vely as continuous functions.

Referring now to Fig 5, as previously mentioned, the false demand output 8 for engine speed comprises an electrical current output from the DEW 10 (see Fig 3) which is operative to control operation of the Fuel System 6. The false output 8 is calculated using the higher of two speed demands generated in upper and lower signal paths 12 and 14 in Fig 5 labelled "Rate limited false demand" and "Minimum level of false demand" respectively.

The two parallel signal paths share a common input signal NH error, representing the difference between a demanded speed signal NIF1d and measured engine speed NH, derived by signal summing circuit 16. The lower signal path 14 generates a minimum level of speed demand as a function of the NH error signal. This is generated by, adding at summing point 18 a bias signal 20 to the pilot's demand signal NHd.

The bias signal 20 is generated by a bias function circuit 22 operation of which is further illustrated in Fig 6. In the present example circuit 22 implements a linear function the value of which is determined by. the magnitude of the NH error signal. The value of this function may be calculated using for example a linear multiplier or, as is preferred in the example, by using a digital look-up table (not shown).

A rate function circuit 24 Jin the second or upper signall- path 1.9 operates to limit the rate of change of false demand, and does so by employing the function illustrated in Fig 7 multiplied by the processor cycle time. Again, the value of the rate of change function is dependent upon the magnitude of the NH error signall. In this example the function is not linear and may involve several changes of gradient at valuen. The funcrion may be a continuous function.

1 t 4Q The rate circuit output signal 26 is multiplied at 218 with a further signal 30 representing the processor cycle time, that is the elapsed time between consecutive updates of the controller output. This yields the maximum change 32 in the false demand -permitted in the processor cycle. In the example being described the ne-v demanded engine speed is "idle" so that the engine has a negative acceleration, NH error is negative and the maximum permitted change has a negative value. Adding the maximum change:rom the signal 32 to the final false demand signal. 34 fl preceding cycle in summing circuit 36 generates a new rate limited signal which gives the lowest demand which does not exceed the rate function. Finally, to find the final false demand signal 8 for a new CVcle the signals in both signal paths are compared by a highest wins circuit 38.

At the conclusion of each cycle the false demand signal 8 is passed to the fuel system 6 and is also stored in a signal level store or memory (not shown) for use in the next cycle as the last deniar. signal 34.

The above example is described in order to provide a better understanding of the invention, and the values mentioned for the variables are not intended to be limiting upon the scope of the claimed --Invention. T tis also to be understood that although the example is based upon control by measurement of engine speed other limiting variables such as pressure or temperature. These limiting or governing control loops may be used as substitutes for, but more commonly as overrides for the main engine control, function. Thus, if the situation should arise that an engine is accelerating at a speed within its acceleration limits but, say, the turbine temperature is approaching a critical value the control loop will override the normal control loop and demand a reduced fuel flow rate so as to maintain the turbine temperature within acceptable limits. Similarly, control may be exercised using a pressure monitor.

1 1 It 1

Claims

CLAIMS:

1. An engine control unit for a turbomachine operative to adjust the level of fuel flow to achieve an engine speed change required by an input demand -includes means for solving an engine control function by an iterative algorithm involving at least two iterations in which in response to a new input speed demand a false demand representing an engine speed change less than that demanded is utilised in the first iteration and said false demand is made to approach the input demand subsequently.

2. An engine control unit as claimed in claim 1 wherein the rate of change of the false demand between subseauent iterations progressively decreases.

3. An engine control. unit as claimed in claim 2 wherein the rate of change of the false demand is a function of the error between instantaneous engine speed and the equivalent input demand speed.

4. An engine control. unit as claimed in any precedirg claim wherein the difference between the false demarid level and the input demand level is a functiOP of the error between instantaneous engine Fpeed and the equivalent input demand speed.

5. An engine control unit as claimed in any preceding clair, which operates cyclically to solve the CID J engine speed control function.

6. An engine control unit as claimed in claim 5 wherein a false deTrand level is calculated by adding a selected bias level to a demanded speed signal.

-l-)-

7. An engine control unit as claimed in claim 6 wherein a rate of change of engine speed demand signal is dete-n-ined by subtracting a new speed demand signal and an old speed demand signal from a previous cycle one from another.

8. An engine control unit as claimed in claim 7 0herein the old speed demand signal is taken from the immediately preceding cycle.

1 1

9. An engine control unit as claimed in claim 7 or claim 8 %.?herein a change of engine speed demand signal is compared with a reference signal representing a maximum change allowed between the old and new cycles.

10. An engine control unit substantially as described with reference to Lhe accompanying drawings.

Published 1989 at The Patent Office. State House, 60 -1 High Holborn, London WCIR 4TP. Further copies maybe obtaJned from The Ta ntofftce Sales Branch, St Mary Cray. Orpirgton, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con. 1/87