FR2997443A1 - CONTROL UNIT AND METHOD FOR CONTROLLING THE AUBES TOP SET - Google Patents

Abstract

A control unit for controlling a clearance between blade tips of a turbine rotor of a gas turbine engine engine and a turbine ring of a casing surrounding the blades, comprising a module of controller (50) configured to determine a control signal (SC50) of at least one valve for acting on a flow rate and / or an air temperature directed to the housing, comprising: - a take-off management module (51) configured to provide a first control signal (SC51) equal for at least a predetermined time after starting the engine, to a first predefined value corresponding to a decrease of the game, - a transient management module (52) configured to provide a second control signal (SC52) corresponding to the placement of the valve in a retracted position for a second duration; - a cruise management module (53) configured to provide a third control signal (SC53) from at least an alt itude (ALT) and a speed (Mach) of the aircraft, and - a selection module (54) configured to select the control signal from the first, the second and the third control signals as a function of a flight speed. engine operation.

Description

BACKGROUND OF THE INVENTION The present invention relates to the general field of turbine engines for aeronautical gas turbine engines. It aims more precisely the control of the game between, on the one hand, the tips of moving blades of a turbine rotor and, on the other hand, a turbine ring of an outer casing surrounding the blades.

The clearance between the top of the blades of a turbine and the ring around them is dependent on the differences in dimensional variations between the rotating parts (disk and blades forming the turbine rotor) and the fixed parts (external casing of which the turbine ring that he understands). These dimensional variations are both of thermal origin (related to variations in the temperature of the blades, the disk and the casing) and of mechanical origin (in particular related to the effect of the centrifugal force exerted on the rotor of turbine). To increase the performance of a turbine, it is desirable to minimize the game as much as possible.

It is known to use an active steering system to control the blade tip clearance of a turbomachine turbine. A system of this type generally operates by directing on the outer surface of the turbine ring air taken for example at a compressor and / or the fan of the turbomachine. Fresh air sent to the outer surface of the turbine ring has the effect of cooling the latter and thus limiting its thermal expansion. The game is therefore minimized. Conversely, hot air promotes the thermal expansion of the turbine ring, which increases the game. Such active control is controlled by a control unit, for example by the full authority control system (FADEC) of the turbomachine. Typically, the control unit acts on a regulating position valve to control the flow rate and / or the temperature of the air directed on the turbine ring. For this purpose, the control unit implements a control loop which comprises the comparison of a set point and an estimate of the actual set of blade tips.

The estimation of the actual set of vane tips requires knowing the thermal state of the engine, especially at startup, which is complex to achieve. In addition, the calculation time required is relatively long. Finally, the estimation of the blade tip set uses many parameters, tables, filters, etc., and the tuning of the regulation loop is therefore complex. The document FR 2 971 291 A1 proposes to control the blade tip clearance with a control loop and a pulse-width modulated discrete-type valve. The regulation loop requires the estimation of the blade tip clearance and therefore has the aforementioned drawbacks. FR 2 960 905 A1 proposes alternatively a game control method, in which the valve is either open or closed, depending on the flight phase of the aircraft. The control of the valve can therefore be relatively simple, and it is possible to use an all-or-nothing type valve. OBJECT AND SUMMARY OF THE INVENTION The main object of the present invention is to overcome the drawbacks of the state of the art requiring in particular the estimation of the blade tip clearance, and proposes for this purpose an improvement of the control mechanism described. in document FR 2 960 905. More specifically, the invention proposes a control unit for controlling a clearance between, on the one hand, blade tips of a turbine rotor of a motor of a motor. gas turbine engine and, secondly, a turbine ring of a casing surrounding the blades, the control unit comprising a control module configured to determine a control signal of at least one valve to act on a flow rate and / or an air temperature directed towards the crankcase.

This control unit is remarkable in that the control module comprises: a management module for take-off, configured to provide a first control signal associated with a take-off phase of the aircraft, and equal for a first predetermined duration following starting the engine, at a first predefined value corresponding to a reduction of the clearance, - a transient management module, configured to provide a second control signal associated with a transient phase of operation of the engine, and equal to a second corresponding value placing the valve, for a determined second time, in a determined fallback position, - a cruise management module configured to provide a third control signal associated with a cruising phase of the engine, from a third value predefined adjustment according to at least one altitude and a speed of the aircraft, and - a selection module c configured to select the control signal from the first, second and third control signals according to an engine operating speed. Correlatively, the invention proposes a method of controlling a game between, on the one hand, blade tips of a turbine rotor of a gas turbine engine engine and, on the other hand, a turbine ring of a housing surrounding the blades, said method being implemented by a control unit, said control method comprising determining a control signal of at least one valve to act on a flow and / or an air temperature directed to the crankcase.

This control method is remarkable in that it comprises, for determining the control signal: a step of supplying a first control signal associated with a take-off phase of the aircraft, and equal for a first predetermined duration after starting the engine, at a first predetermined value corresponding to a reduction of the clearance, - a step of supplying a second control signal associated with a transient operating phase of the engine, and equal to a second value corresponding to the placement of the engine. the valve, for a determined second period, in a determined retracted position, - a step of providing a third control signal associated with a cruising phase of the engine, from a third predetermined value adjusted according to the at least one altitude and a speed of the aircraft, and - a step of selecting said control signal determined by the control unit from among the first, the uxth and the third control signal as a function of a running speed of the engine.

As opposed to the regulation of the aforementioned prior art, which can be called closed-loop regulation because it is based on an estimate of the blade tip clearance, the regulation implemented by the control unit of the invention can be described as open-loop regulation because it does not need to estimate this game. Indeed, the inventors have judiciously observed the behavior of a valve controlled by a closed loop of the prior art, during a mission typical of an aircraft. This mission included the following flight phases: ground idle, take-off and climb, cruise at 30,000 feet, landing at 20000 feet, approach and landing. The inventors have found that the behavior of the valve, during this mission, was relatively simple. They deduced that it was possible, from at least three different modes of management well chosen and adapted to different phases of flight of the aircraft (takeoff, cruise and transient), to determine the necessary valve opening at each instant of a typical mission of an aircraft, from a limited number of input data, and without requiring the estimation of the game. Thus, more specifically, the three management modes implemented according to the invention. mainly aim at: - limiting the opening of the game during the takeoff phase, so as to limit the overtaking (or "overshoot" in English) of the limit set for the temperature of the exhaust gas (EGT Exhaust Gas Temperature ) at the outlet of the high-pressure turbine of the turbojet engine; to avoid the contact between the blades and the turbine casing due to the change of speed, and to a certain extent, a change in temperature of the engine parts (and especially the turbine), during the transient phases; and to limit the clearance at the top of the blades, during the cruising phase, in order to improve the performance of the gas turbine engine.

It should be noted that the invention is not limited to these three management modes, and other management modes, in addition to the three aforementioned, can be envisaged alternatively, such as a mode managing the rise or fall of the aircraft, etc. The open-loop regulation proposed by the invention therefore makes it possible to respond to the need for active regulation of the game, without requiring an estimate of the game, which makes it possible to avoid the difficulties associated with this estimation. In addition, the resources required for calculating the control signal (software resources, computation time, necessary information, etc.) according to the invention are relatively limited. Furthermore, the logic of the regulation operated by the invention, and the architecture supporting this control logic, are advantageously simplified compared to the state of the art, and do not require the choice of a large number of parameters. : the development of the control loop on the engine is relatively simple, and requires few adjustments. In addition, at the level of the controlled valve, the simplification of the control obtained by virtue of the invention makes it possible to improve the service life of the valve, in particular by reducing the wear of the seals of the valve related to its movement. According to one embodiment, the management module for take-off is configured to further provide, after the first predetermined duration, as a first control signal, a fourth predefined value corresponding to an increase in the game.

According to one embodiment, the transient management module is configured to select the second value from two predefined values as a function of a crankcase temperature and a flow temperature. According to one embodiment, the transient management module is configured to select the second value from two predefined values as a function of a time elapsed since the start of the engine. According to one embodiment, the cruise management module is configured to determine the third control signal as a function of an operating speed of the engine. According to the invention, the selection of the control signal is performed by the selection module as a function of the operating speed of the engine. In a preferred embodiment of the invention, this operating speed of the engine is determined from the thrust setpoint transmitted to the engine. This setpoint can be derived from various parameters, such as the position of the joystick (or control) of the aircraft. According to one embodiment, the selection module is configured to select the first control signal when a management parameter of a thrust applied to the engine corresponds to a ground idling situation. According to one embodiment, the selection module is further configured to select the second control signal when the thrust management parameter does not correspond to a ground idle situation and a speed acceleration or deceleration is detected. According to one embodiment, the selection module is further configured to select the third control signal when the thrust management parameter does not correspond to a ground idling situation and no acceleration or deceleration of the engine speed. 'is detected. In other words, the take-off management module, the transient management module and the cruise management module respectively provide the first control signal, the second control signal and the third control signal (for example continuously) to the module. selection, which then operates a selection among the control signals provided as a function of the flight phase in which the aircraft is located determined from the thrust management parameter.

The invention also proposes a gas turbine engine comprising a control unit according to the invention and at least one valve for acting on a flow rate and / or an air temperature directed towards the casing, in which said valve is controlled according to said control signal.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will emerge from the description given below, with reference to the accompanying drawings which illustrate an embodiment having no limiting character. In the figures: - Figure 1 is a schematic view in longitudinal section of a portion of a gas turbine engine engine according to one embodiment of the invention; FIG. 2 is an enlarged view of the engine of FIG. 1 showing in particular the high-pressure turbine thereof; FIG. 3 is a block diagram of a control module of a valve for controlling the blade crown set in the motor of FIG. 1; FIG. 4 is a block diagram of the control module of FIG. takeoff of the control module of FIG. 3; FIG. 5 is a block diagram of the transient management module of the control module of FIG. 3; FIG. 6 is a graph showing a conversion law used by the control module of FIG. FIG. 5; FIG. 7 is a block diagram of the cruise management module of the control module of FIG. 3; FIG. 8 is a graph showing a conversion law used by the module of FIG. 7; FIG. 9 is a graph showing the values of a protection coefficient used by the module of FIG. 7; FIG. 10 is a block diagram of the selection module of the control module of FIG. 3, and FIG. 11 is a graph illustrating the evolution of u game during a mission of an aircraft. DETAILED DESCRIPTION OF AN EMBODIMENT FIG. 1 schematically represents a turbojet engine 10 of the double-flow, double-body type to which the invention applies in particular. Of course, the invention is not limited to this particular type of gas turbine engine. In a well known manner, the turbojet engine 10 of longitudinal axis XX comprises in particular a fan 12 which delivers a flow of air into a primary flow stream 14 and into a secondary flow stream 16 coaxial with the vein primary flow. From upstream to downstream 35 in the direction of flow of the gas stream passing through it, the primary flow flow stream 14 comprises a low-pressure compressor 18, a high-pressure compressor 20, a combustion chamber 22, a turbine high-pressure 24 and a low-pressure turbine 26. As shown more precisely in FIG. 2, the high-pressure turbine 24 of the turbojet engine comprises a rotor formed of a disc 28 on which a plurality of movable blades 30 are mounted. in the flow path of the primary flow 14. The rotor is surrounded by a turbine casing 32 comprising a turbine ring 34 carried by an outer turbine casing 36 by means of fixing struts 37. turbine 34 may be formed of a plurality of adjacent sectors or segments. On the inner side, it is provided with a layer 34a of abradable material and surrounds the vanes 30 of the rotor, making with the apices 30a thereof a clearance 38.

According to the invention, there is provided a system for controlling the clearance 38 by modifying, in a controlled manner, the internal diameter of the outer casing of turbine 36. For this purpose, a control unit 46 controls the flow and / or the temperature of the air directed towards the turbine outer casing 36. The control unit 46 is for example the full authority control system (or FADEC) of the turbojet engine 10. In the example shown, a control unit 40 is arranged around the outer turbine casing 36. This housing receives fresh air by means of an air duct 42 opening at its upstream end in the flow passage of the primary flow at one of the floors the high-pressure compressor 20 (for example by means of a scoop known per se and not shown in the figures). The fresh air circulating in the air duct is discharged on the outer turbine casing 36 (for example by means of a multiperforation of the walls of the control box 40) causing a cooling thereof and therefore a decrease of its inner diameter. As shown in FIG. 1, a valve 44 is disposed in the air duct 42. This valve 44 is controlled by the control unit 46. The valve 44 is here a controlled position valve. The position of the valve 44 may be between 0%, corresponding to a closed valve, and 100%, corresponding to an open valve. When the valve 44 is open (100% position), the fresh air is supplied to the outer turbine casing 36, which has the effect of a thermal contraction of the latter and therefore a reduction of the clearance 38. On the contrary, when the valve 44 is closed (position 0%), the fresh air is not brought to the outer casing of turbine 36 which is heated by the primary flow, which has the effect of thermal expansion of the latter and a increased clearance 38. In the intermediate positions, the outer casing turbine 36 contracts or expands and the clearance 38 increases or decreases, to a lesser extent.

Of course, the invention is not limited to this particular type of control of the dimensions of the housing 32. Thus, another example not shown is to take air at two different stages of the compressor and to control valves to modulate the flow rate of each of these samples to adjust the temperature of the mixture to be directed on the outer turbine casing 36. This variant can be reduced by equivalence to control based on a control signal between 0% and 100% and n is not described in detail. The control of the valve 44 by the control unit 46 is now described in more detail. The control unit 46 presents the hardware architecture of a computer and notably comprises a processor, a non-volatile memory, a volatile memory, etc. The non-volatile memory of the control unit 46 constitutes a recording medium, readable by the processor of the control unit 46 and on which is recorded a computer program comprising instructions for executing the steps of FIG. a control method according to the invention. In known manner, the control unit 46 obtains, by measurement or by calculation, the values of various quantities characterizing the operation of the aircraft and the turbojet engine 10, and in particular the following quantities: RATING, a signal created by the thrust management function of the control unit 46, described in more detail below, Mach, the mach number of the aircraft: this number, in known manner, is representative of the speed of the aircraft (brought back to the speed of sound), - ALT, the flight altitude of the aircraft, 20 - Ni, the low-pressure body of the turbojet engine, - N2, the high-pressure body of the turbojet, Ps3 , the pressure of the combustion chamber 22, TopDecel, a signal representing a detection of a deceleration intention of the turbojet, and - TopAcc, a signal representing a detection of an accelerator intention of the turbojet engine. The signal RATING is used by the thrust management function of the control unit 46, and is known to those skilled in the art.

It reflects here the different positions of the joystick (i.e. piloting) used by the pilot of the aircraft. In the example envisaged here, the signal RATING can take 16 discrete values numbered 0 to 15 reflecting different levels of thrust applied to the turbojet engine 10, from which can be deduced an operating speed of the turbojet engine 10 (eg Ni mode). Table I indicates, by way of illustration, the name of the RATING associated with the numbers 0 to 15, which corresponds here to the value (or the range of values) of the thrust setpoint applied to the turbojet engine for each RATING. The third column of Table I will be described below. Number Title (Setpoint Value Corresponding Thrust Selection) 0 MaxRev Transient 1 2 MinRev Transient 3 Between MinRev and Transient Idle 4 5 6 Transient Idle 7 Between Idle and MCL Cruise or Transient 8 MCL Cruise or Transient 9 Between MCL and MCT Cruise or Transient 10 MCT Cruise or Transient 11 NTO Takeoff 12 Between NTO and APR Takeoff 13 APR Takeoff Table I In this table I: - MaxRev is the maximum thrust available in reverse (reverse thrust) mode; - MinRev is the intermediate thrust available in reverse mode (thrust reversal) (the minimum thrust corresponding to the idle speed); - MCL (for Maximum CLimb rating) means the maximum recommended thrust for climbing phases; Maximum Cruise Rating (MCR) is the maximum recommended thrust for cruising; - NTO (for Normal Take Off) designates the thrust available at takeoff, during normal operation of the turbojet engine; - APR (Automatic Power Reserve) means the maximum thrust certified for takeoff in case of failure of the other turbojet engine of the aircraft; and MCT (Maximum ConTinous rating) means the maximum thrust available for a continuous operation of the turbojet engine, intended initially for emergency situations.

The TopAcc and TopDec signals respectively reflect an intention of the turbojet to accelerate or decelerate. They become active here when the setpoint of the operating mode Ni of the turbojet engine 10 (from the position of the joystick and RATING signal) is sufficiently far from the current Ni regime. These signals reflect the fact that the turbojet engine will experience a transient sufficiently large to change the type of control (ie one goes from a regulation in Ni mode to a regulation in N2 regime until approaching the setpoint in Ni mode, then a regulation is taken in Ni mode). The signal TopDecel is, for example, a binary indicator, determined as a function of the operating speed Ni of the turbojet engine 10. During a sudden decrease in the speed Ni, TopDecel takes the value "1" representative of a deceleration. Similarly, the TopAcc signal is for example a binary indicator, determined mainly as a function of the Ni regime of 14 Between MCT and APR 15 Between MCT and NTO Takeoff Takeoff operation of the turbojet engine 10. During a sharp increase in the Ni rate, TopAcc takes the value "1" representative of an acceleration. The control unit 46 implements a control module 50, shown in FIG. 3, in which a control signal SC50 of the valve 44 is determined. The control module 50, which is described in the form of functional modules 51, 52, 53 and 54, for example corresponds to the execution of the aforementioned computer program by the control unit 46.

More specifically, the control module 50 comprises a take-off management module 51, a transient management module 52, a cruise management module 53, and a selection module 54. The take-off management module 51 determines and provides a control signal for transient SC51 and a selection indicator for takeoff TOP51, according to the signal RATING. The purpose of the take-off management module 51 is to reduce the opening of the game 38 during the take-off of the aircraft in order to limit the overtaking (or overshoot) of the limit set for the temperature of the exhaust gases. (EGT Exhaust Gas Temperature) at the outlet of the high-pressure turbine 24 of the turbojet engine 10 (this limit may be greater than the temperature limit certified by the manufacturer of the turbojet engine 10 (or also known as the "EGT redline") ). The take-off management module 51 is described in more detail below with reference to FIG. 4. The transient management module 52 determines and provides a transient control signal SC52 and a transient selection indicator TOP52, depending on TopDecel, TopAcc, Ps3 and Mach signals. The function of the transient management module 52 is to avoid contact between the vanes 30 and the ring 34 during regime transients. It is described in more detail below with reference to FIGS. 5 and 6. The cruise management module 53 determines and provides a cruise command signal SC53 and a cruise selection flag TOP53, as a function of the RATING signals, Mach. , ALT and Ni. The cruise management module 53 has the function of limiting the clearance 38 during cruising in order to improve the performance of the turbojet engine 10. It is described in more detail below with reference to FIGS. 7 to 9. The selection module 54 selects one of the control signals SC51, SC52 and SC53 respectively provided by the management modules 52, 53 and 54, as a function of the operating speed of the turbojet engine 10, and more specifically in the embodiment described here, as a function of the RATING signals. , TOP51, TOP52 and TOP53. This module is described in more detail below, with reference to FIG. 10. The selected signal is noted SC50. The control unit 46 then controls the opening of the valve 44 as a function of the control signal SC50 thus determined. FIG. 4 represents the management module for take-off 51. This module comprises a comparator 60, a counter 61, and a selector 62.

The comparator 60 compares the RATING signal with the NTO value (see line 11 of Table I) and provides a binary signal S60 representative of a take-off detection. More precisely, if the RATING is greater than or equal to NTO (lines 11 to 15 of Table I), S60 takes the value "1" representative of a detected take-off. If not, S60 takes the opposite "0" value. The counter 61 serves to limit the opening time of the valve 44 during takeoff, by providing a binary signal S61 representative of a take-off detection for a limited time. Thus, when S60 takes the value "1" representative of a detected take-off, the counter 61 provides a signal S61 whose value "1" is also representative of a detected take-off, and begins to count a duration D. When the D duration reaches a predetermined value Dmax (first duration within the meaning of the invention), the counter 61 provides a signal S61 of "0" opposite, even if the signal S60 always indicates a detected takeoff. Of course, when S60 does not indicate a detected take-off (S60 = 0), S61 either (S61 = 0). The selector 62 selects a value between two predefined values, namely a predefined value VFULL and a predefined value VLow, according to the signal S61. The selected value (first value in the sense of the invention) is provided as control signal SC51.

More precisely, when S61 indicates a detected take-off (S61 = 1), the selector 62 selects for the control signal SC51 the value VFuLL corresponding to an opening of the valve 44 at 100%. When S61 does not indicate a detected take-off (S61 = 0), the selector 62 selects for the control signal SC51 the value VLow corresponding to an opening of the valve 44 at full hot flow, that is to say at 0%. The aforementioned S60 value is used by the take-off management module 51 to provide the above-mentioned TOP51 take-off selection indicator.

This indicator TOP51 is a bit indicator which specifies (in particular for the selection module 54) whether the control signal SC51 supplied by the selector 62 can be selected or not as the control signal of the valve 44: thus, T0P51 = 1 means that SC51 can be selected.

In the embodiment described here, T0P51 = S60, in other words, the control signal SC51 supplied by the selector 62 is associated with the take-off phase of the turbojet engine 10. The operation of the take-off management module 51 is therefore as follows. When a take-off is detected (SC60 = 1), SC51 is equal to VFULL during the duration Dmax, and TOP51 indicates that SC51 can be selected (TOP51 = 1). Thus, the opening of the valve 44 at 100% is controlled, which limits the play during this phase. After the expiration of the duration Dmax, the opening of the valve 44 is reduced (S61 = 0), which protects the mechanism of the valve 44.

FIG. 5 represents the transient management module 52. This module comprises logic gates 70 and 71, a comparator 72, a counter 73, a conversion module 74, and a selector 76. In a known manner, when the turbojet accelerates or decelerates in the transient phase, the temperatures of the parts of this turbojet engine 30 change. In order to avoid any contact between the blades and the turbine ring during these transient phases, the valve 44 is placed, in accordance with the invention, in the retracted position by the control signal SC52 during these phases for a sufficient period of time. in order for the clearance 38 to stabilize (this time being able to be determined experimentally, as described later), this folding position being defined so as to allow the injection of hot air on the outer casing 36 in order to expand it to the maximum. More specifically, in the embodiment described here, the selector 76 provides a control signal SC52 equal to a value (second value in the sense of the invention) selected from two predefined values Vo and V1, corresponding to the positioning of the valve 44. in two distinct predefined withdrawal positions These values are defined so as to overcome a hot air flow temperature insufficient to expand the outer casing 36 and thus increase the clearance 38. For example, the value Vo corresponds to a valve 44 in the 35% fallback position (full hot flow) and the value V1 to a valve 44 in the 4% fallback position (low hot flow rate). Different criteria can be applied to select one of the values Vo and V1.

Thus, in a variant, the selector 76 is configured to select the value Vo for a predetermined time elapsed since the start of the turbojet engine 10 (experimentally determined duration), then select the value V1. The inventors have found that this selection criterion allows a good reproduction of the behavior of closed-loop control of the prior art, during the transient phases. In another variant, the selector 76 is configured to select Vo or V1 depending on the temperature Tcase of the casing 36 and the temperature T3 of the air of the primary flow.

More specifically, if Tcase> T3, it means that the air injected at the temperature T3 is colder than the casing 36. In this case, the casing 36 no longer expands and the valve 44 must be placed in the 4% position. In the opposite case, that is to say when Tcase <T3, the air injected at temperature T3 is hotter than the casing 36. Thus, the casing 36 expands. We can keep the valve 44 in the 35% position. The TopAcc and TopDecel signals are combined by the logic gate 70 which is an OR gate. The comparator 72 compares the Mach of the aircraft with a predetermined threshold Machmin (for example Machrnin = 0.3), and delivers a 1 if the Mach of the aircraft is greater than the threshold Machmjn and a 0 otherwise the outputs of the logic gate 70 and comparator 72 are combined by a logic gate 71 which is also an OR gate. The logic gate 71 supplies the counter 73 with a signal S71 which represents a transient detection, the comparator 72 making it possible to avoid such detection on the ground (i.e. the signal S71 is equal to 1 only if the aircraft is in flight). The counter 73 supplies, during a duration d, the transient selection indicator TOP52 of value "1" when the signal S71 corresponds to a transient detection (S71 = 1), so as to keep the valve 44 in the position of decline selected by the selector 76 during this period d (second duration within the meaning of the invention). In the embodiment described here, this duration depends on the pressure Ps3: more precisely, it is taken equal to 3 times the time constant of the rotor of the high-pressure turbine, which itself depends on the pressure Ps3. Indeed, the rotor being the part of the turbine which has the greatest inertia, it can be considered that if this piece has reached (converged to) its normal operating mode, then the turbine game 38 has also stabilized . The duration d is determined here by the conversion module 74 which implements a conversion law indicating a value of the fallback time (expressed in seconds) as a function of the value of the pressure Ps3 (expressed in bar), and of which an example is illustrated in Figure 6. It should be noted that if necessary, this value d determined from the conversion law can be weighted by the magnitude of the acceleration or deceleration of the turbojet engine 10 during the phase transient considered (quantified by the difference between the Ni regime and the Ni regimen at the moment of transient detection (lifting of S71)). Thus, the operation of the transient management module 52 is as follows. When the engine 10 accelerates or decelerates, the temperatures of the parts change. In order to avoid any contact between the blades 30 and the ring 34 during the transient phases, the valve 44 is placed in the folded position (SC52 equal to Vo or V1). Thus, hot air is injected on the outer casing of turbine 36 and the latter expands.

The period of time during which the valve 44 is placed in the retracted position is chosen to allow stabilization of the clearance 38 to be achieved. Here, the retraction time d corresponds to three times the rotor time constant which depends on Ps3, for the reasons mentioned above.

FIG. 7 represents the cruise management module 53. This module comprises a conversion module 80, a multiplier 81, a conversion module 82, an adder 83, comparators 84, 85 and 86 and a logic gate 87. The conversion factor 80 determines a gate opening signal Sgo, as a function of the Mach and altitude ALT. More precisely, the higher the altitude, the higher the value of Sgo corresponds to a valve 44 open. From 30000 feet, the value of Sgo corresponds to a valve 44 open to 100%. The graph of FIG. 8 represents the conversion law implemented by the conversion module 80. This conversion law can be developed by simulating the behavior of a valve controlled by a closed loop of the prior art, for different altitudes and Mach. The conversion module 82 determines a coefficient 582 as a function of the speed Ni. More precisely, below a certain regime, the coefficient S82 is zero. Above a certain regime, the coefficient S82 is equal to 1, and varies linearly between these two levels. An example of conversion law implemented by the conversion module 82 is represented by the graph of FIG. 9. The multiplier 81 multiplies the valve opening signal S80 by the coefficient S82. This ensures the mechanical protection of the valve if the engine idle cruise. Indeed, when the Ni speed is low, it means that the engine is idle and must be a game reserve to anticipate the sudden expansion of the parts during a possible acceleration. Thus, for a low speed (less than 4000 rev / min in the case of FIG. 9), Sgo is canceled and the valve 44 is closed, which makes it possible to increase the clearance 38. The adder 83 adds a minimum value Vm ,, at the output of the multiplier 81, to ensure a minimum opening of the valve 44 in the cruise phase. The output of the adder 83 is the cruise control signal SC53 provided by the management module 53. The comparators 84 and 85 determine whether the RATING is between the values 7 and 10 of Table I, which are the RATING normally used in the cruise phase. In addition, the comparator 86 determines whether the Mach is greater than a MachmaxTo threshold, which ensures that the cruise function is not activated on the ground. The outputs of the comparators 84 to 86 are combined by the logic gate 87 which is an OR gate to provide the TOP53 cruise selection flag. FIG. 10 represents the selection module 54. This module 10 comprises a module 90 and a selector 91. In the embodiment shown, it also comprises a comparator 92 and a logic gate 93, which are however optional. The module 90 determines a selection signal Sgo, as a function of the indicators TOP51, TOP52 (or TOP52 combined with the output of the comparator 92 in the variant represented) and TOP53. These indicators are representative of the operating regime of the turbojet and in particular the current flight phase of the aircraft. The selection signal Sgo indicates to the selector 91 which control signal it should select according to the following Table II: TOP52 TOP52 TOP53 S90 0 0 0 SC52 (transient) 0 0 1 SC53 (cruise) 0 1 Indifferent SC52 (transient) 1 Indifferent Indifferent SC51 (takeoff) Table II Thus, in the control module 50, each of the management modules 51, 52 and 53 determines a control signal that is suitable for a particular situation (takeoff, transient, cruise), and an indicator selection to select which of the control signals is most appropriate given the current operating regime of the turbojet engine. By simulating the behavior of a valve controlled by the control module 50 during a typical mission of an aircraft, the inventors have found that the combination of the management modules 51, 52 and 53 makes it possible to cover all the situations encountered. Thus, a regulation loop based on the estimate of the clearance 38 is not necessary. Indeed, FIG. 11 is a graph which makes it possible to compare the closed-loop regulation of the prior art, and the open-loop control of the control module 50. The curves 100 and 101 respectively represent the set point and the estimated real game of the closed-loop control, and the curve 102 represents the simulated game of the open-loop control. In FIG. 11, the ordinate axis represents the clearance 3 in mm and the abscissa axis represents the time t in seconds, during a typical mission of an aircraft. This mission includes a P1 ground idle phase, a take-off and climb P2 phase, a cruise P3 phase, a P4 descent and landing phase at 20000 feet and a P5 descent and landing phase. Comparing the curves 101 and 102, it can be seen that even if the open loop of the control module 50 does not reproduce exactly the same behavior as the closed loop, the resulting clearance satisfies the needs of the regulation. More precisely, if we examine each phase: 20 - Phase P1 (Slow motion on the ground): In the closed loop, the valve is positioned at 35% then at 4% after a certain duration. In the open loop, the take-off management module 51 is activated. Indeed, the comparator 60 finds that RATING> = NTO and TOP51 25 is therefore equal to 1 (in the absence of manual deactivation). The control signal SC51 is therefore selected. In the case of the graph of FIG. 11, 5C51 always corresponds to an opening of 35% and the behavior of the closed loop is thus partially reproduced. In the case described with reference to FIG. 4, the selection of the VLow value after the duration Dma makes it possible to reproduce more closely the behavior of the closed loop. - Phase P2 (take-off and climb): During take-off, in the closed loop, the valve is closed after a certain time interval. In the open loop, the transient management module 52 is activated. Indeed, the comparator 60 finds that RATING> NTO and TOP51 is therefore equal to 0, while TopAcc is activated and TOP52 is therefore equal to 1. The control signal SC52 is therefore selected. As previously explained, SC52 goes from the value Vo to the value V1 plus closed after a certain time, which makes it possible to reproduce the behavior of the closed loop. During the climb, the closed loop further closes the valve in order to respect the game instruction provided to it. In the open loop, the cruise management module 53 is activated and the control signal SC53, which depends in particular on the altitude, is selected.

Thus, the valve 44 is reduced to 35% and then reopened gradually as the aircraft is gaining altitude. It should be noted that the evolution of the game is safer in the case of the open loop. - Phase P3 (Cruise): The two control loops place the valve in the 100% position (full cold flow). In the open loop, the cruise control module 53 is activated. - Phase P4 (Descent and landing at 20000 feet): The plane descends for a landing at 20000 feet. In the first part of this phase P4, the engine is at idle speed. After half an hour of slow motion, the engine is pushed to the maximum. During the first part, the closed loop detects an increase in the setpoint and places the valve in the low heat flowback position. The game then opens quickly to follow the instructions. In the case of the open loop, the transient management module 52 is activated, the valve is placed in the full hot flowback position and the game also opens. In the second part of this phase P4, the engine is pushed to RATING MCT and full throttle. The closed-loop control detects a drop in the set point and opens the valve to 100% to follow this instruction. In the open-loop control, the cruise management module 53 is activated and the valve is opened in the intermediate position (45%). - Phase P5 (Descent and Landing): During this phase, the engine idles, the plane descends and lands. The two regulations (closed loop and open loop) place the valve in the closed position.

It can thus be seen that the open-loop control implemented by the control module 50 meets the main needs in the takeoff phases and in the stabilized cruising phases. During transients, the closed loop is more responsive but open-loop regulation remains satisfactory. The resources used by the control unit 46 to implement the control module 50, in particular the calculation time, can be reduced compared with the implementation of a closed-loop control. For example, the calculation time can be reduced in a ratio 1 to 9. In addition, the number of parameters used being reduced, the development of the control module 50 is simplified. Finally, at valve 44, the simplification of the control can lead to a longer service life.

Claims (10)

REVENDICATIONS1. Control unit (46) for controlling a clearance (38) between blade tips (30) of a turbine rotor of a gas turbine engine engine and a turbine ring (34) a housing (32) surrounding the blades, the control unit (46) comprising a control module (50) configured to determine a control signal (5C50) of at least one valve (44) for acting on a flow rate and / or an air temperature directed to the housing (32), said control unit being characterized in that said control module (50) comprises: - a take-off management module (51), configured to provide a first control signal (SC51) associated with a take-off phase of the aircraft, and equal for a first predetermined duration (Dmax) following engine start, to a first predefined value (VFuLJ corresponding to a decrease in the clearance (38 ), a transient management module (52) configured to provide a second associated control signal (5C52) a transient operating phase of the engine and equal to a second value corresponding to the placement of the valve (44), during a determined second duration (d), in a determined retracted position, - a cruise management module (53) configured to provide a third control signal (SC53) associated with a cruising phase of the engine, from a third predefined value adjusted for at least one altitude (ALT) and a speed (Mach) of the aircraft, and - a selection module (54) configured to select said control signal (SC50) from the first, second and third control signals (SC51, 5C52, SC53) as a function of a speed (Ni) of engine operation. 30 The control unit (46) of claim 1, wherein the take-off management module (51) is further configured to provide after the first predetermined duration (Dmax), as a first control signal (SC51), a fourth predefined value 35 (VLow) corresponding to an increase of said set (38). The control unit (46) according to claim 1 or 2, wherein the transient management module (52) is configured to select the second one of two predefined values (Vo, V1) according to a crankcase temperature. and a flow temperature. The control unit (46) of claim 1 or 2, wherein the transient management module (52) is configured to select the second one of two predefined values (Vo, V1) based on a time elapsed since starting the engine. The control unit (46) according to one of claims 1 to 4, wherein the cruise management module (53) is configured to determine the third control signal (SC53) according to a regime (Ni). engine operation. 6. Control unit (46) according to one of claims 1 to 5, wherein the selection module (54) is configured to select the first control signal (SC51) when a management (RATING) parameter of a thrust applied to the engine corresponds to a situation of idling on the ground. The control unit (46) of claim 6, wherein the selection module (54) is further configured to select the second control signal (SC52) when said thrust management (RATING) parameter does not match. a ground idling situation and a speed acceleration or deceleration is detected. The control unit (46) of claim 7, wherein the selection module (54) is further configured to select the third control signal (SC53) when said thrust management (RATING) parameter does not match. ground idle and no acceleration or deceleration is detected. 9. A gas turbine engine engine comprising a control unit (46) according to one of claims 1 to 8 and at least one valve for acting on a flow rate and / or a temperature of air directed towards the housing ( 32), wherein said valve (44) is controlled according to said control signal (SC50). A method of controlling a clearance (38) between blade tips (30) of a turbine rotor of a gas turbine engine engine and a turbine ring (34) of a crankcase (32) surrounding the blades, the method being implemented by a control unit (46) and comprising determining a control signal (SC50) of at least one valve (44) to act on a flow rate and / or an air temperature directed towards the housing (32), this method being characterized in that it comprises, for determining the control signal (SC50): a step of supplying a first control signal (5C51) associated with a take-off phase of the aircraft, and equal for a first predetermined duration (Dmax) following engine start, to a first predefined value (VFuLL) corresponding to a reduction of the clearance (38), - a supply step a second control signal (SC52) associated with a transient phase of engine operation, and equal to a second value r corresponding to the placement of the valve (44), for a second determined duration (d), in a determined fallback position, and - a step of supplying a third control signal (SC53) associated with a cruise phase of the from a third predefined value adjusted according to at least one altitude (ALT) and a speed (Mach) of the aircraft, the control signal (SC50) determined by the control unit being selected from the first, the second and the third control signals (SC51, SC52, SC53) as a function of the operating speed of the motor.