Compressor control system

(56) Documents cited

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F1C (/1/ Applicants

Lucas Industries Limited, Great King Street, Birmingham, England (72) Inventors (74) Agents

Marks Er Clerk (54) Compressor control system (57) A control system for a compressor having adjustable inlet guide vanes includes an adjustable, mass airflow gauge means (not shown, but integrated with a bypass valve 21) for monitoring the output of the compressor 16, actuator means 36 for adjusting the setting of the gauge means and the bypass valve'21 and the setting of the compressor inlet guide vanes 18 in use, sensor means 51 producing signals representative of the setting of the inlet guide vanes 18 and a control unit 47 which processes the signals from said sensor means 51 and output signals from said gauge means, and produces an output signal controlling operation of said actuator means 36, to adjust said gauge means and said inlet guide vanes 18 in accordance with the mass airflow output of the compressor 16.

Kenneth Theodore Guy, Rodney Stuart Howard, Kenneth George Standell, Robert James Smith

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GB 2 024 944 A

SPECIFICATION 65

Compressor control system

This invention relates to a control system for a compressor having adjustable inlet guide vanes, particularly but not exclusively a centrifugal γθ compressor intended to be run at constant speed to provide an air suppy for an aircraft.

It is known that a major concern in the running of a compressor is that the compressor should not be allowed to enter a surge condition. A surge 75 condition can arise in a compressor when the demand for air from the compressor is reduced below a critical value. It has been proposed to monitor the mass airflow of the output of a compressor using a venturi gauge, and to use the gg signal produced by the gauge to operate, when necessary, a dump valve for dumping all or part of the outlet of the compressor to atmosphere, or an inlet restrictor which reduces the flow of air entering the compressor, so as to prevent the 35 compressor running into a surge condition.

However control effected by dumping is both wasteful and potentially noisy.

A compressor having adjustable inlet guide vanes, can by appropriate adjustment of the inlet go guide vanes, be operated efficiently over a wider range of mass airflows than can a compressor without adjustable inlet guide vanes. However a venturi gauge as used in the prior proposal mentioned above is not sufficiently accurate over g5 the wider range of mass airflows to provide effective monitoring of the mass airflow in order to provide surge protection over the whole operating range, of the compressor, and it is an object of the present invention to provide a control system for a 1 qq compressor having adjustable inlet guide vanes, capable of minimising the risk of the compressor running into a surge condition throughout the range of mass airflow conditions in which the compressor can operate.

A control system fora compressor having adjustable inlet guide vanes, according to the invention includes an adjustable, mass airflow gauge means for monitoring the output of the compressor, actuator means for adjusting the 110 setting of the gauge means and the setting of the compressor inlet guide vanes in use, sensor means producing signals representative of the setting of the inlet guide vanes, and a control unit which processes the signals from said sensor means and 115 output signals from said gauge means, and produces an output signal controlling operation of said actuator means to adjust said gauge means and said inlet guide vanes in accordance with the mass airflow output of the compressor. 120

Preferably said adjustable, mass airflow gauge means comprises an orifice, venturi gauge, of fixed capacity and an adjustable by-pass valve, the valve permitting differing proportions of the output of the compressor to by-pass the throat of the 125 venturi gauge, the magnitude of the by-passed proportion of the compressor output being determined by the setting of the adjustable bypass valve so that the appropriate adjustment of the setting of the valve the mass airflow through the throat of the gauge can be maintained substantially constant and within the capacity of the gauge, said actuator means adjusting the setting of the by-pass valve.

Alternatively the adjustable, mass airflow gauge means comprises an orifice, or venturi gauge of adjustable capacity, said actuator means effecting an adjustment of the setting of the capacity control mechanism of the adjustable gauge.

Preferably said actuator means comprises a single actuator device, the movable output member of which adjusts both the by-pass valve and inlet guide vanes in use.

Desirably the system includes actuator means for a dump valve whereby a proportion or all of the output of the compressor can be dumped to atmosphere, the dump valve actuator means being operated when the mass airflow demand of the consumer system supplied in use by the compressor falls below that which can be controlled satisfactorily by adjustment of the inlet guide vanes of the compressor to prevent a surge condition arising.

Conveniently, said dump actuator means is said single actuator device.

Preferably said by-pass valve and said dump valve are incorporated in a single unit, and their moving valve members are parts of a common moving member.

Desirably said venturi gauge is also incorporated in said single unit.

One example of the invention is illustrated in the accompanying drawings, wherein:—

Figure 1 is a schematic representation of an auxiliary power unit for an aircraft, incorporating a compressor control system;

Figure 2 is a plan view of part of the control system shown in Figure 1 ;

Figure 3 is a sectional view on the line A—A in Figure 2;

Figure 4 is a sectional view on the line B—B in Figure 3;

Figure 5 is a scrap view in the direction of Arrow G in Figure 3;

Figure 6 is a sectional view on the line C—C in Figure 4;

Figure 7 is a scrap view in the direction of Arrow D in Figure 4;

Figure 8 is a sectional view on the line E—E in Figure 7; and

Figure 9 is a sectional view on the line F—F in Figure 3.

In modern aircraft there is a large demand for auxiliary services for example air conditioning and electrical power for lighting, heating and various actuators. These services may be required both while the aircraft is on the ground, or in flight, and are generally provided by an auxiliary power unit. Where the auxiliary power unit is to supply an aircraft on the ground then it may be mounted as a mobile installation, for example as part of a tractor and trailer unit. However on larger aircraft it is now becoming general practice to provide the

GB 2 023 944 A auxiliary power unit on the aircraft, so that it can power the auxiliary services on the ground and in flight. The auxiliary power unit is generally independent of the main propulsion units of the aircraft, and in a known form comprises a gas turbine engine driving an air compressor and an electrical generator. The compressor provides pressurized air for air conditioning of the aircraft and the generator provides electrical power for the aircraft services. Most large aircraft now use alternating current electrical systems which necessitates driving the alternator at a constant speed. In order to avoid the need to provide a constant speed drive coupling between the prime mover and the alternator it is therefore desirable to operate the prime mover, generally a gas turbine engine, at constant speed. However the air demand for the air conditioning of a large number of services on an aircraft is variable, and it is therefore essential to provide means to prevent gg compressors running into surge conditions which can result from a reduction in air demand.

Referring first to Figure 1 of the drawings, there is shown an auxiliary power unit for an aircraft, which can either be a ground based unit, or can be gg incorporated into the aircraft. The auxiliary power unit includes a prime mover 11 in the form of a gas turbine engine intended to be run at constant speed. The auxiliary power unit includes a prime mover 11 in the form of a gas turbine engine gg intended to be run at constant speed. The output shaft 12 of the prime mover drives an oil pump 14 and an electrical generator 15 by way of a gear ' box 13, and also drives a centrifugal air compressor 16 together with other accessories θθ (not shown). The air compressor 16 has an air intake 17 incorporating adjustable inlet guide vanes 18. The outlet 19 of the compressor carries the air output of the compressor and a drum valve 21 receives the output from the compressor and ^Qg directs it either to the aircraft services by way of a conduit 22, or to atmosphere by way of an exhaust conduit 20. As will be described in more detail hereinafter, the drum valve 21 is capable of diverting part of the output of the compressor to 11 o each of the conduits 22, 20. The generator 15 provides the electrical supply for the aircraft.

The drum valve 21 is shown in more detail in the remaining drawings, and comprises a casing 23 having an inlet passage 24 connected to the ·] ·] g outlet conduit 19 of the compressor 16, and a pair of outlet passages 25,26 connected respectively to the supply conduit 22 and the exhaust conduit 20. The inlet passage 24 of the drum valve 21 is partially obstructed by an internal flange 27 the inner periphery of which carries an integrally formed venturi throat 28. Additionally the flange 27 is formed with an arcuate aperture 29.

Journalled for rotation within the casing 23 is a 125 movable valve member 31 having formed therein a flow passage 32. The flow passage 32 is smoothly curved through 90° and communicates at one end with the venturi throat 28. Thus air flowing into the inlet passage 24 from the 130

120 compressor can enter the flow passage 32 of the valve member 31 by way of the venturi throat 28.

In addition, the movable valve member includes an apertured wall 33 positioned closely adjacent = the flange 27, the aperture 34 in the wall 33 communicating in certain angular positions of the member 31 with the aperture 29 so that part of &lt; the air flowing into the inlet passage 27 can bypass the throat 28 can enter the flow passage 32 by way of the apertures 29 and 34. The flow passage 32 of the member 31 serves to divert the airflow through 90° and the angular position of the member 31 within the casing 23 determines not only the extent of the opening of the by-pass valve defined by the aperture 29, 34 but also the route by which air leaves the casing 23, that is to say either through the supply conduit 25, the exhaust conduit 26 or a proportion through both conduits 25, 26. The open end of the supply passage of the member 31 is shown in Figure 4 at 35.

The movable valve member 31 can be moved angularly within the casing 23 by operation of a rotary actuator 36 conveniently an hydraulically powered actuator, the output shaft 37 of the actuator being coupled to the movable member 31 by way of a gear train 38 carried by the casing 23 or alternatively by some either mechanical drive mechanism for example a belt or chain.

Control of the hydraulic fluid supplied to the actuator is provided a servo valve 40 (Figure 1) which may conveniently be mounted on the casing 23. The servo valve itself may be supplied with hydraulic fluid from the lubricating system of the auxiliary power unit, or the fuel system of the auxiliary power unit if this proves more convenient in use. It is to be understood however that any other suitable source of hydraulic power can be utilised if desired. It will be recognised that if desired the actuator may be electrically powered.

The output shaft of the actuator 36 extends through the rotary actuator, and at its end remote from the gear train coupling the output shaft 37 to the valve member 31, the output shaft 37 is connected to a rotary cam 39.

The rotatry cam 39 is engaged by cam follower 41 and the movement of the cam follower 41 effected by rotation of the cam 39 acts through a convenient linkage 42 to impart rotary motion to a shaft 43 coupled in turn to the adjustment collar 44 of the inlet guide vane assembly 18 of the compressor 16. Thus operation of the rotary actuator 36 serves simultaneously to adjust the ·: position of the movable valve member 31 and the inlet guide vanes 18 of the compressor 16.

Extending through the casing 23 and the flange * 27, in that part of the flange not occupied by the aperture 29, is a first bore 45 which communicates with the orifice of the venturi throat 28. A second bore 46 extends through the wall of the casing 23 and communicates with the air inlet passage 24. The bores 45, 46 are connected to respective pressure transducers which provide an electrical signal representative of the air pressure in the throat 28 and the passage

GB 2 023 944 A respectively it being recognised that a comparison of the pressure at the orifice of the throat 28 and the passage 24 will be indicative of the mass air flow through the throat 28. The electrical output signals of the two pressure transducers are supplied to an electronic control unit 47 (Figure 1) by way of the line 48 shown dotted in Figure 1. Two further electrical signals are supplied to the electronic control unit 47, the two further signals being representative respectively ofthe position ofthe rotary valve member 31 and the adjustment collar 44 of the inlet guide vane assembly 18.

The signal representative ofthe position of the rotary valve member 31 relative to the casing 23 is derived from a rotary potentiometer 49 the rotating component of which is carried by the last gear wheel 38a in the gear train 38 which transmits rotary motion from the actuator 37 to the valve member 31. The signal representative of the angular position of the adjustment collar 44 and therefore representative ofthe operative position of the guide vane assembly 18 is derived from a rotary potentiometer 51 the rotating part of which is carried by the shaft 43. Thus the control unit 47 receives separate signals representative of the angular position of the drum valve movable member 31 and the operative position ofthe guide vanes of the guide vane assembly 18. It will be recognised that since the two are moved always in unison by the same actuator, then in theory a single sensor providing a signal could be utilised. However the connection ofthe rotary cam 39 to the output shaft 37 of the actuator 36 is by way of a shear pin so that in the event that the guide vane assembly 18 becomes jammed for some reason and cannot operate, then the actuator 36 can continue to operate the movable valve member 31, as permitted by shearing of the shear pin connecting the shaft 37 to the rotary cam 39. In such circumstances it is essential that the electronic control unit receives signals independently from the guide vane assembly and the movable valve member 31 so that appropriate remedial acation to accommodate failure ofthe guide vane assembly can be initiated.

The operation ofthe system is as follows. The compressor 16 will be operated at constant speed, and thus in stable conditions will be producing a constant mass air flow by way of its outlet conduit 19. Assuming that there is a heavy demand for air by the aircraft systems then the movable valve member 31 will be in a position wherein the whole of the output of the compressor is diverted into the conduit 22, and then additionally the position of the valve member 31 will be such that there is a large overlap between the aperture 34 and the aperture 29 so that a proportion of the flow from the compressor into the inlet passage 24 will by-pass the venturi throat 28. The venturi throat 28 can of course only accurately monitor a small range of mass air flow, whereas the system may be called upon to provide a much wider range of mass airflow. The arrangement is such that the by-pass valve by-passes a sufficient proportion of

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130 the flow from the compressor to ensure that the venturi throat 28 is operating within its most efficient range.

When the demand for air by the aircraft system is reduced, unless some compensating adjustment is made within the compressor the operating condition of the compressor will start to move towards a surge condition as a result ofthe restriction in the output. The change in mass airflow through the drum valve 21 will be sensed by the venturi throat 28 in conjunction with its two associated pressure transducers and thus there will be a change in the signals supplied to the control unit 47. The unit 47 processes the signals received and produces an output signal controlling the servo valve 40 which it will be recalled, in turn controls the actuator 36. The change in input signal to the control unit 47 causes an output signal to be produced which by way of the servo valve 40 initiates operation of the rotary actuator 36 to move the valve member 31 and the adjustment collar 44 of the inlet guide vane assembly 18. The direction and extent of the adjustment in the position of the adjustable guide vane assembly 18 is such that the inlet guide vanes modify the direction of air flow into the compressor in such a way as to move the surge point of the compressor away from the new operating point. That is to say the compressor operating characteristics are modified so as to move the surge region ofthe operating characteristic away from the new operating point. The compressor may now be operating less efficiently than previously but will have a suitable safety margin against the running into surge conditions. The adjustment in the position ofthe valve member 31 reduces the overlap between the aperture 34 and 29 so as to reduced the proportion of the output of the compressor which by-passes the venturi throat 28. The reduction is sufficient to restore the mass airflow through the throat 28 to the optimum which the throat 28 is intended to monitor. Unless the reduction in demand by the aircraft system has been large, then at this time the valve member 31 although now in a different angular position relative to the casing 23 is still diverting the whole of the flow through the valve 21 to the conduit 25.

Assuming now that the reduction in demand continues to fall progressively then a point will be reached at which further adjustment of the inlet guide vanes of the compressor cannot compensate for the reduced demand and the compressor operating characteristic will therefore once gain, as the demand continues to fall, start to move towards its surge condition. The magnitude of the mass airflow which gives rise to this condition may be approximately the optimum which can be monitored by the throat 28 and so the valve member 31 will have moved sufficiently far for there to be no overlap between the apertures 29, 34 so that none of the output of the compressor by-passes the throat 28. However, in a modification the mass airflow capacity of the throat 28 is lower such that some by-pass is still

GB 2 024 944 A required. This can be achieved by control of the overlapping of the apertures 29, 34 so that as one end of aperture 34 aligns with the end of the aperture 29 the other ends are clear leaving a bypass and/or the axial spacing between the wall 33 and the flange 27 can be arranged to provide a route for the by-pass flow.

In either case movement of the valve member 31 sufficiently far to achieve this situation opens communication between the flow passage 32 of the valve member 31 and the exhaust conduit 26 so that a proportion of the output of the compressor is dumped to atmosphere of by way of the exhaust conduit. Initially however communication still exists between the flow passage 32 and the conduit 25 so that the small demand by the aircraft systems is still being met, but the restriction in the output of the compressor is being released by a proportion of the output being dumped to atmosphere. Thus the tendency for the operating characteristic of the compressor to be moved further towards the surge condition is prevented. In the event that the demand by the aircraft system is zero, then the valve member 31 is moved by the actuator 36 under the control of the unit 47 to a position wherein the whole of the output of the compressor is diverted by the member 31 to the exhaust conduit 26.

It will be recognised that the lift of the cam 39 provides a variable ratio link in the drive system allowing correction for peculiarities of component characteristics and non-linearities of linkages. Furthermore, it allows the actuator to continue movement of the valve member 31 when the limit of movement of the guide-vane adjustment collar 44 has been reached. Such movement is necessary since it is required to progressively open the dump valve port after the limit of guide-vane adjustment has been reached.

It will be recognised that the operation of the system as the demand increases is the reverse of that described above, the change in mass airflow being sensed by the venturi throat 28 and its associated pressure transducers, and the signals applied to the control unit 47 causing the control unit 47 to produce an output signal in turn causing the actuator 36 to move the guide vanes 44 in a direction allowing the increased output of the compressor to be efficiently achieved and moving the member 31 in a direction to increase the overlap of the apertures 34 and 29 to increase the proportion of the now increased output of the compressor which by-passes the throat 28. It will be recognised that when the bypass of the throat 28 is in operation then the tendency always is to restore the mass flow through the throat 28 to a constant value.

It will be recalled that there is mentioned above a situation wherein the guide vane assembly 18 becomes jammed and thus cannot operate to modify the operating characteristics of the compressor. In such a situation there is immediately a discrepancy between the signal representative of the position of the guide vanes and the signal representative of the position of the

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125 valve member 31. In such a situation the electronic control unit immediately initiates movement of the valve member 31 to its extreme position in which the whole of the output of the compressor is dumped to atmosphere. This is a safety procedure in order to ensure that the operating characteristics of the compressor do not move rapidly into the surge condition. Moreover, it will be recognised that a further fault could develop in that the inlet guide vanes could become free of control, as a result of shearing of the connection between the shaft 37 and the cam 39. Again, the discrepancy between the two signals will immediately result in the member 31 being moved to its extreme position to dump the output of the compressor to atmosphere thus preventing a surge condition arising.

It is to be understood that many detailed changes may be made in the manner and mechanisms whereby the inlet guide vanes and the member 31 are operated. For example, the output signal of the control unit 47 could be used to operate a torque motor which in turn drives a selector valve controlling the flow of hydraulic fluid to the rotary actuator 36, the selector valve selecting the direction of movement required by . the valve member 31 and the inlet guide vanes. The actuator 36 would continue to be operated until the pressure signals derived from the pressure transducers associated with the throat 28 and the inlet passage 27 are restored to their values immediately prior to the change which initiated operation of the actuator 36 whereupon the torque motor would restore the selector valve to its original position ceasing operation of the rotary actuator 36. It will be recognised that a lost motion connection is provided in the form of a constant radius region of the cam 29 between the shaft 37 and the adjustment collar 44 of the inlet guide vane assembly in order, in the reduced demand condition, to permit the valve member 31 to continue to be moved after the inlet guide vanes have reached their limit position.

In an alternative construction the venturi gauge and by-pass valve are replaced by a venturi gauge of variable capacity, the capacity of the throat of the variable capacity gauge being controlled by a movable tapered plug member. The actuator 36 thus controls the position of the plug member relative to the throat.