Abstract:
A simplified air bleed balancing control system for a pair of aircraft gas turbine engines reduces the number of pressure transducers and differential pressure transducers. Advantages include lower weight, less expensive system, better total system MTBF (mean time before failure), acceptable differential pressure transducer drift identification and compensation by the digital controller, and fewer maintenance tasks.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       TECHNICAL FIELD 
       [0003]    The technology herein relates to aircraft engine airflow control, and more particularly to bleed airflow balancing control using simplified pressure sensing. Still more particularly, an exemplary illustrative non-limiting implementation provides bleed airflow balancing using a single bi-directional differential pressure transducer and a single pressure transducer. 
       BACKGROUND AND SUMMARY 
       [0004]    As shown in  FIG. 1 , gas turbine engines  10  of the type commonly found on many aircraft include a compressor  20 , a combuster  30  and a turbine  40 . The compressor  20  compresses air which is then mixed with fuel for combustor  30  to ignite. The combustor  30  exhausts gases turn the vanes of the turbine(s)  40 . Power from the rotating turbine  40  operates the compressor  20 . 
         [0005]    Turbine engine compressors  20  can be designed to supply more compressed air than is needed to operate the engine  10 . This additional compressed air from the compressor  20  can be used for tasks other than feeding the combustor  30 . For example, it is common to bleed some of the compressed air from the compressor  30  and route it to other equipment onboard the aircraft such as de-icers, cabin pressurization systems and the like. 
         [0006]    Each of the aircraft engines  10  can be used as a source for compressed bleed air. It is generally desirable to balance the amount of bleed air obtained from each pair of engines  10  to equalize wear and other engine operating conditions. Various techniques have been designed in the past to control such bleed air balancing. 
         [0007]    An exemplary prior art airflow balancing control  100  for a dual engine, dual bleed aircraft is shown in  FIG. 2 . In this prior art exemplary design, hot bleed air flow from the compressor  30  of a first engine  10   a  is regulated by a first pressure regulating shutoff valve  110   a , and hot bleed air flow from the compressor  20  of a second engine  10   b  is regulated by a second pressure regulating shutoff valve  110   b . These two streams of regulated hot bleed air are provided to precoolers  112   a ,  112   b  that receive cold air from the fan or prop (in the case of a turboprop) on the front of the engines  10   a ,  10   b . The amount of cold air is also regulated by valves  114   a ,  114   b . The outputs of precoolers  112   a ,  112   b  are provided to opposite input ports  116   a , 116   b  of a T-configuration bleed air manifold  118 . Upon entering the input ports  116   a ,  116   b , the bleed air flow encounters temperature sensors  108   a ,  108   b  respectively that measure the bleed air temperature. The two different bleed air streams are then passed through respective venturis  120   a ,  120   b  before being combined into a common air supply to aircraft systems available at a manifold output port  122 . Differential pressure transducers  104   a ,  104   b  placed across each venturi  120   a ,  120   b  measures the bleed air flow pressure differential across the venturi&#39;s throat. Pressure transducers  106   a ,  106   b  placed within the manifold  116  before the bleed air flow encounters the venturis  120  measures the absolute pressure of each engine&#39;s bleed air flow. 
         [0008]    It can be observed that digital controller  102  receives the signals provided by the two differential pressure transducers  104   a ,  104   b , the two pressure transducers  106   a ,  106   b  and the two temperature sensors  108   a ,  108   b . Digital controller  102  processes these signals to determine the mass air flow of each bleed air stream. Control law software operating on the controller  102  calculates currents that are delivered to modulate the PRSOV  1  (Pressure Regulating Shutoff Valve)  110   a  and PRSOV  2  (Pressure Regulating Shutoff Valve)  110   b  butterflies that independently control how much air to bleed from the compressors  20  of each of engines  10   a ,  10   b , respectively. In this way, the controller  102  can dynamically balance the mass air flow from the respective bleed air streams to ensure that each engine  10   a ,  10   b  contributes exactly half of the total bleed air pressure provided at manifold output port  112 . 
         [0009]    Generally speaking, the exemplary illustrative non-limiting controller  102  may implement a control law algorithmic process that processes these currents as follows. The pressure delivered to the aircraft systems should stabilize in the desired set point in acceptable settling time; the respective pressure overshoot and undershoot should also be acceptable and under steady state condition; each engine should contribute half of the total air bleed flow. 
         [0010]    While much work has been done in the past, further improvements are possible and desirable. In particular, it would be highly advantageous to simplify the air bleed balancing control system described above to reduce the number of pressure transducers, to reduce system weight and to optimize the bleed balancing control system. 
         [0011]    An exemplary illustrative non-limiting method of controlling bleed air flow may comprise measuring the differential pressure between a first bleed air flow and a second bleed air flow; generating a differential pressure correction signal in response to said measured differential pressure; measuring the pressure of a bleed air flow obtained by combining said first and second bleed air flows; and controlling valves modulating said first and second bleed air flows based at least in part on said measured pressure and said differential pressure correction signal. 
         [0012]    The method may further include measuring the temperature of said first bleed air flow and measuring the temperature of said second bleed air flow, and said controlling includes modulating said first and second bleed air flows in response to said measured temperatures. The differential pressure measuring may comprise measuring the differential pressure between the throat of a first venturi through which said first bleed air flow passes, and the throat of a second venturi through which said second bleed air flow passed. The differential pressure measuring may comprise measuring the differential pressure between two different regions of a manifold used to combine said first and second bleed air flows. The method may further include deriving said first bleed air flow from a first gas turbine engine, and deriving said second bleed air flow from a second gas turbine engine. The generating may includes applying a proportional gain, an integration and a differentiation. The controlling may comprise controlling first and second pressure regulating shutoff valves. 
         [0013]    A dual engine bleed airflow regulator may comprise a single pressure transducer that measures the pressure of combined first bleed air flow from a first engine and a second bleed air flow from a second engine; a single bi-directional differential pressure transducer that measures the difference between the pressure of said first bleed air flow and said second bleed air flow; and a controller responsive to said single differential pressure transducer and said single pressure transducer, said controller generating a first control signal for modulating said first bleed air flow and generating a second control signal for modulating said second bleed air flow. 
         [0014]    Non-limiting exemplary illustrative features and advantages include:
       Reduced number of pressure transducers.   Reduced number of differential pressure transducers.   Lower weight.   Less expensive system.   Acceptable differential pressure transducer drift identification and compensation by the digital controller during the system power-up (when there is zero pressure)   Better total system MTBF (mean time before failure) and, consequently, it requires less maintenance tasks.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    These and other features and advantages will be better and more completely understood by referring to the following detailed description of exemplary non-limiting illustrative embodiments in conjunction with the drawings of which: 
           [0022]      FIG. 1  shows an exemplary illustrative non-limiting aircraft gas turbine engine; 
           [0023]      FIG. 2  is a block diagram of an exemplary illustrative non-limiting prior art dual bleed airflow balancing control; 
           [0024]      FIG. 3  is an exemplary illustrative non-limiting improved bleed mass flow balancing control employing a reduced number of sensors; 
           [0025]      FIG. 4  shows an exemplary illustrative non-limiting pressure regulating shutoff valve control law control law architecture that may be implemented inside a digital controller; and 
           [0026]      FIG. 5  shows exemplary illustrative non-limiting pressure shutoff valve control law architecture for a “venturiless” configuration. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    In the  FIG. 3  exemplary illustrative non-limiting implementation for a pair of engines  10   a ,  10   b , a digital controller  202  processes the information provided by a single differential bi-directional pressure transducer  204 , a single pressure transducer  206  and two temperature sensors  108   a ,  108   b . In this exemplary illustrative non-limiting implementation, temperature sensor  108   a  senses the temperature of the bleed air stream from engine  10   a , and temperature sensor  108   b  senses the temperature of the bleed air stream from engine  10   b  as discussed in connection with  FIG. 2 . However, in this exemplary illustrative non-limiting implementation, the differential pressure transducer  204  is no longer connected or disposed to monitor the pressure drop across either one of venturis  120   a ,  120   b . Instead, the bi-directional differential pressure transducer  204  measures the difference between (a) the pressure after engine  10   a &#39;s bleed air stream passes through the throat of venturi  120   a , and (b) the pressure after engine  10   b &#39;s bleed air stream passes through the throat of venture  120   b . Furthermore, pressure transducers  106   a ,  106   b  shown in  FIG. 2  can be eliminated, and a single pressure transducer  206  is now placed in the manifold output port  122  to sense the pressure of the combined bleed air flow from the pair of engines  10   a  and  10   b.    
         [0028]    In the exemplary illustrative non-limiting implementation, venturis  120   a ,  120   b  are preferably configured identically, so it is assumed that the pressure difference caused by passage through each venture  120   a ,  120   b  is almost or substantially the same. In other words, after the both venturis  120   a ,  120   b , it is assumed that the “T” duct output port  122  has symmetrical dimensions with respect to each of engine  10   a ,  10   b  bleed air flows. It is also assumed that both Fan Air Valve  114   a ,  114   b  control loops are adjusted to the same temperature set point. Therefore, after a transient period is over and the system reaches steady state, both side temperatures can be considered almost the same. 
         [0029]    An exemplary illustrative non-limiting pressure shutoff valve control law architecture or algorithm  300  that is implemented by digital controller  202  is shown in the  FIG. 4 . This exemplary illustrative non-limiting control law architecture  300  (implemented by software running on controller  202  in the illustrative non-limiting exemplary implementation) can be divided into two control loops  302 ,  304 . 
         [0030]    One control loop  302  is the pressure transducer loop. Pressure control loop  302  calculates the average current to modulate the both PRSOV valve butterflies  110   a ,  110   b  in order to reach the desired pressure set point in an acceptable settling time, considering also acceptable pressure overshoots and undershoots. In more detail, the pressure sense output signal of pressure transducer  206  is combined with a pressure set point and is then applied to three different subchannels. The first subchannel  302   a  provides a proportional gain. The second subchannel  302   b  provides a dead zone (hysteresis) based integration gain and discrete time integration. The third subchannel  302   c  provides a discrete gain that is differentiated using a discrete transfer function. These three processed subchannel outputs are available for combining according to a predetermined function. 
         [0031]    The second control loop  304  is the differential pressure transducer loop. This differential pressure control loop  304  is used to annul or correct for the differential pressure between the left and right sides. Its set point is assumed to be 0 PSID in the exemplary illustrative non-limiting implementation, so no set point value combine is needed. After an acceptable transient period, the pressure in each venturi throat  120   a ,  120   b  converges to the same value. At this moment, it can be considered that bleed airflow from both engines  10   a ,  10   b  are symmetrically balanced. The exemplary illustrative non-limiting implementation provides this processing using a proportional gain and an integrator gain that is integrated using a discrete time integration. 
         [0032]    In the exemplary illustrative non-limiting implementation, the outputs of control loops  302 ,  304  are combined and then applied to control each of valves  110   a ,  110   b . Clippers  305   a ,  305   b  may be used to prevent valves  110   a ,  110   b  from being overdriven. 
         [0033]    Considering that an acceptable transducer drift may be identified and effectively compensated by the digital controller  202  during the system power-up (when there is zero pressure) and also considering that the differential pressure transducer loop  304  tries continuously to annul the differential pressure between the both venturis,  120   a ,  120   b , by providing a correction signal to the pressure control loop  302  output, this differential pressure control loop  304  is immune to transducer gain degradation. In other words, the proposed configuration remains robust even as the system ages and the characteristics of differential pressure transducer  104   b  changes. The change in characteristics of differential pressure transducer  104   b  affects the bleed air flow of both engines  104   a ,  104   b  equally. The differential pressure transducer provides a “zero” adjustment in every A/C power up to compensate for any bias in the transducer, thus providing independence from drift. 
         [0034]    The risk of unstable interactions between the different PRSOVs  110   a ,  110   b  (which are designed to have identical characteristics but may not in fact have identical characteristics) is reduced because the pressure transducer loop  302  provides simultaneously the same average current for both valves. 
         [0035]    In a further exemplary illustrative non-limiting implementation, if it is possible to obtain a significant pressure drop in the “T” duct of manifold  116 , then both venturis  120   a ,  120   b  can be removed. In this case, the bi-directional differential pressure transducer  204  shall measure the differential pressure drop between the both sides of the “T” duct. Since the pressure drop increases when the flow increases (the pressure in the venturi throat has an inverted behavior), then the differential pressure signal provided by the differential transducer shall be inverted. An exemplary illustrative non-limiting modified control law architecture  300 ′ shown in the  FIG. 4  includes an additional inverter  304   a.    
         [0036]    While the technology herein has been described in connection with exemplary illustrative non-limiting embodiments, the invention is not to be limited by the disclosure. For example, the system can modified to provide bleed air flow balancing for a system providing more than two engines. The invention is intended to be defined by the claims and to cover all corresponding and equivalent arrangements whether or not specifically disclosed herein.