Apparatus for Decoupling a Torque Loop From a Speed Loop in a Power Management System for Turboprop Engines

The present invention relates in general to power management systems for turbine engines and in particular to a torque loop decoupler for turboprop engines.

Historically, turboprop engines have been controlled via two levers in the cockpit. A speed lever adjusts engine speed and a power lever controls engine torque. First, the pilot would adjust the speed lever to obtain the desired engine speed setting. Then, the pilot would adjust engine torque, which is a function of power lever angle. The pilot would "close the loop" on torque by watching a gauge and moving the power lever angle to the desired torque level.

In turboprop engines, the torque and propeller shaft speed loops are coupled very closely. The torque loop adjusts fuel flow or gas generator speed in an attempt to control engine torque. As fuel flow is being adjusted, the engine power-turbine speed is affected. The power-turbine speed governor regulates propeller blade angle to control power-turbine speed. A change in blade angle directly affects engine torque, thereby affecting the torque control loop.

Among the problems associated with controlling the speed and torque loops simultaneously is that the torque and speed loops react simultaneously to sudden changes in engine speed. For example, an external disturbance such as wind shear can cause a small change in engine speed. The propeller governor speed loop immediately reacts to the external disturbance by adjusting propeller blade angle, and the torque loop immediately reacts by changing fuel flow to compensate for the resulting change in torque. Because speed and torque are controlled together, adverse control-action results.

Therefore, it is an object of the present invention to decouple the torque loop from the speed loop in a power management system for turboprop engines.

The present invention relates to an apparatus which that decouples the torque loop from the speed loop to allow only the speed loop to react to disturbances in engine speed. The apparatus comprises delta speed means, compensating means and modifying means. The delta speed means provides a signal indicative of small, rapid changes in engine speed. In response to the delta speed means, the compensating means generates a compensation signal that varies as a function of the small, rapid changes in engine speed. The modifying means utilizes the compensation signal to a control signal, to which the torque loop. By virtue of the compensation signal, the torque loop does not compensate for small, rapid changes in engine speed.

FIG. 1 is a pictorial view of an aircraft that has twin turbo prop engines;

FIG. 2 is a block diagram of a power management system employing a torque loop decoupler according to the present invention;

FIG. 3 is a logic diagram of a torque loop decoupler according to the present invention; and

FIG. 4 is a block diagram of another of a power management system employing a torque loop decoupler according to the present invention fuel flow to a turboprop engine.

Referring to FIG. 1, there is shown an aircraft 10 having twin turbo prop engines 12. Each engine 12 has a turbine-type power plant that includes a compressor section (not shown), combustion section (not shown) and a turbine section (not shown). These sections are arranged in serial flow relation. A spinner 14 is connected to the aft end of each engine 12. A plurality of propeller blades 16 are circumferentially disposed about the spinner 14 and extend radially therefrom. Air that enters the engines 12 is compressed in the compressor section. The compressed air is discharged to the combustion section, where the air is mixed with fuel. The mixture is ignited to produce hot expanding gases that turn the turbine section, which, in turn, drives the propeller blades 16. The propeller blades 16 move a mass of cold air to create a thrust. The thrust produced by the propeller blades 16 is varied by changing their pitch.

Referring now to FIG. 2, a torque loop decoupler 18 is employed by a power management system 20 such as the one disclosed in applicants' copending application entitled "POWER MANAGEMENT SYSTEM FOR TURBINE ENGINES", U.S. Ser. No. 07/762,322 filed concurrently herewith. This application is incorporated herein by reference.

This power management system 20 simultaneously controls engine speed and engine torque. It includes propeller governor control logic 22, which provides speed commands NPSFLT to a propeller governor. Through a closed speed loop, the propeller governor changes blade pitch to set engine speed to the commanded speed.

The power management system 20 also includes a torque scheduler 24, which provides torque commands QPLAPT. Torque commands QPLAPT are expressed as a percentage of maximum rated power of the engine.

The torque loop decoupler 18 provides a signal indicating decoupled torque QDC that allows the torque loop to be decoupled from the speed loop and thereby allow only the speed loop to react to sudden changes in engine speed. The signal for decoupled torque QDC is indicative of turbine torque compensated for sudden changes in engine speed.

Through a closed torque loop, the torque scheduler 24 controls fuel flow to set engine power to the commanded power. The difference between the torque command QPLAPT and the signal indicating decoupled torque QDC is taken by a subtracter 26, whose output provides an error signal QERR. This error signal QERR is converted to a ratio of fuel flow to compressor discharge pressure (Wf/PCD) by fuel schedules 28. The ratio (Wf/PCD) is supplied to a hydromechanical metering unit (HMU). Compressor discharge pressure (PCD) is provided by the engine 12. Based upon this ratio (Wf/PCD), the hydromechanical metering unit meters the appropriate fuel flow to the turboprop engine 12.

Referring now to FIG. 3, the torque loop decoupler 18 includes a control block 29, which controls engine torque to the turbine torque. The turbine torque Q is derived from the equation

where term QP is indicative of the torque measured at the propeller, the term KJPT is the inertia of the propeller, and the term NPWS is the derivative of propeller speed with respect to time. Torque can be measured at the propeller of the engine 12 by such well known means as phase-shift torque sensing. Thus, the signal indicating measured torque QP comprises a torque Q contributed by the propeller of the engine 12. A signal indicating the derivative NPWS is calculated by a differentiator 30 as a function of propeller speed NP. Propeller speed NP can be measured by such well known means as speed monopoles. To derive the signal indicating turbine torque Q, a first multiplier 32 multiplies a signal indicating the propeller inertia term KJPT by a signal indicating the first derivative NPWS. The inertia term KJPT is an apriori value that is stored in electronic memory 33. The output of the first multiplier 32, which is a signal indicating torque contributed by the propeller, is added to the signal indicating measured torque QP by a first adder 34. An output of the first adder 34 provides the signal indicating turbine torque Q.

A decoupler block 35 provides a compensation signal QCMP, which compensates for changes in turbine torque Q due to small, rapid changes in engine speed. The compensation signal QCMP is derived from the partial derivative DLPQNP of turbine torque with respect to propeller speed times the change in propeller speed. The partial derivative DLPQNP is a function of gas generator speed NGC. Each engine has its own curve of the partial derivative DLPQNP versus gas generator speed. For a particular engine, the curve is derived from engine tests and programmed into a function generator 36. A one-dimensional function generator is well known to those skilled in the art. During flight of the aircraft 10, a signal indicating gas generator speed NGC is supplied to one input of the function generator 36. In response to this signal, the function generator 36 outputs a signal indicating the partial derivative PLPQNP. The signal indicating partial derivative PLPQNP is adjusted for temperature and altitude by a second multiplier 38, which multiplies the signal indicating partial derivative PLPQNP by signals indicating the temperature RTHETA and the altitude DELTA. Temperature RTHETA is calculated as the ratio of ambient temperature (in degrees Rankine) to temperature for a standard day (519 degrees Rankine). Altitude DELTA is calculated as a ratio of ambient pressure (in psia) over pressure at sea level (14.696 psia). This adjustment is desirable because the characteristics of an engine change as a function of altitude and temperature. The output of the second multiplier 38 provides the signal indicating the partial derivative DLPQNP of torque with respect to propeller speed.

A subtracter 40 calculates a signal DELNP indicating change in propeller speed by taking the difference of the commanded speed NPSFLT and the signal indicating measured propeller speed NP. One output of the second subtracter 40 is supplied to a limited integrator 42, which generates a signal DNPTR indicating gradual changes in engine speed. Such changes can occur when the aircraft 10 slowly transitions from climb to cruise. The limited integrator 42 outputs the steady-state component of the signal DELNP indicating rapid changes in engine speed. Thus, the limited integrator 42 behaves as a low pass filter. Terms KTR and TRMLIM indicate gain and rate limit. The limited integrator 42 drives the signal DELNP to zero. An output of the limited integrator 42 is supplied to a subtrahend input of the subtracter 40, which removes large changes in speed from the signal DELNP. Thus, the signal DELNP only reflects only small, rapid changes in engine speed.

A third multiplier 44 multiplies together the signal DELNP and the signal indicating the partial derivative DLPQNP of torque with respect to speed. Thus, the output of the third multiplier 44 provides the compensation signal QCMP, which indicates the torque that results from small, rapid changes in engine speed.

A second adder 46 adds the compensation signal QCMP to the signal indicating turbine torque Q. An output signal of the fourth adder 46 is limited to a value QDCLIM, by an authority limiter 48. The output of the authority limiter 48 provides the signal indicating decoupled torque QDC. The compensation signal QCMP keeps the signal indicating decoupled torque QDC at a constant value for small, rapid changes in engine speed. When a disturbance causes an increase in engine speed, the speed loop causes the blade angle to be increased, thereby reducing engine speed and increasing engine torque. Since propeller speed NP is greater than the speed command NPSFLT for a short duration, the signal DELNP is negative. When signal QCMP is added to the signal indicating turbine torque Q, which is increasing, the net result at the decoupled torque QDC is zero. Thus, the torque loop is decoupled from the speed loop. When a disturbance causes an decrease in engine speed, the speed loop causes the blade angle to be decreased, thereby increasing engine speed and decreasing engine torque. Since propeller speed NP is less than the speed command NPSFLT for a short duration, the signal DELNP is positive. When signal QCMP is added to the signal indicating turbine torque Q, which is decreasing, the net result at the decoupled torque QDC is zero. Once again, the torque loop is decoupled from the speed loop.

When employed in the power management system 20 depicted in FIG. 2, the torque loop decoupler 18 is most conveniently realized through software. The power management system 20 is essentially a microprocessor. Thus, the torque loop decoupler 18 can programmed into the microprocessor. The step of programming can be readily accomplished by a person skilled in the art.

Alternately, the torque loop decoupler 18 can be realized by hardware. Adders, subtracters, limiters and filters (i.e., integrators) are well known to those skilled in the art.

The torque loop decoupler is not restricted to applicants' Power Management System. It can be employed by any power management system that controls speed and torque simultaneously.

Referring now to FIG. 4, a torque loop decoupler according to the present invention is employed by a power management system having a conventional torque loop 100. The torque loop decoupler is identical to the torque loop decoupler 18 shown in FIG. 3, except that the control block 29 and decoupler block 35 have been separated. Torque commands QCMD are provided by a torque scheduler 102. The control block 29 converts a signal indicating torque QP measured at the propeller to a signal indicating torque Q at the turbine. A subtracter 104 takes the difference between the signal indicating measured torque Q and the torque command QCMD to provide a signal DELQ. An adder 106 takes the sum of the signal DELQ and the compensation signal QCMP provided by the decoupler block 35. An output of the adder 106 provides an error signal QERR, which is converted to a fuel command by fuel schedules 108. The compensation signal QCMP keeps the error signal QERR at a constant value during small, rapid changes in engine speed. The torque scheduler 102 and fuel schedules 108 are well known to those skilled in the art. If it is desirable to control to torque QP measured at the propeller torque instead of turbine torque Q, the control block 29 can be eliminated and the signal indicating torque QP can be fed directly to the subtracter 104.

It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such modifications are intended to be included within the scope of the invention as defined in the appended claims.