Continuous flow fuel control system

A fuel control system for providing steady state fuel flow to a gas-turbine engine comprises a pulse-width modulated, solenoid-operated fuel metering valve, an engine speed transducer, and a remote microprocessor communicating with the valve and the speed transducer through an optical fiber. The valve is operated by a direct current signal which is pulsed at a frequency substantially higher than the natural frequency of the valve spring/plunger system, whereby the plunger remains axially displaced from the valve seat to provide continuous flow through the valve. The amount of axial plunger displacement and, hence, the fuel flow rate are prescribed by the pulse width of the signal directed through the valve.

BACKGROUND OF THE INVENTION 
The instant invention relates to a fuel control system providing steady 
state fuel flow to a gas turbine engine of a guided missile in direct 
proportion to engine fuel requirements. The system uses a pulse-width 
modulated, solenoid-operated fuel metering valve to provide a 
substantially linear fuel flow rate in response to control signals derived 
from engine parameters and communicated from a remote processor via fiber 
optic link. 
The prior art teaches the use of solenoid-operated valves wherein fluid 
flow through the valve is regulated by means of a magnetic plunger which 
is normally mechanically biased to seat in a valve body orifice and which 
is cyclically unseated therefrom by energizing a solenoid coil wrapped 
around the valve body. The resulting pulsating flow creates difficulties, 
in the form of stall or flameout, when used for supplying fuel to a gas 
turbine engine requiring a continuous supply of fuel. In U.S. Pat. No. 
4,015,426, Hobo attempts to mitigate the fuel flow discontinuities 
inherent to fuel systems employing such known pulsed solenoid-operated 
valves by supplying thereto pulsed control signals having a constant 
frequency but with a variable pulse width. However, the modification of 
the pulses as taught in Hobo does not, in and of itself, provide for 
continuous fuel flow, as the valve taught therein continues to operate in 
a pulsating manner; rather Hobo relies on the external mechanical damping 
provided by a elastic fuel line acting as a hydraulic accumulator in order 
to smooth the flow of fuel to the engine. Hobo further teaches the use of 
a second valve, 180 degrees out-of-phase with the first valve, to double 
the effective frequency of the fuel pulse to further mitigate fuel flow 
discontinuity. The use of multiple valves and external damping means 
nonetheless remain impractical given the space and cost constraints 
imposed by a guided missile system application. 
U.S. Pat. No. 3,523,676 to Barker teaches a fluid control valve wherein a 
solenoid-operated plunger is offset from the central axis of the valve so 
as to induce radial vibration when the plunger is cycled into contact with 
the valve seat. Such radial vibration is used to reduce the undesirable 
axial rebound inherent in valves which operate in the aforementioned 
cyclical fashion, whereby greater fuel flow accuracy is obtained. However, 
the reduction of axial rebound of the plunger correspondingly further 
defines each fuel pulse generated by the cyclically-operating valve, 
thereby increasing the fuel flow discontinuities experienced by the 
engine. 
Moreover, the Barker valve contemplates application in a stationary 
environment, such as a chemical processing plant, where the corrective 
vibratory action induced by the eccentric plunger is not defeated by 
external vibratory sources. Thus, the Barker valve would not be effective 
in an environment which itself is subject to severe vibratory action, such 
as within a launched missile. 
Thus, in short, systems known to the prior art incorporating solenoid based 
metering valves deliver fuel in a pulsating fashion and require external 
damping and/or internal vibratory control to stabilize fuel flow to an 
engine. Systems of this sort are impractical for use in sensitive fuel 
control applications such as that required for guided missiles in which 
space and cost considerations, as well as reliability of operation, are 
critical factors. 
SUMMARY OF THE INVENTION 
It is an object of the instant invention to provide a fuel control system 
for a gas turbine-powered guided missile, the control of which may be 
accomplished remotely. 
Another object of the instant invention is to provide a fuel control system 
for a guided missile which employs a solenoid-operated valve to meter fuel 
in a proportional, steady state fashion to the engine thereof, as 
determined by operating parameters. 
The fuel control system of the instant invention for metering fuel to the 
engine of a guided missile comprises a fuel metering valve on the missile 
having a tubular valve body, the interior surface of which defines a 
passage extending therethrough, and a magnetic plunger located within the 
passage. The interior surface of the valve body is provided with a 
radially-tapered portion defining the valve seat. The plunger is also 
provided with a radially-tapered end portion to facilitate fluid-tight 
engagement between the plunger and the valve seat. A valve spring 
mechanically biases the plunger towards the valve seat while an O-ring 
seated into an annular groove in the valve seat ensures the formation of a 
seal between the plunger and the valve seat when the plunger is maximally 
biased thereagainst. 
A solenoid coil is wrapped around the valve body which, when energized by a 
constant frequency pulsed direct current signal, produces a biasing 
magnetic field which axially displaces the plunger to allow fuel flow. The 
width of the signal pulses prescribes the axial displacement of the 
plunger relative to the valve seat and, hence, the instantaneous rate of 
fuel flow through the valve. Significantly, the frequency at which the 
signal pulses are directed through the solenoid coil is substantially 
higher than the natural frequency of the valve spring/plunger assembly, 
whereby a dynamic equilibrium is achieved within the valve so as to 
maintain the plunger in a substantially fixed position within the passage 
away from the valve seat. Started another way, the mechanical biasing 
provided by the spring in combination with the inertia of the plunger act 
to mechanically rectify the effect of the pulsating magnetic field 
generated by the solenoid coil. As a result, the valve operates in a 
manner similar to a servomechanism, with the plunger being displaced to 
and maintained in a position corresponding to the average power input 
represented by the signal pulses directed therethrough, whereby continuous 
fuel flow is achieved. The instant fuel control system thus obviates the 
need for the vibratory control and external damping required in prior art 
systems. 
Preferably, the optimal instantaneous pulse width is determined from engine 
parameters in a closed loop control system. In the preferred embodiment, a 
microprocessor generates the fuel flow commands which define the signal 
pulses to be directed through the solenoid coil while receiving critical 
operating data, such as engine speed, from one or more sensors on the 
missile to provide immediate corrective action. For example, where a speed 
sensor is employed to monitor the rotational speed of the engine, the 
microprocessor compares the output from the speed transducer with a speed 
set point and accordingly adjusts the width of the signal pulses directed 
through the solenoid coil. Instantaneous airframe speed and altitude, for 
example, may also be supplied to the microprocessor for use in adjusting 
the width of the signal pulsed directed through the solenoid coil. 
The preferred embodiment of the instant invention further provides for the 
remote positioning of the microprocessor relative to the missile, thereby 
obviating the need for providing an expendable microprocessor for each 
missile. Specifically, the ground-based microprocessor communicates with 
the missile-based speed transducer and solenoid coil via a fiber optic 
cable. The fiber optic link enables the transmission of the high frequency 
signal pulses required for controlling fuel delivery to the high-speed gas 
turbine engines utilized in guided missiles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION 
Referring to FIG. 1, an exemplary fuel control system 10 constructed in 
accordance with the instant invention regulates the flow of fuel 22 from a 
fuel tank 18 through fuel lines 12 and 14 to a gas turbine engine 16 
providing propulsion for a guided missile (not shown). The fuel tank 18 is 
held at constant pressure by compressor bleed air 20 from the engine 16 to 
assist fuel flow through the fuel lines 12 and 14 and a fuel metering 
valve 24. A speed transducer 26 generates an output 78 proportional to the 
rotational speed of the engine 16. The output 78 of the speed transducer 
26 is communicated to a ground-based microprocessor 28 via an optical 
fiber 30 and two signal converters 32 and 34. The microprocessor 28 
compares the output of the speed transducer 26 to a set point 72 provided 
by a missile operator and generates a fuel flow command 74 which, 
subsequent to conversion by signal converters 32 and 34, comprises a 
pulse-width modulated direct current signal 66 for use in controlling the 
valve 24, as described more fully below. In this manner, the speed 
transducer 26 provides feedback information to the microprocessor 28 
regarding engine speed to facilitate precise control thereof. 
More specifically, FIG. 2 shows the fuel metering valve 24 of the instant 
fuel control system 10 as having a tubular valve body 31 with an input 
port 33 at one end thereof communicating with the fuel tank 18 via fuel 
line 12 and an output port 35 at the other end thereof communicating with 
the engine 16 via fuel line 14. The interior surface 36 of the valve body 
31 defines a passage 38 extending therethrough in the direction of fuel 
flow. 
A plunger 40 with a magnetic core 42 is located within the valve passage 
38. The magnetic core 42 acts as a solenoid core 42, with the plunger 40 
moving along the central axis 44 of the valve body 31. A solenoid coil 46 
is wrapped around the valve body 31 so as to be concentric with the 
passage 38 therein. 
The interior surface 36 of the valve body 31 has a radially-tapered portion 
48 defining the valve seat. The plunger 40 has a radially-tapered end 
portion 50 to facilitate fluid-tight engagement of the plunger 40 with the 
valve seat 48. An O-ring 52 is seated into an annular groove 54 in the 
valve seat 48 to further ensure a seal between the plunger 50 and valve 
seat 48 when the plunger 40 is mechanically biased thereagainst by a 
spring 56 engaging with a surface 58 of the plunger 40 and a surface 60 of 
a cap 62. The cap 62 has threads 64 which allow for the spring 56 to be 
pre-loaded when the cap 62 is screwed tight. 
The plunger 40 is electromagnetically biased away from the valve seat 48 by 
the interaction of a magnetic field with the magnetic core 42 of the 
plunger 40. The biasing magnetic field is generated when the solenoid coil 
46 is energized by the pulse-width modulated signal 66 generated by the 
microprocessor 28. Preferably, the rise time of the magnetic pulse 
generated by the solenoid coil 46 is long in comparison to the inertia of 
the plunger 40. A pulse frequency is also selected which is substantially 
higher than the natural frequency of the plunger/valve spring assembly so 
as to minimize plunger 40 travel when the solenoid coil 46 is energized by 
pulses of a varying width, thus simulating a servomechanism response from 
the valve in which plunger 40 is opened to a position corresponding to the 
average power input represented by the pulses. A steady state condition is 
achieved wherein the plunger 40 vibrates slightly about the point of 
average power as related to signal pulse width. The pulse width is 
selected in proportion to desired engine speed and thus fuel requirements. 
Wider pulses cause the plunger 40 to become further axially displaced from 
the valve seat 48, thereby allowing more fuel 22 to flow through the gap 
68 between the O-ring 52 and plunger 40 and causing a greater engine 
speed. Similarly, narrow pulses generate reduced fuel flow and, hence, a 
lesser engine speed. 
As noted hereinabove, the microprocessor 28 regulates fuel flow through the 
valve 24 by generating a fuel flow command 74 which, subsequent to 
conversion by signal converters 32 and 34, provides a pulse-width 
modulated direct current signal 66 for energizing the solenoid coil 46 of 
the valve 24. Under the instant invention, the microprocessor 28 may be 
located on the missile or remotely. In the preferred embodiment 
illustrated in FIG. 1, a ground-based microprocessor 28 communicates with 
the missile-based speed transducer 26 and solenoid coil 46 via the optical 
fiber 30 extending between the microprocessor 28 and the missile. 
Specifically, the digital fuel flow commands 74 generated by the 
microprocessor 28 in response to engine speed feedback 78 from the speed 
transducer 26, as well as inputs 72 from the ground-based missile 
operator, are encoded to a digital word and transmitted through the fiber 
optic cable 30 as an optical signal 76 by signal converter 32, and then 
detected and decoded in the missile to provide the pulse-width modulated 
signal 66 for operating the valve 24. Engine speed feedback 78 from the 
speed transducer 26 on the missile's engine 16 is likewise digitally 
encoded and transmitted in the opposite direction through the fiber optic 
cable 30 as an optical signal 80 by signal converter 34, and then detected 
and decoded by ground-based signal converter 32 for use by the 
microprocessor 28. 
The simultaneous transmission of data in both directions through the fiber 
optic cable 30 is achieved by using encoders and decoders within each 
signal converter 32 and 34 capable of operating on two distinct 
wavelengths. Thus, for example, the output 78 from the speed transducer 26 
is converted to a green light by signal converter 34 for transmission 
through the fiber 30 for ultimate use by the microprocessor 28, and the 
fuel flow commands 76 generated by the microprocessor 28 are transmitted 
through the fiber 30 to the missile as yellow light for ultimate use by 
the valve 24. The specific wavelengths employed for optical signals 76 and 
80 are selected so as to minimize signal attenuation in the fiber optic 
cable 30, in the manner known to one skilled in the art. 
The fiber optic link allows for the transmission of the fuel flow commands 
from the ground-based processor to the missile at the high frequency and 
with the accuracy required for optimum performance. In the preferred 
embodiment, the digital words generated by the microprocessor 28 are 
transmitted through the fiber optic cable 30 at a frequency of between 60 
to 100 Hz to control engine speed within a .+-.3 percent speed tolerance 
bandwidth. It will be readily appreciated, however, that higher 
transmission rates are possible to more tightly regulate engine speed, due 
to the broad bandwidth of the fiber optic cable 30. 
The instant invention permits the repeated use of microprocessor 28 which 
is part of computer facilities (not shown) at a remote location, thereby 
providing a cost savings over missiles having an on-board fuel control 
microprocessor which is lost on any missile flight. It is further noted 
that the output port 35 of the valve 24 of the instant fuel control system 
10 may be directly connected to the engine 16, i.e., without the aid of 
fuel-pulse dampening hoses, due to the continuous fuel flow provided 
thereby. 
While the preferred embodiment of the invention has been disclosed, it 
should be appreciated that the invention is susceptible of modification 
without departing from the spirit of the invention or the scope of the 
subjoined claims. For example, the pulse-width modulated valve 24 on-board 
the missile may be replaced by a rotary valve operated by a proportional 
rotary solenoid or a variable-speed pump. The solenoid-operated rotary 
valve or variable speed pump on-board the missile is controlled by the 
remote processor 28 by using a pulse-width modulated signal as described 
hereinabove or a variable voltage DC source. The instant invention also 
contemplates the use of a variable-opening rotary valve operated by a 
stepper motor under remote processor control to precisely and continuously 
meter fuel to the missile's engine.