It is common among pilots of even small private twin engine aircraft to manually make very minor adjustments to the speed of one of the engines to bring it into close conformity with the speed of the other engine to minimize vibration and noise or "wow." This is typically accomplished by the pilot based on the sound of the engines.

Multi-engine speed control circuits are also well known and illustrated, for example, by U.S. Patents

4,653,981 and 4,659,283 to Harner et al and Niessen et al respectively. In the schemes shown in these patents, the propeller pitch of a "slave" engine is fine-tuned so as to operate at the same speed as the "master" engine and eliminate any "wow" between the two (or more) engines.

This fine-tuning is accomplished according to master and slave speed input information as well as master and slave phase input information. In particular, the phase difference between the master and slave is compared to a reference phase difference to generate a phase error signal which is used to impart a very fine amount of propeller speed control to the slave engine bringing it into close conformity with the speed of the master engine. Niessen et al effect a rough control by comparing the speeds of the master and slave engines followed by a fine-tuning based on the phase difference between the master and slave engines. In these two patented arrangements, the "phase sensor " is of an unknown type. While propeller pitch is controlled in the above mentioned patented arrangements, fuel flow could also be controlled to effect fine-tuning of the speed of one or more engines and eliminate "wow". Computer control of gas turbine engines is well established with the Ralston et al

4,716,723 patent being but a single example. Ralston et al employ a FADEC (Full Authority Digital Electronic Control) to control the flow rate of fuel to an engine. While the primary Ralston et al purpose is to prevent overspeeding, a system such as that of Ralston et al can be adapted according to the present invention to effect a phase comparison based fine-tuning of engine speed as well,

Finally, engine speed measurement systems and measured speed based fine tuning of engine speed is well known and is typified by the Thomas et al 4,434,470 and Cording et al 4,485,452 patents. In each of these speed control systems, engine speed is determined from an engine driven toothed wheel rotating past a magnetic pickup to generate a pulse train the repetition rate of which is related (by the number of teeth on the wheel) to engine speed.

Applicant's assignee currently manufactures the . EH-AB2 Full Authority Digital Electronic Control (FADEC) which includes an Event Frequency Interface (EFI) integrated circuit chip and may be used as part of a speed control system in controlling for example, a pair of turbo fan engines, turboshafts or turboprops. This commercially available product incorporates many of the principles of the last two mentioned patents. While systems according to these last two patents work well in practice as electronic speed governing devices, it would be desirable to increase their accuracy so as to also be able to employ them as fine-tuning circuits to synchronize one or more slave engines to a master engine or to an independent master speed signal source.

Among the several objects of the present invention may be noted the provision of a phase difference computing circuit; the provision of an engine control circuit which "fine tunes" the difference in speed between two engines; the provision of an engine control circuit in accordance with the previous object which effects the fine tuning based on a determination of successive differences

in phase between two engine speed indicative signals. The provision of an engine control circuit which "fine tunes" the difference in speed between two or more engines based on a determination of successive differences in phase between engine speed indicative signals and a master reference signal; and the provision of an engine speed control circuit in which all engine fuel control circuits are fine-tuned to bring the engines into close harmony according to a phase angle comparison with an independently generated master signal. These as well as other objects and advantageous features of the present invention will be in part apparent and in part pointed out hereinafter.

In general, an engine control circuit for making small adjustment in the difference in speed between two or more engines based on a determination of successive differences in phase between two engine speed indicative signals has a first engine driven encoder for providing a first pulse train the repetition rate of which is indicative of the speed of one engine and a second engine driven encoder for providing a second pulse train the repetition rate of which is indicative of the speed of another engine. An independently generated master signal may be used instead and all engines made to conform to a speed indicated by that independent signal. A first counter for counting pulses in the first pulse train over a predetermined time interval and a second counter for counting pulses in the second pulse train over the same predetermined time interval provide two pieces of information for computing the average phase difference between the two signals over the predetermined time interval. A third counter for determining the time between the last pulse of the first pulse train counted during the predetermined time interval and the end of that time interval and a fourth counter for determining the time between the last pulse of the second pulse train counted during the predetermined time interval and the end

of the time interval provide two additional pieces of information. The frequencies of the first and second pulse trains are computed (one is known in the case of an independent reference) and these computed frequencies and the counts determined by the first, second, third and fourth counters are used to compute the average phase difference between the first and second pulse trains over the predetermined time interval. A fuel control regulator is responsive to the computation to control the flow of fuel to one of the engines to modify the speed of that engine to bring it into closer conformity with the speed of the other engine. The repetition rate of the first and second pulse trains are each the same integral multiple n of their respective engines' speed, Fm is the frequency of the first pulse train, Fs is the frequency of the second pulse train. Tern is the first pulse count, Tcs is the second pulse count, Tvm is the time between last pulse of the first pulse train and the end of the time interval, Tvs is the time between the last pulse of the second pulse train and the end of the time interval, and the phase difference between the first and second pulse trains is determined by iteratively maintaining a total pulse count Tct by interactively computing Tct = Tct + Tern - Tcs with multiples of n added or subtracted to maintain it between ± n, (where n is the number of pulse counts per 360° revolution) and then computing the phase difference P from the equation

P = (Tvm - Tvs + Tct/Fm) * Fm * 360 n BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is a schematic block diagram illustrating an engine speed control system in which the concept for determining the phase difference between master and slave input signals may be incorporated; Figure 2 is a block diagram of a known Event

Frequency Interface integrated circuit chip which forms a part of the circuit of Figure 1;

Figure 3 is a schematic block diagram of an engine speed control system incorporating the concepts of Figure 1 and in which two engine speeds are conformed to an independent master signal; and Figure 4 illustrates two input waveforms for the circuit of Figure 1.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawing. The exemplifications set out herein illustrate a preferred embodiment of the invention in one form thereof and such exemplifications are not to be construed as limiting the scope of the disclosure or the scope of the invention in any manner. DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 depicts an engine speed control circuit in which the present invention finds particular utility. A master engine speed indicative signal on line 11 and a slave (controlled) engine speed indicative signal on line 13 are shaped or conditioned and supplied to an interface 19. The interface communicates with a central processing unit 21 to determine engine speed by known prior techniques, and the speed of the slave engine is controlled by regulating the fuel supply at 23. The event frequency interface (EFI) 19 of Figure 1 may be implemented as shown in Figure 2. The signal conditioners

15 and 17 may be separate from the chip of Figure 2 or may be integrated therewith as the indicated digital filters

16 and 18. Both may in some instances be used. This known chip incorporates the CPU 21 interface logic for two frequency inputs on lines 11 and 13 and one "event" input. A bus interface (buffer) is used to read data and status bits from the chip and all chip inputs and outputs are TTL compatible. In Figure 3, a pair of engines 25 and 27 such as

General Electric CFE738 Turbo Fan engines are coupled to encoders 29 and 31 which, for example, may be toothed wheels or gears with adjacent magnetic or optical pick-up

devices for providing speed indicative pulse trains as taught in the above mentioned Thomas et al patent. Either one of these signals may be considered to be the master signal and the other the slave as in the discussion of Figure 1. As an alternative, an independent master frequency signal source 41 operating at a selectable fixed frequency independent of the encoders may be used and each engine speed signal compared thereto. In Figure 3, the signal on line 43 is the master signal analogous to the signal on line 11 in Figure 1 and the signal on line 45 is the slave signal analogous to the slave signal on line 13 of Figure 1. These two signals are supplied to a FADEC (Full Authority Digital Electronic Control) unit 35 which includes a central processor unit as well as input-output buffers, and in particular, the EFI chip of Figure 2.

The engine control circuit of Figure 3 is adapted for making small adjustments in the difference in speed between two (or more) engines 25 and 27 based on a determination of successive differences in phase between engine speed indicative signals and a master reference signal from frequency generator 41. The reference pulse train generator 41 provides a first pulse train while one or more engine driven encoders 29 and 31 provide one or more second pulse trains the respective repetition rates of which are indicative of the speed of the respective engines. FADECs 33 and 35 each include a CPU and an EFI similar to that of Figure 2. A first counter 20 counts the pulses in the first (reference) pulse train over a predetermined time interval as set by signals on strobe line 50. A second counter 22 counts the pulses in the second engine speed indicative pulse train over the same predetermined time interval. A third counter 24 determines the time between the last pulse of the first pulse train counted during the predetermined time interval and the end of that time interval while a fourth counter 26 determines the time between the last pulse of the second pulse train counted during the predetermined time interval and the end of that time interval. The CPU

computes the frequencies of the first (if necessary) and the second pulse trains. The phase difference is then computed utilizing the computed frequencies and the counts determined by the first, second, third and fourth counters to compute the average phase difference between the first and second pulse trains over the predetermined time interval. Regulators 39 and 37 are controlled accordingly to supply more or less fuel to the corresponding engine in responsive to the phase differences to modify the speed of that engine to bring it into closer conformity with a speed indicated by the frequency of the reference pulse train or that of the other engine.

As seen in Figure 3, each engine is provided with an encoder 29 or 31 as well as with first, second, third and fourth counters on the EFI chip in FADEC 33 or 35 as well as the CPU's for computing engine speed indicative pulse train frequencies and phase difference computing means and all engines are provided with fuel control circuits 37 and 39, all of these fuel control circuits are responsive to their respective phase difference computing circuits for fine-tuned their respective speeds to bring the engines' speeds into close harmony according to phase angle comparisons with the independently generated master signal. The FADEC 35 which, by way of a fuel flow regulator 39, controls the speed of engine 27, may be substantially the same as taught in the above mentioned Ralston et al patent. Regulator 39 may, for example, be a torque motor that regulates fuel flow. Synchronization of the speed of engine 25 with the signal from independent source 41 is accomplished in the same manner.

In Figure 4, a pair of illustrative engine speed indicative pulse trains of 400 and 333 Hz respectively are illustrated. The phase difference P between these two

frequencies using the master frequency as reference is determined by the equation

P = Tsm * Fm * 360 = (Tvm - Tvs + Tct/Fm) * Fm * 360 n n where Tct is computed iteratively from Tct = Tct + Tern - Tcs with multiples of n added or subtracted to maintain it between + n,

Tsm = Tvm - Tvs + Tct/Fm, and where Fm = frequency of the master signal Fs = frequency of the slave signal

Tsm = time between slave and master teeth Tct = tooth counts total (initialized to zero) Tern = tooth counts master (assume zero for the first iteration) Tcs = Tooth counts slave (assume zero for the first iteration)

Tvm = time between last master tooth and iteration strobe

Tvs = time between last slave tooth and iteration strobe n = number of pulses per revolution. Tvm and Tvs are computed from the vernier count at 24 and 26 respectively in Figure 2 while Tcm and Tcs are from counters 20 and 22. While the above refers to "tooth counts", it is clear that where a master signal is independently generated it is really a count of the number of pulses which is being considered.

While other fixed time intervals may be used, in Figure 4, this computation is repeated every 16 ms. The following are illustrative computations for Figure 4 at times a, b, c and d with encoders having 8 teeth (n = 8). a) Tct = 0 + 0 + 0 = 0

P = (1/1000 - 0.5/1000 + 0/400) * 400 * 360 = 9 degrees

8 b) Tct = 0 + 6 - 5 = 1

P = (2/1000 - 1.5/1000 + 1/400) * 400 * 360 = 54 degrees

8

c) Tct = 1 + 7 - 5 = 3

P = (0.5/1000 - 2.5/1000 + 3/400) * 400 * 360 = 99 degrees

8 d) Tct = 3 + 6 - 6 = 3 P = (1.5/1000 - 0.5/1000 + 3/400) * 400 * 360 = 153

8 degrees

Substantially the same computation may be stated in a number of different ways. For example, the following equation may be executed at the same fixed iteration rate the speed signals are read (once between each strobe interrupt signal on line 50 in Figure 2) according to the following relationship:.

P = [(Tvm - Tvs) * Fm/Vf + Tct] * 360

where Vf = Vernier counter frequency. The method of operation of the invention should now be clear. The speed on an aircraft engine to bring that engine speed into close conformity with a reference signal includes generating a reference pulse train (which may be independent such as 41, or may be derived from an encoder such as 29 on a "master engine" . An engine speed indicative pulse train is generated by an encoder 31, the repetition rate of which is an integral multiple of the speed of the engine 27. Circuitry in FADEC 35 counts the number of pulses in the reference pulse train on line 43 over a predetermined time interval as well as counting the number of pulses in the engine speed indicative pulse train on line 45 over the same predetermined time interval. Measures of the time between the last reference pulse counted during the predetermined time interval and the end of that time interval and the time between the last engine speed indicative pulse counted during the predetermined time interval and the end of that time interval are also generated. The two pulse counts and the two time measurements are then utilized to determine the average phase difference between the two generated pulse trains over the predetermined time interval. This average

phase difference is then used to control the fuel supply to the engine. Each of these steps may be repeated during a subsequent equal time interval to determine the average phase difference between the two generated pulse trains over the subsequent predetermined time interval, and the fuel supply to the engine varied according to the difference between the two average phase differences. If the subsequent phase difference is unchanged, no change in engine speed is called for.

In summary, the invention has a number of advantages over known prior schemes and in particular allows for a "fine tuning" of the difference in speed between aircraft engines.

From the foregoing, it is now apparent that a novel aircraft engine control arrangement has been disclosed meeting the objects and advantageous feature set out hereinbefore as well as others, and that numerous modifications as to the precise shapes, configurations and details may be made by those having ordinary skill in the art without departing from the spirit of the invention or the scope thereof as set out by the claims which follow.