GB2135796A - Variable trim-engagement system - Google Patents

Abstract

When trim engagement is commanded, a variable trim-engagement system delays trim engagement until the trim position has had a sufficient time to acquire correspondence with the stick position. The delay is based on the difference between the stick position and the trim position when trim engagement is commanded. Thereby a trim reference is acquired with a minimum of delay. Both dedicated hardware (Figs. 1, 2, 3) and digital (Fig. 4) embodiments are disclosed. <IMAGE>

Description

SPECIFICATION Variable trim-engagement Technical field This invention relates to aircraft automatic flight control systems and more particularly to establishing a trim reference.

Background art Aircraft attitude is controlled by the stick position of controls which affect the positions of positionable aerodynamic surfaces. In the case of a helicopter, these positionable aerodynamic surfaces are the main and tail rotor blades. A variable trimengagement system applicable to helicopters is disclosed herein, but the teachings have relevance to fixed-wing aircraft.

A cyclic stick is movable in two axes. Forward and aft (i.e., longitudinal) movement of the cyclic stick controls the pitch attitude of a helicopter (nose down/nose-up). Right and left (i.e., lateral) movement of the cyclic stick controls the roll attitude of the helicopter as measured from wings-level. A coliective stick controls the lift of the helicopter. The pitch, roll, and lift of the helicopter are controlled by affecting the main rotor blade cyclic and collective pitch. Movement of yaw pedals controls the heading (yaw) of the helicopter by affecting the tail rotor blade collective pitch.

The blade pitch of the main and tail rotors may also be affected by an automatic flight control system (AFCS). An outer-loop AFCS has trim actuators that position the flight controls to maintain the heading, attitude and altitude of the helicopter.

Typically, the desired flight characteristics are achieved by manual control and then the outer-loop trim-is engaged. For simplicity, pitch attitude control is particularly discussed henceforth, but it should be understood that the invention is applicable to roll, yaw and lift.

An electro-hydraulic trim actuator works through an override spring to position the cyclic stick. When trim is engaged,the trim actuator position (trim position) is maintained through a trim feedback loop. Trim position corresponds to stick position.

Therefore, when trim is engaged, stick position is maintained. A given stick position nominally relates to a given attitude. However, attitude and stick position may not always correspond, depending on prevailing and transient environmental factors.

Therefore, attitude is also maintained by the AFCS sensing changes in attitude from the desired attitude and providing an error signal (autopilot command) based on the difference. The trim actuator responds to the error signal and biases the trim position to maintain attitude. With trim engaged, the trim actuator must be able to acquire a trim position quickly in response to the error signal to maintain a desired attitude (trim reference). A pilot may move the stick against trim (trim engaged) or without trim (trim released). Trim is engaged and released with a trim switch. For instance, a pilot desiring to acquire a new attitude releases trim, maneuvers to the desired attitude, and then re-engages trim to establish a trim reference. Or, he may maneuver against trim and wish to acquire a new trim reference quickly before another maneuver.This is done by releasing and re-engaging trim.

When trim is released, the trim actuator tracks the stick rather than positioning it. Therefore, it is known to provide the trim actuator with a series damper.

Consider the following. A maneuver against trim is made. The override spring is elongated or compressed since the trim reference is maintained. Trim is then released. But for the series damper, the trim actuator would rapidly accelerate to acquire correspondence with the stick position, and could overshoot. The force felt through the override spring would vary correspondingly and may move the stick.

This is called "ringing".The series damper limits the rate of change of the trim position to correct this problem. However, due to the lag imposed on the trim position in acquiring correspondence with the stick position, when trim engages, an erroneous trim reference may be acquired. Therefore, it is known to delay the engagement of trim based on the worst possible case - this being the time it would take the trim position to acquire correspondence with the stick position when the stick is at one extreme, such as forward, and the trim actuator is at the other, such as aft. Depending on the damper characterisics, this delay can be significant.This requires the pilot to maintain a stick position from the time he commands trim engagement until trim is actuaily engaged, thus causing a high pilot workload and a delay in establishing and maneuvering from a trim reference. Furthermore, when the delay is based on the worst case, the pilot must hold the stick position for significantly longer than may be required to acquire trim position correspondence. This is especially true when, as is typical, small maneuvers are made.

Disclosure of invention Therefore, it is an object of this invention to minimize the delay between trim engagement command and trim engagement. It is a further object to provide for quickly establishing a trim reference, such as immediately after a maneuver either against or without trim, and immediately before conducting a subsequent maneuver against trim. Thereby pilot workload is reduced.

Accordingly, the variable trim-engagement system of this invention includes a delay circuit responsive to the pilot command (switch closure) to engage trim and delays the engagement of trim based on the difference between the trim position and the stick position at the moment that the trim engage command is given. The invention is described as it applies to a pitch channel, but the teachings are applicable to other AFCS channels.

Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof.

Brief description of the drawings Figure 1 is a simplified schematic of an AFCS pitch channel.

Figure2 is a simplified schematic of a trim feedback loop and pertinent helicopter components in association with a variable trim-engagement portion of this invention.

Figure 3 is a simplified schematic of a delay circuit.

Detailed decription In Figure 1 is shown a simplified pitch channel of an AFCS. A pitch gyro 10 provides a pitch signal (P) on a line 11. A track-store circuit 13 tracks the pitch signal (P) when a switch 12 is closed. The switch 12 is closed when a trim command signal (R) is at logic ZERO. The track-store circuit 13 provides a stored pitch signal (Ps) on a line 14 indicative of the value of the pitch signal (P) at the moment the switch 12 is opened. The switch 12 is opened when the trim command signal (R) is at logic ONE (trim engage commanded). A summer 15 provides a pitch error signal (Per) as the difference of the pitch signal (P) and the stored pitch signal (Ps). At the moment that the pitch channel is turned on, the pitch signal (P) agrees with the stored pitch signal (Ps) and the pitch error signal (Per) is ZERO.For undisturbed flight, the pitch error signal (Per) is nominally ZERO. As the aircraft pitch is disturbed, the pitch signal (P) is driven out of agreement with the stored pitch signal (Ps) and the pitch error signal (Per) is non-ZERO.

The pitch error signal (Per) is appropriately shaped (proportional, rate, and integrater paths) in a shaping circuit 16 to provide an autopilot command (Pac).

The AFCS controls the aircraft in response to the autopilot command (Pac) and thereby maintains a selected pitch attitude (trim reference).

In Figure 2 is shown a prior art trim position feedback loop 101. The trim loop 101 is enabled (trim engaged) by a trim engage signal (C). When the signal (C) is logic ONE, trim is engaged. When the signal (C) is logic ZERO, trim is released. The provision of the signal (C) is discussed hereinafter (Figure 3).

A longitudinal trim actuator 60 comprises a piston 61 and a cylinder 62. The piston 61 defines two fluid containing chambers 63,64 within the cylinder 62.

Fluid, under pressure, is provided by a hydraulic supply 71 to an electro-hydraulic servo value 68 that is operable to modulate the pressure to the chambers 63, 64 via inlets 69,70 in response to a control signal (P) on a line 28. By modulating the fluid pressure on either side of the piston 61, the servo valve 68 can position the piston 61 based on the control signal (P).

A bypass circuit 67 is disabled when, in response to the signal (C=ONE) a valve 65 is closed. The valve 65 opens when the signal (C) is ZERO, and fluid pressure on either side of the piston 61 is equalized.

Therefore the piston is free to be moved by displacement of fluid from one chamber (e.g., 63), through the bypass 67, to the other chamber (e.g., 64).

However, a series damper, such as an orifice 66 restricts the flow of fluid through the bypass 67 and dampens the movement of the piston 61. A position sensor, such as a linear variable-displacement transformer 30, senses the position of the piston 61 and provides a signal TRM POSN) on a line 25 indicative of the trim position (i.e., trim actuator position).

When trim is released (C=ZERO), a switch 20 is closed and a track-store circuit 22 tracks the value of the trim position signal (TRM POSN). When trim is engaged (C=ONE), the switch 20 is open and the track-store 22 provides a stored trim position signal (TRMs) to a summer 24. The trim position signal (TRM POSN) agrees with the stored trim position signal (TRMs) at the moment that trim is engaged.

The summer 24 provides a trim error signal (TRMer) to an amplifier 27 indicative of the difference between the trim position signal (TRM POSN) and the stored trim position signal (TRMs). The trim error signal (TRMer) is amplified by a gain K1 and is provided as the control signal (P) on the line 28 to the valve 68 to position the piston 61. Thereby, a feedback loop is established. Trim reference is maintained by the trim loop 101 as the trim position when trim is engaged.

The pitch autopilot command (Pac) is added to the trim error signal (TRMer) at the summer 24 and biases the trim error signal (TRMer) in response to aircraft perturbations. This biases the trim position and, consequently, the stick position to maintain attitude.

The trim piston 61 acts upon the stick 34 through a stick linkage 33 and an override spring 31. With trim engaged, the pilot may move the stick 34, causing the override spring 31 to elongate or compress, but the trim position does not change. The stick 34, controls the pitch attitude of the aircraft. The stick 34 is movable, and its position determines the blade pitch of a main rotor 44. The blade pitch is nominally related to pitch attitude. The blade pitch of the main rotor 44 is also affected by the stick 34 in its roll mode 39 and by a collective stick 40. The positions of the cyclic stick 34 in its pitch and, roll modes, and the position of the collective stick 40 are mixed by a mixer 41, and, through servos 42 and a swash plate 43, affect the blade pitch of the main rotor 44. Yaw pedals affect the blade pitch of a tail rotor (not shown).

The cyclic stick 34 has a handle 35 in which is disposed a trim switch 36. The trim switch 36 provides a trim command signal (R) on a line 37. The signal (R) is logic ZERO (trim released) when the trim switch 36 is operated (e.g., depressed). When the trim switch 36 is released, the command signal (R) is logic ONE and initiates trim engagement. However, trim is not immediately engaged when the command signal (R) becomes logic ONE, but is delayed, as follows.

In Figure 3 is shown a trim engagement delay circuit of the prior art. When the trim command signal (R) is logic ONE, a timer 47 starts timing and provides a time signal (Z) to a comparator 49. When the value of the time signal (Z) reaches T seconds, as provided by a reference 74, the comparator49 provides a logic ONE on a line 19 to an AND circuit 17. The AND circuit 17 is also responsive to the command signal (R). When both the trim command signal (R) and the output of the comparator 49 are logic ONE, the trim engage signal (C) is provided at logic ONE to engage trim. Thus, the trim engage signal (c=logic ONE) lags the command signal to engage trim (R=logic ONE) by Tseconds. It is known to establish the reference T for the "worst case", a discussed hereinbefore.

When the trim switch (36 of Figure 2) is operated, the trim command signal (R) becomes ZERO, indica tive of trim release. The signal (R=ZERO) is com plemented by an inverter 45 to provide a logic ONE to reset the timer 47. The time signal (Z) immediately becomes ZERO, the output of the comparator 49 becomes ZERO and the trim engage signal (C) becomes ZERO and trim is released. Thus, there is no delay in actual trim release when the signal (R) becomes ZERO.

Returning to Figure 2, there is shown the variable trim-engagement portion 102 of this invention. A stick position signal (STICK POSN) on a line 50 is provided by a position sensor, such as a linear variable-displacement transformer 32. A summer 51 provides a position difference signal (Pd) as the difference between the stick position signal (STICK POSN) and the trim position signal (TRM POSN). The position difference signal (Pd) is nominally ZERO with trim engaged. The position difference signal (Pd) is converted to time units by an amplifier 53 having a gain K2. The gain (K2) is based on the "worst case" position disparity and the time it would take the trim position to acquire correspondence in the "worst case". The absolute value of the amplified position difference signal is provided by an absolute circuit 55, on a line 56, to a summer 57.At the summer 57 a small bias (t) 58 is added to the signal on the line 56 to provide a reference signal (Rs) on the line 59. The bias (t) is added to provide a minimum delay for closure of values, etc. Thus, the reference signal (Rs) varies directly in proportion to the lack of correspondence between the trim position and the stick position, at all times. In a maneuver against trim, the position difference signal (Pd) is non-ZERO and may be quite large. In a maneuver without trim (trim released) the position difference signal (Pd) is non-ZERO during the time required for the trim piston to acquire correspondence with the stick position.

When the signal (R) equals ZERO, indicative of trim release, a switch 72 provides the reference signal (Rs) to a track-store circuit 73. When trim is commanded (R=ONE), the switch 72 opens and the reference 74 is established at T seconds, according to the lack of correspondence between the trim position and the stick position at the moment that trim engage is commanded. The output of the track-store 73 becomes (T), the reference 74, to control the delay between the trim command signal (R=ONE) and actual trim engagement (C=ONE). The reference 74, as established in the variable trimengagement portion 102 is provided to the delay circuit of Figure 3 to delay trim engagement based on trim position and stick position when trim is commanded.

In a maneuver against trim, the trim position remains constant while the stick position changes.

Therefore, the difference signal (Pd) can become large. If the pilot momentarily depresses the trim switch 36 (R=ZERO), the trim position will graduaily gain correspondence with the stick position. The longer trim is released, the greater the correspond ence. Given any operation of the trim switch 36 longer than a "blip" it is likely that the correspond ence of trim position and the stick position will be great, thereby minimizing the delay between trim command (R=ONE) and trim engage (C=ONE).

Similarly, in a maneuver without trim, the trim position lags, but may substantially correspond to the stick position, depending on the movement of the stick relative to the trim position. Correspond ence is enhanced if the pilot holds the stick at a position for more than a moment. Typically, the lack of correspondence of stick and trim positions in such a maneuver is very small. Therefore, the ultimate delay when trim engage is commanded is proportionally small, and in most cases, the variable trim engagement system of this invention provides for trim engagemet which, given "human factors", seems instantaneous to the pilot.

The foregoing description is in simplified block form. Many of the functions can obviously be achieved in a simpler fashion by using more true and complement outputs and fewer inverters; in many instances the positive logic disclosed may readily be reworked into inverting logic to be more suitably applicable to available hardware chips. The description is, therefore, principally in terms of function achieving blocks, and it should be understood that numerous variations may be utilized for achieving the same or equivalent functions and combinations of functions within the skill of the art.

In Figure 4 is shown a flowchart of a digital routine for implementing the variable trim engagement system. The routine is accessed at an entry port 76.

In a first test 77 it is determined whether trim engagement is commanded. if not, the pilot is operating the trim switch (36 of Figure 2) and the signal (R) is ZERO. When the signal (R) is ZERO, a delay counter is reset at a step 78. Then the difference between the trim position and the stick position is calculated at a step 79. Then the trim engagement delay (T) is calculated based on the absolute value of the position difference times a constant (the gain K2 of the amplifier 53 of Figure 2).

Then the routine is exited at an exit port 83. It is important to note that the delay (T) is continually calculated during trim release (R=ZERO), and is fixed in value as soon as trim engage is commanded (R=ONE).

When trim engage is commanded, such as at the end of a maneuver, the signal (R) equals ONE and the routine proceeds to a test 81. There it is determined whether the delay counter has counted for T seconds, based on the duty cycle of the computer. If not, at a step 82 the delay counter is incremented and the routine exits. If the delay counter has incremented to T seconds, trim is engaged at a step 84 by setting a signal (C) to logic ONE. Then the routine is exited at the exit port 83.

Since the routine does not allow for trim engagement (step 84) in the same duty cycle as the DELAY acquiring a value of T (step 82), there is always a small lag between trim command and trim engage, based on the duty cycle. The desirability of some minimal lag is discussed hereinbefore with respect to the hardware embodiment.

It should be understood that the digital routine as described herein may easily be integrated into an existing routine that contains other tests and functions based on the relative stick position and trim position, such as a fault indicator.

The routine of Figure 4 digitally performs some of the functions the hardware of Figure 2. It should be understood that much more of the hardware of Figure 1 and Figure 2, such as summing junctions, amplifiers, etc. may be implemented digitally in a matter known in the art.

The digital function indicated by the simplified flowchart of Figure 4 is straighfforward and capable of implementation in even the simplest of digital computers, in either simplex or multicomputer systems, of a variety of architectural types, employing well known programming techniques or in a dedicated digital device.

The foregoing description of this invention is intended to enable those skiiled in the art to practice the invention. Although the invention has been shown and described with respect to exemplary embodiments, it should be understood that various other embodiments and modifications as are suited to the particular use contemplated will become apparent upon examination and practice of the invention and various changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the invention.

Claims (2)

1. In an AFCS that has a trim position sensor for providing a signal indicative of trim position, and a trim loop responsive to a trim engage signal for maintaining a trim reference based on the trim position signal, and that includes a trim switch to provide a trim command signal to command trim engagement and to release trim, and that also has a delay circuit for providing the trim engage signal to the trim loop at a time after the trim command signal to engage trim is provided, said time based on a reference signal indicative of a period of time, the improvement comprising: a stick position sensor for providing a signal indicative of stick position; and signal processing means for providing the reference signal to the delay circuit based on the difference between the stick position and the trim position as indicated by the stick position signal and the trim position signal.

2. A variable trim-engagement system, for an aircraft having an outer-loop AFCS, said AFCS having a trim actuator operable to position a flight control, and establish a trim reference in response to a trim engage signal said variable trim-engagement system comprising: a first position sensor for providing a signal indicative of the flight control position; a second position sensor for providing a signal indicative of the trim actuator position switch means for providing a signal to release trim and a command signal to engage trim; ; signal processing means, connected for response to the first position sensor, the second position sensor, and the switch means, for providing the trim engage signal to establish the trim reference at a time afterthe command signal to engage trim is sensed, said time based on the difference between the trim position and the stick position as indicated by the stick position signal and the trim position signal.