Limiting Aircraft Vertical Acceleration Response

An appendix containing a software flow diagram, code writing standards, and an exemplary computer program is appended hereto.

Aircraft autopilot devices are typically engineered to control the aircraft during most typical flight operations and in more complex systems include roll and yaw stabilization, heading control, navigation control and pitch stabilization.

The pitch rate sensor, typically a rate gyro, senses the rate of pitch change (with reference to the aircraft longitudinal axis, nose-to-tail angle relative to the horizontal axis), and outputs a signal useful to generate a correction to control the pitch rate of the aircraft.

A problem is noted in that the performance of the aircraft is a function of the servos controlling the respective control surfaces of the aircraft. Their relative torque factors coupled through gearing to provide pitch rates acceptable at slower speeds, can be entirely unacceptable at higher speeds due to the excessive vertical acceleration forces on the airframe during a pitch rate command output that would otherwise be acceptable. Accordingly, the pitch rate has typically been limited by an airspeed factor input by a sensor to the autopilot. This factor reduces the allowable pitch rate command output as the aircraft speed increases.

This system works well in most instances; however, it requires an airspeed input to the autopilot so that pitch rate output can be varied accordingly.

It is an object of the present invention to provide a variably limited pitch rate command output in an aircraft to limit vertical acceleration, without requiring an airspeed input.

It is a further object of the present invention to provide a vertical acceleration limited pitch command output apparatus for an autopilot without any requirement for an airspeed sensor or airspeed compensation.

Briefly, and in accordance with the present invention, an autopilot pitch command apparatus having a specified maximum pitch rate control output comprises first means for receiving a sensed vertical acceleration signal, second means for receiving a sensed pitch rate signal, third means coupled to said first means for lead compensating said sensed vertical acceleration signal, and fourth means coupled to the second means and to the third means for detecting when said lead compensated acceleration signal exceeds the absolute value of a first predetermined factor and for requiring the autopilot pitch command apparatus to limit the maximum pitch control output to the presently-sensed pitch rate as a function of said detection. This requires a vertical acceleration sensor, a pitch rate signal, and a lead compensation signal coupled to the vertical acceleration signal input. The lead compensated acceleration signal is input into a logic detector fourth means which senses when the aircraft is approaching maximum vertical acceleration and causes the pitch command apparatus to select the sensed present pitch rate as the maximum. Because this is invariably less than the design pitch rate maximum and requires the aircraft to maintain the present pitch rate, the maximum vertical acceleration is thereby limited. A situation wherein the vertical acceleration is rapidly approaching the design maximum for the aircraft is transitory and brief in duration. By use of the present apparatus, the prior art requirement for an airspeed input to vary the maximum allowable pitch rate as a function of airspeed is entirely eliminated. The hypothesis that the airspeed will be constant throughout this control period holds very well, since the vertical acceleration as lead compensated is precisely the variable desired to be controlled, and further since the problem is essentially transitory.

As the sensed vertical acceleration (lead compensated) drops back away from the design maximum for the aircraft, the maximum allowable pitch rate command output for the autopilot is reinstituted as the operational maximum. This reestablishes the full performance potential of the aircraft.

The use of this apparatus allows higher torque servos and gearing to produce a very responsive performance characteristic for the aircraft, and still allows limiting of the maximum allowable pitch to control vertical acceleration within specified limits.

Additionally, since the actual pitch rate output is not changed, but merely limited, this apparatus may be engaged and disengaged with no control signal discontinuities or other noticable physical control deviations beyond the smooth control of the vertical acceleration forces.

The novel aspects of the present invention may be more readily ascertained by reference to the specification in combination with the drawings in which:

FIG. 1 is a block diagram of a typical autopilot pitch control apparatus in accordance with the present invention.

FIG. 2 is a block diagram of the lead compensated acceleration input coupled to the level detection logic for controlling the maximum pitch command rate during excessive vertical acceleration periods.

FIG. 3 and 3a are function circuit diagrams to further aid in the understanding of the invention.

Referring now to FIG. 1, the configuration for a pitch control apparatus capable of incorporating the present invention is shown wherein an outer loop pitch command is provided to a magnitude limit block 110, and thereafter to the rate limit block 111. Rate limit block 111 provides the maximum command rate limit output to summer 112 as a function of the input 119 from the pitch command rate limit calculations at block 120. Summer 112 incorporates a pitch feedback 116 and pitch rate feedback through inner loop damping block 118, to provide signal 117, which is thereafter summed with signal 116 in summer 115. This feedback signal is thereafter subtracted from the outer loop pitch command, having its magnitude and rate limited into summer 112 to provide a signal to the inner loop forward gain block 113 and thereafter to the elevator servo 114 for actual control of the elevator. The feedback pitch rate signal 121 is also provided to the pitch command rate calculation block 120 as is the actual normal acceleration (A.sub.z) on line 130 after being converted from pitch rate through a true airspeed conversion block 123 and lowpass filter 122. The normal acceleration is a sensed signal and the scale factor V.sub.T /573 and the lowpass filter 122 represent aircraft dynamics, for this configuration. It can be seen therefore that this autopilot pitch command apparatus is adaptable to a multitude of aircraft without significant airframe-dependent changes required.

The pitch command rate limit calculations block 120 contains the apparatus shown in FIG. 2, having normal acceleration 130 input as well as pitch rate 121 and outputs a rate limit command 119 to rate limit block 111.

Referring now to FIG. 2, the sensed normal acceleration 130 is provided to lowpass filter 210 (essentially for noise effect suppression), and thereafter provided to the lead compensator 212 for detecting when excessive A.sub.z is anticipated. This signal is then output to the level detection logic 214 and a control signal is output on line 215 to set the present pitch rate to the commanded rate maximum when the level is exceeded. On line 217, a sample-and-hold circuit 216 is useful in determining the actual present pitch rate 121 subsequent to its being filtered in lowpass filter 213, to maintain a constant pitch rate maximum output on line 119. When lead compensator 212 detects a decrease in normal acceleration, the level detection logic 214 switches back to the maximum command rate to normal operation.

The following is an approximation of the relationship between pitch rate and normal acceleration, wherein: ##EQU1## where: A.sub.z =normal acceleration (beat/second/second),

V.sub.t =true airspeed (feet/second),

t=acceleration lag time constant of the aircraft dynamics (seconds),

.theta.=pitch rate (degrees/second). Thus, it can be seen in FIG. 2 that the calculations used in setting the actual pitch rate command as a function of sensed excessive vertical acceleration is useful in directly controlling the desired parameter, normal acceleration. No input for actual airspeed is required and the normal acceleration data is filtered to eliminate noise. The normal acceleration is then lead compensated to account for the lag inherent in the relationship between the normal acceleration and pitch rate expressed in the equation above. When this signal exceeds the desired acceleration, the present pitch rate is first processed by lowpass filter 213 to compensate for noise and variations in the lag time constant and then loaded into the rate limiter as a temporary maximum. When the processed acceleration returns to a value somewhatlower than the original trip point, the rate limit is returned to its normal maximum value. In actual operation, the difference between the trip point and the return point provides a hysteresis allowing a smooth transition between maximum command rate and present pitch rate.

Significantly, the flight control performance is not compromised by limiting the servo torque as in a typical approach to this problem. Rather, the maximum servo output is directly controlled by sensed normal acceleration and therefore higher torque servos can be used to increase performance.

Additionally, under normal operations the autopilot inner loop stability is not adversely affected since the rate limit is normally set at a maximum command rate.

This technique is broadly applicable to a variety of aircraft types since the fundamental basis for this method of command rate limiting is direct sensing of the parameter to be controlled (normal acceleration) and therefore increased simplicity and improved performance for the aircraft are provided. The actual time of implementation under typical circumstances will be less than three (3) seconds, and therefore the requirement for any sensed true airspeed input to the control is eliminated. This significant advantage further enhances the simplicity of implementation and breadth of application of the present invention.

Implementation of the present embodiment was in the software program of a Collins digital autopilot. The appendix accompanying this specification contains the actual software program, a softwar functional block diagram, and a terminology glossary for the software labels actually used. The novel technique discussed herein, however, is readily adapted to a variety of hardware and software combinations or purely hardware logic implementation by one of ordinary skill in the art, by reference to FIGS. 1 and 2 in conjunction with this specification.

In this embodiment, the lead compensation maintains a 3:1 ratio in block 212, and the level detection logic 214 is set to trip in at 0.4 g's vertical acceleration and to trip out at 0.2 g's. The pitch rate lowpass filter 213 is set at a 1.0 second lag and the maximum command rate is set at 3.75.degree. per second, which will provide for 0.41 g's vertical acceleration at approximately 120 knots. It can be seen therefore that the actual values can be changed to coincide with desired aircraft characteristics, including maximum allowable vertical acceleration and maximum pitch rate command to suit a particular design specification.

While the present invention has been described with respect to a specific embodiment disclosed herein, it is not intended that this description be construed in a limiting sense. Various modifications of the illustrative embodiment disclosed herein, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications of embodiments as fall within the true scope of the invention. ##SPC1##