GB2292195A

GB2292195A - Brake control system e.g.for an aircraft

Info

Publication number: GB2292195A
Application number: GB9514815A
Authority: GB
Inventors: Michael James Bartram
Original assignee: Dunlop Ltd
Current assignee: Dunlop Ltd
Priority date: 1994-08-05
Filing date: 1995-07-19
Publication date: 1996-02-14
Anticipated expiration: 2015-07-19
Also published as: GB9514815D0; GB2292195B

Abstract

A brake control system e.g. for an aircraft has means for providing signals 20, 24 related respectively to required and actual deceleration, and processor means 25 to operate with a variable characteristic on an error signal related to the difference between the signals 20, 24 and to provide an output signal 27 that can cause an increased or decreased braking effect. The variable characteristic can be a function of wheel speed, brake torque, brake temperature, brake material, and/or aircraft mass or can be related to the ratio of required or actual deceleration to required or actual brake pressure. The required deceleration can be determined by averaging signals obtained from movement of left and right brake pedals, and the actual deceleration can be determined from wheel speeds or from an inertial system. Switch 50 switches the system from the deceleration control mode to a pressure control mode at low aircraft speeds. <IMAGE>

Description

CONTROL SYSTEM FOR AIRCRAFT BRAKING This invention relates to a control system operable to cause selective operation of at least one brake in response to an operator command. The invention relates in particular, though not exclusively, to a control system for selective operation of aircraft brakes of a kind comprising friction discs of carbon material.

Carbon brakes are known to exhibit a considerable variation in the amount of torque generated (i e deceleration effect) per unit of hydraulic fluid pressure acting on the brake pistons. It is believed that this variation of torque/pressure gain is due to the variable friction/temperature characteristics of carbon material of the kind used for the friction discs of carbon brakes. An additional gain variation would be caused by aircraft mass variation. A consequence of this variation of gain is that the pilot may experience unpredictable braking effects.

In an aircraft having a conventional arrangement of brakes on the main undercarriage wheels disposed either side of the fuselage, and at least one forwards (unbraked) nosewheel an unpredictable braking effect of the main undercarriage wheels can result. In the case of an unduly high brake torque being applied to the main undercarriage wheels, high loading on the nosewheels and associated aircraft structure can result.

The present invention seeks to provide a control system suitable for enabling a pilot more accurately to control aircraft deceleration, and which preferably still maintains a differential braking capability. It seeks also to provide a system which prevents unnecessary antiskid operation at all speeds, and which facilitates a soft onset of brake torque.

In accordance with the present invention there is provided a brake control system comprising first means for providing a signal related to required deceleration, second means for providing a signal related to actual deceleration, and processor means to operate on error signal related to the difference between the signals from said first and second means, said processor means providing an output signal in response to which the control system is operable to increase or decrease the braking effect, wherein the processor means provides a variable characteristic between output and input signals.

The variable characteristics provided by the processor means preferably compensate for any variable torque/pressure characteristics of the brakes, e g carbon brakes. The overall characteristics of the processor means may be a function of one or more of the aircraft mass changes, wheel speed, brake torque, brake temperature, rate of change of brake temperature or brake material. The characteristics may be varied in relation to measurements of aircraft mass and/or the ratio of required or actual aircraft deceleration to average required or actual brake pressure.

Typically, for the purpose of retarding or manoeuvring an aircraft, wheel brakes will be present to the left and right-hand sides of the centreline of the aircraft and will be pilot controlled respectively by left and right-hand pedals. In that case the pilot demand of required deceleration may be determined by averaging signals obtained from or related to movement of the left and right-hand pedals. Similarly, the actual aircraft deceleration may be obtained by averaging the rate of change of a pseudoaircraft velocity, produced from the rotational speeds of the left and righthand wheels.

The invention envisages that the control system may take account of the actual pressure of hydraulic fluid acting on a brake piston/cylinder assembly to avoid brake pressure imbalance between respective servo valves operating left and right-hand brakes when the pilot is demanding symmetric braking and the servo valve pressure/current (P/I) characteristics are dissimilar. This is achieved by individual pressure control loops around each of the left and right-hand servo valves thereby to avoid any unpredictability arising from tolerance band variations of left and right servo valves.

The control system may feature the ability to switch between a deceleration control braking means and a pressure control braking means eg automatic switching from deceleration control to pressure control at low aircraft speeds.

Optionally a ramp characteristic or other transition means may be provided in the control system for smoothness of operation when switching between deceleration and pressure control operating modes and vice versa.

The control system may be responsive to other information such as whether or not an air brake, nose wheel brakes, parachute or other retarding facility has been deployed.

One example of the invention is described below, by way of example only, with reference to Figure 1 which is a schematic diagram of a deceleration controller in accordance with the present invention.

CONTROLLER DESCRIPTION Referring to Figure 1. The aircraft deceleration AV DEC 20 can be derived in a number of ways and therefore no particular detailed procedure is described, e g antiskid signals 21 of wheel speed memory which can be averaged and then differentiated or aircraft inertial reference signals 22 of velocity or acceleration can be utilised.

The pilot pedal inputs, L PED 10 and R PED 11 are derived from the respective left and right pedal transducers which can be of load-, pressureor stroke-measurement type.

The outputs of the existing right and left cranked pedal functions 13,23 are factored by constant K and then averaged to derive a deceleration requirement, REQ DEC 24. The constant K shown in Figure 1 preferably is to be set to a value which results in a suitable maximum deceleration at maximum pedal input. K may be a function of aircraft speed and this may be beneficial e g to prevent skids at low speeds in dry conditions, for which purpose it could be set to give a maximum deceleration of say .3 g at low speeds.

A deceleration controller 25 acts as the aforedescribed processor means and is responsive to the difference between REQ DEC 24 and AV~DEC 20. This could comprise a single integrator, proportional plus integral or a full PID (proportional integral differential) controller, whichever provides the best control performance. The characteristics of this controller 25 can be modified dynamically to compensate for the variations in brake torque/pressure gain change and aircraft mass change. In this system combination of the aircraft mass and the brake torque/pressure gain changes can be detected in consequence of both pressure and deceleration control being employed since the ratio of required or actual deceleration to the average required or actual pressure gives an indication of the prevailing plant (i e braked aircraft) gain.This can be used to effect a change in the characteristics of the deceleration controller in order to obtain optimal control.

Further aspects of this control system embodiment will now be described for one brake on the left hand side of an aircraft (since in these respects the right hand control system is identical to that described). Said aspects may be provided separately for use with each brake on the left hand side of the aircraft. Identical systems will be provided for use with brakes on the right hand side of the aircraft.

Similarly it will be clear to those skilled in the art that the invention applies equally to nose wheel brakes and brakes fitted to a landing gear on the centre line of the aircraft.

Alternatively, the control system for each brake could be provided within a central processing system supplying dedicated output signals to provide for individual control of each of a plurality of brakes.

The output 27 of the deceleration controller is factored (using multiplier 28) by the output of the divider X. The divider output (or quotient) 33 is the pedal deceleration demand L~DEC~R 26 divided by the average deceleration demand REQ~DEC 24 (numerical value between 0 and 2). The case when REQ~DEC is zero will need to be protected against e g by adding a small offset to REQ~DEC prior to the division.

The resulting left deceleration control pressure demand 34 is transmitted to switch 50. When in deceleration control mode the switch 50 directs signal 34 to the output 29 which is then mixed with the antiskid signal 38 using device 37 to produce the left pressure demand 34. The switch 50 switches to the cranked pedal demands 32 to provide pressure control in response to pilot pedal inputs at a threshold level, for example speeds below the deceleration controller dropout (disabling) speed.

The output 29 of switch 50 is mixed with the antiskid dump signal 36 and constant MAX~PED Y to prevent pedal demand variations having an effect on brake pressure when the antiskid is operating i e in the case when pedal or deceleration demands are above the skid level pressure. This is achieved by adding the antiskid dump signals 36 to the selected pedal or deceleration pressure demand output 29 and then subtracting a constant MAX PED Y, which corresponds to the maximum value of the pressure or deceleration demand. The output 30 of this summation is then limited by means 35, the minimum value of which is normally zero, so that an output is not possible until antiskid is operating. The output 38 of the limited summation is then subtracted from the individual pedal or deceleration pressure demand 29 by means 37.Therefore, the variable effect from the pilot pedals is eliminated when antiskid is operating, i e only antiskid demand variations are seen at the brakes when pedal demands are above skid level pressure.

The output L PRESS DEM 34 is fed to the left pressure controller 31, together with left measured brake pressure 40, and is also factored at Z by constant A. The factored pressure demand 41 is then summed with the output 42 of the pressure controller at means 39 to produce the servo valve drive signal 43 SV L CMD. This pressure control circuit therefore enables a feed forward path via the factored pressure demand 41 which can be of benefit to the antiskid response if the pressure control is slow, e g the pressure control can be configured such that it is primarily used for the brake control situation which is applicable prior to antiskid action.

The pressure controller 31 receives an input of left pressure demand 34 and the measured brake pressure 40 BRK~PRESS~L~SIG. The pressure controller 31 may be of a conventional type known per se and may incorporate a combination of proportional, integral and differential elements which provides the desired pressure control for this application. However, the output from the controller can be limited to limit the allowable change or authority of the controller. By virtue of the feed forward path via the factored pressure demand the overall controller L~PRESS~DEM to SV~L~CMD could be anything from a full authority pressure controller through limited authority pressure controller to zero authority pressure controller (i e a situation which may exist if pressures were not measured and deceleration control was still required).

Aerodynamic effects could prevent brake pressure application if the aircraft deceleration with no wheel braking was above the pilot's demand.

This could be perceived by the pilot as a loss of braking, but would reduce brake energy input in the same way as a conventional autobrake system when low or medium deceleration demands are selected by the pilot. To take account of this it is proposed that the aircraft deceleration selected by the pilot pedal inputs and/or the deceleration error may be displayed in the cockpit.

Claims

CLAIMS:

1. A brake control system comprising first means for providing a signal related to required deceleration, second means for providing a signal related to actual deceleration, and processor means to operate on an error signal related to the difference between the signals from said first and second means, said processor means providing an output signal in response to which the control system is operable to increase or decrease the braking effect, wherein the processor means provides a variable characteristic between output and input signals.

2. A brake control system in accordance with claim 1, wherein the variable characteristic provided by the processor means compensates for variation in the torque/pressure characteristics of the brakes.

3. A brake control system in accordance with claim 1 or claim 2 for the control of aircraft brakes, wherein the characteristic of the processor means is a function of one or more of wheel speed, brake torque, brake temperature, rate of change of brake temperature, brake material and change of aircraft mass.

4. A brake control system in accordance with any one of claims 1 to 3 for the control of aircraft brakes, wherein the characteristic of the processor means is related to a measurement of the aircraft mass.

5. A brake control system in accordance with any one of claims 1 to 4, wherein the characteristic of the processor means is related to the ratio of required or actual deceleration to required or actual brake pressure.

6. A brake control system in accordance with any one of the preceding claims for the control of the brakes of an aircraft having wheel brakes present to the left and right hand sides of the centreline of the aircraft, said wheel brakes being pilot controlled respectively by left and right hand pedals.

7. A brake control system in accordance with claim 6, wherein a pilot demand of required deceleration is determined by averaging instantaneous signals obtained from or related to movement of the left and right hand brake pedals.

8. A brake control system in accordance with claim 6 or claim 7, wherein actual aircraft deceleration is obtained from the rate of change of a pseudo aircraft velocity produced from the rotational speeds of the left and right hand wheels.

9. A brake control system in accordance with claim 6 or 7 wherein the actual aircraft deceleration is determined by means comprising an inertial reference system.

10. A brake control system in accordance with any one of the preceding claims, wherein the brakes are hydraulically actuated and the control system is responsive to the actual pressure of hydraulic fluid acting on a brake piston and cylinder assembly.

11. A brake control system according to claim 10 wherein said response of the control system is achieved by pressure control loops around left and right servo valves.

12. A brake control system in accordance with any one of the preceding claims, wherein means is provided to enable the control system to switch automatically between a deceleration control mode and a pressure control mode.

13. A brake control system in accordance with claim 12 for the control of aircraft brakes, wherein the control system is adapted to switch to the pressure control mode at low aircraft speeds.

14. A brake control system in accordance with claim 12 or claim 13 and comprising transition means operable to provide smoothness of operation when the control system switches between deceleration and pressure control modes.

15. A brake control system in accordance with the claim 1 and substantially as described herein.

16. An aircraft braking system comprising a brake control system in accordance with any one of the preceding claims.

17. An aircraft braking system in accordance with claim 16, wherein the control system is responsive to information relating to deployment of one or more of an air brake, nose wheel brake, parachute or other retardation facility.

18. An aircraft braking system in accordance with claim 16 or claim 17, wherein the control system controls the operation of brakes of a kind comprising friction discs of carbon material.

19. An aircraft braking system in accordance with any one of claims 16 to 18, wherein the brakes are hydraulically actuated under control of the control system.

20. An aircraft braking system in accordance with claim 16 and substantially as described herein.