GB2189078A - Electromagnetic actuator - Google Patents

Abstract

The actuator, which is especially for use in a hydraulic servo system for moving a control surface of an aircraft, comprises an armature 39 and a stator 42 one of which is ironless, and independent windings 50, 51, 52 to make the actuator failsafe. A transmission 56 is provided through which drive is transmitted from the armature to a member 58 driven by the actuator. <IMAGE>

Description

SPECIFICATION Electrically operable actuator The invention relates to an electrically operable actuator and is particularly but not exclusively concerned with an actuator for use in a hydraulic servo system for moving a control surface of an aircraft.

In the field of aircraft control, it is common to use hydraulics to control movement of ailerons, elevators, etc. Atypical system is shown in Fig. 1 of the accompanying drawings where a joystick, autopilot or autostabilizer operates on an armature 1 of a miniature force motor 2. The armature 1 is fast with a finger 3 which extends between opposed nozzles 4, 5 of fluid outlet passages 6,7 of a hydraulic flow control system 8. Fluid from a supply 9 is fed to inlets a, b, c, d, of a spool valve 10, flow of fluid through the valve to outlets e, f being controlled by a slidable spool 11. The spool includes a pair of central lobes 12 which locate therebetween a feedback spring 13 connected to the finger 3. A valve of this type is described in detail in U.K.Patent No.923,839. Movement of the armature 1 causes the finger 3 to move closer to one of the nozzles, e.g.

nozzle 4 thereby causing in that case pressure in passage 6 to increase and pressure in passage 7 to decrease. In that way, the spool 11 is caused to move axially to uncover an appropriate inlet port, e.g. port d, thereby allowing fluid to flow at a metered rate to outlet f. The spool movement is fed back to the armature 1 via feedback spring 13 thereby ensuring that movement of the spool will be proportional to the force applied to the armature.

Outlets, e,f are connected to inlets 17, 18 of a servo 20 having a piston 22 connected, e.g. to an elevator horn 23. Fluid fed to the servo causes the piston 22 to move to the right or the left to control the elevator. A further electrical feedback is taken from movement of the piston 22, e.g. by potentiometer 24 to cancel the incoming signal to the force motor 2.

In the present day aircraft, it is common to use multiple systems 8 and servos 20, e.g. three or four, in order to provide redundancy in any event of failure. The miniature force motors 2 rely on flow through the nozzles 4, 5 to provide an effective amplification of the minute forces of the force motors. The aggregate of such flows can be a considerable volume per second bearing in mind that twenty or more systems 8 may be in use. Such flow represents power loss and generation of heat which needs to be dispersed and this presents a problem. Moreover, compared to the robust and generally satisfactory components such as the spool valve 10 and servo 20, the miniature force motor 2 has only a feeble output very sensitive to variations in hydraulic and electrical supply and to imperfections in its various mechanical components.

Consequently, it has been proposed as in Fig. 2 to control the valve 13 directly by means of a solenoid 30 operable in response to the joystick 10. It is desirable to exert sufficient axial load on a valve spool having pistons 15, to enable a chip of metal to be sheared if it becomes lodged in a port 15a, 16a of the valve in the path of the valve spool. If the weight of the solenoid is to be kept low, it is not possible to produce both the required force and a large axial movement of the solenoid armature. In practice, such movement may be as low as +0.016" (0.406 mm). In view of such small movements, it is necessary for the outlet ports 15a, 16a in the valve 13 to be of increased width Was in Fig. 3 to reduce resistance to flow, thereby complicating construction of the valve.Moreover, even though attempts are made to keep the weight of a solenoid to a minimum, the amount of iron required in the solenoid for efficient operation makes the solenoid an inherently heavycomponent. Also the mass of the armature can result in poor response to input signals.

An object of the present invention is to provide an improved electrically operable actuator.

According to one aspect of the invention, there is provided an electrically operable actuator comprising an armature and a stator one of which is ironless and comprises a plurality of independent windings, and means through which drive is transmitted, in use, from the armature to a member driven by the actuator.

According to another aspect of the invention, there is provided an electrically operable actuator in combination with a valve for operating a servo arranged to move a control surface of an aircraft, the actuator comprising an armature and a stator one of which is ironless and comprises a plurality of independent windings, and means through which drive is transmitted, in use, from the armature to a member of the valve to enable operating fluid to pass through the valve to said servo.

By using an ironless armature or stator, the weight of the actuator can be kept to a minimum.

In order to provide a failsafe system, the windings may be arranged so that if one or more of the windings becomes defective, the remaining windings or winding substantially compensate for the or each defective windings.

Preferably, the armature is ironless and each winding of the armature may be associated with a respective commutator. Each commutator may be supplied with current via a pair of associated brushes. All of the brushes may be supplied with electric current simultaneously. The commutators may be axially spaced apart.

The armature may be cylindrical with the stator coaxially within the armature. Preferably the armature is rotatable. Where rotary movement is being utilised to effect the movement of the member, it is possible to effect a much greater axial movement of the member.

The means through which drive is transmitted may comprise a gearing having an output member which effects axial movement of the member to be driven by the actuator. The output member may be rotary and may include an eccentric element which effects said axial movement of the member or may include a screw threaded portion which engages an element for effecting said axial movement of the member to be moved by the actuator.

Sensor means may be provided for sensing the position of the member. The sensor may pick up the position of the member directly by sensing the position of an element of the transmission means, e.g. the output member.

An electrically operable actuator in accordance with the invention will now be described with reference to the following accompanying drawings in which: Fig. 4 is a longitudinal cross section through a preferred form of actuator in accordance with the invention; Fig. 5 is an elevation of the actuator of Fig. 4 together with a reduction gearing; and Fig. 6 is a view of the actuator and gearing of Fig. 5 looking in the direction of arrow VI in Fig. 5.

The actuator has a housing 40 supporting a permanent magnet stator 42. The stator is cylindrical and has a reduced diameter section 43 formed with an axial bore 44. The stator 42 carries two bearings 45 which rotatably support a shaft 46.

The left hand end of the shaft 46 as viewed in Fig. 4 projects from the bore 44 and supports tubular sections 47,48,49 for three independent sets of coaxial armaturewindings 50,51,52 respectively forming an armature 39.

Each tubular section 47,48,49 includes a commutator associated with a respective set of brushes 53,54, 55. Each of the windings 50,51, 52 may of a self supporting basket type construction to avoid the need for an iron core. Preferably, in each winding the turns lie closely adjacent each other (preferably without a space therebetween). A motor having an ironless armature is described in a technical publication produced by Portescap entitled Think Escap 3. However, in the motor described in that publication, the armature includes only a single winding and is not, therefore, failsafe.

The actuator of the present invention includes three independent windings which are fed simultaneously with electric current. In the event that one winding fails (open circuit) the remaining two windings will maintain the actuator in an operational condition. If a second circuit fails in the same way, the third winding remains operational. If one or more windings fail (closed circuit) the or each remaining winding is arranged to resist any tendency for the armature speed to increase. Therefore, the actuator is fail safe for up to failure of two windings. If necessary, two independentwindings or four or more independent windings could be used. Rotation of the armature 39 turns the shaft 46. As shown in Fig. 5 a gearbox 56 containing reduction gearing (not shown) is carried by the right hand end of the housing 40.The right hand end of the shaft 46 is formed with a spline S which engages a complementary spline on a drive input member of the reduction gearing. The gearing has an output shaft 57 which is drivably connected to an arm 58.

As shown in Fig. 6, the arm 58 is connected to one end of a link 59 which is connected at its opposite end to a rod 60 connected to an axially movable valve member (not shown) of a valve 62. The valve 62 controls flow of the fluid to and from a servo 63 which may be used to operate a control surface 64 of an aircraft.

To move the joystick by a desired amount, a joystick 65 is shifted so as to operate a potentiometer which enables electrical current fed to the armature to be varied. The armature 39 rotates and causes the arm 58 to turn so as to open the valve 62. With such an arrangement, there is far less constraint on the amount of available movement than with a solenoid as proposed hitherto. Theoretically, it is possible to obtain substantially + x amount of movement (Fig. 5) of the valve member from a datum with the proposed valve and that can be varied by altering the length of arm 58. The reduction gearing also provides a high mechanical advantage to enable the valve member (or a spool associated therewith) to shear chips of metal which may become jammed in a port of the valve 62.If desired, the output from the joystick controlled potentiometer can be fed to a computer which will modify the current fed to the armature 39.

The position of the valve member can be determined by counting the revolutions of the shaft 46 by a device 62 which includes an optical encoder on the left hand end of shaft 46. The signal from the device 62 can be used to cut off the current supply to the armature when the valve member reaches the desired position. Alternatively, the movement of the valve member may be sensed by a potentiometer, e.g. of the inductive type.

Instead of using a permanent magnet type stator 42, an electromagnetic type could be used. Also, instead of using an ironless armature, the stator could be ironless and the armature could comprise a permanent magnet. However, in view of the mass of the magnet, such an armature would have much greater inertia than the ironless type shown in Fig. 1 and would be slower to respond to input signals.

If all three windings fail, the aircraft control surface will in any event take up its normal failsafe position.

Claims (13)

1. An electrically operable actuator comprising an armature and a stator one of which is ironless and comprises a plurality of independent windings, and means through which drive is transmitted, in use, from the armature to a member to be driven by the actuator.

2. An electrically operable actuator in combination with a valve for operating a servo arranged to move a control surface of an aircraft, the actuator comprising an armature and a stator one of which is ironless and comprises a plurality of independent windings, and means through which drive is transmitted, in use, from the armature to a member of the valve to enable operating fluid to pass through the valve to said servo.

3. An actuator according to Claim 1 or 2 in which the windings are arranged so that if one or more of the windings becomes defective, the remaining windings or winding substantially compensate for the or each defective winding.

4. An actuator according to Claim 1,2 or 3 in which the armature is ironless and each winding is associated with a respective commutator.

5. An actuator according to Claim 4 in which the commutators are axially spaced apart.

6. An actuator according to any preceding claim in which the armature is cylindrical and the stator is arranged coaxially within the armature.

7. An actuator according to any preceding claim in which the armature is rotatable.

8. An actuator according to any preceding claim in which the means through which the drive is transmitted comprises a gearing having an output member which effects axial movement of the member to be driven by the actuator.

9. An actuator according to Claim 8 in which the output member is rotary.

10. An actuator according to Claim 9 in which the output member includes an eccentric element which effects said axial movement of the member.

11. An actuator according to Claim 9 in which the output member includes a screw threaded portion which engages an element for affecting said axial movement of the member to be moved by the actuator.

12. An actuator according to any preceding claim in which means is provided for sensing the position of the member to be moved by the actuator.

13. An actuator constructed and arranged substantially as described herein with reference to Figs. 4,5 and 6 of the accompanying drawings.