Multiple servomotor actuator

[57] ABSTRACT

Four servomotors are coupled together in a velocity summing arrangement to produce a single motion output. Two of the four servomotors are coupled together through a differential gear set that has a single rotary output. The second servomotor pair is also coupled together through a differential gear set that produces a single rotary output. These two rotary outputs of the differential gear sets are in turn coupled through a third gear set that also has a single rotary output. This rotary output may be converted into a linear motion by means of a rotary-tolinear motion transducer, or used as a rotary motion. Each of the four servomotors is controlled individually by a separate generated control signal in a system that includes a tachometer for generating velocity feedback signals and a linear variable differential transformer for generating position feedback signals.

Claims, 5 Drawing Figures

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PATENTEDJUL25 1972

3.679,956

SHEET 1 OF

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FIG. 2

INVENTOR:

WILLIAM G. REDMOND

ATTORNEYS

PATENTED Jut 2 5 isre

3,679,956

SHEET 2 OF 3

TACHOMETER

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FIG. 5

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/·( (Ol'Hf C.

PATENTED JUL 251972

3,679.956

SHEET 3 OF 3

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Al ΙΜ'ΙΠ &lt;·,

3,679,956 ¹ 2

MULTIPLE SERVOMOTOR ACTUATOR

This invention relates to a multiplex actuator and more particularly to a redundant channel, velocity summing, multiplex actuator.

It was early realized that as aircraft increase in size and 5 speed that conventional cable and mechanical linkage control mechanisms are inadequate and there is a need for electrical flight control systems. There has, however, been some reluctance to accept the electrical flight control system because it is thought that mechanical systems are more relia- 1C ble. To improve the reliability of electrical control, a system of redundant parallel channels has been implemented.

Heretofore, approaches for providing redundancy have typically resulted in double control chains or channels in which a failure in one channel hopefully would permit the other chan- * 5 nels to carry on the necessary command functions. Such a system, depending upon the particular failure suffered, generally experienced at least degradation of control when the failed channel must be “dragged” by the operating channel or channels. 20

When redundant control channels are employed to improve the system reliability the several channels must eventually terminate at a single command that positions the control surface of the aircraft. The several channels may be brought together at the single command either in a “force summing” configura- ²⁵tion or a “displacement summing” configuration. In the force summing configuration, the outputs of the several channels are connected to a common summing point. The forces developed by each of the channels are summed at this common point into a single force motion. Upon a failure of any ³⁰one of the several channels, the remaining channel will continue to position the aircraft controls through the full operating range. However, the remaining active channel must “drag” the failed channel. One of the major disadvantages of the force summing system is that a “jam” in any one of the ³several channels may result in a catastrophic failure.

When the several channels of a redundant control system are “displacement summed,” the output of each channel is combined in a series arrangement. Displacement summing (series summing) has the advantage that the remaining active channels do not have to drag the failed channel. In other words, the remaining operative channels do not fight each other force wise, as they can in the force summing configurations. In the conventional displacement summing system, the 45 several redundant channels are combined to produce the total desired range of movement for the aircraft control surface. Upon failure of one of these channels, some loss of stroke of the master power drive results with the attendant loss in the range of movement of the control surface. 50

An object of the present invention is to provide a multiplex actuator in a redundant control system wherein the actuator has full stroke output capabilities upon a failure of all but one of the redundant channels. Another object of this invention to to provide a multiplex actuator in a redundant control system 55 wherein the failure of one of the several channels does not produce a drag on the remaining active channels. A further object of this invention is to provide a multiplex actuator in a redundant control system capable of delivering full force output upon a failure of all but one of the several control chan- 60 nels. Still another object of this invention is to provide a multiplex actuator in a redundant control system capable of producing a full stroke output with all but one of the several channels jammed in a fixed position. A still further object of this invention is to provide a velocity summing multiplex ac- 65 tuator in a redundant control system.

In accordance with this invention, a multiplex actuator for positioning aircraft controls in accordance with generated control signals includes a plurality of servomotors each responsive to a separate generated signal. The servomotors 70 are coupled together to produce a single rotary output from several of the servomotors. The single rotary output from each motor group is combined with the single output of another motor group or with the output of another of the several servomotors. Velocity couplers having inputs connected to either 75 a servomotor group, a servomotor and another velocity coupler, or two other velocity couplers are employed to develop the single rotary output from a plurality of inputs. The output of the last velocity coupler in the chain is converted into a control signal that varies in accordance with the generated signals.

In accordance with a more specific embodiment of the invention, a multiplex actuator for producing a position output in accordance with generated control signals includes a first servomotor pair with each motor responsive to a separate generated signal. A velocity coupler having two inputs individually connected to one of the servomotors of the first pair combines the motor outputs and produces a single rotary output. A second servomotor pair with each motor responsive to one of the generated signals not connected to said first servomotor pair is similarly coupled together by means of a velocity coupler having inputs individually coupled to one of the servomotors of the second pair. The velocity coupler for the second servomotor pair also produces a single rotary output. A third velocity coupler having one input connected to the output of the first velocity coupler and a second input connected to the rotary output of the second velocity coupler develops a rotary output related to the two inputs connected thereto. A position control signal that varies in accordance with the pilot generated signals is produced by converting the rotary output of the third velocity coupler.

A more complete understanding of the invention and its advantages will be apparent from the specification and claims and from the accompanying drawings illustrative of the invention.

Referring to the drawings:

FIG. 1 is a schematic of a redundant control system including a multiplex actuator having a linear output coupled to a servo-valve for controlling a power ram connected to an aircraft control surface;

FIG. 2 is a block diagram of a quadruplex input velocity summing actuator producing a linear output for coupling to a servo-valve;

FIG. 3 is a schematic of a quadruplex input velocity summing actuator employing three differential gear sets to produce a single rotary output;

FIG. 4 is a cross section of a multiplex actuator in accordance with the present invention; and

FIG. 5 is a mechanical schematic of the full multiplex actuator of FIG. 4.

Although the invention will be described with reference to a quadruplex velocity summing actuator, it should be understood that other degrees of multiplexing may be employed to produce a single motion output in a velocity summing arrangement. Further, the system to be described employs servomotor coupling in pairs by means of a differential gear set. By employing other gearing arrangements, the servomotors may be coupled together in other configurations. It should be further understood that although the output of the multiplex actuator described is in the form of a linear displacement, the actuator may produce a rotary output as a control signal.

Referring to FIG. 1, there is shown a multiplex actuator 10 having four electrical signals applied to input terminals 11 through 14. These control signals may be generated in a conventional manner by a stick transducer that converts a mechanical input from a pilot’s control stick into electrical signals. The pilot’s control input, converted into electrical signals by the stick transducer, is transmitted to the terminals 11 through 14 by a parallel arrangement of wires which may be located at different paths in the air frame to minimize the possibility of a disruption of all the pilot generated control signals to the actuator 10. In addition to pilot generated signals, the electrical signals on terminals 11 through 14 may be received from autopilot sensors, a stability augmentation system, or from other systems, such as a navigation control.

The multiplex actuator 10 produces a linear motion output on a connecting rod 16; the linear motion varies in accordance with the electrical control signals on the terminals 11 through

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4 in a manner to be described. Coupled to the output of the actuator 10 is a dual tandem servo-valve 18 providing fluid pressure-flow signals to a dual tandem power ram 20. The servo-valve 18 includes a cylinder 22 having a spool valve 24 slidably disposed therein and including interconnected lands 25 through 31. Conduits 32 and 34 interconnect the first section of the tandem valve 18 to a pressure supply and reservoir (neither shown), respectively. Similarly, conduits 36 and 38 connect the second section of the valve 18 to a source of fluid pressure and a fluid reservoir (neither shown), respectively. Conduits 40 and 42 interconnect the first section of the valve 18 to the first section of the power ram 20 on opposite sides of a piston 44. Conduits 46 and 48 similarly interconnect the second section of the valve 18 to the second section of the power ram 20 on opposite sides of a piston 50.

Pistons 44 and 50 of the power ram 20 are interconnected on a piston rod 52 that has an external coupling to a link 54. Link 54 is intended to represent the mechanical linkage between a power ram and one of the control surfaces 56 of an aircraft. The piston rod 52 is in the form of a hollow shaft and is positionable over a linear voltage differential transformer 57 that generates four separate but equal position feedback signals on lines 59 through 62. Feedback signals on the lines 59 through 62 are applied to the multiplex actuator 10 to balance the pilot generated signals on the terminals 11 through 14 to stop the motion of the control surface 56 at a new desired position.

Referring to FIG. 2, there is shown a quadruplex velocity summing actuator wherein the electrical control signals are applied to the terminals 11 through 14. At present, quadruplex actuators provide the most favorable degree of redundancy for many actuator applications, but any degree of redundancy may be used with the velocity summing system of the present invention.

Referring to FIG. 2, the system consists of four sets and includes electric servomotors 66 through 69, tachometers 71 through 74, gear drives 75 through 78, and electronic circuitry, all combined to develop a linear motion for driving the dual tandem servo-valve 18 to provide positioning signals to the power ram 20. Output shafts of the motors 66 and 67, through the respective gear drives 75 and 76, are coupled to a differential 80 and the output shafts of the motors 68 and 70, through the respective gear drives 77 and 78, are fed into a differential 82. The single rotary motion output of the differentials 80 and 82 are fed to a differential 84, the rotary output of which is converted by a rotary-to-linear motion transducer 86 into linear motion for driving the dual tandem servovalve 18 through the connecting rod 16. A four-channel linear voltage differential transformer (LVDT) 58 is illustrated responsive to movement of the servo-valve 18 through a linkage 88 that is intended to represent the internal feedback loop of the actuator 10. The four-unit LVD transformer 57 is used to provide electrical signals which are proportional to the position of the connecting rod 16 for the internal follow up or inner feedback loop. Typically, a four-unit LVDT is a cluster of four separate transducers in a common housing. Thus, the actuator output is measured directly to produce a position feedback signal and the position of the control surface 56 is measured to produce system feedback signals.

In operation, an electrical control signal at the terminal 11 is applied to a summing amplifier 90 having an output for energizing the servomotor 66. Tachometer 71 responds to the speed of the motor 66 and generates a velocity feedback signal applied to a synchronizer circuit that includes an integrator 92 for first order lag feedback and a feedback resistor 94 for velocity feedback. The output of the synchronizer circuit is applied to a second input of the summing amplifier 90. The tachometer 71 provides velocity feedback to accomplish two functions. First, a velocity signal is sent back to the summing amplifier 90 through the first order lag (synchronizer) circuit in order to reduce steady state motor speeds that is, the tachometer velocity signal provides channel synchronization. The velocity feedback signal also permits use of a higher loop gain than a system with no rate feedback. This higher loop gain in turn enables better positioning accuracy and higher closed loop frequency response (i.e., bandpass). The signal on the input terminal 11 thus produces a rotary motion at the output of the gear drive 75 at a desired velocity which continues until a feedback signal from the LVDT 58 neutralizes the effect of the input terminal signal. This is accomplished when the position of the connecting rod 16 is at the command location.

An electrical command signal appearing at the terminal 12 is applied to one input of a summing amplifier 96 having an output for driving the servomotor 68. The tachometer 73 responds to the speed of the motor 68 to produce a velocity feedback signal to a synchronizer circuit consisting of an integrator 98 with a feedback resistor 100. The output of the synchronizer circuit is applied to a second input of the summing amplifier 96. This channel is similar to die channel responding to the signal on the terminal 11. An electrical control signal on the terminal 12 thus provides rotary motion at the output of the gear drive 77 at a desired velocity until the feedback signal from the LVDT 58 neutralizes the input terminal signal, as explained.

An electrical control signal at the terminal 13 is applied to one input of a summing amplifier 102 having an output for energizing the servomotor 67. The tachometer 72 responds to the speed of the servomotor 67 to produce a velocity feedback signal applied to a synchronizer circuit consisting of an integrator 104 having a feedback resistor 106. An output of the synchronizer circuit is applied to a second input of the summing amplifier 102. Similarly, an electrical control signal on the terminal 14 is applied to one input of a summing amplifier 108 having an output for energizing the servomotor 69. The tachometer 74 generates a velocity feedback signal applied to a synchronizer circuit including an integrator 110 and a feedback resistor 112. The output of this synchronizer circuit is connected to a second input of the summing amplifier 108. Those skilled in the art will recognize that synchronizers other than tachometers and integrator circuits may be used in the system of FIG. 2. For example, a total electronic synchronization scheme may replace the tachometer-integrator circuit illustrated.

A fourth input to each of the summing amplifiers 90, 96, 102 and 108 is the position feedback signal from the LVD transformer 57. The summing amplifiers each produce an output signal related to the inputs applied thereto.

Electrical control signals at the terminals 11 through 14 produce rotary motion at the output of the gear drives 75 through 78, respectively, at a desired velocity. Outputs of the gear drives 75 and 76 are combined in the differential 80 to produce a single rotary motion at a velocity related to the inputs thereto. Outputs of the gear drives 77 and 78 are combined in the differential 82 to produce a single rotary motion output at a velocity related to the two inputs thereto. The single rotary outputs of the differentials 80 and 82 are combined in the differential 84 which in turn produces a rotary motion output at a velocity related to the pilot generated signals at the terminals 11 through 14. An output of the differential 84 is converted into linear motion in the transducer 86 to position the servo-valve 18, as explained. When the commanded output position of the actuator output has been attained, the feedback signals from the LVD transformer 58 neutralizes the electrical commands at the terminals 11 through 14, and the motors 66 through 69 stop.

Referring to FIG. 3, there is shown a quadruplex velocity summing actuator wherein gear sets are employed as differentials for velocity coupling four channels into a single velocity output. Output shafts from each of the gear drives 75 through 78 are coupled to respective input bevel gears 114 through 117 of differentials 80 and 82. In the differential 80, bevel gears 118 and 120 are rotatably mounted to a spider carrier 124 to combine the output of the gear drives 75 and 76 and impart a rotary motion to a gear 126. Similarly, in the differential 82, bevel gears 128 and 130 are rotatably supported

3,679,956 on a spider carrier 132 to combine the output of the gear drives 77 and 78 into a rotary motion imparted to a gear 134. The gear 126 is coupled to an input bevel gear 136 ofthe differential 84 by means of a shaft 138. A second input bevel gear 140 of the differential 84 is connected to the gear 134 by means of a shaft 142. Rotary motion imparted to the gears 126 and 134 is combined in the differential 84 by means of bevel gears 144 and 146 rotatably mounted to a spider carrier 148. The spider carrier 148 is rotary output motion. This motion may be converted into linear movement by a crank or jackscrew or other such devices.

With an understanding of mechanical differential gear sets, it will be evident that a jamming or locking or otherwise stopping rotation of any one of the four channels will not interfere with the operation of the remaining active channels or degrade the output performance ofthe actuator by restricting the output stroke or force. For example, referring to HG. 3, assume the channel containing the servomotor 66 has failed and the bevel gear 114 is held in a fixed position. Operation of the servomotor 67 is not impaired and the bevel gear 115 will rotate at a velocity determined by the electrical control signal coupled to terminal 13. The rotary motion of the bevel gear 115 is transferred by means of the bevel gears 118 and 120 to the gear 124 and then to the gear 126. The velocity of the gear 126 will be summed with the velocity ofthe gear 134 in the differential 84 as explained. Even though one ofthe channels has failed, the motion of the output gear on the spider carrier 148 will not be affected.

The motion of the output gear of the spider carrier 148 will likewise not be affected by a failure of both servomotors coupled to the differential 80. Upon the occurrence of such a failure, the gear 126 will have a zero velocity and be held in a fixed position. The gear 134, however, rotates at a velocity determined by the servomotors 68 and 69, and this velocity is transferred to the output gear of the planet carrier 148 by the differential 84.

This failure analysis can be further carried to a failure of three of the four servomotors. Assume that only the servomotor 69 remains operative, then the velocity of the bevel gear 117 will be transferred to the gear 134 and in turn transferred to the output gear of the planet carrier 148 through the differential 84.

By sensing a failure in any one of the four channels, a brake may be applied to the output shaft of the affected gear drive or ' to the servomotor, thereby holding the corresponding bevel gear in a fixed position. This prevents the failed servomotor from being dragged by the remaining operative motor of the affected motor pair. Where the inertia of the servomotor, tachometer and gear drive combination is sufficiently high, ' and no steady state output forces must be maintained, no braking action is required to hold the bevel gear of the failed channel in a fixed position.

Mechanical differentials, either with bevel gears as illus- , trated or spur gears, provide a suitable system for summing the ' velocity output of each of four independent control channels. In addition to differential gear sets, other velocity summing devices may be employed to combine the outputs of several servomotors into a single rotary motion output at a velocity related to the servomotor control signals. Where an even number of servomotors are employed in a system, they may be velocity summed in pairs. Where an odd number of redundant channels are employed, the sum of one servomotor pair may be combined with the output of a single servomotor.

Referring to FIG. 4, there is shown in cross section a quadruplex velocity summing actuator. A servomotor, tachometer and brake unit 150 attaches to a housing 152 and has a gear cut shaft 154 extending through one end plate of .... «vuumdci _ullu urase units i the housing and engaging a spur gear 156. The spur gear 156 is 70 along with the units 150α and 160α (not shown i: direct coupled to one of the input bevel gears of a differential '' —

158. A servomotor, tachometer and brake unit 160 attaches to a bearing plate 162 that forms one end of the housing 152.

Unit 160 includes a gear cut shaft 164 engaging a spur gear ______________{Ilao 11IUU}

166 that is direct coupled to a second input bevel gear of the 75 motion by operation of the lead differential 158. A similar arrangement of two motor, tachometer and brake units having gear cut shafts coupled to the input bevel gears of a differential are included in the cutaway half of the actuator not illustrated in FIG. 4.

Differential 158 includes two output bevel gears mounted on a shaft 168 as part of a spider carrier. The spider carrier is mounted to rotate with a shaft 170 that carries an output spur gear 172. By operation of the differential 158, the velocity of the shafts 154 and 164 is summed and appears as a single rotary motion imparted to the output spur gear 172. This summing action is accomplished by the interaction of the input and output bevel gears ofthe differential 158.

Spur gear 172 engages a spur gear 174 and transfers rotary motion thereto at a velocity equal to the sum of the velocities ofthe shafts 154 and 164. Spur gear 174 is direct coupled to an input bevel gear of a differential 176 that has a second input bevel gear coupled to a spur gear 178. Gear 178 engages the output spur gear corresponding to the spur gear 172 for transferring the velocity sum of the two motor units not shown in HG. 4 to the differential 176. Differential 176 also includes two output bevel gears rotatably mounted on a shaft 180. Shaft 180 is part of a spider carrier that rotates with a shaft 182 journaled in a bearing 184. With the velocity sum of the two motor units not shown imparted to the spur gear 178, and the velocity sum of the units 150 and 160 imparted to the’spur gear 174, the output of the differential 176 equals the velocity sum of the four motor units.

This velocity summation appears as rotary motion at the shaft 182 which engages a lead screw 186 of a ball screw assembly. The lead screw 186 is rotatably supported in the housing 152 by means of bearings 188 and 190. In a conventional manner, a ball nut 192 is fitted to the lead screw 186. The ball nut 192 is restrained from rotation in the housing 152 by means of retaining pins 194 and 196 and is fixed to an extension shaft 198 that terminates in a rod end 200. The rod end 200 will be coupled to a servo-valve through mechanical linkages, as illustrated by the rod 16 in FIG. 1 connected to the servo-valve 18, or to a load directly where the power requirements are small. Thus, the operation of the lead screw 186 and the ball nut 192 functions to convert the rotary motion of the shaft 182 into linear motion at the rod end 200. This then comprises one form of the transducer 86 of FIG. 2. LVD transformers (not shown) are located outside the housing 152 and connect to the rod end 200 to sense and feedback the actuator output position (such as illustrated in FIG. 2 except that the LVDT 58 is shown connected to the servo-valve 18).

In case of a failure of the actuator of FIG. 4, a centering assembly positions the rod end 200 to a neutral position. The centering assembly includes a carriage 202 fastened to the extension 198 by means of a nut 204. The carriage 202 moves with the ball nut 192 during normal operation. Upon a failure ofthe actuator, a spring 206, maintained in a housing 208 by means of spring guides 210 and 212, forces the ball nut 192 to a center position. To allow the spring 206 to the actuator output, at least one brake must be released to allow back-driving the mechanism.

Energizing signals for the motor and brake portion of the units 150 and 160 and tachometer signals from these units are coupled to the actuator by means of electrical connectors 214 and 216. Two additional connectors (not shown) provide for connecting signals to the units not shown; such an arrangement maintains four channel independence. These connectors 65 carry the electrical signals to and from the amplifiers and synchronizer circuits of HG. 2.

Referring to FIG. 5, there is shown a mechanical schematic of the actuator of FIG. 4. The velocity of the output shafts of the servomotor, tachometer and brake units 150 and 160 _____________in FIG. 4) are summed by differential gear sets 158,176 and 158α, the latter not shown in HG. 4. The velocity sum of the four motor units appears as a single rotary motion at the output shaft 182 of the differential 176. This rotary motion is converted into a linear . . -----------J screw 186 and the ball nut

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192. In a typical embodiment, there are 20 gear teeth on the output shaft of the motor units 150, 160, 150α and 160α. These shafts engage spur gears 156, 166, 156α and 166α, respectively, having 180 teeth. The output spur gear 172 of the differential 158 and the output spur gear 172α of the differential 158 a, along with the spur gears 174 and 178 have 119 gear teeth. In this typical embodiment, a 0.250 inch lead screw 186 was coupled to the shaft 182. Using 10,000 RPM motor units, the linear velocity of the rod end 200 for each motor unit can be calculated as follows:

1 119 1 0.250 in. I min.

^{V =} 10.000 REV X-jgjjXjXj^XjX REV ^X60 sec.

= 1.58 in/sec/motor. (I)

The total linear velocity of the rod end 200 would be equal to the above figure times the number of motor units operating. Thus, even if all the motor units but one jammed or became inoperative for any reason, the rod end 200 is positionable over its full stroke, and the actuator delivers the same force to position any device as if all four channels were active and operating.

As previously explained, a multiplex actuator may include an odd number of servomotors, the velocities of which are to be summed. Assume that only three motor units were considered for the actuator of FIG. 5. If units 150, 160 and 160α are to be velocity summed, the units 150 and 160 are connected as illustrated. Unit 160α, on the other hand, is connected directly to the spur gear 178 through the output shaft. The velocity of this unit is then summed with the velocity of the spur gear 172 of the differential 158. If more than two motor pairs are to be velocity summed, additional differential gear sets are required. The number of differential gear sets required in a system is equal to one less than the number of servomotor outputs to be summed. The end result in any case is a single rotary output at the shaft 182.

While only one embodiment of the invention, together with modifications thereof, has been described in detail herein and shown in the accompanying drawings, it will be evident that various further modifications are possible without departing from the scope of the invention.