TWO MOTOR REDUNDANT DRIVE MECHANISM

Abstract: A redundant drive mechanism (10) comprises a platform (16) and a shaft (30) rotatably mounted with respect to the platform (16). A first motor (42) is stationarily mounted with respect to the platform (16). The first motor (42) drives a first pinion (50). A ring gear carrier (56) is mounted on the shaft (30) and is rotationally fixed to the shaft (30). A ring gear (62) is rotatably mounted on the ring gear carrier (56). The first pinion (50) is in gear-tooth engagement with the ring gear (62). A second motor (44) is mounted on the ring gear carrier (56). The second motor (44) drives a second pinion (52) which, in turn, is in gear-tooth engagement with the ring gear (62). Energization of either the first or second motor (42, 44) causes rotation of the shaft (30) with respect to the platform (16).

Description: This invention relates to a redundant drive mechanism for an antenna of a satellite comprising: A platform; a first motor stationarily mounted with respect to the platform and a first pinion, the first motor being connected to drive the first pinion for rotation about a first pinion axis; a ring gear being in gear-tooth engagement with the first pinion; a second motor and a second pinion, the second being connected to drive the second pinion for rotation about a second pinion axis and the second pinion also being in gear-tooth engagement with the ring gear. A mechanism of the aforementioned kind is known from document US-A-3,624,656. In more general terms the invention is directed to a redundant drive mechanism where two motors have an input to an all-spur gear mechanism where either motor can be energized to drive the output shaft. Reliability is achieved by conservative design wherein parts are physically loaded to stresses far below their yield point and wherein duplicate mechanisms are provided for those parts which have the highest failure rates, even with conservative design. In some environments, the weight associated with conservative design is also a negative factor. The sheer cost of the additional material and the machine time involved in handling the additional material is only a starting additional cost. In those systems where the mechanism will be involved in transportation, the additional weight will be a continuing penalty. Whether on highway, rail or flying vehicles, the excess weight of conservative design poses a penalty. The penalty becomes quite large in air flight and space flight. As a result of such penalties, original design must incorporate factors which maximize reliability but minimize weight. US-A-3,624,656, mentioned at the outset, discloses a drive mechanism of the kind described above; however, this prior art drive mechanism is intended for use in a radar antenna, especially in airport surveillance. A common problem in such applications is that downtime due to repair is extremely undesirable and thus should be as short as possible. For this purpose, US-A-3,624,656 provides a bull gear for rotation of the antenna assembly, the bull gear being rotated by means of two engaging pinions respectively driven by electric motors. DE-A-33 41 320 relates to a rotatable drive mechanism for military equipment, e.g. rotatable cannon-bearing towers. Three rings are provided, wherein the outer ring is connected with the lower part of the apparatus and contacts the middle ring. The middle ring, in turn, contacts the inner ring which is connected with the rotatable upper part (e.g. tower). Two driving motors are in fixed connection with the upper and lower part of the apparatus, respectively. Their pinions engage the middle ring via appropriate teeth. Thus, in case both motors are driven in the same direction at the same speed, the middle ring is permanently rotating, without any rotational movement of the upper part. However, any variation in the speed, or the direction, of one of the motors causes the upper part to rotate. It is, therefore, an object of the present invention to provide a redundant drive mechanism which is light of weight and capable for space application. According to the drive mechanism mentioned at the outset this object is achieved by the following feature: A shaft rotatably mounted with respect to the platform for rotation about a shaft axis with respect to the platform, wherein the first and second pinion axes are substantially parallel to the shaft axis; a ring gear carrier mounted on the shaft and rotationally fixed to the shaft, wherein the ring gear is rotatably mounted on the ring gear carrier for rotation about the shaft axis and wherein the second motor is mounted on the ring gear carrier; so that energization of either the first or second motor causes rotation of the shaft with respect to the platform. The present two-motor redundant drive mechanism is thus a low weight mechanism with two redundant drive motors and is configured to provide reliable drive for a number of different mechanical devices. In space, it is capable of raising an antenna from a stowed to a deployed position. In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to two-motor redundant drive mechanism wherein each motor can individually drive the output shaft by spur gear engagement. It is, thus, a purpose and advantage of this invention to provide a two-motor redundant drive mechanism which is constructed with spur gear engagement for ease of manufacture and elimination of thrust loads, and reliably drive an output shaft. FIG. 1 is a side-elevational view of a satellite, showing one application for the two-motor redundant drive mechanism in accordance with this invention. FIG. 2 is an elevational view, as seen generally along line 2-2 of FIG. 1, with parts broken away. FIG. 3 is a perspective view of the drive mechanism. FIG. 4 is an enlarged section through the two-motor redundant drive mechanism of this invention, as seen generally along 4-4 of FIG. 1. FIG. 5 is a transverse section through the main shaft of the mechanism, as seen generally along the line 5-5 of FIG. 4, with parts broken away. The two-motor redundant drive mechanism of this invention is indicated at 10 in FIGS. 1 and 2. As a particular example of utility, the mechanism 10 is shown in connection with the satellite 12, although it is useful with other devices, as described above. The satellite 12 includes a rotating body 14 which may carry on the surface thereof a plurality of solar cells. In order to provide directional capability, the satellite carries a despun platform 16 which can be rotated with respect to the body 14 so that the despun platform can carry equipment which can point in a desired direction. Such equipment may include an antenna reflector 18 which faces an antenna feed (not shown) on the platform. Antenna 18 is shown in its erected position in full lines in FIGS. 1 and 2 and is shown in its stowed position in dashed lines in FIG. 1. In the stowed position, the antenna 18 forms a compact, easily transportable structure and can be secured in place by various clamps to give it additional vibration-resisting strength. In the deployed position, antenna 18 is directed at the desired target. Ribs 20 and 22 are secured to the back of the antenna and support the antenna in the erect position. Bearing housing 24, see FIGS. 3 and 4, is in the form of a cylindrical tube and is mounted on feet 26 and 28. These feet are secured to platform 16 by means of bolts which extend up through the platform into the bolt holes shown in dashed lines in FIG. 4. Shaft 30 is the main antenna support shaft of the mechanism and is rotatably mounted within bearings 32 and 34 within housing 24. Flange 36 is secured on the main shaft 30. Flange 36 engages against tube 35 which engages against bearing 34 to serve as an axial thrust bearing for the shaft 30. Flange 36 also carries bolted thereto the antenna rib 22. On the other end of main shaft 30, flange 38 is carried on hub 40, which is secured on main shaft 30. Tube 37 bears against bearing 32 as a thrust bearing for shaft 30 facing in the opposite axial direction. In this way, main antenna support 30 is constrained radially and axially to minimum play and is permitted only rotational motion about its axis to raise the antenna from stowed to deployed position. Two motors 42 and 44 are connected to rotate shaft 30 and thus raise the antenna. In the embodiment shown, the motors are preferably step motors so that angular movement can be accurately controlled. The motors respectively carry gear reducer housings 46 and 48 which contain reduction gears which respectively drive spur gear pinions 50 and 52. The reduction gears are of a high enough ratio so they can only be driven from the motor end. Housing 46 is fixed to foot 28 by means of bracket 54. Ring gear carrier 56 is fixed to and is formed as part of hub 40 and flange 38, and thus rotates therewith. Ring gear carrier 56 is circular on its outer periphery and carries a ball bearing raceway therein. A plurality of balls is positioned around this raceway, and balls 58 and 60 are shown in FIG. 3. Ring gear 62 is a circular ring with external spur teeth. Ring gear 62 has a ball raceway on its inner diameter, with its raceway engaged upon the balls 58 and 60, and the additional balls. In this way, ring gear 62 is rotatably mounted on ring gear carrier 56. Spur gear pinions 50 and 52 are in gear-tooth engagement with the spur tooth ring gear 62, as seen in FIGS. 3 and 5. Gear Housing 48 is mounted on bracket 64, and the bracket is in turn mounted on ring gear carrier 56, see FIGS. 4 and 5. Suitable mounting bolt holes are shown in dotted lines in both of those figures. It could be mounted on shaft 30 or something fixed to shaft 30 such as hub 40. Motors 42 and 44 are any suitable motors and are connected to a control supply. They may be stepper motors, where the speed and total number of revolutions can be readily controlled. The gear reductions in the gear housings are sufficiently large that the gear trains cannot be back-driven. Thus, torque at either pinion will not back-drive the other. When motor 42 is powered and motor 44 is not, pinion 50 rotates ring gear 62 with respect to the platform and pinion 52 and bracket 64 lock the ring gear to the ring gear carrier 56. Thus, rotation of motor 42 causes rotation of shaft 30. When there is no power to motor 42, the ring gear 62 is fixed with respect to the platform. Power to motor 44 causes rotation of ring gear carrier 56 and shaft 30 with respect to the platform. In this way, powering of either motor causes deployment of the antenna. By the use of spur gears, the mechanism 10 is more tolerant of misalignment and changes in center distance, as compared to bevel gear structures. In addition, it must be noted that both thrust loads and radial loads of the antenna on the mechanism are carried directly through the bearings 32 and 34 onto the platform, and only rotational loads are carried by the gearing. An additional advantage of this structure is seen in the fact that those portions carrying the gear teeth are quite small, and the balance of the mechanism can be made of light alloys, such as aluminum to minimize weight and minimize differential expansion due to temperature changes.

Claims: Redundant drive mechanism (10) for an antenna (18) of a satellite (12) comprising: a platform (16); a first motor (42) stationarily mounted with respect to the platform (16) and a first pinion (50), the first motor (42) being connected to drive the first pinion (50) for rotation about a first pinion axis; a ring gear (62) being in gear-tooth engagement with the first pinion (50); a second motor (44) and a second pinion (52), the second motor (44) being connected to drive the second pinion (52) for rotation about a second pinion axis and the second pinion (52) also being in gear-tooth engagement with the ring gear (62); characterized by a shaft (30) rotatably mounted with respect to the platform (16) for rotation about a shaft axis with respect to the platform (16) wherein the first and second pinion axes are substantially parallel to the shaft axis; a ring gear carrier (56) mounted on the shaft (30) and rotationally fixed to the shaft (30), wherein the ring gear (62) is rotatably mounted on the ring gear carrier (56) for rotation about the shaft axis, and wherein the second motor (44) is mounted on the ring gear carrier (56); so that energization of either the first or second motor (42,44) causes rotation of the shaft (30) with respect to the platform (16). Redundant drive mechanism (10) according to claim 1, characterized by irreversible drive means between either the first motor (42) and the first pinion (50), or between the second motor (44) and the second pinion (52), for driving the pinions (50,52), respectively, so that torques on either of the pinions (50,52) cannot rotate the associated motor (42,44). Redundant drive mechanism (10) according to claim 1 or 2, characterized by a bearing housing (24) mounted on the platform (16), wherein the shaft (30) is rotatably mounted on the bearing housing (24) for rotation about the shaft axis. Redundant drive mechanism (10) according to claim 3, characterized by mounting means on each end of the shaft (30) exterior of the bearing housing (24) for attachment of members to be moved by rotation of the shaft (30) about the shaft axis. Redundant drive mechanism (10) according to any of the preceding claims, characterized by a first gear reduction between the first motor (42) and the first pinion (50), a second gear reduction being between the second motor (44) and the second pinion (52) so that each of the pinions (50, 52) turns slowly with respect to the speed of rotation of its corresponding motor (42,44). Redundant drive mechanism (10) according to any of the preceding claims, characterized by each of the first and second pinions (50,52) and the ring gear (62) being a spur gear to eliminate axial loads on the shaft (30) due to pinion torque or gear-tooth engagement. Redundant drive mechanism (10) according to any of claims 3 to 6, characterized by first and second bearings (32,34) being mounted in the bearing housing (24), the shaft (30) being mounted in the first and second bearings (32,34) for rotation with respect to the platform (16), the bearings (32,34) being arranged to resist both radial and axial loads from the shaft (30) onto the bearings (32,34). Redundant drive mechanism (10) according to claim 7, characterized by the bearings (32,34) rotatably supporting the shaft (30) comprising sleeve bearings, one adjacent each end of the bearing housing (24), each of the sleeve bearings having a radial face and an axial face, respectively, for carrying axial loads and radial loads, respectively, of the bearings (32,34) with respect to the bearing housing (24). Redundant drive mechanism (10) according to any of claims 4 to 8, characterized by the mounting means for the second pinion (52) comprising a bracket (64) mounted on the ring gear carrier (56), the bracket (64) carrying the second motor (44) and the second pinion (52). Redundant drive mechanism (10) according to any of the preceding claims, characterized by ribs (20,22) being attached to the shaft (30), the ribs (20,22) being support ribs (20,22) for the antenna (18).

Inventors: GRANT, RICHARD W.
Applicants: HUGHES AIRCRAFT COMPANY
Application year: 1988
Main classification: H02K7
IPC codes: B64C13, H02K16, F16D65, H01Q3, H02K7, F16H37, F16H1