This invention relates to a rotary actuator.

It is often desired to provide rotary actuators, which provide rotating relative movement between two members. In particular situations, it is desirable to provide an electrically driven mechanical actuator within a relatively compact space. There are a large variety of applications for such an actuator including the replacement of hydraulic operated devices.

Furthermore, it is generally desirable to produce compact gear mechanisms which can be used in high-torque situations.

According to a first aspect of the invention, we provide a rotary actuator comprising:

• a housing; and

• a first gear arrangement comprising:

o a first annular gear having a first axis about which it is rotatable in a reaction member;

o a pair of second annular gears, each being fixed relative to the reaction member;

o a set of planet gears which each engage the first annular gear and the second annular gears; and

o a sun gear rotatable about the axis, and which engages each of the planet gears;

• an input, coupled to the sun gear of the first gear arrangement; and

• an output, coupled to the first annular gear.

Thus, this provides for a compact rotary actuator that can be used in high ratio, high torque situations. The loads on the common planet wheels are balanced by the use of the two annular gears. This reduces the loads on the planet gears such that, rather than requiring a planet carrier and bearings supporting the planet gears on the planet carrier, all that is required is a pair of simple rolling rings. This allows the gearbox to be lighter and more compact than in prior art arrangements. This actuator can provide a rotary torque actuator with up to 330 degrees of operation, typically driven by a high-speed electric motor with position feedback, via this gearbox arrangement, embodied in a compact structural body that allows significant torque, moments and vertical loads to be imposed on the actuator.

Typically, the rotary actuator will further comprise a second gear arrangement comprising an epicyclic gear comprising a sun gear coupled to the input, an annular gear fixed relative to the reaction member and a plurality of planet gears, the planet gears being carried on a planet carrier coupled to the sun gear of the first gear arrangement.

As such, the first gear arrangement can act as a high torque stage of the gear mechanism, and the second gear arrangement as a low torque stage.

The pair of second annular gears, and also potentially the first annular gear may be symmetrical about a plane perpendicular to the axis. The pair of annular gears will typically have internal teeth; the teeth of one of the pair of annular second gears will typically be aligned circumferentially about the axis with the teeth of the other of the pair of second annular gears.

This allows the load on the planet gears to be symmetric, as the pair of second annular gears can be arranged symmetrically about the first annular gear. Typically, the first annular gear and each second annular gear will be parallel to each other. A notable feature of the split annular gears is they can be reacted elastically and symmetrically to maintain planet tooth alignment under high torque applications.

Each of the first and second annular gears may have teeth by means of which they engage the planet gears. The first annular gear may have a different number of teeth to each second annular gear, even if the first and second annular gears have the same internal meshing radius. This can be achieved by the teeth of the first annular gear having a different correction factor to the teeth of each second annular gear. The difference in number of teeth may be equal to the number of planet gears.

The input may comprise an input for a motor, such an electric motor; the actuator may comprise a motor coupled to the input. The output may comprise a moving part which moves relative to the housing. Typically, the housing will comprise a flange (which may be fixed relative to the reaction member), and the moving part will comprise a flange, with the flange of the output being drive for movement relative to the flange of the housing by rotation of the input. The actuator can be controlled by measuring the housing position relative to the reaction housing and driving the motor appropriately.

There now follows, by way of example only, description of embodiments of the invention, described with reference to the accompanying drawings, in which:

Figure 1 shows a schematic view of a gear mechanism for use in a rotary actuator in accordance with a first embodiment of the invention;

Figure 2 shows a cross section through the gear mechanism of Figure 1 ;

Figure 3 shows a perspective view of a rotary actuator using the gear mechanism of Figure 1 ; and

Figures 4 to 6 show plan views of the rotary actuator of Figure 3 in different positions.

Figures 1 and 2 of the accompanying drawings show a gear mechanism 2 for use in a rotary actuator 1 as shown in Figures 3 to 6 of the accompanying drawings, all in accordance with a first embodiment of the invention.

The gear mechanism comprises an input 3 , coupled to an electric motor 50. The gear mechanism is mounted within housing forming a reaction member 6, which is coupled to a first flange 5 1. The output 4 of the gear mechanism is coupled to a second flange 52, so as to drive the first 5 1 and second 52 flanges for relative rotational motion as shown in Figures 4 to 6. As such, the gear mechanism comprises a first, low torque stage 61 and a second, high torque stage 62.

The low torque stage 61 comprises an epicyclic gear arrangement, comprising an annular gear 10 fixed relative to housing 6 but coaxial with axis 16. Inside the annular gear 10, there is provided a planet carrier 1 1 , carrying a set of planet gears 12 which mesh with the internal gearing of the annular gear 10, the planet carrier 1 1 being rotatable about axis 16 in the housing 6. Inside the planet carrier 11 is provided a sun gear 13 that is coupled to the input 3.

The high torque stage 62 comprises a first annular gear 7 coupled to the output 4. The annular gear 7 is rotatable in the housing 6 about axis 16. The high torque stage 62 also comprises a pair of second annular gears 8, mounted either side of the first annular gear 7 coaxially with the axis 16, but which are fixed relative to the housing

Inside the annular gears 7, 8 there are provided common planet gears 9 which mesh with the internal gearing of both first and second annular gears 7, 8. Each of the planet gears 9 can rotate about an axis parallel to axis 16, the axes (and so the planet gears 9) being equally spaced around the axis 16. The planet gears 9 are supported in the housing 6 by a pair of rolling rings 19. Because the second annular gears 8 are symmetrical, the loads on those planet gears 9 are sufficiently low that they can be reacted by the rolling rings 19 rather than requiring a planet carrier or planet bearings supporting the planet gears on a planet carrier.

Inside the planet gears 9 is a sun gear 18 which meshes with the planet gears 9 and is coupled to the planet carrier 11 of the low torque stage 61.

As such, this embodiment can take a low torque, high speed input and provide a high torque low speed output. In one particular embodiment, the various gears have the following numbers of teeth per gear:

• Low Torque stage:

o Sun gear 13: 22 teeth

o Planet gears 12: 44 teeth

o Annular gear 10: 110 teeth

• High torque stage:

o Sun gear 18: 22 teeth

o Planet gears 9: 57 teeth

o First annular gear 7: 140 teeth

o Second annular gears 8: 137 teeth In this case, if the electric motor 50 provides the input 3 with 0.330 Newton metres (Nm) of torque at 2752.15 revolutions per minute (rpm), then the output 4 (and so the first 51 and second 52 flanges relative to each other) will rotate at 1.36rpm but with 667.3Nm of torque. This gives the gear arrangement a ratio of 2022.

In order to achieve this, we have been careful to ensure that the teeth of the two second annular gears 8 are precisely aligned with each other, and that the angular elastic deflection of each of those second annular gears 8 is the same under load, to maintain tooth alignment. As such, the high torque stage 62 is symmetrical about a plane perpendicular to the axis through the centre of the first annular gear 7.

This rotary actuator can achieve up to 330 degrees of operation, driven by a high speed electric motor with position feedback, embodied in a compact structural body that allows significant torque, moments and vertical loads to be imposed on the actuator.