Compact actuator arrangement

1. An actuator comprising:

a motor comprising a housing and a drive shaft, the motor arranged to rotate the drive shaft relative to the housing about a drive shaft axis;

a first torque transfer device comprising a first drive member on the drive shaft and a first driven member on a second shaft, the first drive member being arranged to transfer torque to the first driven member, such that the first torque transfer device is arranged to transfer torque from the drive shaft to the second shaft, the second shaft being rotatable about a second shaft axis spaced from the drive shaft axis; and

an output torque transfer device comprising an output drive member on the second shaft and an output driven member, the output drive member being arranged to transfer torque to the output driven member, such that the output driven member rotates about an output axis,

wherein a torque ratio of the output torque transfer device is smaller than or equal to a torque ratio of the first torque transfer device, and

wherein (a) the motor housing is arranged at least partially inside the output driven member or (b) the output driven member is fixed to the motor housing.

2. The actuator of claim 1 , wherein the torque ratio between the output of the motor and the output of the output member is at least 12:1.

3. The actuator of claim 1 , wherein the motor housing has a diameter of at least 40 mm.

4. The actuator of claim 1 , wherein the first and second torque transfer devices and the motor are arranged within a cylindrical volume, the cylindrical volume having a diameter less than three times the diameter of the motor housing, optionally wherein the cylindrical volume has a diameter of 100 mm or less.

5. The actuator of claim 4 , wherein the cylindrical volume has an axial extent less than three times the axial extent of the motor housing.

6. The actuator of claim 4 , wherein the actuator comprises a housing, the housing defining the cylindrical volume, and wherein the motor, the first torque transfer device and the second torque transfer device are within the housing.

7. The actuator of claim 1 , wherein the first driven member overlaps the output driven member in a direction viewed along the drive shaft axis and/or the second shaft axis.

8. The actuator of claim 1 , wherein the second shaft extends from the first driven member to the output drive member in a first direction and wherein the drive shaft extends from the motor housing to the first drive member in a second direction opposite to the first direction.

9. The actuator of claim 1 , wherein the motor housing is arranged at least partially inside the output driven member.

10. The actuator of claim 1 , wherein the output drive member is arranged within a cylindrical sector volume, the cylindrical sector volume being defined by a cylindrical sector subtended at the drive shaft axis and having an internal angle at the drive shaft of less than 100°, optionally less than 60°.

11. The actuator of claim 1 , wherein at least one of the first torque transfer device and output torque transfer device is a pulley-based system.

12. The actuator of claim 1 , wherein the output driven member is fixed to the motor housing.

13. A robotic joint comprising:

a first member;

a second member pivotably coupled to the first member; and

the actuator of claim 1 ;

wherein the actuator is arranged to rotate the second member relative to the first member.

14. An actuator comprising:

a motor comprising a housing and a drive shaft, the motor arranged to rotate the drive shaft relative to the housing about a drive shaft axis;

a first torque transfer device comprising a first drive member on the drive shaft and a first driven member on a second shaft, the first drive member being arranged to transfer torque to the first driven member, such that the first torque transfer device is arranged to transfer torque from the drive shaft to the second shaft, the second shaft being rotatable about a second shaft axis spaced from the drive shaft axis;

an output torque transfer device comprising an output drive member on the second shaft and an output driven member, the output drive member being arranged to transfer torque to the output driven member, such that the output driven member rotates about an output axis;

a first connection flange fixed to the motor housing at an axial end of the motor housing opposite to the end of the motor housing where the drive shaft extends; and

a second connection flange arranged such that the drive shaft is between the second connection flange and the motor housing, the second connection flange being fixed to the motor housing via a cross member,

wherein (a) the motor housing is arranged at least partially inside the output driven member or (b) the output driven member is fixed to the motor housing.

15. The actuator of claim 14 , wherein the cross member is shaped as an extruded arc.

16. The actuator of claim 15 , wherein the arc has an arcuate extent of less than 180°.

17. A robotic joint comprising:

a first member;

a second member pivotably coupled to the first member; and

an actuator comprising:

a motor comprising a housing and a drive shaft, the motor arranged to rotate the drive shaft relative to the housing about a drive shaft axis;

a first torque transfer device comprising a first drive member on the drive shaft and a first driven member on a second shaft, the first drive member being arranged to transfer torque to the first driven member, such that the first torque transfer device is arranged to transfer torque from the drive shaft to the second shaft, the second shaft being rotatable about a second shaft axis spaced from the drive shaft axis; and

an output torque transfer device comprising an output drive member on the second shaft and an output driven member, the output drive member being arranged to transfer torque to the output driven member, such that the output driven member rotates about an output axis,

wherein the actuator is arranged to rotate the first member relative to the second member, wherein the motor is coupled to the second member, and wherein the second shaft is supported by the first member, and

wherein the first member is arranged to pivot relative to the second member about the drive shaft axis.

18. The robotic joint of claim 17 , further comprising a third member, arranged to rotate with the first member relative to the second member, the first and third members being arranged to support the second shaft.

19. The robotic joint of claim 17 , further comprising a second cross member, arranged to couple the first and third members to one another to provide further support to the second shaft.

20. The robotic joint of claim 17 , wherein a torque ratio of the output torque transfer device of the actuator is smaller than or equal to a torque ratio of the first torque transfer device of the actuator.