Stabilized gimbal system with unlimited field of regard

1. A gimbal system, comprising:

a gimbal structure with four or more axes; and

a control system comprising:

one or more processors; and

memory storing instructions that, when executed by the one or more processors, cause the control system to:

read inputs from a set of encoders on the four or more axes and a set of inertial sensors mounted to the gimbal structure;

read command input comprising a desired inertial rate for the gimbal structure;

compute a set of motor commands with a first feedback control loop to achieve the desired inertial rate; and

update the set of motor commands with a second feedback control loop to prevent the gimbal structure from:

obscuring a line-of-sight of a payload mounted to the gimbal structure; and

entering a gimbal lock configuration.

2. The gimbal system of claim 1 , further comprising:

a rotating joint fitted to one or more axes of the gimbal system.

3. The gimbal system of claim 1 , further comprising:

one or more inertial sensors that provide inertial angular rate measurements for stabilization of the payload by the gimbal structure and the control system.

4. The gimbal system of claim 3 , wherein the memory further stores instructions that, when executed by the one or more processors, cause the control system to:

use the inertial angular rate measurements to estimate a horizon and control a roll of the payload relative to the horizon.

5. The gimbal system of claim 3 , wherein the memory further stores instructions that, when executed by the one or more processors, cause the control system to:

use the inertial angular rate measurements to estimate the attitude of the payload and point the payload relative to a coordinate system.

6. The gimbal system of claim 1 , wherein the payload comprises a camera.

7. The gimbal system of claim 1 , wherein the four or more axes comprise:

a first axis;

a second axis that is perpendicular to the first axis;

a third axis that is perpendicular to the second axis; and

a fourth axis that is perpendicular to the third axis.

8. The gimbal system of claim 7 ,

wherein the fourth axis comprises a large-diameter hollow mechanism within which the payload is mounted, and

wherein the third axis has limited rotation.

9. The gimbal system of claim 7 ,

wherein the third axis has a large-diameter hollow mechanism, and

wherein the fourth axis has limited rotation.

10. The gimbal system of claim 7 , wherein the gimbal structure further comprises:

one or more additional axes of rotation fitted within or outside the first, second, third, and fourth axes of the gimbal structure.

11. The gimbal system of claim 1 , wherein the control system further comprising:

a relative rate feedback filter that generates commands to one or more motors coupled to the four or more axes through an inverse dynamics transform.

12. The gimbal system of claim 1 , wherein the memory further stores instructions that, when executed by the one or more processors, cause the control system to:

stabilize the gimbal system based on inertial rates from one or more inertial measurement units coupled to the gimbal system.

13. The gimbal system of claim 1 , wherein the updating the set of motor commands with the second feedback control loop comprises:

centering an axis of the gimbal system to prevent the gimbal lock configuration in the gimbal system and obscuration of the line-of-sight of the payload by the gimbal system.

14. A method for controlling a gimbal structure with four or more axes, comprising:

reading inputs from a set of encoders on the four or more axes and a set of inertial sensors mounted to the gimbal structure;

reading command input comprising a desired inertial rate for the gimbal structure;

computing, by a processor, a set of motor commands with a first feedback control loop to achieve the desired inertial rate; and

updating the set of motor commands with a second feedback control loop to prevent the gimbal structure from:

obscuring a line-of-sight of a payload mounted to the gimbal structure; and

entering a gimbal lock configuration.

15. The method of claim 14 , further comprising:

inputting an estimated attitude of the gimbal structure into the second feedback control loop prior to updating the set of motor commands with the second feedback control loop.

16. The method of claim 14 , wherein the inputs comprise:

a relative angular rate measurement from the set of encoders; and

an inertial rate measurement from the set of inertial sensors.

17. The method of claim 16 , wherein computing the set of motor commands to achieve the desired inertial rate comprises:

applying an inverse dynamics transform to the relative angular rate measurement to produce a motor torque for achieving an angular acceleration of an axis in the four or more axes.

18. A non-transitory computer-readable storage medium containing instructions embodied therein for causing a computer system to perform a method for controlling a gimbal structure with four or more axes, the method comprising:

reading inputs from a set of encoders on the four or more axes and a set of inertial sensors mounted to the gimbal structure;

reading command input comprising a desired inertial rate for the gimbal structure;

computing, by a processor, a set of motor commands with a first feedback control loop to achieve the desired inertial rate; and

updating the set of motor commands with a second feedback control loop to prevent the gimbal structure from:

obscuring a line-of-sight of a payload mounted to the gimbal structure; and

entering a gimbal lock configuration.

19. The non-transitory computer-readable storage medium of claim 18 , the method further comprising:

inputting an estimated attitude of the gimbal structure into the second feedback control loop prior to updating the set of motor commands with the second feedback control loop.