Hexapod pose knowledge improvement by joint location calibration with individual strut length differential measurements

1. A process for calibrating a hexapod structure, comprising:

measuring a head pose of the hexapod structure relative to a base of the hexapod structure;

for each strut, independently increasing a length of each strut by a predefined amount from an original length and repeating the measuring of the head pose relative to the base;

for each strut, independently decreasing a length of each strut by a predefined amount from the original length and repeating the measuring of the head pose relative to the base;

moving each strut back to the original length; and

estimating joint errors for each strut prior to calibrating of the hexapod structure.

2. The process of claim 1 , wherein the estimating of the joint errors comprises

estimating upper joint location errors and lower joint location errors for each strut using a difference between a predicted pose and a measured pose.

3. The process of claim 2 , wherein the estimating of the upper joint location errors and lower joint location errors comprises

predicting an upper joint location in a base frame using initial estimates of the head joint location coupled with a measured pose.

4. The process of claim 3 , wherein the estimating of the upper joint location errors and lower joint location errors comprises

with the predicted upper joint location in the base frame, computing predicted strut lengths from upper joint position and lower joint position in a base frame.

5. The process of claim 4 , wherein the estimating of the joint errors further comprises

when a measured strut length is available, estimating the joint error by using a difference between the measured strut length and the predicted strut lengths.

6. The process of claim 4 , wherein the estimating of the joint errors further comprises

when a measured strut length is not available, estimating the joint error by using strut length differentials.

7. A computer program embodied on a non-transitory computer readable medium, the computer program is configured to cause at least one processor to execute:

measuring a head pose of the hexapod structure relative to a base of the hexapod structure;

for each strut, independently increasing a length of each strut by a predefined amount from an original length and repeating the measuring of the head pose relative to the base;

for each strut, independently decreasing a length of each strut by a predefined amount from the original length and repeating the measuring of the head pose relative to the base;

moving each strut back to the original length; and

estimating joint errors for each strut prior to calibrating of the hexapod structure.

8. The computer program of claim 7 , wherein the computer program is further configured to cause at least one processor to execute:

estimating upper joint location errors and lower joint location errors for each strut using a difference between a predicted pose and a measured pose.

9. The computer program of claim 8 , wherein for the estimating of the upper joint location errors and lower joint location errors, the computer program is further configured to cause at least one processor to execute:

predicting an upper joint location in a base frame using initial estimates of the head joint location coupled with a measured pose.

10. The computer program of claim 9 , wherein for the estimating of the upper joint location errors and lower joint location errors, the computer program is further configured to cause at least one processor to execute:

with the predicted upper joint location in the base frame, computing predicted strut lengths from upper joint position and lower joint position in a base frame.

11. The computer program of claim 10 , wherein for the estimating of the joint errors, the computer program is further configured to cause at least one processor to execute:

when a measured strut length is available, estimating the joint error by using a difference between the measured strut length and the predicted strut lengths.

12. The computer program of claim 11 , wherein for the estimating of the joint errors, the computer program is further configured to cause at least one processor to execute:

when a measured strut length is not available, estimating the joint error by using strut length differentials.

13. An apparatus, comprising:

memory comprising a set of instructions; and

at least one processor, wherein

the set of instructions with the at least one processor is configured to cause the apparatus to execute:

measuring a head pose of the hexapod structure relative to a base of the hexapod structure;

for each strut, independently increasing a length of each strut by a predefined amount from an original length and repeating the measuring of the head pose relative to the base;

for each strut, independently decreasing a length of each strut by a predefined amount from the original length and repeating the measuring of the head pose relative to the base;

moving each strut back to the original length; and

estimating joint errors for each strut without influence of errors associated with other struts prior to calibrating of the hexapod structure.

14. The apparatus of claim 13 , wherein the set of instructions with the at least one processor is further configured to cause the apparatus to execute:

estimating upper joint location errors and lower joint location errors for each strut using a difference between a predicted pose and a measured pose.

15. The apparatus of claim 14 , wherein for the estimating of the upper joint location errors and lower joint location errors, the set of instructions with the at least one processor is further configured to cause the apparatus to execute:

predicting an upper joint location in a base frame using initial estimates of the head joint location coupled with a measured pose.

16. The apparatus of claim 15 , wherein for the estimating of the upper joint location errors and lower joint location errors, the set of instructions with the at least one processor is further configured to cause the apparatus to execute:

with the predicted upper joint location in the base frame, computing predicted strut lengths from upper joint position and lower joint position in a base frame.

17. The apparatus of claim 16 , wherein for the estimating of the joint errors, the set of instructions with the at least one processor is further configured to cause the apparatus to execute:

when a measured strut length is available, estimating the joint error by using a difference between the measured strut length and the predicted strut lengths.

18. The apparatus of claim 17 , wherein for the estimating of the joint errors, the set of instructions with the at least one processor is further configured to cause the apparatus to execute:

when a measured strut length is not available, estimating the joint error by using strut length differentials.

19. The apparatus of claim 18 , wherein for the estimating of the joint errors by using the strut length differentials, the set of instructions with the at least one processor is further configured to cause the apparatus to execute:

moving the strut from its nominal position to a position further up and to a position further down, allowing for three strut positions or poses; or

measuring three poses of each strut such that lengths of each strut are computed as a function of the three poses, and forming two length differences for each strut.

20. The apparatus of claim 18 , wherein for the estimating of the joint errors by using the strut length differentials, the set of instructions with the at least one processor is further configured to cause the apparatus to execute:

measuring one or more N number of poses of each strut with N number of lengths of each strut, where N is greater than 3; and

forming N−1 number of length differences for each strut.