Counterbalance assembly

1. A counterbalance assembly for a rotatable mechanical arm comprising:

a first force generating device interacting with an associated first cam; and

a second force generating device interacting with an associated second cam, wherein:

the first and second cams are fixed eccentrically relative to a pivot of a joint about which said mechanical arm rotates,

said first force generating device and said second force generating device are offset by 90 degrees,

said first and second force generating device are first and second springs,

and the relationship of the first force generating device to the second force generating device and the first cam to the second cam is preserved throughout rotation of the mechanical arm.

2. The counterbalance assembly of claim 1 , wherein compression of the first force generating device is adjustable.

3. The counterbalance assembly of claim 2 , wherein compression of the first force generating device is adjustable such that the torque exerted by the first force generating device and associated first cam offsets torque exerted by the mechanical arm about the joint, when the mechanical arm is in a position to exert its greatest torque about the joint.

4. The counterbalance assembly of claim 1 , wherein compression of the second force generating device is adjustable.

5. The counterbalance assembly of claim 1 , wherein the longitudinal axis of the first force generating device is substantially perpendicular to a line extending between the pivot and the center of the associated first cam, and the longitudinal axis of the second force generating device is substantially parallel to a line extending between the pivot and the center of the associated second cam, when the mechanical arm is in a position to exert its greatest torque about the joint.

6. The counterbalance assembly of claim 1 , wherein the first force generating device applies a compressive load to the associated first cam.

7. The counterbalance assembly of claim 1 , wherein the second force generating device applies a compressive load to the associated second cam.

8. The counterbalance assembly of claim 1 , wherein the first force generating device applies a compressive load to the associated first cam and the second force generating device applies a compressive load to the associated second cam.

9. The counterbalance assembly of claim 1 , wherein the second force generating device applies a compressive load or a tensile load to the associated second cam depending on the rotational position of the mechanical arm.

10. The counterbalance assembly of claim 1 , wherein at least one of the first and second cams is set within the circumference of the joint.

11. The counterbalance assembly of claim 1 , wherein at least one of the first and second cams is set outside the circumference of the joint.

12. The counterbalance assembly of claim 1 , wherein the first force generating device at least offsets the weight of a payload of the mechanical arm, when the mechanical arm is in a position to exert its greatest torque about the joint.

13. The counterbalance assembly of claim 12 , wherein the second force generating device offsets the linear change in force of the first force generating device.

14. The counterbalance assembly of claim 1 , wherein the first and second force generating devices abut the associated first and second cams, respectively.

15. The counterbalance assembly of claim 1 , wherein the first and second force generating devices are coupled to the associated first and second cams, respectively.

16. The counterbalance assembly of claim 1 , further comprising a third force generating device in the form of a third spring.

17. The counterbalance assembly of claim 16 , wherein the third force generating device interacts with the second cam.

18. The counterbalance assembly of claim 16 , wherein the third force generating device interacts with the first cam.

19. The counterbalance assembly of claim 16 , further comprising a third cam that interacts with the third force generating device.

20. The counterbalance assembly of claim 16 , further comprising a fourth force generating device in the form of a fourth spring.

21. The counterbalance assembly of claim 1 , wherein the product of the spring rate of the first spring and the square of the distance between the center of the first cam and the pivot is substantially equal to the product of the spring rate of the second spring and the square of the distance between the center of the second cam and the pivot.

22. The counterbalance assembly of claim 21 , wherein the spring rate of the first spring is substantially equal to the spring rate of the second spring.

23. The counterbalance assembly of claim 1 , wherein at least one of the first and second springs is a compression spring.

24. The counterbalance assembly of claim 1 , wherein at least one of the first and second springs is a tension spring.

25. The counterbalance assembly of claim 1 , wherein the mechanical arm is a guide apparatus for supporting a medical tool payload.

26. The counterbalance assembly of claim 25 , wherein the guide apparatus comprises a crank, a link hingedly coupled at one end thereof to the crank and a medical tool holder coupled to an opposite end of the link.

27. The counterbalance assembly of claim 26 , further comprising a counterweight to offset the weight of the medical tool payload.