Spring counterbalance with failure detection

1. A spring counterbalance system comprising:

a load;

a spring;

a cable having a first protrusion fixed to the cable midway between two ends of the cable, the two ends being fixedly coupled to one of the load and the spring;

a cable attachment that is coupled to the other of the load and the spring, the cable passing through the cable attachment such that the first protrusion is captive in the cable attachment and is movable between two stops in the cable attachment, the cable forming two segments each of which couples the spring and the load;

a pulley over which the two segments of the cable pass; and

a sensor to sense rotation of the pulley.

2. The spring counterbalance system of claim 1 wherein the spring is compression spring and the cable passes through the spring and is coupled to the spring such that the load causes the spring to be compressed.

3. The spring counterbalance system of claim 1 wherein the cable attachment includes a U shaped slot, the central portion of the slot being large enough to receive the first protrusion and allow it to slide within the slot, the two end portions of the slot being large enough to receive the cable and small enough to form the two stops that limit the movement of the protrusion.

4. The spring counterbalance system of claim 1 wherein: the cable has a second protrusion fixed to the cable; the two stops of the cable attachment comprise a first stop at an end of a first channel and a second stop at an end of a second channel; the first protrusion slides within the first channel until reaching the first stop at the end of the first channel; and the second protrusion slides within the second channel until reaching the second stop at the end of the second channel.

5. The spring counterbalance system of claim 1 wherein the sensor is a Hall effect array.

6. The spring counterbalance system of claim 1 wherein the sensor senses a rotational position of the pulley.

7. The spring counterbalance system of claim 1 wherein the sensor senses one of angular velocity and angular acceleration of the pulley.

8. A spring counterbalance mechanism comprising:

a spring;

a cable having two ends, a midpoint, and a first protrusion fixed to the cable substantially at the midpoint;

means for coupling the two ends of the cable to a load;

means for loosely coupling the midpoint of the cable to the spring through which the cable passes such that the protrusion is held captive and is movable between two stops such that two segments of the cable couple the spring to the load;

a pulley over which the two segments of the cable pass; and

means for sensing rotation of the pulley.

9. The spring counterbalance mechanism of claim 8 wherein the spring is a compression spring and the cable passes through the spring and is coupled to the spring such that the load causes the spring to be compressed.

10. The spring counterbalance mechanism of claim 8 wherein the means for loosely coupling the midpoint of the cable to the spring includes a U shaped slot, the central portion of the slot being large enough to receive the first protrusion and allow it to slide within the slot, the two end portions of the slot being large enough to receive the cable and small enough to form the two stops that limit the movement of the protrusion.

11. The spring counterbalance mechanism of claim 8 wherein: the cable has a second protrusion fixed to the cable; the two stops of the means for loosely coupling the midpoint of the cable to the spring comprise a first stop at an end of a first channel and a second stop at an end of a second channel; the first protrusion slides within the first channel until reaching the first stop at the end of the first channel; and the second protrusion slides within the second channel until reaching the second stop at the end of the second channel.

12. The spring counterbalance mechanism of claim 8 wherein the means for sensing rotation of the pulley is a Hall effect array.

13. The spring counterbalance mechanism of claim 8 wherein the means for sensing rotation of the pulley senses a rotational position of the pulley.

14. The spring counterbalance mechanism of claim 8 wherein the means for sensing rotation of the pulley senses one of angular velocity and angular acceleration of the pulley.

15. A method for balancing a load with a spring, the method comprising:

coupling two ends of a cable to the load;

loosely coupling a midpoint of the cable to the spring such that two segments of the cable couple the spring to the load;

passing the two segments of the cable over a pulley;

maintaining the coupling of the midpoint of the cable to the spring with only one of the two cable segments; and

sensing rotation of the pulley when the coupling of the midpoint of the cable to the spring with only one of the two cable segments first occurs.

16. The method of claim 15 wherein the spring is compression spring and the method includes passing the cable through the spring and coupling the cable to the spring such that the load causes the spring to be compressed.

17. The method of claim 15 further comprising providing the cable with at least one protrusion adjacent the midpoint and supporting the protrusion against a stop to maintain the coupling of the midpoint of the cable to the spring with only one of the two cable segments.

18. The method of claim 15 further comprising providing a Hall effect array for sensing rotation of the pulley.

19. The method of claim 15 wherein sensing rotation of the pulley senses a rotational position of the pulley.

20. The method of claim 15 wherein sensing rotation of the pulley senses one of angular velocity and angular acceleration of the pulley.