Spring counterbalance with failure detection

A spring counterbalance mechanism balances a load with a spring. A cable has a protrusion fixed to the cable midway between two ends of the cable. The two free ends of the cable are fixedly coupled to one of the load or the spring, and a cable attachment is coupled to the remaining member. The cable passes through the cable attachment such that the protrusion is captive in the cable attachment and is movable between two stops in the cable attachment. The cable forms two segments each of which couples the spring to the load. The two segments of the cable pass over a pulley. A sensor coupled to the pulley senses rotation of the pulley allowing cable breakage to be detected by the anomalous rotation of the pulley.

BACKGROUND

Embodiments of the invention relate to the field of counterbalance mechanisms; and more specifically, to spring force counterbalance mechanisms with failure detection.

In load positioning systems the effects of gravity on the load may be compensated for with a spring counterbalance. Failures of the spring counterbalance may cause the load to move, which may be an undesirable failure mode. The reliability of a spring counterbalance may be increased by introducing redundancy in the mechanism so that a single failure does not leave the load unbalanced. However, a failure in a redundant system may go unnoticed leaving the system without protection of the redundant element.

It would be desirable to provide a redundant structure for a spring force counterbalance mechanism that detects failures of the redundant elements.

SUMMARY

A spring counterbalance mechanism balances a load with a spring. A cable has a protrusion fixed to the cable midway between two ends of the cable. The two free ends of the cable are fixedly coupled to one of the load or the spring, and a cable attachment is coupled to the remaining member. The cable passes through the cable attachment such that the protrusion is captive in the cable attachment and is movable between two stops in the cable attachment. The cable forms two segments each of which couples the spring to the load. The two segments of the cable pass over a pulley. A sensor coupled to the pulley senses rotation of the pulley allowing cable breakage to be detected by the anomalous rotation of the pulley.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known devices, structures and techniques have not been shown in detail in order not to obscure the understanding of this description.

FIG. 1shows a front view of a spring counterbalance mechanism100with a load102balanced by a spring118.FIG. 2shows a side view of the spring counterbalance mechanism100.FIG. 3shows a top view of the spring counterbalance mechanism100.

A cable120has a protrusion122fixed to the cable midway between two ends of the cable. The two ends of the cable120are fixedly coupled to either the load or the spring. In the embodiment shown inFIGS. 1-3, the two ends of the cable120are fixedly coupled to the spring118by a cable fixing device124. A cable attachment is coupled to the other of the load or the spring. In the embodiment shown inFIGS. 1-3, the cable attachment126is coupled to the load102.

The cable120passes through the cable attachment126such that the protrusion122is captive in the cable attachment and is movable between two stops in the cable attachment. In the embodiment shown inFIGS. 1-3, the cable attachment126includes a U-shaped slot or groove through which the cable120passes. The central portion of the slot is large enough to allow the protrusion122to slide freely. The two end portions of the slot are large enough to allow the cable120to slide freely but small enough to provide the two stops that limit the movement of the protrusion122. A retaining plate128may be secured to the cable attachment126with a screw130or other fastener to retain the cable120within the slot of the cable attachment. The retaining plate is not shown in the side view ofFIG. 2to allow the cable120and protrusion122to be seen within the slot of the cable attachment126.

The cable120forms two segments, each of which couples the spring118and the load102. The protrusion122will be located approximately midway between the stops in the cable attachment126. The two segments of the cable120pass over a pulley104. A sensor108is provided to sense rotation of the pulley104.

In normal operation the load102is supported by both of the two segments of the cable120.FIG. 4shows the cable attachment126and a portion of the cable If one of the segments of the cable breaks420as shown inFIG. 4the entire load102will be supported by the remaining cable segment120causing the protrusion to slide in the cable attachment126until it hits one of the stops. For the purposes of this description, the coupling of the cable120to the cable attachment126in a manner that allows a limited amount of movement of the cable relative to the cable attachment may be referred to as a loose coupling. This loose coupling will cause an anomalous rotation of the pulley104if one of the redundant cable segments breaks. By sensing this anomalous rotation with the sensor106, the likelihood of a cable breakage having occurred can be detected. This may be important because the redundant cables prevent a large movement of the load if a cable breaks which may allow the loss of redundancy in the counterbalance mechanism to go unnoticed.

In one embodiment, the sensor108senses an angular velocity of the pulley104. In another embodiment, the sensor108senses an angular acceleration of the pulley. An unexpectedly high velocity or acceleration may be used to indicate the likelihood of cable breakage. In yet another embodiment, the sensor108senses a rotational position of the pulley. The rotational position may be compared to an expected position based on other sensors, such as a sensor of the load position. This may allow cable stretching as well as cable breakage to be detected. Cable stretching may indicate an impending cable breakage.

FIG. 5shows a front view of another spring counterbalance mechanism500with a load502balanced by a spring518. In this embodiment, the spring518is compression spring and the cable520passes through the spring and is coupled to the spring such that the load502causes the spring to be compressed. A compression spring may be considered more reliable than a tension spring for some embodiments of the invention. If a tension spring breaks, it would then be unable to provide any force to counterbalance the load. If a compression coil spring breaks, the coil at the broken point will move slightly to rest on the next coil in the spring. This slight movement may only change the load balancing force by a small amount (e.g., 5-10%), which may be computed by multiplying the space between coils by the spring rate. This slight movement may be detected as an anomalous movement of the pulley504is the same manner as previously described for detection of cable breakage.

In the embodiment shown inFIG. 5, a cable520has a protrusion522fixed to the cable midway between two ends of the cable. The two ends of the cable520are fixedly coupled to the load502by a cable fixing device524. A cable attachment526is coupled to the load502. The cable520passes through the cable attachment526such that the protrusion522is captive in the cable attachment and is movable between two stops in the cable attachment. In the embodiment shown inFIG. 5, the cable attachment526is configured to engage the distal end of the coil spring518and have the cable pass through the spring and emerging at the proximal end of the coil spring which is supported by the frame506of the spring counterbalance mechanism500. The frame506may provide a cylindrical chamber in which the coil spring518is confined. The cable attachment526may include low friction guides530, such as rollers or plastic skids, that center the distal end of the cable attachment in the cylindrical chamber.

The cable attachment526includes a U-shaped slot through which the cable520passes and is retained in a manner similar to the cable attachment described above in connection withFIGS. 1-3. A retainer528may be fastened to the cable attachment526to retain the cable520and protrusion522.FIG. 6shows a side view of the cable attachment526with a portion of the cable520that includes the protrusion522. The retainer is not shown to allow the configuration of the cable520and protrusion522in the U-shaped slot of the cable attachment526to be seen.

FIG. 7shows a cross-section of the pulley504and coupled sensor508taken along section line7-7ofFIG. 5. The pulley504may be rotatably supported by a pulley support510. A magnet512may be fixedly coupled to the pulley504such that the magnet rotates in unison with the pulley. The sensor508detects the rotation of the magnet512to detect anomalous movements of the pulley504that may indicate a breakage of the cable520or the spring518. The sensor508may be a Hall effect array that detects changes in the angular position of the magnet512.

FIG. 8shows an exploded pictorial view of another cable attachment842that embodies the invention. The cable attachment842may include a generally U-shaped channel or slot through which the cable814passes. A retaining plate828may be fastened to the cable attachment842, such as with a screw832, to retain the loop of cable814in the channel. Rollers or skids830may be provided to allow the cable attachment842to move freely along the length of an enclosing structure.

In this embodiment, the cable814has two protrusions844,846fixed to the cable midway between two ends of the cable. The two protrusions844,846are spaced apart by a short cable segment848. The cable814passes through the cable attachment842such that each of the two protrusions844,846are captive in the cable attachment. Each protrusion may be retained in an enlarged straight portion of the channel in the cable attachment842through which the cable814passes. An end of the of the enlarged portion of the channel may be reduced in size to permit the cable814but not the protrusion844,846to pass through and thereby form a stop850,852that limits the movement of the protrusion in the cable attachment842. The short cable segment848between the two protrusions844,846may make a 180 degree turn in the U-shaped channel. An end of the of the enlarged portion of the channel opposite the stop850,852may also be reduced in size to permit the cable814but not the protrusion844,846to pass through and thereby form an additional stop that limits the movement of the protrusion in the cable attachment842. as in the previously described embodiments, the cable attachment842provides a redundant coupling of a load to a spring counterbalance that allows a small amount of cable motion in the event that one of the redundant cable segments breaks.

FIG. 9shows a cutaway view of a portion of a set-up joint arm that may be used to support a surgical robotic manipulator and provide initial positioning of the manipulator as disclosed in U.S. patent application Ser. No. 11/627,934, which is included herein by reference. The set-up joint arm may include rotational joints930,932,934that connect links in the arm.

The link shown connecting the second932and third934rotational joints includes a parallelogram linkage structure900. The parallelogram linkage structure900includes an idle link902, a counter balancing link904, a proximal bracket906, and a distal bracket908. The idle link902is pivotally coupled to the proximal bracket906at a pivotal joint936and to the distal bracket908at a pivotal joint938. The counter balancing link904is pivotally coupled to the proximal bracket906at a pivotal joint912and to the distal bracket908at a pivotal joint940. The pivotal joints936,912,938,940are located at the corners of the parallelogram linkage structure900. The counter balancing link904includes a substantial portion of a spring-cable-pulley balancing mechanism910that generally operates around a pivotal joint912. The pulleys are arranged such that the spring provides tension in the cable that acts on the parallelogram linkage structure900to counterbalance a load supported by the third rotational joint934.

The spring-cable-pulley balancing mechanism910includes a doubled cable814coupled to the set-up arm that is wrapped over a plurality of pulleys916-818and tensioned by a compressible spring assembly920. The cable814may couple to the set-up arm by coupling to the set-up joints or the counter balancing link904. In the embodiment shown inFIG. 9, the cable814may have a first end that is fixed to the counter balancing link at a pin or post922, wraps over the plurality of pulleys916-818in one direction, passes through a U-shaped channel in cable attachment842, then route back and wrap over the pulleys916-918in a reverse direction, and have the second end of the cable fixed to the counter balancing link at the pin or post. Wrapping the cable through the cable attachment842in this manner is a convenient way to have segments of a single cable act like a redundant pair of cables. The cable attachment842may be similar to the cable attachment shown inFIG. 8. The cable814includes two protrusions844,846that are retained in the cable attachment842such that the cable can slide only a short distance within the cable attachment.

In one embodiment of the invention, the plurality of pulleys916-918are of equal diameter. Each of the pulleys916-918may include one or more tracks in which the cable814is wrapped and guided to substantially maintain alignment. Pulley918is concentric with the pivotal joint912coupling to a shaft at the pivotal joint. With the cable814wrapped over it, the pulley918does not rotate relative to the counter balancing link904. However, the counter balancing link904and the pulley918rotate together about the pivotal joint912with respect to the bracket906. Pulley917is rotatably coupled to an adjustable mount926that is coupled to the bracket906. The adjustable mount926may slide in the bracket906to adjust the position of pulley917and further adjust the tension in the cable814and spring924during set-up and maintenance. However, the adjustable mount926is rigidly fixed in placed during operational periods so that the position of the pulley917rotatably coupled to the adjustable mount926does not change. Pulley916is rotatably coupled to the housing of the link904and thus pivots with the link about the pivotal joint912. The center points or center point positions of the pulleys916-918are the corners or vertices of a triangle. Changing the length of one of the sides of the triangle adjusts the counter-balancing mechanism for variations in spring rate or the amount of weight being balanced.

A sensor is coupled to one of the pulleys916-918to sense anomalous rotation of the pulley caused by a breakage in the redundant structure of the cable or in the coil spring. In this way, the breakage can be detected even though the slight drop of the counterbalanced load might otherwise go unnoticed.