Detecting cable breakage on cable driven tools
A surgical robotic tool used with a surgical robotic system can include cables that effect movement in the surgical robotic tool. A brake in any of these cables can be detected by checking a plurality of conditions. A process can compare a) a tension error against a first threshold, b) a rate of change of the sensed tension of the cable against a second threshold, and c) a rate of change of a cable extension error against a third threshold. If all thresholds are exceeded, the process can disable the respective actuator.
1 . A method performed by a surgical robotic system, the method comprising:
determining, for a cable that couples an actuator to an end effector of a surgical robotic tool, a first tension in which the end effector is in a relaxed position and a second tension of the cable in which a force is applied onto the cable by the actuator in order to effect movement of the end effector;
determining 1) a tension error as a difference between the first tension and the second tension and 2) a rate of change of the tension error of the cable;
detecting that an imminent break of the cable is going to occur or that a break of the cable has occurred responsive to determining that 1) the tension error exceeds a first threshold and 2) the rate of change of the tension error exceeds a second threshold that is different than the first threshold; and
responsive to detecting that the imminent break of the cable is going to occur or that the break of the cable has occurred, disabling the actuator to stop the movement of the end effector.
2 . The method of claim 1 , wherein the first tension is a pre-defined force value at which the cable is at rest and is coupled to the end effector and the actuator.
3 . The method of claim 1 further comprising causing the actuator to apply the force onto the cable to effect the movement of the end effector, wherein determining the second tension comprises receiving, responsive to the applied force, sensor data from a sensor that is coupled to the cable.
4 . The method of claim 1 further comprising detecting that the imminent break is going to occur or that the break of the cable has occurred in response to determining that the rate of change of the tension error is over a period of time comprises a spike that exceeds the second threshold over a time period that is less than the period of time.
5 . The method of claim 1 , wherein determining the second tension of the cable comprises receiving, from a tension sensor coupled to the cable, a sensed tension of the cable a moment before or when the break of the cable occurs.
6 . The method of claim 1 , wherein the cable is a first cable and the actuator is a first actuator, wherein the surgical robotic tool further comprises a second cable coupled to the end effector and a second actuator, wherein each actuator effects a different movement of the end effector, wherein the method further comprises disabling the second actuator along with the first actuator responsive to detecting that the imminent break of the first cable is going to occur, or that the break of the first cable has occurred.
7 . The method of claim 1 is performed continuously during a surgical operation.
8 . The method of claim 1 further comprising:
generating, responsive to a user instruction, a command for the actuator to cause the movement of the end effector; and
determining a cable extension error of the cable as a difference between a modeled cable extension based on the command and a measured cable extension of the cable responsive to the applied force,
wherein detecting that the imminent break of the cable is going to occur, or that the break of the cable has occurred comprises determining that 1) the tension error exceeds the first threshold, 2) the rate of change of the tension error exceeds the second threshold, and 3) the cable extension error exceeds a third threshold.
9 . The method of claim 8 , wherein the measured cable extension is determined using a measured cable position based on the applied force, the second tension, and a stiffness of the cable.
10 . The method of claim 1 , wherein the surgical robotic system comprises a surgical robotic arm that is coupled to a proximal end of the surgical robotic tool, wherein the proximal end includes the actuator, wherein the surgical robotic tool comprises a distal end that includes the end effector.