Surgical Instrument Velocity Control for Force-Limited End Effectors
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Solution Overview
Problem
Minimally invasive surgical instruments with small end effectors face challenges in maintaining stiffness and stability during procedures like grasping, clamping, and stapling due to limited stiffness, leading to flexing and splaying, which can result in failed operations and irregular velocity profiles due to tissue variations.
Innovation Solution
A computer-assisted surgical instrument system that adjusts the velocity set point of the actuator based on applied force or torque thresholds, reducing velocity when force or torque exceeds a first threshold, increasing it when below a second threshold, and setting it to zero when above a maximum threshold, with a state machine controlling these transitions to manage the operation smoothly and prevent excessive force or torque.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If the end effector size is kept small for minimally invasive procedures, then the invasiveness is reduced, but the stiffness and stability deteriorate leading to flexing and splaying
Solution Approach 1:
The system dynamically adjusts the velocity set point of the actuator based on real-time monitoring of applied force or torque. When force/torque exceeds a first threshold, the velocity set point is reduced; when it exceeds a maximum threshold, the velocity set point is set to zero. This dynamic control compensates for the limited stiffness of small end effectors, preventing flexing and splaying during surgical operations.
2Reliability
If a simple torque limit is applied to the actuator, then the safety is improved, but the velocity profile becomes irregular and operations fail due to tissue variations
Solution Approach 1:
The system implements a feedback control mechanism where the actual applied force or torque is continuously monitored and compared against threshold values. Based on this feedback, the velocity set point is dynamically adjusted: reduced when force/torque exceeds a first threshold, increased when below a second threshold, and set to zero when exceeding a maximum threshold. This feedback loop ensures both safety and consistent velocity profiles by adapting to tissue variations in real-time.
3Stability of the object's composition
If the velocity set point is reduced to prevent excessive force, then the stability is improved, but the operation speed decreases
Solution Approach 1:
The system employs dynamic velocity adjustment rather than a fixed velocity limit. The velocity set point is continuously adapted based on the monitored force or torque levels. When force/torque is within acceptable ranges, higher velocities are maintained for efficient operation. When force/torque exceeds thresholds, the velocity is dynamically reduced only to the extent necessary to maintain stability, allowing optimal speed-stability balance throughout the operation.
Data Source
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AI summary
A system and method of variable velocity control of a surgical instrument in a computer-assisted medical device including an end effector located at a distal end of the instrument, an actuator, and drive mechanisms for coupling force or torque from the actuator to the end effector. To perform an operation with the instrument, the computer-assisted medical device sets a velocity set point of the actuator to an initial velocity, monitors force or torque applied by the actuator, reduces the velocity set point when the applied force or torque is above a first threshold, increases the velocity set point when the applied force or torque is below a second threshold, decreases the velocity set point to zero when the applied force or torque is above a maximum threshold, and drives the actuator based on the velocity set point. The first and second thresholds are lower than the maximum threshold.