Four-Cable Wrist Mechanism for Minimally Invasive Surgery
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Solution Overview
Problem
Conventional robotically controlled surgical instruments with wrist mechanisms require a large number of cables for three degrees of freedom, complicating miniaturization and increasing costs, while also needing pulleys to manage cable friction and electrical isolation for cauterizing tools.
Innovation Solution
A wrist mechanism using only four cables to control pitch, yaw, and grip, with stiffer gripping cables and a backend mechanism connecting these cables to motor-driven axes, eliminating the need for pulleys and enabling efficient electrical isolation through brush-type contacts and insulating materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional wrist mechanisms use six cables for three degrees of freedom, then reliable control is achieved, but device complexity and manufacturing cost increase
Solution Approach 1:
The patent combines multiple cable functions into a reduced four-cable system. Cables are routed to control multiple degrees of freedom simultaneously, merging the function of six separate cables into four integrated cables that maintain reliable control while reducing system complexity
Solution Approach 2:
Each cable in the four-cable system is designed to serve multiple functions and control multiple degrees of freedom. The cables are universally configured to work across pitch, yaw, and grip movements, eliminating the need for dedicated one-to-one cable mappings
2Measurement precision
If more cables are used for precise control, then control precision improves, but instrument diameter increases
Solution Approach 1:
The patent merges cable routing paths to reduce the overall instrument diameter. By combining multiple cable functions into fewer cables and optimizing their routing through the wrist mechanism, the instrument achieves precise control with a smaller cross-sectional area
3Loss of energy
If pulleys are added to reduce cable friction, then cable friction decreases, but device complexity and part count increase
Solution Approach 1:
The patent extracts and eliminates pulleys from the cable system. Instead of using pulleys to manage cable friction, the design employs direct cable routing through the wrist mechanism with friction-reduced surfaces, removing the pulley components entirely while maintaining low friction
4Reliability
If electrical isolation components are added for cauterizing tools, then electrical isolation improves, but device complexity increases
Solution Approach 1:
The patent uses insulating materials such as PTFE coatings and plastic components to provide electrical isolation for cauterizing tools. These thin film and shell-based solutions achieve reliable electrical isolation without adding complex isolation mechanisms
Data Source
AI summary
A wrist mechanism for an instrument used in minimally invasive medical procedures employs only four drive cables for control of the pitch, yaw, and grip of an effector. To reduce part count, no pulleys are used. Instead, solid surfaces of clevises in the wrist mechanism cradle the drive cables and are curved in multiple directions to maintain a fixed moment arm for cable torques and eliminate the need for large cable wrap angles. Further, the stiffness of each of the four drive cables can be controlled for improved clinical performance. In particular, the stiffness of the two drive cables that cause grips to close can be greater than the stiffness of the remaining two drive cables that cause the grips to open. The arrangement can minimize cable slack, improving fidelity of motion.