Passive Joint Cam Mechanism for Compact Weight Compensation
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Solution Overview
Problem
Conventional passive joint devices in medical robots face challenges in reducing downward force on the abdominal wall without enlarging the joint size, and power transmission to the distal end is hindered due to the passive joint mechanism.
Innovation Solution
A passive joint device with a cylindrical cam member and spring mechanism that reduces downward force by using cam surfaces and a spring to provide upward rotational force, combined with a cable guide and power transmission mechanism using a worm gear system to transmit power without enlarging the joint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a passive joint is provided in the robot arm, then the robot can be supported with a degree of freedom with the abdominal wall as a fulcrum, but the weight of the body on the distal side of the joint is applied to the abdominal wall
Solution Approach 1:
A counterweight member is provided that applies an upward force to the rotation-side member to compensate for the downward force caused by the weight of the distal side body. The counterweight member rotates about the horizontal axis and is positioned to generate a counterbalancing moment that offsets the gravitational load on the abdominal wall.
2Force
If a very large mass counterweight is used to compensate the self-weight, then the downward force is reduced, but the joint size becomes enlarged which is not practical
Solution Approach 1:
The counterweight mechanism is segmented into multiple components: a cylindrical cam member with cam surfaces, a pedestal with cam followers, and a spring mechanism. This segmentation allows the weight compensation function to be achieved through a distributed system rather than a single large mass, reducing the overall joint volume while maintaining the counterbalancing effect.
Solution Approach 2:
The counterweight system uses a dynamic spring mechanism that adjusts the upward force based on the rotational position. The spring biasing force varies as the cam surfaces interact with the cam followers during rotation, providing adaptive weight compensation that reduces the required counterweight mass compared to a static system.
3Device complexity
If a passive joint is provided, then the robot structure is simplified, but power cannot be transmitted to the distal end side from the joint
Solution Approach 1:
A power transmission mechanism is introduced as an intermediary between the fixed-side member and the rotation-side member. This mechanism includes a drive source on the fixed side that transmits rotational force through the cam-follower interaction to the rotation-side member, enabling power transmission while preserving the passive joint's structural simplicity.
4Force
If the spring mechanism is used to compensate self-weight as disclosed in Japanese Patent Laid-Open No. 2011-115906, then the downward force is reduced, but the size of the joint portion becomes enlarged
Solution Approach 1:
The spring mechanism is arranged along the horizontal axis rather than occupying vertical space. The cylindrical cam member rotates about the horizontal axis, and the spring is disposed inside the horizontal axis, utilizing the radial dimension of the rotation-side member. This dimensional reorganization reduces the joint portion size by efficiently packing the spring mechanism within the existing rotational geometry.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces the burden on the patient during surgery by minimizing downward force and enables power transmission to the distal end of the joint, maintaining a compact joint size.
Implementation Method 1
a spring disposed inside the horizontal axis fixed to the rotation-side member, the spring biasing the pedestal toward the fixed-side member along the horizontal axis
Implementation Method 2
a cylindrical cam member having a cylindrical surface centered on the horizontal axis and fixed to the fixed-side member, the cam member having a pair of cam surfaces symmetrically arranged on the cylindrical surface about the horizontal axis and formed obliquely along the horizontal axis
Implementation Method 3
a power transmission mechanism using a worm gear system to transmit power without enlarging the joint
Data Source
AI summary
A passive joint device for supporting a rotation-side member rotatably about a horizontal axis in a vertical direction with respect to a fixed-side member, includes: a cylindrical cam member having a pair of cam surfaces symmetrically arranged about a horizontal axis, a pedestal slidably disposed along the horizontal axis fixed to the rotation-side member, the pedestal having a pair of cam followers that contact with each of the pair of cam surfaces, a spring disposed inside the horizontal axis and biasing the pedestal toward the fixed-side member along the horizontal axis, wherein the spring force causes the pair of cam followers to come into contact with the pair of cam surfaces, and provide upward rotational force to the rotation-side member to reduce the downward rotational force of the rotation-side member.