Surgical Robot Arm Control Using Friction-Based Motion Modeling

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Solution Overview

Problem

Existing surgical robot arms used in surgery require higher stability and precision due to intricate operations within the human body, but current mass, spring, damper models result in poor stability margins and oscillations, leading to drifting setpoints.

Innovation Solution

Implementing a Coulomb and viscous friction model to control surgical robot arms, using a backward Euler approximation to solve an equation of motion, which models frictional forces and calculates desired velocities to achieve stable movement in response to external forces.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If a mass, spring, damper model is used to control the surgical robot arm, then the arm can respond to external forces, but the system exhibits poor stability margins and oscillations leading to drifting setpoints

Engineering Contradiction:

Improveresponse to external forceVSAvoidposition stability

Core Design Contradiction:

Adaptability or versatilityVSStability of the object's composition

Solution Approach 1:

The patent changes the control model parameters by transitioning from a mass-spring-damper model to a Coulomb and viscous friction model. This parameter change fundamentally alters the system dynamics, eliminating oscillations and improving stability margins while maintaining the ability to respond to external forces during compliant mode operation.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent substitutes the traditional mass-spring-damper mechanical model with a Coulomb and viscous friction model. This replacement changes the underlying mechanical assumptions from elastic-restoring forces to friction-based resistance, which better represents the desired compliant behavior and eliminates the oscillatory characteristics of the original model.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Ease of operation

If a mass, spring, damper model is used to control the surgical robot arm, then the arm can move in response to external forces, but the control precision deteriorates due to oscillations and drifting setpoints

Engineering Contradiction:

Improvecompliant mode operationVSAvoidposition precision

Core Design Contradiction:

Ease of operationVSManufacturing precision

Solution Approach 1:

By changing the control model parameters from mass-spring-damper to Coulomb and viscous friction characteristics, the system achieves both compliant operation and high position precision. The friction-based model provides stable, non-oscillatory response that maintains setpoint accuracy while allowing natural movement in response to external forces.

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If traditional control models are used for surgical robot arms, then the system structure remains simple, but the stability and precision requirements for intricate surgical operations are not met

Engineering Contradiction:

Improvecontrol model structureVSAvoidsurgical operation reliability

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The patent modifies the control model parameters to use Coulomb and viscous friction characteristics instead of mass-spring-damper parameters. This change improves reliability for surgical operations by eliminating oscillations and stabilizing setpoints, while the implementation remains integrated within the existing controller architecture, minimizing structural complexity increases.

Inventive Principle:

Principle #35Parameter changes

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

Improves stability and precision by eliminating oscillations, ensuring the robot arm maintains position stability and follows desired behavior, particularly in surgical environments.

Implementation Method 1

calculate a desired velocity of the first part of the surgical robot arm for a time subsequent to a current time by evaluating (i.e. solving) an equation of motion modelling Coulomb and viscous frictional forces

Methodology Applied

Scientific EffectCoulomb friction: Friction

Implementation Method 2

Methodology Applied

Scientific EffectViscous friction: Viscous Damping

Data Source

PatentUS20260026899A1Control of a surgical robot arm

Publication Date: 2026.01.29 CMR SURGICAL LTD

AI summary

A controller for moving a first part of a surgical robot arm, the surgical robot arm comprising a plurality of arm segments separated by a plurality of driven joints, in response to an external force being imparted on a second part of the surgical robot arm, the controller being configured to: determine a torque at each of the plurality of joints which results from the force imparted on the second part of the surgical robot arm; calculate from the determined torques a resultant force which acts on the first part of the surgical robot arm as a result of the external force being imparted on the second part of the surgical robot arm; calculate a desired velocity of the first part of the surgical robot arm for a time subsequent to a current time by evaluating an equation of motion modelling Coulomb and viscous frictional forces using a backward Euler approximation, the equation of motion having inputs of the calculated resultant force which acts on the first part of the surgical robot arm; and the current velocity at the current time of the first part of the surgical robot arm; and drive the surgical robot arm in accordance with the calculated desired velocity.