Surgical Robot Joint Control Without Torque Sensors
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Solution Overview
Problem
Current robotic joint control systems require complex algorithms and torque sensor data to account for the non-linear stiffness of drivetrains, which increases complexity and stability limitations, especially in systems with backlash.
Innovation Solution
A controller that calculates input torque by comparing configuration inputs from sensors at the drive source and joint, using a dynamic torque observer and spring coefficient selection based on elongation values, without relying on torque sensor data.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If multiple feedback terms from torque sensor data are incorporated into the control algorithm to account for non-linear stiffness, then the accuracy of joint configuration control is improved, but the complexity of the controller increases and stability limitations are introduced
Solution Approach 1:
The patent extracts and eliminates the torque sensor and its associated feedback terms from the control system. Instead of using multiple feedback terms including torque sensor data, the invention uses only position feedback from the drive source and joint position sensors, thereby reducing controller complexity while maintaining control accuracy through a simplified control algorithm that accounts for non-linear stiffness effects.
Solution Approach 2:
The patent creates a virtual model or copy of the drivetrain's non-linear stiffness characteristics and incorporates this model into the control algorithm. This allows the controller to compensate for elastic effects and non-linear behavior without requiring physical torque sensors, thus reducing hardware complexity while maintaining control precision.
2Measurement precision
If torque sensor data is used as input to the control algorithm, then the control accuracy for joints with non-linear stiffness is improved, but the stability of the system deteriorates
Solution Approach 1:
The patent removes torque sensor data from the feedback loop entirely, using only position feedback from sensors at the drive source and joint positions. This extraction of the problematic torque feedback path eliminates the stability issues associated with multiple feedback terms while maintaining control accuracy through the simplified control algorithm that models non-linear stiffness behavior.
3Device complexity
If a stiff drivetrain model is used in theoretical modelling, then the relationship between input torque and output torque becomes linear and simple, but the model does not accurately represent real-world drivetrains with elasticity
Solution Approach 1:
The patent changes the modeling approach from assuming constant stiffness to using variable stiffness parameters that depend on drivetrain operating conditions. The control algorithm incorporates non-linear stiffness characteristics through position-based feedback and a simplified control law that adapts to the actual elastic behavior of the drivetrain, achieving both model accuracy and control simplicity.
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
Simplifies joint control by reducing the need for torque sensor data, improving stability and accuracy in systems with non-linear stiffness and backlash, while maintaining precise control of robotic joint configurations.
Implementation Method 1
In reality, the components of the drivetrain that actuate a robotic joint have an associated level of elasticity. Thus, the stiffness of the joint cannot be accurately modelled using a linear relationship between input torque and output torque.
Implementation Method 2
The value of output torque may be represented by the equation τo=kφ(qi−qo), where qi is the first input, qo is the second input and kφ is a spring coefficient of the drivetrain.
Data Source
AI summary
A controller for controlling the configuration of a joint in a surgical robot, the joint being driven by a drivetrain which transfers power from a drive source to the joint, wherein the controller is configured to: receive a first input indicating a configuration of the drive source; receive a second input from a first sensor, the second input indicating a measured configuration of the joint in the surgical robot; calculate a value of output torque about the joint using the first input and the second input; and calculate, using the value of output torque, a value of input torque to be applied to the joint in the surgical robot by the drive source.