Robotic Wrist Friction Estimation for End Effector Tracking Error
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Solution Overview
Problem
Existing surgical robotic systems face challenges in accurately estimating joint friction and tracking error, which affects the precision and reliability of robotic end effector movements.
Innovation Solution
A method is developed to estimate joint friction in the robotic wrist by measuring force through the transmission coupling the wrist to its actuator, and using these estimates to calculate tracking error of the end effector.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If transmission stiffness is increased to reduce tracking error, then manufacturing precision improves, but device complexity increases
Solution Approach 1:
The patent changes the physical parameter of transmission stiffness to directly address tracking error. By increasing the stiffness of the transmission elements (cables, linkages), the system reduces elastic deformation and compliance, thereby minimizing the difference between commanded and actual end effector positions. This parameter change provides a direct solution to improving manufacturing precision while accepting the trade-off of increased device complexity.
2Manufacturing precision
If friction torques are reduced to improve tracking accuracy, then manufacturing precision improves, but reliability may worsen due to increased wear
Solution Approach 1:
The patent converts the harmful effect of friction into a beneficial measurement signal. Rather than simply reducing friction through lubrication or material changes, the system uses friction torque measurements to actively compensate for tracking errors. The friction model, combined with compliance measurements, allows the control system to calculate and correct for positioning deviations, thereby improving tracking accuracy while maintaining reliable friction-based joint operation.
Solution Approach 2:
The patent implements a feedback mechanism where friction torque measurements and compliance data are continuously used to update tracking error estimates. The control system receives feedback about actual friction conditions and uses this information to adjust commanded positions, compensating for the effects of friction on tracking accuracy. This closed-loop approach maintains precision without requiring friction to be minimized.
3Measurement precision
If force measurement through transmission is implemented to estimate joint friction, then measurement precision improves, but device complexity increases
Solution Approach 1:
The patent uses cable force measurements as an intermediary to indirectly measure joint friction. Rather than placing sensors directly at the friction-prone joint interfaces, the system measures tension in the transmission cables and uses this data, combined with a friction model, to estimate joint friction torques. This intermediary measurement approach provides the needed precision while avoiding the complexity of direct joint friction sensing.
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
This approach enables the designer to understand and reduce tracking errors by selecting appropriate transmission stiffness or reducing friction torques, thereby improving the precision of robotic movements.
Implementation Method 1
mechanical compliance (or equivalently stiffness or elasticity) of the transmission
Implementation Method 2
joint friction that may resist the commanded motion of the end effector
Data Source
AI summary
A computerized method for estimating joint friction in a joint of a robotic wrist of an end effector. Sensor measurements of force or torque in a transmission that mechanically couples a robotic wrist to an actuator, are produced. Joint friction in a joint of the robotic wrist that is driven by the actuator is computed by applying the sensor measurements of force or torque to a closed form mathematical expression that relates transmission force or torque variables to a joint friction variable. A tracking error of the end effector is also computed, using a closed form mathematical expression that relates the joint friction variable to the tracking error. Other aspects are also described and claimed.