Iterative Machine Learning for Precision Trajectory Tracking with Series Elastic Actuators
This work addresses positioning accuracy issues in robots with SEAs, which is crucial for safe operation in unknown environments, but it is incremental as it builds on existing iterative learning and modeling methods.
The paper tackled the problem of reduced positioning precision in series elastic actuators (SEAs) by developing an iterative machine learning technique using complex-valued Gaussian Process Regression to learn feedforward commands, demonstrating successful convergence and error reduction in a 2-DOF robotic arm.
When robots operate in unknown environments small errors in postions can lead to large variations in the contact forces, especially with typical high-impedance designs. This can potentially damage the surroundings and/or the robot. Series elastic actuators (SEAs) are a popular way to reduce the output impedance of a robotic arm to improve control authority over the force exerted on the environment. However this increased control over forces with lower impedance comes at the cost of lower positioning precision and bandwidth. This article examines the use of an iteratively-learned feedforward command to improve position tracking when using SEAs. Over each iteration, the output responses of the system to the quantized inputs are used to estimate a linearized local system models. These estimated models are obtained using a complex-valued Gaussian Process Regression (cGPR) technique and then, used to generate a new feedforward input command based on the previous iteration's error. This article illustrates this iterative machine learning (IML) technique for a two degree of freedom (2-DOF) robotic arm, and demonstrates successful convergence of the IML approach to reduce the tracking error.