Cascade Control of Antagonistic VSA-An Engineering Control Approach to a Bioinspired Robot Actuator

Area of Science:

• Robotics
• Control Systems Engineering
• Mechanical Engineering

Background:

• Variable stiffness actuators (VSAs) are essential for compliant robotic systems, but controlling both position and stiffness simultaneously presents significant challenges.
• Tendon-driven mechanisms introduce complexities like elastic transmission and potential tendon slackening, requiring precise control strategies.

Purpose of the Study:

• To develop and evaluate a cascade control structure for the simultaneous position and stiffness control of antagonistic tendon-driven VSAs.
• To ensure stable operation, minimize oscillations, and prevent tendon slackening in VSAs.

Main Methods:

• Implementation of a cascade control architecture with inner-loop controllers for motor position and outer-loop controllers for actuator position and stiffness.
• System dynamics identification using a closed-loop auto-regressive with exogenous input (ARX) model.
• Tuning of outer-loop controllers (I-PD and I-P) based on experimentally identified transfer functions and a controller bank approach.
• Development of analytical formulas for controller tuning using a single parameter (λ) for independent loop adjustment.

Main Results:

• The cascade control structure successfully achieved simultaneous position and stiffness control in a laboratory setup.
• The proposed controllers (I-PD, I-P) demonstrated smooth responses without overshoot for step changes, effectively preventing tendon slackening.
• Decoupled position and stiffness loops allowed for independent performance adjustment via the parameter λ.

Conclusions:

• The presented cascade control approach is simple to implement and provides robust performance for tendon-driven actuators.
• This method ensures the pulling constraint is met, laying a foundation for advanced bioinspired antagonistic VSAs.
• The control strategy enhances stability and reduces oscillations, making it suitable for compliant robotic applications.