Actuator Dynamics Models

In Spring quarter 2021 I took actuator dynamics (ROB 542) which was focused around gaining a deep understanding of the dynamics of actuators. We developed models and simulations of actuators to perform useful tasks and gain understanding of the benefits and drawbacks of adding passive dynamics to a system and how they affect its ability to achieve force and position control as well as bandwidth considerations.

The course was open ended and project focused in nature. We were free to pursue any system and task of our choosing.

Spring Mass Damper Systems

The foundation for these concepts starts with a classic spring mass damper system. A block falls onto the toe mass which is at the end of a spring and damper connected to a PD controlled base mass.

Depending on the stiffness and damping coefficients as well as the difference of the 3 masses, starting height of the falling mass and spring length the system behaves wildly differently

Force Control: Stabilizing Coffee in a Car

These foundational concepts can be extended to more useful tasks such as force control problems. In this example the blue mass represents a traveling car that receives an impulse from a bump in the road. The spring/damper and toe mass (orange) are your arm attempting to apply a constant force on a cup of coffee (yellow). If proper force application is not maintained then the coffee accelerates out of the cup and into your lap...

Position Control: Chattering Machine

Passive dynamics play an important role in position control tasks as well, such as this simulated machining process where a target sinusoid is being tracked while under external vibrations

Spring Mass Walking

Further extension of these concepts results in the classic locomotion model of a spring-mass walker. The characteristics of the gait are defined by the systems parameters such as spring and damping coefficients, masses and leg lengths as well as the desired touchdown angle.

Improper tuning of these parameters for the simple controller I employed can result in the system becoming unstable.

Direct and Quasi-Direct Drive BLDC Motors

For class projects each group chose a particular type of actuator to derive a high fidelity dynamic model for. Our team (of 2) chose to investigate the differences between direct and quasi-direct drive brushless DC motors.

The dynamics of these models were used in the place of the previous simple spring-mass-damper systems to analyze their performance to achieve similar force and position control tasks.

Animation credits: Kyle Mathenia

Direct Drive Actuator

Quasi Direct Drive Actuator

Monopod Hopping Robot

Using our dynamic models of DD and QDD actuators we created a simulated monopod hopping robot with a controller that harnesses the systems passive dynamics and injects energy during stance to launch the hopper to a desired height.

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