MR Actuator Current Control for Faster Prosthetic Braking Response
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Solution Overview
Problem
Magnetorheological (MR) actuators in lower-limb prosthetics face challenges in achieving fast response times and optimal braking torque due to the latency in changing the magnetic field and apparent viscosity of the MR fluid, which is exacerbated by the complex dynamic characteristics of the magnetic coil and the need for refined control strategies.
Innovation Solution
A control system employing gain scheduling and closed-loop current regulation with adjustable gains and configurable duty cycles to minimize the time required for current changes in the magnetic coil, ensuring rapid changes in the magnetic field and actuator properties, and using a Field Programmable Gate Array (FPGA) for high-bandwidth control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If conventional current control methods are used in MR actuators, then the control system is simple, but the response time is slow due to latency in changing magnetic field and apparent viscosity
Solution Approach 1:
The control system proactively drives the current and resulting magnetic field before the actuator properties need to change, compensating for the inherent latency in the MR fluid's apparent viscosity change. This preliminary action reduces the overall response time by anticipating and preparing the system state in advance.
Solution Approach 2:
The control system dynamically adjusts current based on real-time actuator state and operational requirements, transitioning from static to dynamic control. This enables the system to optimize response time across different operating conditions by adapting control parameters on-the-fly.
2Force
If the magnetic circuit size and MR fluid volume are increased, then the actuator can generate sufficient braking torque, but the latency between current change and actuator property change increases
Solution Approach 1:
The control system changes electrical parameters (current magnitude and timing) to compensate for the physical constraints of the magnetic circuit and MR fluid volume. By adjusting current parameters proactively, the system achieves faster effective response despite the fixed physical dimensions.
Solution Approach 2:
The closed-loop control system continuously monitors actuator state and adjusts current accordingly, creating a feedback mechanism that reduces latency by responding to actual system conditions rather than relying solely on open-loop timing.
3Reliability
If the magnetic coil is designed for strong coupling like DC motors or solenoids, then the current time-response is well-defined, but the actuator performance spectrum is reduced due to different MR actuator characteristics
Solution Approach 1:
The control system applies different control strategies and parameters tailored to specific MR actuator characteristics rather than using a universal approach. This localized control optimization maintains reliability for each specific actuator design while preserving the performance spectrum across different operating modes.
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 significantly reduces the time required for the actuator to respond to changes, enhances the consistency and repeatability of braking performance, and maintains optimal performance throughout battery discharge, providing improved responsiveness and user experience.
Implementation Method 1
the magnetic coil in a shear-type MR actuator does not behave like a pure theoretical inductor would do
Implementation Method 2
the braking is achieved through a change in the apparent viscosity of the MR fluid which results in increased friction between the thin discs mounted on the device rotor and stator
Data Source
AI summary
A prosthetic or orthotic device has an elongate frame that houses electronics and an actuator rotatably mounted to the frame. The actuator can rotate in an anterior-posterior direction about a medial-lateral axis and includes magnetorheological (MR) fluid and a coil operable to selectively apply a magnetic field to the MR fluid to vary its viscosity and thereby vary a torsional resistance of the actuator about the medial-lateral axis. Circuitry controls an amplitude of a current applied to the coil, and employs a gains schedule to accelerate a change in the current amplitude based on an error amplitude between a current set point and a measured current to reduce a response time for varying the torsional resistance of the actuator.