Despite advances in assistive technology, existing prosthetic knees still have some limitations, such as weight, low active and braking torque, and high energy consumption. This paper presents an active magnetorheological knee (AMRK) actuator developed for transfemoral prostheses. The system consists of a motor unit comprising an EC motor, harmonic drive and magnetorheological (MR) clutch. The motor unit provides active motion, working in parallel with an MR brake. With this configuration, the AMRK possesses multiple functions; it can work as a motor, clutch, or brake, reproducing movements similar to those of a healthy knee in different activities. All components of the prosthetic knee are protected to avoid risk of accidents and to provide an aesthetically appropriate structure. To reduce weight, energy consumption and volume, the MR clutch/brake geometric design was optimized using a particle swarm optimization algorithm. A prototype was fabricated and tested to evaluate the AMRK performance. Dynamic models of the MR clutch, MR brake and motor unit were analysed, and torque control was implemented. The results show that the AMRK is promising for the proposed applications, which require multiple functions with compact size, low weight, low energy consumption, high active and braking torque, and quick response time.

This study analyses the energy consumption of an active magnetorheological knee (AMRK) actuator that was designed for transfemoral prostheses. The system was developed as an operational motor unit (MU), which consists of an EC motor, a harmonic drive and a magnetorheological (MR) clutch, that operates in parallel with an MR brake. The dynamic models of the MR brake and MU were used to simulate the system’s energetic expenditure during over-ground walking under three different working conditions: using the complete AMRK; using just its motor-reducer, to operate as a common active knee prosthesis (CAKP), and using just the MR brake, to operate as a common semi-active knee prosthesis (CSAKP). The results are used to compare the MR devices power consumptions with that of the motor-reducer. As previously hypothesized, to use the MR brake in the swing phase is more energetically efficient than using the motor-reducer to drive the joint. Even if using the motor-reducer in regenerative braking mode during the stance phase, the differences in power consumption among the systems are remarkable. The AMRK expended 16.3 J during a gait cycle, which was 1.6 times less than the energy expenditure of the CAKP (26.6 J), whereas the CSAKP required just 6.0 J.

In general, actuators are built to be as stiff as possible to increase the bandwidth. When a robot works in a structured environment, its automation is easier than in a nonstructured environment in which case its modeling is quite difficult and presents a high computational effort. To overcome this difficulty, series elastic actuator SE" has been applied in compliant robotic grasping. Unlike rigid actuators, a SE" contains an elastic element in series with the mechanical energy source. Such an elastic element gives SE"s tolerance to impact loads, low mechanical output impedance, passive mechanical energy storage, and increased peak power output. The spring has to be able to support the loads, but it cannot be too stiff otherwise, system impedance will be high. This chapter describes a comparison between two types of SE", an electric series elastic actuator ESE" and a hydraulic series elastic actuator HSE" , for four-legged dynamic robot application. The parameters employed in the comparison are bandwidth, output impedance, time response, power density, and dynamic range. The results indicate that HSE" is a better actuator than ESE" for a weight carrying four-legged dynamic robot because of its higher power density and dynamic ratio with desirable output impedance, time response, and bandwidth.

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Lower limb exoskeletons (LLE) have been successfully used in robotic-assisted rehabilitation to reduce the burden of locomotor impairment of disabled people. However, the design limitations of LLE mechanisms, such as the lack of kinematic compatibility relative to the user’s joints and the use of high stiff or heavy actuators, limit the outcomes of treatment and increase the risk of injury. To address these shortcomings, in this work we present the design of the MRKneeExo, a highly backdrivable and kinematically compatible active knee exoskeleton. The powertrain of the system is composed of a BLDC 70 W motor associated with a harmonic drive gearbox and a customized magneto-rheological (MR) clutch. To improve kinematic compatibility with the user’s knee, a crossed four-bar linkage mechanism (FBLM) was optimally designed to follow the trajectory of the instant center of rotation (ICR) of the knee projected in the parasagittal plane where the joint is placed. The MR clutch is used to decouple the motor-reducer from the FBLM, thus enabling high backdrivability. The results showed a small error (<3 mm) between the FBLM and the knee ICR. Furthermore, the MR clutch allowed for low back-drive torque (1.28 N m) compared to the torque to back-drive the motor-reducer (18.51 N m). This paper is presented in a framework that can be generalized to support the design of other knee exoskeletons.