Search

Wearable robots are emerging as a viable and effective solution for assisting and enabling people who suffer from balance and mobility disorders. Virtual prototyping is a powerful tool to design robots, preventing the costly iterative physical prototyping and testing. Design of wearable robots through modelling, however, often involves computationally expensive and error-prone multi-body simulations wrapped in an optimization framework to simulate human–robot–environment interactions. This paper proposes a framework to make the human–robot link segment system statically determinate, allowing for the closed-form inverse dynamics formulation of the link–segment model to be solved directly in order to simulate human–robot dynamic interactions. The paper also uses a technique developed by the authors to estimate the walking ground reactions from reference kinematic data, avoiding the need to measure them. The proposed framework is (a) computationally efficient and (b) transparent and easy to interpret, and (c) eliminates the need for optimization, detailed musculoskeletal modelling and measuring ground reaction forces for normal walking simulations. It is used to optimise the position of hip and ankle joints and the actuator torque–velocity requirements for a seven segments of a lower-limb wearable robot that is attached to the user at the shoes and pelvis. Gait measurements were carried out on six healthy subjects, and the data were used for design optimization and validation. The new technique promises to offer a significant advance in the way in which wearable robots can be designed. [ABSTRACT FROM AUTHOR]

Despite being widely used in several applications, commercially available spool valves, both servovalves and proportional valves, are inefficient components because they cause high power consumption due to the large pressure drops across the metering orifices. A recent research field aims at substituting spool valves with on/off valves having high switching frequency (changing state between open and closed in a few milliseconds or less) and producing low pressure drops, in order to make the so-called digital hydraulics possible. In spite of the advantages that it could provide, digital hydraulics does not have significative industrial applications yet, because of the difficulty in manufacturing such high frequency on/off valves. Hence, this paper performs a feasibility study of an on/off poppet-type valve actuated directly by a commercially available ring stack, which is a multilayer piezo-actuator capable of generating very high actuation forces needed for this application. Modulation of the average flow can be achieved by changing the duty cycle of the pulse width modulation (PWM) signal driving the piezostack. An inertance tube could also be used to smooth flow pulsation. The simulations obtained using a detailed Simulink model show that high switching frequency and very effective flow modulation can be obtained with this valve architecture along with low pressure drops and high flow rates, thus making it potentially suitable for digital hydraulics. The disadvantages of this single stage architecture are the large dimensions of the piezo stacks, and the high current generated because of both the high capacitance of the piezo stack and the high frequency switching. However, large-scale production of these components could help to reduce the costs, and the simulations show that limiting the maximum current to 10 A still provides good regulation.

Digital hydraulics is a novel alternative to proportional or servovalve-controlled systems in fluid power engineering, providing hydraulic systems with high-energy efficiency, good controllability, and insensitivity to contamination. Switched inertance hydraulic converters (SIHCs) are new digital hydraulic devices that can adjust flow and pressure by digital switching instead of throttling the flow. In this paper, an efficient closed-loop control system is proposed for SIHCs subject to time-varying loading conditions in which the load pressure and/or flow varies with time. The control system is designed to operate SIHCs at optimized switching frequencies and ratios that maximize system efficiency when the load varies. With the proposed controller, the SIHC can effectively adapt to the time-varying load and has achieved up to 10% efficiency improvement and up to 65% pressure ripple reduction without affecting the system's dynamic responses, compared with using a nonoptimized controller. The work shows the feasibility and advantages of simultaneously controlling the switching ratio and switching frequency of SIHCs with a time-varying load. As time-varying loading conditions are commonly found in hydraulic applications, the research outcomes constitute an important aspect in the design and development of highly efficient SIHCs and their practical use in hydraulic machinery. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the idea of using, in place of recent linear force motors, amplified piezo-stack actuators for the actuation of direct drive servovalves, in order to exploit the fast response of piezoelements accompanied by the increased displacement ensured by mechanical amplification systems. Some possible architectures are proposed in this paper for the direct actuation of four-way three-position (4/3) servovalves using one or two commercially available amplified piezo-stack actuators having a diamond amplification mechanism. The simplest architecture, which employs only one actuator, is assessed using wellestablished equations implemented in Simulink, allowing the hydraulic, mechanical and electrical parts of the valve to be accurately simulated. Three spools of different size are considered in the simulations in order to obtain performance predictions for different valve flow ratings. From the analysis of the inherent characteristics of the amplified piezo-stack and from the results of the simulations, advantages and disadvantages of this possible architecture are drawn and discussed in detail. Among the advantages, there are the simplicity of construction and the high potential in terms of step response speed and frequency response; the large dimensions, low chip shear force capability and high costs are the main disadvantages.

This paper proposes a modelling approach for simulating mechanical ventilators for intensive care units (ICUs). The shortage of ventilators during the coronavirus disease 2019 (COVID-19) pandemic has focused attention on their design and performance. The proposed modelling approach consists in using the Mathworks® Simulink software tool and the SimScape Fluids (gas) library, so as to use well-established subroutines to simulate all the pneumatic components of typical ventilators for ICUs, such as the pressure reducing valves, pressure relief valves, check valves, tanks, ON\OFF and proportional directional valves, etc. The patient is simulated by setting the values of lung compliance and pressure losses occurring in the trachea. The proposed modelling approach is used in this paper to simulate a pneumatic scheme employed in some commercial ventilators. The model allows a very accurate prediction of fundamental parameters, such as the inspiratory flow rate, the inspiratory pressure, the end-expiratory pressure. Since the software interface is user-friendly, it can easily be used by manufacturers to correctly choose the geometrical and operating parameters of the components during the design stage or to assess different scenarios.