Abstract
Semi-active knee orthosis (SAKO) is a kind of wearable lower-limb exoskeleton that uses actuators to support the regular biomechanical functions. It is much better than conventional knee orthosis (CKO) devices because of its high torque to volume ratio (TVR) and low mass. Magnetorheological (MR) brake is one of the smart actuators that can be used as an active resistance device in SAKO. It has advantages of fast response, low power consumption, and low vibration operation. This smart brake also has wide applications in the robotic and automotive industries. However, the electromagnetic setup in MR brakes has a hysteresis problem. This paper aims to turn this hysteresis problem into an advantage to save the power consumption of MR brake. Since the SAKO needs precise torque control, this research studied the hysteresis effect on the torque performance of MR brake. A less energy-consuming PWM actuation signal is proposed to activate the MR brake. The effects of frequency and duty cycle of PWM actuation signal on MR brake performance are also investigated. The electromagnetic (EM) and mechanical models of the MR brake were developed to simulate performance. Initial validation of these models is done by simulating the MR brake model with the DC actuation signal in finite element analysis software. For the final validation, the model simulation results are compared with experimental results. The factors affecting the steady torque and the response time of the MR brake are studied to find the optimal frequency and duty cycle for the applied PWM signal. This study revealed that the proposed new PWM actuation signal with a 5 kHz frequency and 60% duty cycle can power the MR brake to maintain steady torque. By turning hysteresis into an advantage, it saves 40% power consumption of MR brake compared to DC signal.
Highlights
IntroductionWearable robotic exoskeletons have been extensively researched in the past decade
Published: 15 October 2021Wearable robotic exoskeletons have been extensively researched in the past decade.They play a vital role in assisting patients with mobility problems [1]
Initial validation of the Simulink model is done by comparing the Simulink results with ANSYS simulation results
Summary
Wearable robotic exoskeletons have been extensively researched in the past decade. They play a vital role in assisting patients with mobility problems [1]. Robotic exoskeletons are used in various joints of the body. The actuators of the exoskeleton can be designed and modified as per the location and mobility requirement of the body [2]. The mechanical design of the exoskeleton can have multiple degrees of freedom (DOF) depending on the type of joint and comfort. They are classified based on the type of actuator used [3]. The friction in the robotic exoskeletons is modeled to extend its use in stiffness applications [4]
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