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.

Variable valve timing (VVT) and variable valve lift (VVL) are two promising methods for improving gasoline engine performance. VVL improves part-load performance, and VVT reduces low-speed fuel consumption. Automobile industries and researchers have developed several mechanical, hydraulic, and electronic devices to implement these variable valve functions in engines. In this study, a control strategy is developed for a new compact and low-energy-consumption magneto-rheological valve train (MRVT) to effectively accomplish the variable valve functions and achieve superior engine performance. A non-throttle single cylinder SI engine dynamic- model is established to simulate the engine performance by using the flexibility of this new valve train. A six-mode strategy using VVT and VVL is proposed under different engine running conditions of speed and load. Dynamic simulations were conducted for investigating the six-mode-strategy based engine performance. The results indicate that the combination of VVT and VVL in the corresponding engine mode can effectively give about 15%-20% improvement in the brake fuel efficiency during low and medium speeds. Moreover, by using VVL, about 10-14% improvement in brake specific fuel consumption can be achieved at part-load conditions. According to this computational investigation, the overall engine efficiency and performance can be improved significantly by using a controllable magneto-rheological valve and strategically changing the engine VVL and VVT.

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