By preparing a sensor system around isolated exceptional points, one can obtain a great enhancement of the sensitivity benefiting from the non‐Hermiticity. However, this comes at the cost of reduction of the flexibility of the system, which is critical for practical applications. By generalizing the exceptional points to exceptional surfaces, it has been theoretically proposed recently that enhanced sensitivity and flexibility can be combined. Here, an exceptional surface is experimentally demonstrated in a non‐Hermitian photonic sensing system, which is composed of a whispering‐gallery‐mode microresonator and two nanofiber waveguides, resulting in a unidirectional coupling between two degenerate counter‐propagating modes with an external optical isolator. The system is simple, robust, and can be easily operated around an exceptional surface. On the one hand, sensitivity enhancement is observed by monitoring the resonant frequency splitting caused by small perturbations. This demonstration of exceptional‐surface‐enhanced sensitivity paves the way for practical non‐Hermitian sensing applications. On the other hand, the suppression of frequency splitting around the exceptional surface is also shown for the first time.
To solve the problems with the existing active fault-tolerant control system, which does not consider the cooperative control of the drive system and steering system or accurately relies on the vehicle model when one or more motors fail, a multi-input and multi-output model-free adaptive active fault-tolerant control method for four-wheel independently driven electric vehicles is proposed. The method, which only uses the input/output data of the vehicle in the control system design, is based on a new dynamic linearization technique with a pseudo-partial derivative, aimed at solving the complex and nonlinear issues of the vehicle model. The desired control objectives can be achieved by the coordinated adaptive fault-tolerant control of the drive and steering systems under different failure conditions of the drive system. The error convergence and input-output boundedness of the control system are proven by means of stability analysis. Finally, simulations and further experiments are carried out to validate the effectiveness and real-time response of the fault-tolerant system in different driving scenarios. The results demonstrate that our proposed approach can maintain the longitudinal speed error (within 3%) and lateral stability, thereby improving the safety of the vehicles. Appl. Sci. 2019, 9, 276 2 of 16 distribution [5] and machine learning [6,7]. In the area of fault-tolerant vehicle control, recent research studies have shown that the fault-tolerant control on the electric drive systems focuses mainly on the diagnosis of motor faults and failure of the motor [8]; however, these studies have not paid sufficient attention to the fault-tolerant control methods of electric vehicles.Certain researchers have proposed simultaneously turning off the failed motor and opposite side motor of the 4WID electric vehicle [9,10], so that in the case of a single wheel failure or coaxial two-motor failure, it can still provide a partial driving force and maintain vehicle stability. This method is easy to implement but does not carry out real-time distribution of the vehicle wheel torque according to the real-time vehicle state, thereby reducing the longitudinal drive capability of the vehicle. Nakano researched the front and rear wheel independent drive system with a redundant structure. After the motor fails, the front/rear wheel of the faulty motor becomes a freewheel and only the normal rear/front wheel drives the entire vehicle [11] but the method does not fully utilize the four-wheel independent drive of the electric vehicle. The research team led by Wang from the Ohio State University studied the fault diagnosis of such a redundant actuator configuration system for 4WID electric vehicles [12]. Moreover, Wang proposed a method that uses adaptive fault-tolerant control and active fault diagnosis for the fault-tolerant control of the 4WID electric vehicle drive system, in order to isolate and evaluate faults accurately [13]. This method does not consider the impact of uncertainty on the system and thus requires the est...
The cavity ringing phenomenon is vital in both fundamental research and applications such as in the rapid detection and transient sensing. Here we report a novel method -the combination of the optothermal scanning method and Raman gain to achieve and control the ringing effect in a silica optical microsphere. Comparing to the usual method requiring laser sweeping, this method does not require sweeping the probe laser, which makes it easier and more stable in applications. The ringing strength can be easily controlled by adjusting the frequency detuning of the probe laser and changing the scanning speed of the pump laser. In particular, our dynamic models can fit well with the experimental results. This letter shows that the optothermal control of ringing is robust to external fluctuations, which can enhance the performance and stability of the transient microcavity sensors.
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