Networked control systems (NCSs), in which controllers and actuators are connected by communication networks, have been utilized in both industrial and consumer applications. In addition, sleep-based energy-efficient network interfaces have been developed for constructing a green networked society. The combination of the NCSs and energy-efficient network interfaces is a promising approach to reduce the power consumption of NCSs. However, sleeping of the network interface results in loss of data to be transmitted between a controller and an actuator. The sleep-induced data loss causes degradation of control performance. Moreover, NCSs originally include network delay for both forward and feedback paths, which also degrades control performance. This paper proposes a time-delay and dataloss compensation method using a modified communication disturbance observer (MCDOB) for NCSs with sleep-based energy-efficient interfaces. The proposed MCDOB can simultaneously compensate for not only the network delay but also sleep-induced data loss as a disturbance, whereas a conventional communication disturbance observer (CDOB) considers only network delay or network-induced data loss and cannot be directly introduced into the NCSs with sleep-based energy-efficient interfaces. Experimental results show that the proposed method with MCDOB outperforms the conventional method without the MCDOB in terms of integral square error (ISE) and communication rate, which indicates the power consumption of network interfaces. INDEX TERMS Communication disturbance observer, data loss, energy-efficient network, networked control system, time delay.
The energy consumption of network interfaces in networked control systems (NCSs) must be reduced for sustainability. The sleep mechanisms of energy-efficient network interfaces for networked motor control systems, in which controller and motor devices are connected by communication networks, have attracted much interest in recent years. They reduce the energy consumption of network interfaces by making the transmitters installed on the controller and motor devices enter a sleep period when no data are to be transmitted. In a conventional send-on-delta (SoD)-based sleep control method, the network interfaces decide when to enter the sleep period based on the variations of the control input and response values. Although this method effectively reduces the communication rate for reducing energy consumption, it does not explicitly consider the quality of performance (QoP) or control performance such as tracking errors. In high-precision motion control systems, ensuring a certain QoP level is crucial. This study proposes a QoPaware sleep control method for energy-efficient networked motor control systems to maintain the QoP at a certain level while operating the sleep mechanism. Here, the QoP is defined as the tracking error between the command and the response values. In the proposed method, network interfaces decide when to enter the sleep period based on the tracking error. Experimental results confirm that the QoP-aware sleep control method outperforms the SoD-based sleep control method in terms of the tracking errors (i.e., integral square error and steady-state error) at the expense of increased communication rates.INDEX TERMS Data transmission, energy-efficient network, motion control, networked control system, remote control, quality of performance.
In networked control systems, the quality of performance (QoP), such as in terms of consistent tracking errors, must be maintained at a constant level. A QoP-aware sleep mechanism in network interfaces can reduce the amount of traffic or the communication rate while maintaining the QoP. However, in a QoP-aware sleep mechanism with a fixed sleep period, the sleep period cannot be entered effectively when the length of the steady-state period is changed; e.g., when the command frequency is changed. This study proposes a variable sleep period control method to reduce the communication rate while adapting to the tracking error. The simulations confirm that, when compared to methods with a fixed sleep period, the proposed method reduces the communication rate regardless of the command frequency.
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