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In recent times the explosive growth in computing and networking abilities has occurred. This has caused an increased reliance on distributed computing and process operations across the networks. The main aspect in the process control via network is the network delay, which depends on the network type and protocol. This paper gives the literature r...
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... the total network delay includes also computational delays as computational delay in the controller τ kc , but they can be embedded in the delays τ and τ . Besides that the total network delay includes transmission delays and computational delays it includes also delays due to the network load and failures. The length of the delay is varying, and it depends on the network used. The network can be cyclic or random access (RAN). The cyclic networks are based on token passing and TDMA (Time Division Multiple Access) as Token Ring, Token Bus and Profibus. In these networks the delay is caused primarily from the waiting time i.e. the time the node has to wait until the network is idle for sending the message. The delays in cyclic networks can be periodic or deterministic. Ethernet, CAN (Controller Area Network) and Internet belong to RAN networks. In these networks the delay behaves randomly due to the CSMA (Carrier Sense Multiple Access) technology. If the transmission occurs over several networks as in Internet, the waiting times in switches and routers cause also additional delays . For a sample-data control system it is natural to sample the process output equidistantly with a sample period. It is also clear to keep the control delay as short as possible. The reason is that the delays give rise to phase lag, which often degenerates system stability and performance. This motivates to use a system set up with event-driven controller node and event-driven actuator node. The calculation of the new control signal takes place as soon as possible the new signal arrives from the sensor node to the controller node. The timing diagram is presented in Figure 2. Ethernet is one of the most used local area network (LAN) technologies. It transmits data with the speeds 10, 100 or even 1000 Mbit/s. Ethernet is not intended for real- time communications. However, the large number of installed Ethernets will make it attractive for use in real- time control systems. Ethernet uses a bus access method called CSMA/CD (Carrier Sense Multiple Access with Collision Detection) which means that before sending to the network the station listens to the channel, and when the channel is idle, transmission starts. If several stations start sending to the bus the collision is detected, and the colliding stations back off, and try a retransmission after a random time. An almost unlimited number of stations can be connected to Ethernet. The switched Ethernet differs from the conventional Ethernet in two ways. Firstly, the operation of the hub where the stations are connected is different. In Ethernet the hub is a passive device, but in the switched Ethernet the hub, called a switch is an active device. It identifies the destination ports and relays the frame only to those. Figure 3 presents the transmission methods of Ethernet and switched Ethernet. The conventional Ethernet uses a half-duplex link, while the switched Ethernet uses a full-duplex link. This means that the switched Ethernet is free of frame collisions and the delay can be minimized. Typical method of switching technology is a “store and forward” method as shown in Figure 4. In the store and forward method the switch receives the frame from a source station and checks if the reception line of a destination station is idle. If it is idle, it forwards the frame. If not, it stores the frame into its buffer and waits until the line is idle. If several frames with the same destination address are received simultaneously, the switch stores the frames into the buffer and sends them to the destination one by one. According to the research by K.C.Lee and S.Lee [6] the length of the network delay in the conventional Ethernet and in the switched Ethernet differs remarkably. The length relation of delays between Ethernet and the switched Ethernet is 103. It means that the switched Ethernet applies better for control purposes Network delays are usually varying in a random fashion, and they can have different sources ...
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... means that the switched Ethernet is free of frame collisions and the delay can be minimized. Typical method of switching technology is a "store and forward" method as shown in Figure 4. In the store and forward method the switch receives the frame from a source station and checks if the reception line of a destination station is idle. ...
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... Networked control, be it wired or wireless, sets some new challenges for the design and performance of the control systems, network delay among others. Depending on the network structure, media and protocol, the delay can be constant, time varying or even random, and it occurs when sensors, actuators, controllers and humans exchange data over the network [3]. Available constant timedelay control methodologies may not be directly suitable for controlling a system over the network since the delays are usually time-varying, especially in the Internet [4] and in wireless systems, such as WSNs [5]. ...
Full text available at: http://link.springer.com/chapter/10.1007%2F978-3-642-34156-4_24#page-1
Networked control systems (NCS) could be utilised in several industrial applications. However, the variable time delays introduced by the network impair the NCS performance, resulting even in the instability of the controlled process. To mitigate the delay problems, the advantage is taken from model-based, adaptive controllers. This calls for an efficient approach for on-line analysis of measurements applied to update the controller state in NCS. The paper introduces a new adaptive Model Predictive Controller (MPC) capable of compensating for variations in measurement and actuating delays. Weighting factors for delayed measurements and actuators are adjusted based on normalised version of mutual information that is calculated using a procedure described in the paper. The method is superior compared with other, more usual, metrics.
... Waiting time delay is the delay, of which a source has to wait for queuing and network availability before actually sending a frame or a packet Frame time delay is the delay during which the source is placing a frame or a packet on the network Propagation delay is the delay for a frame or a packet travelling through a physical media In the networked control loop of Fig. 5, the network delay consists of the following transmission delays communication delay between the sensor and the controller, τk sc communication delay between the controller and the actuator, τ k ca Fig. 5. Block diagram of the network-based control system with different delays (Yliniemi & Leiviskä, 2006). ...
... In Ethernet, CAN (Controller Area Network) and Internet the delay behaves randomly due to the CSMA technology (Yliniemi & Leiviskä, 2006). Fig. 6 compares experimental process responses with a sample frequency 1 Hz across Ethernet, Internet, FUNET, and without network (Yliniemi & Leiviskä, 2006). ...
... In Ethernet, CAN (Controller Area Network) and Internet the delay behaves randomly due to the CSMA technology (Yliniemi & Leiviskä, 2006). Fig. 6 compares experimental process responses with a sample frequency 1 Hz across Ethernet, Internet, FUNET, and without network (Yliniemi & Leiviskä, 2006). As the Figure shows, the control across Ethernet is similar to the control without the network. ...
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This report presents some simulation results of basis weight control in paper machine. In particular, the dilution actuator width, actuator response width and the effect of scanner measurement uncertainty are studied. Also different controller configurations in machine direction control are tested.
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