A Wireless Networked Control Systems review

A Wireless Networked Control Systems Review Yury A. Mill´an, Francisco Vargas, Fernando Molano, and Eduardo Mojica

Abstract— A Wireless Networked Control Systems review is presented. It begins with a brief history about NCS (Networked Control Systems), recognizing the fundamental features, its advantages over other classical control systems, and the protocols that can be used to do control over the wireless network. Also, we deal with the different problems that can affect the network behavior; including interferences and delays that take place when the control acts. The behavior of the system in different environments is shown through simulations using TrueTime blocks, included in a Simulink/MATLAB box. Index Terms— Networked Control systems (NCS), Wireless Networked Control Systems (WNCS), Network Scheduling, Delays.

I. INTRODUCTION In different topics of science and technology, the control systems explain some of the behaviors in different ecosystems, i. e. animals, biology, neural networks, socials, among others. It is an important evolution on how to keep the control on these areas and to understand its features [9]. Therefore, the control systems have a connection with the communication systems, and how it can keep a controlled system. The communications can be represented for the interconnection between the different cities in a country, areal connection routes between countries, power distribution lines, among others.[12]. Networked control systems arise from the need to cover troubles with different alternatives of a specific network system. It converges to the necessity of to related the feedback control systems theory with theories about network control systems and the diverse methods that it uses. II. BACKGROUND The development of different control systems for applications in different fields has played an important role through history. With the help of the progress of diverse technologies, it is looked for the control of different systems becomes every time more reliable, optimal, and efficient. Based on a feedback system, began the development of the first control systems. It is called classic control. Since 1950’s, with the digital computers us controllers in the feedback system, it is obtained the control systems in discrete time [24]. These control systems were related with wired connections and were designed to take information from sensors to local exchange, where systems conditions were monitored and the decisions were took depending of the plan requirement through actuators. The digital control became an important research when the microprocessors started to be used in Universidad Cat´olica de Colombia Electronic and Telecommunications Engineering Program

1970’s. Although the network control appears in 1980’s, it has become used only in the last years. Due to the advance of the wireless communications the idea was reaching to the network control [24]. Within NCS there are some kinds of communications: • CAN: It is a communication protocol designed for applications that require high data integrity and data rates to 1 Mb/s. • Ethernet: It is a good alternative due to low cost network components and compatibility with existing Ethernet infrastructure, but it is affected when the size of the NCS increases. The cost, installation time required for the maintenance, and the large number of cables typically requires in this environment are drawbacks. • Wireless NCS (WNCS): It has evolved by the necessity of mobile operations, flexible installations, and rapid deployment for many applications. When wireless systems are included, the reliability and the time are more difficult to satisfy, because the harmful properties of the radio channels. The majority of control systems assume that the data collected are accurate, timely, and lossless. Those are challenges of WNCS. The difference with the network control is that the control is remote by microprocessors, and the information can be sent reliably through the shared digital networks, even wireless connections. In the distribution systems on a wireless network, different sensors can have different locations and its measurements need to be encoded and sent one by one over the different links of wireless networks. III. BASIC CONCEPTS NCS are systems in which I/O information is exchanged with the system components through a shared communications medium, as shown in Fig. 1. Actuators, sensors, and controllers shares the information through the network media. The main features of a NCS is provide a communication system appropriated to obtain real-time data. There must be taken into account the network protocol, congestion, routing, and ensure efficient communication data. Also, the networked control is used to design control strategies to minimize problems that might found respect the network performance, delays and parameters that affect system performance [8]. Multiple closed-loop control systems could coexist over the same network. The purpose is that I/O information be exchanged with system components, through of shared communication environment. The rise of new technologies

consistent transfer data keeping the system synchronized and low maintenance. 2) Synchronization in the control loop: An approach of synchronization control loop is clock-driven and eventdriven. The nodes sensor/actuator is clock-driven and controller is event-driven, or might be handled according to system. In this method, communication between sensor/actuator and controller is periodic according to sample time. In case of packet loss, the controller waits the next control signal, i.e., it has the ability to take control actions as soon as feedback is available. Until it receives data, the controller action is taken as zero until the next time interval [5]. See Fig. 2.

Fig. 1.

Typical design of an NCS [8].

implies that the long distance control in remote locations make necessary the implementation of control systems over wireless communication networks. A. Wireless Technologies In wireless communication industry the main standards used are: WLAN 802.11, WPAN Bluetooth 802.15.1, and ZigBee 802.15.4. These protocols work in the 2.4GHz ISM band, and might coexist in current industrial deployments. The Table I illustrates the key features of these standards. Characteristic Range Data rate Number of devices for network Power consumption Periodic data

Transmissions FEC

Bluetooth/ IEEE 802.15.1 10 (50100m) 723 Kbit/s

ZigBee/ IEEE 802.15.4 10m

WLAN/ IEEE 802.11a/b/g

2 to 65000

125 Kbit/s 7

30.6 Mbit/s (Ethernet), 2.6 Mbit/s (60 bytes payload) 40

low

very low

medium

yes (depending on polling algorithm) yes available

yes

DCF: no; PCF: yes (with some jitter); HCF: yes

yes no

yes no

50-100m

TABLE I C OMPARISON OF WIRELESS TECHNOLOGIES [4].

1) Control architecture on the network: The wireless network connections use compatible protocols for data transmissions as TCP/IP. It is a connection-oriented protocol, ensuring the delivery of information between nodes, as it handles an algorithm that allows for retransmissions of packets lost during data transfer. UDP is a connectionless-oriented protocol, but there are no provisions for retransmission and the data are not received. This protocol is preferred for networked control in communications of small sampled data packets, because retransmitting lost data allow frequent and

Fig. 2.

Time and synchronization diagram [5].

3) Wireless network problems: The problems in networks wireless are presented by the conditions of transmission media. Due to this, control systems are focused on security network, robustness, and efficiency. It is necessary to implement control strategies according to these needs. The interference is one of the main problems [11]. Because they are free bands can be used by multiple devices, the interference can be generated by a single WNCS devices with other applications. So, should be considered in that channel is working and how far are the neighboring networks; this can be produced by small-scale effects, i.e., devices of the same WNCS, or largescale effects, i.e., devices of others applications. WNCS also are affected by signal attenuation, obstacles in the radio-path, reflections, propagation delays, fading, and phase shift. Also, system delays (sensor/controller delay and controller/actuator delay) occur while data exchange on devices connected to the shared medium [3]. In networked case, some packets suffer delays and may be lost during transmission or be discarded. To avoid this, it is important that the work channels, distance with neighbors, and capability devices, among others features should be considered to obtain good network performance [20]. B. Networked delays NCS has limitations of flexibility and mobility because the network is wired. WNCS is an alternative to solve those problems, due it has simple configuration and shared network to access media. Depending on the type of software and hardware on the network, there are three models of networked delays:

Constant delays, Random delays, Random delays with Markov chains1 . By these models with others complements might be design appropriate delays for specific system. 1) Constant delays: The simplest model of the network delay generates cycles of constant control for all network transfer. These delays are efficient if the time-scale in the process is much larger than the delay introduced by the communication. Otherwise, it could be affect the stability and efficient of the system because data accumulate are not processed and wrong conclusions can be drawn regarding system information [15]. 2) Random delays: Random delays could be applied by operation when network is inactive. If messages are pending to be sent, delay might include transmission of the waiting messages. Also retransmissions can be needed in case of errors, and can include a random wait to avoid a collision at the next try. As nodes are not synchronized with each other, accumulated data send, resulting in the randomness delays to obtain the information. This randomness make the model delays take the form of probabilistic distribution [15]. 3) Markov chains: It is a statistical model that describes a probabilistic distribution over an infinite number of possible sequences. The system has unknown parameters that can represent the loading on the network and lead to delays induced by the same network. These are determined from observable parameters, each state emits observable parameters according to emission probabilities, and the states are interconnected by the transition probability of states. Given an initial state, sequences of states are generated by passing from one state to another by the probabilities of transition to a final state. Each state issues according to observations indicating the probability distribution of emission, creating a sequence of observations so that the value of the hidden variable at time t only depends on the value of the hidden variable at time (t − 1) and this is called the Markov property, just as the value of the variable observed in a single moment depends on the value of the hidden variable [25],[20]. C. IEEE 802.11 Protocol An important feature to consider when the control is implemented over a WLAN (Wireless Local Area Network) is the real time control, due to the data packets between controllers. Sensors and actuators must to transmit periodically, minimizing any loss or error in the transmission. Although the IEEE 802.11 protocol was not designed to WNCS applications, its implementation can be useful on the network control when sets the DCF (Distributed Control Function). In this protocol, the decision of which station can transmit is taken between the nodes of the WLAN. Before transmission, every station must check the channel status to avoid collisions. If the channel is idle in a specific time, the station can transmit in an interval of random backoff time. The receiver checks the CRC and sends an ACK message to the transmitter. When it receives the ACK, there is a 1 Named after the mathematician Andrei Andreyevich Markov (18561922), who introduced them in 1907.

successful transmission; if the transmitter doesnt receive the ACK, it has to transmit the frame after a random backoff delay until the ACK is received [6]. D. IEEE 802.15.4 Protocol ZigBee is a high level standard based on IEEE 802.15.4 that works in Wireless Personal Area Networks (WPAN). It defines the physical and medium access control layers of wireless networks with low rates of data transmission. ZigBee is designed for low speed, because the channel bandwidth in the 2.4GHz is 250Kbps. It emphasizes the cost of communication with nearby nodes without a complex infrastructure for low power consumption. A primary requirement of the IEEE 802.15.4 applications is the long battery life. When ZigBee is used for Wireless Networks, sensors and controllers dont need a large bandwidth due to the low latency and low power consumption with long battery life. That is because it can work in active mode (transmit/receive) or sleep mode. In RTC (Real Time Control) the packets should be small due to short transmission time. The packets are sent continuously, so is acceptable the loss packet and the retransmissions are not required [23]. This protocol is an alternative to set the control over the network. It proposes three ways of working, beacon-enable mode, non beacon-enable mode and beacon-enable mode with GTS (Guaranteed Time Slot). There are models for GTS that make different functions, including some models to assign the number of GTSs, the application mode of Markov‘s chain to study the behavior of the device assignation of GTS [18],[10],[7],[20]. E. WirelessHART Industrial environments are harsher for wireless applications in terms of interferences and obstacles than office environment. WirelessHART is a secure and TDMA based wireless mesh networking technology operating in the 2.4GHz ISM radio band, specifically aimed at wireless instrumentation for the factory automation industry [22]. WirelessHART is based on the IEEE 802.15.4, and WirelessHART instruments use a pseudo-random channel hopping sequence to reduce the chance of interference with other networks, such as IEEE802.11b/g (Wi-Fi) which operates in the same ISM frequency band. Using techniques such as channel hopping, time division multiple access, low power, mesh networking, and direct sequence spread spectrum coding allow a WirelessHART network to maintain high data reliability and at the same time minimize, if not eliminate, any effect it has on other overlapping networks [1][20]. WirelessHART is compatible only with HART devices. F. ISA100 Like WirelessHART, ISA100 is based on the IEEE 802.15.4, and is intended to provide reliable and secure wireless operation for non-critical monitoring, supervisory control, and alerting. Key features are low power consumption, robustness against interference, and interoperability

with other devices [13]. ISA100.11a has the ability to support different protocols through a single wireless infrastructure and is a TDMA based wireless mesh network too. ISA100.11a standard supports many applications with security requirements [19], and addresses wireless sensor networks in the operating plant environment. Also, ISA100.11a allows the flexibility and scalability, and allows addresses coexistence with other wireless devices, such as mobile phones and other devices based in other relevant standards [2][13]. The main feature of WirelessHART and ISA100 is the channel hopping. Transmissions are scheduled on 1 of the 15 logical channels, which are mapped onto the physical channel by a channel hopping sequence, and the transmission scheduling is organized into multiple superframes[20]. WirelessHART use superframes with a specific time slot and a specific channel, while ISA100 uses different superframes that can use different time slot lengths [20]. Otherwise, a feature of ZigBee and ISA100 is the low energy consumption and use routers to optimize battery consumption, but also investigates the use of alternative energy to avoid the shutdown of the nodes. IV. RESULTS The importance of explaining how systems control and communications are related implies that they are generated around many variables that can not easily determine, which leads to not take into account different phenomena that degenerate systems. It is used design tools to describe the behaviors that occur in any WNCS environment, some programs are used to obtain information about the failures that can affect the behavior of a WNCS. 1) True Time: True Time simulation block 2 has been designed for the development of wired and wireless network applications. It allows to observe the behavior that can have a network including the conditions that can affect the system (delays, interference). This block is based on Simulink/MATLAB motor development using execution blocks with code functions, and block stand-alone that do not require scripts or code function. The Fig. 3 shows the different blocks used to simulate applications. These blocks can interact with the Simulink blocks allowing easy handling of this tool. The True Time Kernel Block enable the use of scripts and code functions in the MATLAB environment, also algorithms in C ++ implemented in MATLAB through MEX-files command execution. The kernel block simulates a real-time kernel executing user-defined tasks and interrupt handlers [21]. 107 (1) s3 + 11s2 + 91s + 108.3 The simulation performs a feedback control system with a DC motor through a wireless network 802.11b. The system is designed to block stand-alone and the control loop is

Fig. 3.

True Time Simulation Block [21].

composed of two nodes. At node 1, this is a PID controller. At node 2, the sensor, actuator, and the transfer function of the DC motor described by (1) [14]. Through simulation is observed the behavior of the system when the interference is on the network and how it can affect the control loop stability. We used two simulation environments; the first shows an ideal control system where there are no interference in the network and have constant delays, the second shows the same control system but now affected by some noise present in the exchange of data between nodes. A. Results 1) WNCS without interference: On the control system shown in Fig. 4, is implemented a PID controller with values P=1.2, I=1.4, D=0.1 [14], it works with event-driven, i.e., a sensor sends network schedule pulses about a clock-driven [16]. The True Time Wireless Network block is designed to work with the 802.11b protocol and contains the coordinates X and Y used for the application. At the moment, nodes in the True Time framework only have X and Y coordinates, but if a direction was to be introduced this function could also be used to model directive effects in the antenna behaviour [21].

Fig. 4.

WNCS Block diagram without interference.

G(s) =

2 True time 2.0 beta 6 Block Library Copyright (c) 2010 Lund University. Written by Anton Cervin, Dan Henriksson and Martin Ohlin. Department of Automatic Control LTH, Lund University. Sweden [email protected]

These nodes communicate through the wireless network in an environment where there are not interferences. Hence the system can remain stable (See Fig. 5). Also it generates graphics of the network scheduling, where there are symmetric pulses with a constant period from node 2, and a control pulse equivalent to the output signal received from the sensor node, this is shown in Fig. 6.

Fig. 5.

WNCS response without interference.

Fig. 8.

WNCS with interference node.

pulses have a minimum interference, which does not affect such pulses. We also observe the behavior of the interference with respect to network scheduling.

Fig. 6.

WNCS network scheduling without interference.

2) WNCS with interference: Based on the previous control system, it is implemented node interference that may affect the control signal and thus the system response. This simulation shows that the network is being affected by random interference, but it is still possible to maintain a proper and efficient control over it.

Fig. 7.

Fig. 9.

WNCS network scheduling with interference node.

In the following results, it is changed the location of interference (coordinates), in order to see how it affects the system. As an effect of this simulation, it is observed that the interference affects the information transmitted, it means that the data are erroneous, and lost control of the system. Fig. 10 can see the output degradation of the output process with respect to reference signal.

WNCS Block diagram with interference node.

As it is shown in Fig. 7, the initial control loop (Fig. 4) includes a block that generates interferences at random times in the system, the interference node contains a periodic task that generates random interfering traffic over the network [17], allowing analysis of how it affects data transmission between elements of the control loop. Fig. 8, is obtained from the simulation result of interference node system, which can be drill that due to its location with respect to the coordinates of nodes 1 and 2. It does not affect the communication and the system continues stable. Similarly, in Fig. 9, we see that the network scheduling

Fig. 10.

WNCS with interference node near to main nodes.

In Fig. 11, there are three interference signals corresponding to the sensor and controller respectively. It can be seen how the sensor reading is directly affected by interference, which leads to, based on erroneous data, a degradation of the system, becoming unstable. An important characteristic to consider with respect to

Fig. 11. nodes.

WNCS network scheduling with interference node near to main

interference distance is found in the control network. Keeping the interference at a given distance, we can control the system and make the system stable, despite of the presence of interference in the wireless networks. V. CONCLUSIONS This paper presentes a brief analysis of the principal characteristic of the NCS, their general aspects and the different problems in a WNCS. Also, it is shown the techniques used to apply delays on the network and make it controllable, and it is described some of the protocols used in the WNCS. MATLAB simulations shows the behavior of a WNCS. Also, simulation worked with the synchronization of control clock-driven and event-driven. Moreover, the block standalone showed that it can have only constant delays. This shows that the system can be affected for interference and can affect the system depending of their location. Depending on the amount of data to transmit, the system can be also affected. To perform a application with random delay is necessary to generate a function or script, combined with the True Time Kernel block, it can generate these delays. For future work is necessary to include control-oriented standards, to research and develop autonomous sensors to improve network performance, optimize the energy source of the sensors, and environment connection and transmission. Otherwise, wireless sensor networks require advanced security protocols to be a more robust system, and can be used on Smartgrids and other projects. R EFERENCES [1] Hart Communication. [2] ”Wireless Systems for Industrial Automation: Process control and related applications”. ISA, 2009. Whitepaper. [3] Garcia-Rivera M.; Barreiro A. , ”Analysis of Networked Control Systems with Drops and Variable Delays”. Automatica, Vol. 43(No. 12):pp. 2054–2059, 2007. [4] Willig A.; Matheus K.; Wolisz A. , ”Wireless Technology in Industrial Networks”. Proceedings of the IEEE, Vol. 93(No. 6):pp. 1130–1151, June 2005. [5] Ploplys N.J.; Kawka P.A.; Alleyne A.G. , ”Closed-loop Control over Wireless Networks”. IEEE Control Systems Magazine, Vol. 24(No. 3):pp. 58–71, June 2004. [6] Guosong Tian; Yu-Chu Tian; Fidge C. , ”Performance Analysis of IEEE 802.11 DCF based WNCS Networks”. Proceedings of The 35th IEEE Conference on Local Computer Networks, pages 496–503, October 2010.

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