Application of LonWorks Distributed Control Technology in Greenhouses

Transcription

1 Application of LonWorks Distributed Control Technology in Greenhouses Gilberto A. Pereira a, Carlos E. Cugnasca b a Agricultural Automation Laboratory, Escola Politécnica, Universidade de São Paulo, Brazil, professor.gilberto@uol.com.br b LAA, Brazil, carlos.cugnasca@poli.usp.br Abstract Greenhouses are becoming more important every day due to the requirements of modern agriculture. The market is becoming increasingly competitive, demanding better quality products at reasonable costs. Optimization of production processes inputs and environmental preservation are also important aspects that must be considered. In order to attend to these necessities it is important to improve the greenhouse control systems. Lately, there has been an increase in the use of distributed control systems that provide more flexible, scalable and cost effective solutions. This paper highlights the features of typical greenhouse automation systems and proposes a solution for greenhouse automation using the LonWorks distributed control technology. Key words: Greenhouse, Distributed Control, Automation, Control Network, LonWorks 1 Introduction Typical commercial and industrial supervision and control systems present as main characteristics centralized architecture, and proprietary hardware and software. Manufacturer dependency, lack of flexibility, incompatible communication protocols and the need of a high performance central processor are examples of problems of this kind of solution. Nowadays, a new control system approach based on control networks and open solutions has been increasingly considered. A control network with controllers, sensors, actuators, and other devices with intelligence and communication capabilities (called nodes) implements a new supervision and control system with important advantages, such as flexibility, expandability of the control elements, standardization and implementation of complex control algorithms. Other important aspects are: optimization of costs, cabling reduction, scalability, and remote supervision and operation. LonWorks is a technology suitable for distributed control systems. Although it is wide used scale in building automation area, other applications such as greenhouses and livestock production facilities can be attended by this technology. Created in the beginning of the 90 s by Echelon Corporation, USA (Echelon Corporation, 1999), LonWorks is an open technology, allowing the development of interoperable devices and flexible systems. The diversity of communications media supported by LonWorks is also very important to applications such as greenhouses. Single twisted-pair cable, power line, radio frequency (RF), and fiber optics are examples of available media. 2 Modern greenhouse automation control system characteristics The use of computers is essential in modern greenhouse control systems (Serôdio, C et al., 2001). Similarly to industrial line production, the greenhouse process must be fully controlled. However, 1349

2 greenhouse control algorithms sometimes can be quite complex compared to industrial applications. The control of the whole greenhouse process, from climate control to inputs administration and agricultural defensives, is very important in order to optimize production, increase quality and reduce costs. Some important features of greenhouse automation control systems are: a) Flexibility: A variety of crops are cultivated in greenhouses requiring frequent changes in the position of sensors. Optimization procedures can also suggest new sensors or actuators. Components replacement is also important and should be easily done (Serôdio, C et al., 2001). Mahalik and Lee (2002) discuss a new generation of control architecture for industrial automation systems in response to high productivity and flexibility needs. These concepts could also be applied to greenhouses. b) Interoperability: The availability of compatible devices from different manufacturers has important economic and technical consequences, being a desirable characteristic. Interoperability provides the means to change, upgrade, and increase the current system while keeping the existing devices and equipment. System integrators can choose devices from different vendors, building solutions based on best quality or cheapest cost. c) Scalability: Is the capability of a system to increase performance when resources are added. Scalable systems can be implemented with reduced investments. They can later be easily expanded according to new necessities or the availability of financial resources. d) Remote supervision and operation: The ability to remotely monitor greenhouse automation systems is becoming an important feature since they are usually located in remote sites. e) Use of complex algorithms of control: There is a trend towards the research of new models of greenhouse environment normally complex. For instance, research on CO 2 enrichment and growing strategies requires the use of powerful software and hardware tools (Serôdio, C et al., 2001). f) Redundancy: Failure in the greenhouse control system may lead to a total loss of the production. The implementation of redundancy increases the reliability of the control system. g) Support for several communication media: Greenhouses can be used for the cultivation of several kinds of crops. Therefore flexible positioning of sensors is a desirable feature. The availability of several kinds of communication medias such as radio frequency allows the development of more flexible solutions (Serôdio, C et al., 2001). h) Use of digital technology: In a control network all the devices such as sensors and actuators have intelligence and communication capabilities. The analog signals from sensors are close to the node that digitalizes them, minimizing the effects of interference. Mahalik and Lee (2002) relate the trend to implement digital technology. This is important in large-scale greenhouses. i) Open Architecture: Many companies are moving to open systems due to the benefits that can be obtained, which normally includes cost reduction. Close and proprietary systems restrict the automation solution and future upgrades. 3 Automation technologies Nowadays several automation technologies are available. Some of them that can be applied in greenhouses are discussed in the following paragraphs. Proprietary Systems Typical automation systems have a proprietary nature. These systems usually adopt proprietary hardware, software and communication bus, forcing the customer to deal with its manufacturer in case of maintenance or expansion. Programmable Logical Controller (PLC) It was first developed by the automobile industry to replace the relay logic in manufacturing lines, and currently it allows complete automation solutions, including digital and analog inputs and outputs and communication capabilities, normally employed to implement 1350

3 an human-machine interface. PLCs are normally robust and their advantages include easiness of maintenance and programming. Systems based on PLCs normally present the traditional centralized arquitecture, requiring complex cabling, and powerful CPUs in case of complex process. PLCs are proprietary equipments, with the disadvantages already mentioned. Controller Area Network (CAN) Developed by Robert Bosch GmbH for connecting electronic control units (ECUs), CAN is a multicast shared serial bus standard. CAN was designed to be robust in noisy environments.although initially created for automotive purposes, nowadays it is also used in embedded control applications. The maximum bit rate allowed is 1 Mbit/s for networks with length below 40 m, decresing with longer network distances (e.g., 250 kbit/s at 250 m). The CAN data link layer protocol is standarized in ISO , comprising OSI model layers 1 and 2; the other protocol layers have to be defined by the designer (proprietary solutions). CAN is an economic and robust solution but the CANbased systems are limited by communication media (only twisted pair cable is allowed), bit rate and network distance, and the amount of ECUs (KVASER Web Site, 2005). BACnet (Building Automation and Controls) It is an ANSI standard adopted by the building automation industry. It describes with great detail how to create an automation and control system, which can interoperate with other BACnet systems. This standard has great potential to be used in greenhouse automation because of the similarity between the environment of buildings and greenhouses. (BACnet, 2005). Jewett et al. (1998) present a study involving the data exchange of greenhouse environment control systems. They discuss the lack of standardization in this area and use BACnet to solve this problem. LonWorks It is an open standard developed by Echelon Corporation (1999) to be used in residential, building and industry automation. It is an automation and control technology, which uses distributed intelligence. Its idea, as the BACnet s, is to propose a standard of communication among its components called nodes. The main elements of a LonWorks control network are: the LonTalk protocol, Neuron chips, and transceivers. LonTalk is the protocol that allows communication among nodes. It follows the ANSI/EIA standard and is media independent, supporting multiple physical layer protocols. The supported physical communication media are: optical fiber, twisted pair, radio frequency, power line, coaxial cable and infrared. There is a specific transceiver for each one. Typically a LonWorks node is composed of: a microprocessor, a transceiver and input/output interface circuits. Echelon developed a low-cost device called Neuron Chip that provides networking and processing capabilities required in the implementation of each node. It comprises three processors, two of which implement the LonTalk protocol (layers 1 to 6 of the OSI model) and the other implements the application layer (layer7 of the OSI model) (Fig. 1) (Echelon Corporation, 1999). A typical LonWorks system does not require a central control device since intelligence is fully distributed among the nodes. The application programs that implement this intelligence are based on network variables, a new concept implemented in LonTalk. This concept simplifies programming and allows the development of interoperable devices. In order to ensure the interoperability LonWorks manufacturers founded The LonMark Interoperability Association that has the objective of standardizing network variable types, called Standard Network Variable Types (SNVTs). There are two main network topologies: free topology (Fig. 2) and bus topology (Fig. 3). In free topology there is no restriction on the shape of the network, and requires only one terminator. In a bus topology the same cable connects all devices in a daisy chain configuration, requiring two terminators. The BACnet standard is object based, and there are many similarities between the BACnet and the LonMark objects. However they are not interchangeable and a gateway is required to connect them. BACnet imposes its own application layer instead of the LonWorks application layer, and its objects do provide services that support the data-intensive operations (Echelon Corporation, 1999). 1351

4 XTAL POWER SUPLY SENSORS/ ACTUATORS Neuron Chip I/O CIRCUIT TRANSCEIVER LONWORKS EPROM NETWORK Fig 1. Typical LonWorks Node 4 A proposal of an experimental greenhouse automation control system Designing closed control system has been a traditional approach followed in many areas as industrial and agricultural. Some important disadvantages such as manufacturer dependency, lack of flexibility, incompatible communication protocols and the need of a high performance central processor can be identified in this approach, stimulating the search for new solutions. A new solution is being developed at Universidade de São Paulo, Brazil, based on LonWorks, a suitable technology for distributed and open control systems. Flexibility and scalability can be distinguished as the main goals of this work. LonWorks is a technology suitable for distributed control systems; it is an important open solution for many areas, such as building and industrial, with thousands of applications worldwide, stimulating its use in other areas such as the agricultural. The main variables typically used in greenhouse control systems such as temperature, humidity and brightness (photo-period), are being considered in the initial phase of the project. Fig. 4 shows the architecture of the proposed system that has two separate LonWorks control networks: the wireless sensors network and the actuators network based on AC power line cable. The mobility of sensors and actuators is an important feature to simplify the customization of the system to each new use of the greenhouse. Sensors are being developed with low power circuits in order to save their batteries. Using the AC power line cable as the actuator s network simplifies cabling and reduces costs. All sensors and actuators are linked to LonWorks nodes. A router is used to link the actuators and sensors networks. Fig. 2 Bus Topology (daisy chain) Fig. 3 Free Topology 1352

5 Fig. 4 Proposed LonWorks Distributed Control The software design is based on a new paradigm of control system. It consists of spreading the control logic across the network. Each controller can be simpler and les expensive. This concept allows the design of information-based control systems rather than command-based ones (Echelon Corporation, 1999). The LonWorks protocol is based on network variables (NVs) that simplify the designing of the applications programs and allow interoperability with products from multiple vendors (in this case standard network variable types SNVTs - must be defined) (Echelon Corporation, 1999). A control network with controllers, sensors, actuators, and other devices with intelligence and communication capabilities (called nodes) implements a new supervision and control system with important advantages, such as flexibility, expandability of the control elements, and standardization. Other important aspects are: cost optimization, cabling reduction, scalability, and remote supervision and operation. The main objective of this proposed automation control is to implement typical environment control in greenhouses: temperature, humidity and brightness (photo-period). The devices are separated in two networks: sensors network and actuators network. The first is a wireless network and the last a power line (power line carrier) network. This proposal includes a supervisory system located in a TCP/IP network. There is a gateway linking the TCP/IP network to the Lonworks network, linking TCP/IP network with LonWorks network. 5 Discussion The distributed control approach in greenhouse automation brings important advantages to the system such as the increase of reliability, flexibility, interoperability and scalability. The redundancy of sensors and actuators can be adopted in the future, increasing the reliability of the system. The control network technology allows the implementation of this kind of solution to be easier than a traditional one. 1353

6 LonWorks is a digital technology and each sensor and actuator node has intelligence and communication capabilities. The effects of interference and noise in analog signals are minimized. Another important feature is the device s mobility. It is desirable that the sensors can be moved easily from one place to another. This is easily accomplished with the use of wireless technology. Actuators normally do not require this feature, but the cabling necessary in a traditional approach is complex and expensive. Power line communication is an interesting option to simplify cabling and reduce costs. Interoperability and scalability are natural advantages of using an open technology, allowing the choice of different devices from different manufacturers. A gateway allows the connection of the system to a TCP/IP network, so that a microcomputer can perform the supervision and operation tasks. Remote supervision and operation can be easily considered and is an important feature since greenhouses are usually located in remote sites. 6 Conclusion LonWorks is a technology wide used in building automation, which is similar in several aspects to greenhouse automation. The proposed greenhouse control system architecture is robust and flexible, allowing the development of a great variety of systems, from simple to complex one, which will be useful to implement flexible experiments. The scalability of the architecture is an important feature: the system can be gradually expanded, with proportional investments. Interoperability allows the use of several devices from different manufacturers and is important to obtain cost-effective solutions. The distributed architecture, with intelligence distributed among specialized nodes is important for easy installation and maintenance, and cabling reduction. Unlike typical hierarchical systems, control network based systems are more robust: they can operate even the supervision microcomputer is disconnected or some node is damaged (in this case with performance degradation). Redundancy of the critical devices can be incorporated easily and is a point of future studies in this work. 7 References Echelon Corporation, Introduction to the LonWorks System ( A). Echelon, USA. Serôdio, C et al., 2001 A networked platform for agricultural management systems. Computers and electronics in agriculture. Amsterdam: Elsevier Science B.V. v.31. p Jewett, T. J. et al., BACnet Data Communication Protocol: Automating Data Exchange with Greenhouse Environmental Control Systems. In: International Conference on Computers in Agriculture, 7., Orlando, Proceedings. Orlando: ASAE. 1 CD-ROM. Mahalik, N. G. P. C., Lee, S. K A study on production line automation with LonWorks control networks. Computer Standards & Interfaces, Volume 24, Issue 1, p BACnet, BACnet Overview. EUA. Retrieved on May 8, 2005, from Microchip Data Sheets. EUA Retrieved on May 8, 2005, from < KVASER Web Site, CAN Physical Layers, Retrieved on May 8, 2005, from