DISTRIBUTED CONTROL NETWORK FOR ... - MIcroLab

DISTRIBUTED CONTROL NETWORK FOR AGRICULTURAL APPLICATIONS

Loukas V. Hadellis, Vassilios D. Kapsalis

Laboratory of Microcomputers, TEI of Patras, Patras, HELLAS

Abstract: This paper presents the design and development of a distributed and integrated control network for irrigation and other agricultural applications employing the LonWorks technology standard. A single control network infrastructure integrates irrigation, pumping and optionally fertilizing, syringing and environmental control systems that interoperate and can be programmed, monitored and optionally controlled over the same PC. Emphasis was given to the communication capabilities to external PSTN and TCP/IP networks; a LonWorks to PSTN gateway was developed. Keywords: Control applications, Networks, Integration, Agriculture, Distributed control.

1. INTRODUCTION Control Technology is moving to the 4th generation of distributed I/O interconnected via control networks, where no central PLC is required any more (Kapsalis, et al., 1996; Pinto, 1996; Pinto, 1997). These systems consist of programmable and intelligent I/O control network nodes, with sensors and actuators directly connected to them, implementing all measurement and control algorithms locally and independently without requiring a central controller (Madan, 1996; Madan, 1997; Raji, 1994). A PC can be used for programming (application download, node/network configuration), monitoring and optionally control. LonWorks, developed by Echelon, is a de facto standard for control networks employing distributed architecture (Lockareff, 1996). Its concept is based on a democratic structure where individual network nodes run independently their own object-oriented control algorithms (Schneider, et al., 1997). These nodes are connected to the communication network infrastructure and exchange messages by means of a highly reliable protocol called LonTalk that employs event-driven and request-response techniques (LonTalk Protocol Specification, 1994).

LonTalk protocol supports a bi-directional network communication based on the 7-layer ISO/OSI standard optimized for control, incorporating error detection/correction that increases reliability and provides real-time feedback about the actual control implementation (Kapsalis, et al., 1997). LonWorks technology is a complete platform for interoperable control systems. Its fully distributed and open architecture along with interoperability offers certain major advantages compared to the older PLC / centralized control systems such as: • No single point of failure due to highly decentralized structure. • The system computing power is exponentially increasing as the number of nodes increases. • Heterogeneous control system integration within the same control network and programming or monitoring through the same PC. • Highly robust communications network with true free topology physical layer which allows bus, star, ring, tree and mixed configurations. • Reliable communication over wired (TP, PL, fiber) or wireless (RF) media. • Multiple connectivity devices (routers, repeaters, gateways, bridges, net-interfaces).

• Easy integration with standard enterprise-wide Ethernet networks and Intranets. • True multivendor interoperability since products from different vendors can be easily combined in a single system. • Reduced programming and associated debugging time due to modularization of the control functions. • A single network infrastructure where new devices can be added to later when required. • Lower initial wiring costs and less need to install excess nodes for spare I/O capacity at initial installation phase. • An interoperability certification process by the LonMark association. In spatially distributed systems such as agricultural systems distributed control is much more efficient than central control concerning reliability, cost, expandability and real integration capability. Distributed control allows computer intelligence required for control and instrumentation to be placed as physically close, to the point of actuation or measurement, as feasibly possible (Wall, 1997). Reliable communication links the numerous points of control with points of instrumentation. Converting measurements to digital signals before transmission reduces sensitivity to noise and transmission losses. In this way noise immunity can be achieved and signal quality will be preserved. A LonWorks system can be extended to tens of thousands of nodes and can be easily integrated within a larger control network that includes several heterogeneous systems. These certain characteristics can minimize irrigation time, energy requirements, maintenance costs and wiring while allowing easy integration and expansion; thus the total system installation, H/W, S/W and wiring costs can be seriously reduced. A typical set of requirements that an agricultural application may impose to a control system could be the following: • Irrigation depending on weather conditions. • Different irrigation schedule and requirements per area. • Covering of long distances. • Cooperation among irrigation, water pumping and other control systems. In this paper the design and development of a distributed control network for agricultural applications is described. The current approach integrates an irrigation control subsystem with other subsystems that incorporate capabilities such as water pumping and optionally fertilizing, lighting, syringing control, environmental measuring, etc., in the same network. Various communication capabilities to external networks are provided.

2. DESCRIPTION OF THE SYSTEM The whole system (illustrated in Fig.1) consists of the following: • A power-line Lonworks network PL-20 type, Smart Rain controllers (SRC) that measure temperature/moisture and control the sprinkler electrovalves (EV), a power line coupler that couples the PC Lonworks PCNSS interface card to the 24 V AC power line, the 220 V AC to 24 V AC transformer / filter and a Lonworks PL-20 to FTT-TP router. • A free topology twisted pair LonWorks network, FTT-TP, Smart Control controllers (SCC) with their I/Os connected to sensors and actuators and an SLTA (Serial LonTalk Adapter) device. • The sprinkler electrovalves (EV), water pipes and flowmeters. • The interface to external networks (PSTN or TCP/IP) for communication to remote PCs or touch-tone telephones. The irrigation subsystem measures the soil temperature, moisture and optionally the hydraulic pressure, atmospheric temperature, wind speed, water flow and activates the sprinkler electrovalves according to predefined scenarios. It communicates to the other subsystems in order to cooperate for achieving proper water flow, fertilization or other control procedures within the area. The whole system can communicate either by means of an SLTA device to a remote PC via the PSTN or by means of a local PC to a remote PC via PSTN or TCP/IP and to a remote touch-tone telephone via PSTN.

2.1 Irrigation control sub-system The irrigation control sub-system consists of the Smart Sensor nodes supplied by Smart Rain that use a special stainless steel probe coupled with a new patent-pending electromagnetic principle, for measuring permittivity and bulk conductivity of the soil and determining thresholds for watering and fertilizing. In addition to soil moisture and temperature measurement, a 4-20 mA input is provided for an additional parameter monitoring (e.g. hydraulic pressure, water flow, air speed). Each Smart Sensor is capable of conducting 1.5 Amps; thus it can drive 1 to 6 typical valve-in-head sprinklers or electrovalves based on the measured soil conditions and its daily schedule. Smart Sensors are able to withstand and detect shortcircuit or open-circuit conditions. Smart Rain provides a flexible and intelligent soak and repeat feature that operates as follows: The user inputs a minimum time-off and a maximum time-on.

TCP/IP or PSTN Network

pumps, tanks, motors, lights, alarms, sensors, etc.

Remote PC

I/O

SCC

SLTA

Twisted-Pair Lonworks network

Touch-Tone Telephone Local PC 24 V AC

I/O

SCC

( TP-FTT )

router

PL coupler Power-Line Lonworks network

(PL-20)

220 V AC SRC

EV

SRC

EV

Water Pipe

EV Pressure Transducer

Fig. 1. The irrigation distributed control network When the irrigation system begins irrigation, it will cycle through the maximum time-on and the minimum time-off, as defined by the user, until the sum of the maximum time-on equals the required irrigation time. Then the soil water level is measured. If the water level is at the upper level defined for that site, irrigation will cease. On the other hand, if the water level is measured to be below the maximum level defined for that site, soak and repeat will continue. When the sum of the soak cycles reaches the user defined maximum on time, or the water level is at the maximum level as defined for that site, the system will stop watering. The advantages are that the maximum time-on and minimum time-off are set by the user to be any amount of time. There is virtually no limit to the number of cycles that can be programmed and thus the total watering time is independent of the soak and repeat cycle. This provides the flexibility to apply water at the soils intake rate, thus eliminating exceeding the field capacity and wasting water. Configuration of the network nodes, monitoring of the measured parameters is achieved by using a central PC and an appropriate graphical user interface. Communication among all the network nodes and the PC takes place directly through the power lines which are used for powering the sprinkler electrovalves.

2.2 Control sub-systems The control sub-systems receive input messages, called network variables (NVs), by the irrigation sub-system and control appropriately the local processes based on the values of these variables and on the algorithms loaded. They provide status information to the PC and to external networks about several critical parameters (e.g. power failure, overheating, etc.) in order to help locating potential problems of the pump station. For example, a long power failure is transmitted to the PC (as a network variable) triggering a dial-out to a predefined location in order to report the fact to authorized personnel. These sub-systems consists of two programmable controllers (both supplied by Smart Controls): • S-ADR112-F which incorporates 2 universal inputs, 2 analog inputs, 2 analog outputs, 2 digital I/Os or contact closure inputs, 3 relay outputs. • S-D80-F which incorporates 8 digital I/Os. These controllers (connected through 78 Kbps Free Topology channel) enable control and sense programs to be written, debugged and downloaded over the LonWorks network. The programs are stored in reliable flash non-volatile memory enabling their permanent store even when the modules are powered-down.

Analytically, the S-ADR112-F controller can measure analog signals from thermistors, RTDs, resistive type sensors and other 4-20 mA or 0-10 V signals with configurable inputs concerning single ended or differential transmission, gain adjust and filtering. The analog outputs can control valves, speed control devices, variable position devices and dampers. The digital channels can be used for alarm input / outputs, over-pressure safety switches, encoder position inputs or other digital functions. The relay outputs can be used to drive small motors, valves, alarm outputs, lights, or other loads that does not exceed the rated value. The S-D80-F controller provides alarm inputs/outputs and switch inputs. It can be used for special digital functions such as pulse counting, encoder input, PWM outputs and other timer/counter functions. The high current outputs can be used to drive displays, valves, relays and other DC powered devices.

2.3 PC interface and application program Referring to Fig.1, the Power Line LonWorks network is connected to the PC through a PCNSS card equipped with a PLT-21 SMX transceiver and a power-line coupler. The connection to the pumping station system is achieved through a Power Line to Free Topology (PT-to-FT) router. New application programs developed and debugged with Echelon’s LonBuilder or NodeBuilder development tools, can be downloaded to the network nodes by using a network management tool like Echelon’s LonMaker (used in this development) or a custom application based on LNS (LonWorks Network Services). In order to monitor several operational parameters and to control the functions of the network controllers, an application program was developed in Microsoft’s Visual Basic 5.0. The user interface provides the capability for a) graphical representation of the monitored values, b) control through graphical objects, c) configuration of the system parameters and d) archiving of historical data. The mapping of the network variables (NVs) to the appropriate objects (text boxes, labels, images, etc.) of the graphical user interface took place through the use of the Echelon’s DDE Server.

•

•

•

•

SLTA and a remote PC is established (initiated either by the SLTA or the remote PC) and status or command information is transferred between the LonWorks network and the remote PC. This is a cost-effective solution for PSTN access when an on-site PC is not necessary or not available. A local PC running (besides the custom application and the DDE Server) one of the commercially available remote control programs which enable a remote PC (running the same remote control program) to control all the functions of another PC through dial-up connection, TCP/IP or IPX protocols. This solution can be used in both dial-up and network connections. A custom application (running at the local PC) acting as a TCP/IP Server which can route the network variables of the LonWorks network through TCP/IP Sockets to a remote PC running a Client application. This can be used if the local PC is connected in a local network and possesses a static IP address. A Web server running at the local PC which maps the network variables to Java applets. A static IP address is required for the local PC while the remote access can be achieved by any PC with a Java enabled browser. An application which exploits the Microsoft Telephony Application Programming Interface (TAPI) and uses the communication features and services available on a telephone network. The required hardware on the local PC can be a TAPI compliant voice modem or Dialogic voice boards.

2.5 LonWorks to Telephone Gateway (LTG) There are several alternative options available for the remote connection of monitored systems to PCs and telephones via the PSTN. The most common is by using a data/voice dialer which, in the case of an alarm, dials one or more remote PCs or telephones and delivers a data report or a user-recorded voice message corresponding to the particular alarm, respectively. Furthermore, it enables a user to call the system (by a PC or a telephone) for checking the status and/or control the installed equipment. These dialers provide a cost-effective solution for applications requiring limited point-to-point connected I/Os (usually digital) in the range of 4 to 40 points.

2.4 Communication sub-system Remote access of the system can be achieved by using one of the following configurations (Lund, 1996): • SLTA and modem (without a local PC connected to the system). Under this configuration, a dial-up connection between the

The high degree of availability of telephone equipment compared with PCs makes this solution quite attractive for the remote access of networks through the PSTN. In order to overcome the inherent expansion limitation of legacy hardwired data/voice dialers a custom application based on Microsoft TAPI was considered as the most

appropriate for the particular implementation. This application which manages calls (places outgoing calls, receives incoming calls, gets digits entered by a touch tone telephone, etc.) can be an excellent interface for remote monitor and control for non-PC users. The LTG application running on the local PC (besides the DDE Server) has been developed in Artisoft’s Visual Voice Pro 4.0 and Visual Basic 5.0 and consists of three logical parts as illustrated in Fig.2: • The first part is Microsoft’s TAPI (Telephony Application Programming Interface) which consists of a library and supporting services that allow 32-bit application development using the telephone network communication features and services. • The second part, developed in Visual Voice Pro 4.0, handles the interaction with the TAPI compliant voice modem. This part places outgoing calls whenever a predefined event takes place (e.g., a value of a network variable exceeding a predefined setpoint), detects rings, receives incoming calls and interprets the received digits entered by a touch tone telephone. • The third part (Event Interpreter), developed in Visual Basic 5.0, maps the interpreted events to the NVs and messages handled by the DDE Server. For example, an incoming event that is interpreted as a status request for the soil temperature of the first zone (measured by the Smart Sensor #1) results in a vocalized response corresponding to the current soil temperature value. The LTG is capable of handling practically an unlimited number of events, NVs and messages between touch tone telephones and the LonWorks network, making it an excellent solution in applications requiring the handling of many network variables and unlimited expandability capabilities.

3. SYSTEM INTEGRATION The interconnected sub-systems cooperate for achieving a total control strategy incorporating irrigation, pumping and various control operations. The irrigation sub-system nodes (Smart Sensors) provide values of the soil moisture and temperature for each measured zone, the hydraulic pressure and the air speed. The measured values are used for the local control of each sprinkler electrovalve and are sent to the LonWorks network in the form of NVs. The NVs from all the Smart Sensors are used as inputs to the control algorithms of the pump station controllers and the application running at the PC.

NVI

LonWorks Network NVO

Event Interpreter

VisualVoice

TAPI

Voice

Tones PSTN

Touch-Tone Telephone

Fig. 2. The LonWorks to Telephone Gateway The pump station controllers perform several operations which are described briefly at the following section: • Each controller accumulates start/stop and runtime data for the corresponding pump and can detect failure conditions by comparing the pump’s actual run-time with an operator-entered reference time period. Each pump should run for about the same amount of time, and should have the same number of stops and starts. A processing of these accumulated data help locating potential problems and performance reports can be generated by the PC as a part of a preventive maintenance procedure. • A UPS backup system provides power to the pump controllers (not the pumps) and the PC for keeping the control system running in the event of a power failure. Each power loss exceeding a predefined time (and any subsequent recovery) is reported vocally through the LTG to authorized personnel. • The pump activation by electro-mechanical relays, which can only turn the pump motors on or off, can wear mechanical equipment, such as motors, seals and bearings. Instead of running

the pumps sporadically at full speed, the AC drives can drive the pump speed and gradually increase or decrease the flow rate. Thus, the water tank can be filled at the same rate at which it is draining, reducing the number of starts and stops and the associated wear on the system’s mechanical equipment. Each AC drive is controlled by an analogue output (4-20 ma) of the S-ADR112-F controller; this analogue output depends on the water draining rate measured by a flow meter at the tank output. Furthermore, the variation of pump motor speed based on the water draining rate reduces the amount of energy the pumps consume. • Each Smart Sensor is connected with a 4-20 ma pressure transducer in order to measure the hydraulic pressure of each water pipe. These measured values are transmitted to the LonWorks network in the form of NVs and are constantly monitored by the pump station controllers. If a pipe breaks and its hydraulic pressure falls under a setpoint, then the corresponding electrovalve that feeds this faulty pipe must turn off. This is achieved by using an analog output from each pump station controller. Also, the event of a fault pipe is illustrated in the local PC and a vocal message corresponding to the particular fault is transmitted through the LTG. • Optionally, any sensors and actuators related to fertilizing and other environmental procedures can cooperate with the whole system in order to achieve an integrated control tailored to the specific application requirements. Operational parameters, such as pump start and stop setpoints can be changed by a technician through the graphical user interface or through a touch tone telephone. In addition to the pump station operations described above, the controllers are used for security and optionally automatic light control. Any intrusion (detected by magnetic contacts and PIR sensors) activates an external siren and dials authorized security personnel.

4. CONCLUSIONS An integration of agricultural processes over a mixed media single network infrastructure employing the LonWorks technology was developed. This control system is flexible, reliable and scaleable. It brings all the advantages previously enjoyed by the computer world to the control world. Its networking communication capabilities extend over a wide range providing remote access even with a touch-tone telephone via the LTG.

REFERENCES Kapsalis,V.D., S.A.Koubias and H.C. Haralabidis, New hybrid Mac-layer for real-time bus networks, IEE Proc. on Communications, Vol. 141, Number 5, pp.325-333. Kapsalis,V.D.,S.A.Koubias and G.D.Papadopoulos, Implementation of a MAC-Layer Protocol (GITCSMA/CD) for Industrial LAN’s and its experimental Performance, IEEE Transactions on Industrial Electronics, Vol. 44, Number 6, pp.825-839. Lockareff, M. (1996). LONWORKS Technology and the LONMARK Standard, Echelon Corporation. LonTalk Protocol Specification V.3, Echelon Corporation 1994. Lund, J.J. (1996). From the Internet and Intranets to “Infranets”- Global Infrastructure Control, Networks, Echelon Corporation. Madan, P. Overview of Control Networking Technology, Echelon Corporation Madan, P. (1996). Device Bus? Field Bus? Or Sensor Bus?…Is this Segmentation Obsolete?, Echelon Corporation. Pinto, J.J. (1996). FieldBus : A Neutral Instrumentation Vendor’s Perspective, Action Instruments, Inc. Pinto, J.J. (1997). Networked, Intelligent I/O The Truly Distributed Control Revolution, Action Instruments, Inc. Raji, R.S. (1994). Smart networks for control, IEEE Spectrum, Vol. 31, Number 6, June 1994, pp.49-55. Schneider, E. and V. Thamburaj (1997). Using LonWorks Controller-Objects in Distributed Nodes, TLON GmbH. Wall, R.W. (1997). Agricultural Irrigation System Control and Data communications for RealTime Variable Water Application, LonUsers Echelon Conference, Spring 1997.