Remote monitor and control application for thermal processes using TCP/IP

Transcription

1 Remote monitor and control application for thermal processes using TCP/IP S. E. Oltean 1, M. Abrudean 2, M. Dulau 1 1 Petru Maior University of Targu Mures, soltean@yahoo.com, agligor@upm.ro 2 Technical University of Cluj Napoca, Mihai.Abrudean@aut.utcluj.ro Abstract - This paper presents a practical approach for remote monitor and control of the electrically heated oven temperature. The digital system contains two main parts: a local digital controller and a server-client application. The digital controller, which controls the temperature of the electrically heated oven, is implemented on a PIC16F628 microcontroller. The server-client application is realized in LabWindows/CVI language and uses TCP/IP protocol for monitoring and controlling the parameters of the temperature local digital controller. I. GENERAL ASPECTS REGARDING SLOW PROCESS CONTROL Slow processes are characterized by large time constants or by large time constants and dead time. The control of a technological process can be expressed by the necessity to attain and to maintain a regime of the process in which the quality outputs have imposed values, which are considered optimal. In order to maintain the desired state of such processes we need to keep constant, at prescribed values, such variables as flows, pressures, levels, temperatures, concentrations etc., which demands the design of automatic control systems for them. On the one hand the control systems for these variables differ from one another through their structure and behavior in dynamical state. On the other hand, the automatic control of one of these variables presents, in principle, the same problems, no matter what the process associated with it is, therefore the control systems are treated as subclasses associated with these physical measures: flow, pressure, level, temperature or concentration [1]. Each of these automatic control systems needs to be considered both from the point of view of its behavior in stationary and dynamic state as well as from the point of view of the possible technical solutions. II. REMOTE MONITOR AND CONTROL APPLICATION FOR THERMAL PROCESSES Controlling the temperature is a classic problem studied in the field of Control Engineering, with various industrial or educational applications. For this paper we proposed a special practical approach: to monitor and to control the electrically heated oven temperature, locally and remotely from a personal computers (PCs). The application developed has two main parts: - local digital controller built around the PIC 16F628 microcontroller (on microcontroller board); - server-client application realized in LabWindows/CVI language that uses the TCP/IP protocol for network communication and remotely temperature monitor and control. The server interacts with client using TCP/IP socket, and the communication is made on a single thread. The remote monitor and control application for electrically heated oven temperature has a block diagram presented in Fig. 1. Figure 1. The block diagram of the remote monitor and control application /06/$ IEEE

2 The microcontroller board has three primary functions: controlling locally the process functioning as thermostat, reading the digital temperature from the LM75 sensor and serial communication with a computer, named server. The remote monitor and control is done using a Local Area Network (LAN ). Server-client application allows the master client (first client connected to server) to control and to monitor the temperature using the server functions and the other clients to record and to monitor the process evolution. Combination between microcontroller PIC family and Labwindows/CVI program assures desired performance, simplicity, flexibility and reliability for remote temperature monitor and control. III. MICROCONTROLLER BOARD (LOCAL CONTROLLER) The Fig. 2 shows the block diagram of the microcontroller board. Figure 2. Microcontroller board (local digital controller) The microcontroller board contains four modules: - PIC16F628 microcontroller is a high performance RISC CPU with Harward architecture, 2k words program memory, 224 bytes CMOS memory, 128 bytes EEPROM memory, builtin oscillator, three timers (two 8 bit and one 16 bit), USART module, capture, compare and PWM module [2]; - digital temperature sensor with I2C interface, a precision of ±2 C for a range of temperatures between -25 C and 100 C, or a precision of ±3 C for temperatures between -55 C and 125 C, 0.5 C resolution and a 100ms conversion time [3]; - serial transmission/reception interface, with MAX 232 integrated circuit; - control relay to satisfy the needs for high switching power, the cheapest and simplest solution; - signalizing module consist of power LED, sensor connected LED, serial read LED, serial write LED, I2C transmission LED and control relay LED. The microcontroller board is connected using RS232 to a personal computer (PC) on which the server application runs. This PC accepts TCP/IP connections from the client applications. Desired temperature (reference) is introduced using a PC keyboard locally or remotely. This value is transmitted using the serial port from the computer to the microcontroller. For serial communication we used the standard Non Return to Zero format (NRZ), also known as 8N1, using 8 bits for data, no parity, one stop bit and baud rate. Microcontroller also reads the temperature binary (in fact measured temperature) from the sensor using the I2C interface. I2C serial master-slave communication is made on two-wire line. When the I2C bus is free the wire are at high potential. All the equipments connected on the I2C line must have opendrain or open-collector pins. The source code for the microcontroller was written in C language and compiled using HITECH PICCDemo The controlling and processing of the signal read on RS232 is done in the main loop, and the temperature reading and the command is done in an interrupt function. The command for temperature control is computed each time the temperature is read, on every 250 ms. The electronic circuit of the microcontroller board circuit and printed circuit board PCB was obtained using Protel 99SE Trial application. IV. SERVER-CLIENT APPLICATION FOR REMOTE MONITOR AND CONTROL Server-client application were written in Labwindows/CVI 4.01 and runs on Windows 98, 2000, XP [4]. The network protocol we used is TCP/IP. The hardware control we employed, RTS/CTS, uses two wires. The local digital controller can only be accessed directly by the server application, which doesnt have an active part in the control of the heated oven temperature. The master client accesses indirectly the local digital controller. Both, server and client, only control and monitor the process, because the temperature thermostat regulator is implemented on PIC microcontroller. To connect to a TCP server from a LabWindows/CVI program, you must have some information about the application to which you would like to connect (like hostname and port number). Because TCP protocol accepts more server applications, they are differentiate by hostname (or known IP address) and theirs unique port number (greater then 2000). Network communication using the CVI TCP library involves a client and a server in each connection. Once registered, with the CVI RegisterTCPServer function, the server waits for clients to request connection to it. Every connection request from a client is processed by a special callback function. The server can send and receive information to and from a client application using a single thread for TCP communication. Sending and receiving information is possible using ServerTCPWrite and ServerTCPRead CVI function. When the server application is closed server must be unregistered. The server application implemented using Labwindows/CVI has the following features (Fig. 3): - accepts connections from client application on a configurable TCP/IP socket (ex. socket no. 2800); - displays the state of the server; - displays the number of connected clients; - sets the local digital controller board parameters;

3 - selects the serial port for RS232 communication; - displays and transmits the state of the controller and the process parameters. The server interacts with client using TCP/IP socket and the communication is made on a single thread. Actually, CVI is a programming language for the single thread applications. So, if we want to accept more than one client we must use handles to differentiate them. Figure 3. Server application user interface Server allows maximum 16 connections on the same socket; the first client connected gets the master handle. The master client can control the parameters of the digital local controller. Other clients connected after master client get different handles and they will only monitor the evolution of the process. Server application (with user interface shown in Fig. 3) sends three types of messages: - broadcast message; - update message; - special rights message. Broadcast message is sent to all connected clients and contains process and digital controller states. Update message is sent only to the master client for updating the starting values of the local controller. Special rights message is sent from the server to the first connected client. This message gives the rights to set the local controller parameters. The possible clients can request connection to a pre-existing server, on the configurable TCP port (with the CVI ConnectToTCPServer function). After the connection to the server, the TCP clients can send and request data to and from a server application (ClientTCPWrite and ClientTCPRead functions). The implemented client application assures: - connection to the server using its IP and TCP socket; - displaying the information it gets from the server (state of the controller and process parameters); - saving and loading data; - exporting data to Microsoft Excel through Dynamic Data Exhcange (DDE) for printing. Figure 4. Client application user interface.

4 Client application sends three types of messages: - reading time message; - local controller initial values message; - update message. Reading time message set the updating time for the digital controller and process states. This message contains a float number representing the updating time. Local controller initial values message contains the values for the process and controller states. Update message is transmitted only from the master client for updating the local controller parameters. V. EXPERIMENTS AND CONCLUSIONS The remote monitor and control application for thermal processes was tested with two plants. First process is a first order plant and the second, also a first order plant, has a death time. Fig. 5 shows the local automatic control scheme. The digital system consists of server-client application and local controller board. The first slow process tested with the remote monitor and control application has the transfer function (2). k H s. (2) T s 1 Where the plant parameters are: amplification coefficient k and time constant T. After process simulation the response recorded by the master client application for a plant without dead time is shown in Fig. 7. For printing, recorded data was exported and saved to Microsoft Excel through Dynamic Data Exchange (DDE) facilities. Figure 5. Automatic temperature control scheme. In Fig. 5 the functional signals are: digital reference ref (prescribed by the server or client), digital measured output y, plant real output y, command u. The local digital controller has as inputs: reference signal and measured signal. The output is the command. This command depends on the error e. Figure 7. The response for first order plant. The resulting time constant using a 2 seconds sample time is around 60 samples, which means 2 minutes. The recorded command for this plant is shown in Fig. 8. e / ref y. (1) The regulator implemented in the local digital controller board (on microcontroller board) is a relay with non-linear characteristic. A relay outputs on or off value by comparing the input (error) with the specified thresholds. The on and off states are not affected by the input between the upper and lower limits. This regulator is specific to slow processes and has the statical characteristic presented in Fig. 6 [1]. Figure 8. The command for first order plant without dead time. The experiments indicate a command with greater switching frequency for smaller difference between the lower and upper thresholds of the relay. The second slow process tested was the first order plant with dead time. Transfer function for this plant is: H s T k s 1 e t s. (3) Figure 6. Statical characteristic for a relay. Where the plant parameters are: amplification coefficient k, time constant T, dead time t. The command is shown in Fig. 9.

5 Figure 9. The command for first order plant with dead time. The response recorded by the master client for a plant with dead time is shown in Fig. 10. Figure 10. The response for first order plant with dead time. Remote monitor and control of the electrically heated oven using PIC F16F628 microcontroller and Labwindows/CVI server-client application assures: - on-line display of the results for the temperature measuring process; - the possibility to change the parameters on-line at any moment; - the possibility of remote interfacing and intervention; - high measuring precision; - low energy demands; - high switching power; - low oscillation domain for the controlled parameter; - low cost; - high reliability. REFERENCES [1] I. Dumitrache, Tehnica regl rii automate, Ed. Didactica si Pedagogica, Bucuresti, [2] Microchip PIC16F628 FLASH Microcontroller Specifications. [3] National Semiconductors LM75 Specifications, February [4] [5] R. B lan, Microcontrolere: structur i aplica ii, Todesco, Cluj Napoca, [6] K. Agoston, Senzori în automatiz ri industriale, Ed. Univ. Petru Maior, Tg Mures, [7] E. Geampana, Echipamente periferice si interfete, Ed. Univ. Petru Maior, Tg. Mures, [8] Phillips I2C Bus Specifications, version 2.1, January [9] Labwindows/CVI Programmer Reference Manual, NI [10] A. Tanenbaum: Computer Networks, Prentice-Hall, Englewood Cliffs, N. J., 1989.