REMOTE TEMPERATURE MONITORING SYSTEM VIA INTERNET, USING A DS18B20 SENSOR

Transcription

1 REMOTE TEMPERATURE MONITORING SYSTEM VIA INTERNET, USING A DS18B20 SENSOR Irinel GEORGESCU *, Bogdan VASILESCU ** *Petroleum-Gas University, Bd. Bucureşti 39, Ploiesti , irinelgeorgescu@gmail.com **Petroleum-Gas University, Bd. Bucureşti 39, Ploiesti , vasilescu@gmail.com Key words: 1-Wire, sensor, DS18B20, temperature. ABSTRACT This paper presents a robust application of a temperature sensor with variable resolution, remotely controlled via Internet, using the 1-Wire network protocol, to implement a temperature monitoring system of an enclosure. The specificity of the proposed system is represented by the use of modern, computer-assisted technologies, for the interaction with the temperature sensor, both in terms of hardware and monitoring and control.net Web interface. INTRODUCTION The 1-Wire network protocol is a robust way to interface a wide range of small devices, such as digital switches, sensors and analogue-to-digital converters to a PC or microprocessor. Every 1-Wire device contains a unique identifier preset at the factory. As the name suggests, the devices are networked together by a single wire that acts as a physical medium to communicate between a host and a large number of devices attached along the wire. It is also capable of powering these devices through the same wire [1]. Among its applications, weather measurement in general and temperature measurement in particular is one of the most common [2,3]. The current paper presents the remote temperature monitoring system accessed and controlled via Internet and developed by the authors, The instantaneous value of the temperature is acquired from a Dallas Semiconductor DS18B20 sensor via a USB adapter and stored in a database on a server computer. Communication is carried out on a single wire through the 1-Wire network protocol. The web interface displays the data gathered, the graphical representation of the history of temperature for a given period, and other parameter values. Due to the modularity and customizable structure, the system can be easily extended by adding new functionalities, like humidity monitoring by installing a humidity sensor on the 1- Wire bus as well as other 1-Wire-compatible sensors, transforming it into a genuine weather station.

2 BACKGROUND The 1-Wire technology is based on a serial protocol that uses a single data line plus ground reference for communication. A 1-Wire master initiates and controls the communication with one or more 1-Wire slave devices on the 1-Wire bus (Figure 1). Every 1-Wire slave device is identified and addressed on the 1-Wire bus by a unique, unalterable, factory-programmed, 64-bit identification number, in which an 8-bit subset represents the family code and identifies the device type and functionality. Usually, 1-Wire slave devices require a voltage range of 2.8V (min) to 5.25V (max) in order to operate over. Alternatively, 1-Wire slave devices can be powered directly the 1-Wire bus (parasitic supply), without having a pin for power supply. The Dallas Semiconductor 1-Wire bus is a simple signalling scheme that performs half-duplex bidirectional communications between a host/master controller and one or more slaves sharing a common data line (Figure 1). Both power and data communication for slave devices are transmitted over this single 1-Wire line. For power delivery, slaves capture charge on an internal capacitor when the line is in a high state and then use this charge for device operation when the line is low during data transmission. A typical 1-Wire master consists of an open-drain I/O port pin with a resistor pull-up to a 3V to 5V supply. This clever communication scheme also allows to efficiently add memory, authentication, and mixedsignal functions at any time. [4] Figure 1. The 1-Wire master/slave configuration The DS18B20 digital thermometer communicates with the central microprocessor via a 1- Wire bus, requiring only one data line (plus ground). This sensor (DS18B20) has a customizable resolution, from 9-bit to 12-bit, for Celsius temperature measurement. It provides an alarm function with non-volatile user-programmable upper and lower trigger points. The DS18B20 sensor covers an operating temperature range of -55 C to +125 C, providing an accuracy of ±0.5 C over the range of - 10 C to +85 C. [5] The unique 64-bit serial code allows to connect multiple DS18B20 devices on the same 1- Wire bus, so it is possible to monitor a large DS18B20 distribution with just one microprocessor. In the middle of the DS18B20 architecture there is a direct-to-digital temperature sensor. with a customizable resolution of 9, 10, 11, or 12 bits (the default resolution at power-up), corresponding to increments of 0.5 C, 0.25 C, C, and C. When DS18B20 is powered-up it is in a low-power idle state. For a temperature measurement and a conversion for analog-to-digital, the master device should launch a ConvertT [44h] command. The resulting thermal data acquired is stored in the 2-byte register temperature in the scratchpad memory. After finalizing the conversion operation, the DS18B20 will return to its idle state.

3 The DS18B20 provides the output temperature in degrees Celsius. In case of a Fahrenheit desired output, it is necessary to use a conversion routine or a more rudimentary method, such as a lookup table. The output temperature data is stored in the temperature register (see Figure 2) as a 16-bit sign-extended two s complement number. [5] Figure 2. Temperature Register Format The sign bits (S) indicate if the temperature is positive or negative, with a value of S = 0 for positive numbers and S = 1 for negative numbers. In the default case of a 12-bit resolution, all bits in the temperature register will contain valid data. Similarly, for 11-bit resolution bit 0 is undefined, for 10-bit resolution bits 1 and 0 are undefined, and for 9-bit resolution bits 2, 1, and 0 are undefined. In case of an alarm trigger, the temperature value is compared to the user-defined two s complement alarm trigger values stored in the 1-byte TH and TL registers (Figure 3), with a similar sign bit. The trigger values are stored in non-volatile (EEPROM) TH and TL registers so they are retained even when the device is powered down. [5] Figure 3. TH and TL Register Format. Since TH and TL are 8-bit registers, only bits 11 through 4 of the temperature register are used in the TH and TL comparison. If the measured temperature is lower than or equal to TL or higher than or equal to TH, an alarm condition exists and an alarm flag is set inside the DS18B20. This flag is updated after every temperature measurement; therefore, if the alarm condition goes away, the flag will be turned off after the next temperature conversion. The master device can check the alarm flag status of all DS18B20s on the bus by issuing an Alarm Search [ECh] command. Any DS18B20 device with a set alarm flag will respond to the command, so the master can determine exactly which ones have experienced an alarm condition. If an alarm condition exists and the TH or TL settings have changed, another temperature conversion should be done to validate the alarm condition. [5] DEVELOPMENT AND IMPLEMENTATION OF THE PROPOSED SYSTEM From a hardware point of view, the DS18B20 sensor represents the kernel of the proposed system (Figure 4a), connected (as shown in Figure 4b) to a 1-Wire-to-USB adapter (similar to the one in Figure 4c). The software consists of a.net Web application, developed using ASP.net for scripting, C# for communicating with the sensor through the OneWireAPI.NET function library and SQLServer for data storage.

4 Figure 4 - a. DS18B20; b. Connecting to the adapter; c. USB adapter. The initialization sequence consists of pulling the DQ pin low for about 500µs, floating the pin on input, waiting for at least 80µs and reading the pin. If the sequence was correct, the sensor responds with a presence pulse, meaning a low line. (Figure 5). Alternatively, a high line corresponds to a misconnected sensor. Figure 5. Initialization times The temperature read and conversion operation begins with a sequence of commands, Skip ROM [CCh], ConvertT [44h], a 1-2 seconds wait period, another initialization as above together with a Skip ROM [CCh] command. After a Read Scratchpad command [Beh], the sensor sends the 9 data bytes over the 1-Wire bus, one bit at a time, starting with the least significant bit of byte 0. The OneWireAPI.NET function library offers an elegant means of interfacing the sensor, having a DSPortAdapter object as middleware. The following C# code illustrates the initialization sequence: private void Initial() string adaptername = null, adapterport = null; key = Registry.CurrentUser.OpenSubKey("SOFTWARE\\Dallas Semiconductor\\OneWireAPI.NET\\owio", true); if(key!=null) adaptername = "DS9490"; adapterport = "USB2"; adaptername = "DS9490"; adapterport = "USB2"; try if (adaptername!= null && adapterport!= null) adapter = OneWireAccessProvider.getAdapter(adapterName, adapterport); else adapter = OneWireAccessProvider.getDefaultAdapter(); adapter.beginexclusive(true); catch(exception ex)

5 addtext("failure: " + ex.message, Color.Red); adapter = null; Reading the temperature value from the sensor's memory can be accomplished with the following C# code sequence: try adapter.setspeed(dsportadapter.speed_regular); // First initialization sequence adapter.reset(); adapter.putbyte(0xcc); // Skip ROM command adapter.putbyte(0x44); // Temperature conversion // Second read sequence adapter.reset(); adapter.putbyte(0xcc); // Skip ROM command adapter.putbyte(0xbe); // Read Scratchpad // Read the temperature sbyte[] block = new sbyte[2]; for (int i = 0; i < block.length; i++) block[i] = -1; adapter.datablock(block, 0, block.length); string temp = com.dalsemi.onewire.utils.convert.tohexstring(block); // Reverse MSB and LSB string tempbuff = temp[2].tostring() + temp[3].tostring() + temp[0].tostring() + temp[1].tostring(); temp = tempbuff; // Transform into bit string temp = System.Convert.ToString(System.Convert.ToInt32(temp, 16), 2).PadLeft(16, '0'); // Save the sign bit char[] semn = new char[5]; temp.copyto(0, semn, 0, 5); // Save the integer part - next 7 bits char[] intregg = new char[7]; temp.copyto(5, intregg, 0, 7); string intreg = ""; for (int i = 0; i < intregg.length; i++) intreg += intregg.getvalue(i).tostring(); // Save the floating part - next 4 bits char[] fractiee = new char[4]; temp.copyto(12, fractiee, 0, 4); string fractie = ""; for (int i = 0; i < fractiee.length; i++) fractie += fractiee.getvalue(i).tostring(); temperatura += System.Convert.ToDouble(System.Convert.ToInt32(intreg, 2)); for (int i = 0; i < fractie.length; i++) temperatura+=int.parse(fractie[i].tostring())*1/system.math.pow(2,(double)i); catch (Exception ex) addtext("failure: " + ex.message, Color.Red); The data acquired from the sensor at user-customizable intervals is stored into a temperature database. The instantaneous value is displayed in the web interface, along with the graphical representation of the history of temperature values for a certain period. The interface also allows the user to set the alarm trigger values. Figure 6 presents a case when the

6 temperature was out of bounds and the alarm was triggered. The plot can also be exported into pdf format. Figure 6. Screenshot from the developed Web application CONCLUSIONS This paper presents a modern implementation of a basic weather station Web application, having the DS18B20 sensor as its core. This approach offers numerous advantages, including scalability, extensibility, efficiency and robustness. REFERENCES [1] What is 1-wire?, [2] Building and Hacking Your Own 1-Wire Weather Station, [3] 1-Wire Weather Instruments, [4] Application Note 3989: Add Control, Memory, Security, and Mixed-Signal Functions with a Single Contact, Maxim, [5] DS18B20 Programmable Resolution 1-Wire Digital Thermometer Data Sheet,