Electronics Engineering: Thermocouples as Temperature Sensors

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The sensing characteristics of the proposed system can be seen through the components included in such system. The temperature sensors, in that regard, are mostly based on sensing the changes in materials, as the temperature of the materials change ¹. Accordingly, the method for the implementation of temperature sensing features vary as well, including bimetallic strips, liquid and gas expansion devices, e.g. thermometer, and thermocouples. The sensing elements are only a part of a measurement unit, which if put in sequenced order are followed by signal conditioning element, signal processing element, data presentation element, and the output value.

Thermocouples as temperature sensors are mostly used in industrial settings, where the range of temperatures is related to the metals used. The work principle in thermocouples is based on measuring the differences in temperature that produce specific measurable voltages. Another important measuring principle can be seen through the usage of Zener diodes. Such diodes rely on the avalanche effect in a way that applying increasing reverse partial discharge (p.d.) to the diode there is no current the avalanche until the avalanche p.d. is reached. Such diodes and their corresponding principle are implemented in the LM135/LM235/LM335 and LM135A/LM235A/LM335A series of popular precision temperature sensors produced by National Semiconductor. In such sensors, the breakdown voltage is “directly proportional to the absolute temperature, where the different models of the series merely differ in their temperature range.

The approaches used to sense gas and smoke vary as well, where different approaches are proposed in the literature. One of the approaches proposed in Albornoz, Munoz, Moreno, Berga, and Anton (2008) is the usage of resistance-to-frequency conditioning circuits (p. 91). Such topology provides several advantages, among which are the price, the simplicity of the microcontroller and digital interfaces, and the absence of a necessity for analogue and digital converters. However, it should be noted that there are other typologies that might be used as well, such as resistance-to-time typology and frequency converters-based typologies. Another approach outlined in Rostedt et al. (2009) is the usage of particle concentration measurement. Although such approach is mostly used to measure specific gases, e.g. engines’ exhaust, aerosol, etc, the main principles can be used to measure any concentration of particles, where experimental measures might be used to set the threshold of the system. The sensor type in such system might include opacimeter and filter smoke number (FSN), and a more modern technique such as laser induced incandescence.

In terms of light sensors, such devices are usually one of the cheapest and the simplest, which inclusion can be seen in many different products. The principles of light sensors work vary as well, where the main purpose of such devices is measuring light intensity. Light sensors fall into two main categories, generating electricity when illuminated and changing property under the influence of light. The ones generating electricity -photovoltaic sensors, are mainly used in solar cells, and the main disadvantage of such method is that the voltage produced is not related “linearly to the incident light intensity”. Sensors that change properties -photoconductive, include such types as photodiode, phototransistor and a light-dependent resistor (LDR). Changing the resistance is a suitable approach for light sensing functions in measurement systems, where the response to different wavelengths is similar to the human eye. The principle of LDR implementation can be seen from the description in Olwal (2006), where LDRs are interface with PIC micro controllers, PIC18F4550 in this case. The exposure to a particular light reports the voltage level for the triggered sensor to the device. The device in this case might be a PC, with interfaces ranging from universal serial bus (USB) to wireless protocols, e.g. Bluetooth and Wi-Fi.

The choice of microcontrollers can be directly related to the ability to program such microcontrollers. In that regard, Arduino microcontroller boards are gradually becoming a standard in prototyping. The usage of the combination of Arduino board, as a data acquisition unit, and a measurement system was outlined in specifically in the context of being easily programmed, the availability of different interfaces, e.g. USB and Bluetooth, and the possibility of connecting to it different sensors and actuators. With large scientific community supporting the Arduino project, the projects based on such boards are easily modified. The microcontroller is programmed using Arduino programming language, based on wiring, C language, and in cases, other programming environments might be supported. The latter is connected to the usage of Firmata, a protocol developed for communicating microcontrollers from compatible software on computers, the reference of which is included in Arduino library.

With the latter being concerned mostly with the sensor techniques and the configuration of microcontrollers, the interfaces for sensors received increased importance as well. Such aspect can be specifically important in such cases where the location of the sensor and the controllers might be dispersed. The common approach is the usage of field buses, mostly used in field devices such as sensors or transducers, while might controllers use Ethernet. Thus, the interconnection of such interfaces in a measurement system can be obvious, where such system contains both controllers and sensors. The suitability of using sensors along with internet-based protocols was tested in Flammini, Ferrari, Sisinni, Marioli, and Taroni (2002), a study in which three interfaces, i.e. Profibus-DP, CANbus2.0B and Ethernet IEEE802.3, were implemented, tested and compared. The results of the study showed that internet based sensors can be comparable in costs, data throughput and complexity to field buses, providing accessibility advantages. Nevertheless, it should be noted that such approach implies using UDP protocols, rather than high-level protocols such as TCP/IP or HTTP due to the resources required for implementation. The usage of wireless networks to transfer such protocols was investigated in Das, Popa, Ballal, and Lewis (2009), outlining the use of wireless sensor networks (WSN). Addressing the potential deficiencies with communication medium, such as the redundancy of collected data, a portion which is only used, the study proposed the implementation of combined data-logging and supervisory control framework (DSC) for the management of the data transmitted from the sensors through a wireless protocol. The approach in using radio frequency modules (RF) to control the monitoring process is becoming widely popular, where RF communications are based on electromagnetic waves. An important issue in such communications is the size of the antenna and the reduction of energy consumption with modulation. Nevertheless, there several advantages of using RF modulation that includes “ease of use, integrality, and [being] well established in the commercial marketplace, which make it an ideal testing platform for sensor node”. Accordingly, the suitability of using wireless communication can be seen through other advantages such as “reliability, accuracy, flexibility, cost-effectiveness, and ease of deployment”.

Another essential element in measurement and control system is the instruments used for data acquisition. Traditionally, the instruments used for such purpose were vendor defined and hardware-based devices, such as oscilloscopes and waveform generators. With the technology incorporated in modern PCs reaching a level that allows emulating all the hardware functions, the demand for virtual instruments had risen. One of the popular packages in that regard is Labview, a virtual instrument developed by the company National Instruments. The advantages of using virtual over traditional instruments can be seen in the provided flexibility by the software, which is neither vendor-defined, nor function specific. Additionally, the usage of Labview allows designing http-based client-server solutions in order to control the environments easily.

The need for measurement and sensor systems can be indicated in several diverse markets. On the one hand, the environmental concerns established a certain market for devices measuring the concentration of specific gases, either on global or local basis, i.e. measuring the concentration and/or the emission of gases in the work place. Accordingly, a totally different market can be seen for smoke and fire detectors, which can be distinguished by different types of technologies implemented, such heat detection devices, photoelectric smoke detection devices, UV/IR detection devices, etc. In all of those technologies the main principles can be seen the same, i.e. a sensor, and a controller. The difference might be seen in that the output value in such devices is not measured, rather than triggers a certain process, e.g. alarm, as soon as the value reaches a specific limit. In that regard, the present project can be seen different is several aspects. On the one hand, the academic purpose for the developed system is measuring the values and thus, it is concerned with a specific output format. On the other hand, it can be stated that industrial measurement systems used in manufacturing facilities use an output format as well, where the main purpose is monitoring, rather than detecting. Accordingly, the developed system can be easily modified to include other parameters for monitoring, whereas the systems in the market are concerned with predefined parameters. In laboratory settings, the parameters measured, in addition to light, temperature, and smoke, might include humidity, gas concentration, and others. In that regard, the modular implementation of the system might allow adding different system and upgrading the firmware of the controllers to make the system appeals to different audiences.

It can be stated that the market for sensor systems implied in this paper mostly refers to the measurement of non-electrical parameters such as temperature, humidity, light, etc. Accordingly, there is an existing market for sensors measuring electrical parameters, such as “voltage, current, active and reactive power, frequency phasor measurements, harmonics, arks detection, ground impedance, electromagnetic fields and transients”. The market for both categories of sensors can be differentiated based on the scales of their implementation, used in developed, researched and used in governmental organizations and industrial and scientific settings.

The advantages of the system proposed in this study can be seen through its purpose, i.e. monitoring. Devices in the market with similar purposes are provided as complete solutions, with little possibility for manual modification of the parameters monitored. In small scale settings, such as the laboratory, adding a parameter to be monitored can be a tiresome and an expensive experience using such solutions. On the other hand, the proposed system uses a bare bone that is capable of being modified with no need to substitute all of the hardware. The inclusion of Arduino board provides the possibility for the system to be modified in term so the connection interfaces, the sensors used, and the desired output format. Accordingly, a large support of the scientific community, as well as the fact that it is an open-source project, the developed system is not limited only by sensor and measurement features. Additionally, the usage of the developed system as an easily modified prototype can be confirmed by the fact that there are many sensor components available in the market, where “Huge manufacturers as Analog Devices, Honeywell coexist with small companies focused on innovative solutions”. In that regard, such fact provides an advantage over complex solutions from the manufacturers in the market. Therefore, changing the purpose of the system can be as easy as changing the type of the sensor used. The latter can be specifically outlined through the usage of Labview as a system for data acquisition, where in addition to the general advantages of using virtual instruments such as the maximum economics of scale and the minimization of development and maintenance costs, there other advantages specific to LabView. Such advantages include the modular programming language, and a large library of mathematical and statistical functions. Such factor contributes to the ability for the developers to modify and create new programs to adjust the control and the monitoring process. Combining the aforementioned factors, the proposed system allows creating a modifiable solution at much lower costs that the solutions available in the market.

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Electronics Engineering: Thermocouples as Temperature Sensors

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