Abstract
Many real-world applications depend on temperature sensing and/or control. This includes a wide range of industrial processes, chemical reactors, and SCADA systems, in addition to other physical, mechanical, and biological systems. With the advancement of technology, it became possible to produce a new generation of smart and compact temperature sensors, which are capable of providing digital outputs that are more accurate, robust, and easily interfaced and integrated into measurement and control systems. This chapter first surveys traditional analog temperature sensors, such as RTDs and thermocouples, to provide a strong motivation for the need to adopt better and smarter techniques that mainly rely on digital technology (e.g., CMOS designs). Different interfacing techniques that do not need ADCs are introduced, including the programmable Arduino microcontrollers. Different applications will be explored that include automotive accessories, weather forecast, healthcare, industrial processing, firefighting, and consumer electronics. Both wired and wireless technologies, including the IoT, will be investigated as means for transmitting the sensed data for further processing and data logging. A special case study to provide information redundancy in industrial SCADA systems will be analyzed to illustrate the advantages and limitations of smart temperature sensors. The chapter concludes with a summary of the design effort, accuracy, performance, and cost effectiveness of smart temperature sensors while highlighting future trends in this field for different applications.
TopIntroduction
Temperature is perhaps the far most important physical quantity to be measured and controlled. Many industrial processes, with physical, chemical, mechanical, electrical, and biological nature, exhibit dependence, some way or another, on temperature. In the last few decades, a huge effort has been made to produce compact, cost-effective, and more accurate temperature sensors that can produce outputs in digital format for easy integration in the new computer-based and microcontroller-based measurement and control systems.
Many analog sensors are used to measure temperature. Among them are the resistance temperature detectors (RTDs), thermistors, and thermocouples. Despite the rapid advances in technology, these analog sensors are still in operation and are widely used in laboratories and many fields in the industry (Huynh, 2015). RTDs are sensors that contain resistors, which change value, according to changes in temperature, almost linearly. Besides having a long life, they are famous for being accurate, stable, and capable of offering good repeatability and interchangeability. They have a wide temperature range from -50°C to 850°C, as reported by Dames (2008). Platinum RTDs are considered the most accurate and can follow two different standards; the European standard, DIN/IEC 60751 that requires the RTD to have a nominal electrical resistance, R0, of 100.0 Ω at 0°C and a temperature coefficient of resistance (TCR) of 3.85 mΩ/Ω/°C between 0 and 100°C. The American standard, used mostly in North America, has a resistance of 100.0 ±0.10 Ω at 0°C and a TCR of 3.92 mΩ/Ω/°C between 0 and 100°C. Equation (1) depicts the temperature dependence of the resistance.
RTDs can also have resistances of 200, 500, 1000, and 2000 Ω at 0°C. These RTDs have the same TCR as the 100.0 Ω RTD, but because of their higher resistances at 0°C, they exhibit greater resolution. RTDs are found in many different classes, e.g. A, B, C, and AA, depending on the temperature range, TCR, and the tolerance associated with it. There are many different connections for the RTDs that make use of a bridge circuit for measuring the resistance-dependent voltage, as shown in Figure 1. Usually, the measured voltage is set to zero, through a calibration process; changes in the temperature will result in equivalent changes in the resistance that will cause the voltage to linearly change, accordingly (azom.com, 2010).
Figure 1. Different wiring configurations for RTDs
RTDs have two main constructions; the thin film and the wire wound, as show in Figure 2, in (a) and (b), respectively. The thin film is available only in the DIN/IEC 60751 standard, in addition to a special TRC of 3.75 mΩ/Ω/°C for appliances industry. The most commonly used wire wound RTDs have a wire packaged inside a ceramic or glass. RTDs can be effectively used to measure surface temperature, in addition to temperature of liquids and gases, including air.
Figure 2. Different constructions of RTDs (omega.com, 2017)
RTDs suffer from the self-heating problem that causes deviation from the correct readings, which puts a limitation to their usage. In addition, they have a slow response time, poor sensitivity to small changes in temperature, and can’t be used for very high temperatures; above 660°C (Dames, 2008; azom.com, 2010; and omega.com, 2017).
Key Terms in this Chapter
DCS: Distributed control systems, a computerized control system, with a large number of decentralized control loops, in which autonomous controllers are distributed throughout the system. It provides higher reliability and reduced installation cost.
DTMOST: Dynamic threshold MOS transistor, a special type of MOSFET transistors that have a controlled threshold voltage. It can be used to replace traditional BJTs for better accuracy, when measuring temperature.
CMOS: Complementary metal-oxide semiconductor, a technology that is used in manufacturing microprocessors, microcontrollers, static RAM, and other digital logic circuits. It provides high noise immunity and low static power consumption.
HDL: Hardware description language, is a specialized computer language used to describe the structure and behavior of electronic circuits, and most commonly, digital logic circuits. It has many variants, such as VHDL and Verilog. It is widely used in mixed analog/digital codesigns.
CSTR: Continuous stirred-tank reactor, a continuous agitated-tank reactor, in chemical engineering that assumes perfect mixing, so that the output composition is identical to the composition of the material inside the reactor, which is a function of residence time, temperature, pressure, and rate of reaction.
BCTS: Bio-compatible temperature sensors, low cost and high-performance range of temperature sensor that can be used in biological applications
ADC: Analog-to-digital conversion, a system that converts analog continuous-time signals to equivalent digital signals that are binary based, with different accuracies and resolutions.
TCR: Temperature coefficient of resistance, a measure of the sensitivity to changes in the electrical resistance, due to changes in temperature. It is positive in metals and negative in semiconductors. Usually, it provides a simple linear mathematical relationship for easy temperature measurements.
SCADA: Supervisory control and data acquisition, a special type of DCS, with an architecture that uses computers, networks, data communications, and graphical user interfaces for high-level process supervisory management. It also uses other peripheral devices such as PLCs and PID controllers.
MOSFET: Metal-oxide semiconductor-field-effect transistor, a type of field-effect transistors (FETs), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate; whose voltage determines the conductivity of the device.
RTD: Resistance-temperature detector, a special type of temperature sensors that relay on the effect of the increase in the electrical resistance, due to the increase in temperature. It has high accuracy and repeatability and can be used for different ranges of the temperature, depending on its material.
PLC: Programmable logic controller, a special type of digital computers, which has been adapted for the control of manufacturing processes in industry, such as assembly lines, robotic devices, or any activity that requires high reliability control and ease of programming and process fault diagnosis.
BJT: Bipolar-junction transistor, a special type of transistors that use both electron and hole charge carriers. They are usually manufactured in PNP and NPN types, and can be used as switches or amplifiers, via controlling the current at the base terminal.
DAQ: Data acquisition, the process of sampling signals that measure real world physical conditions and converting them into digital signals that can be manipulated by a computer. It usually includes a collection of sensors, signal conditioning units, and ADCs. It is mostly software-controlled.
EMI: Electro-magnetic interference, a disturbance generated by an external source that affects an electrical circuit by electromagnetic induction, electrostatic coupling, and/or conduction.
Phidgets: Physical widgets, an easy to use set of building blocks for low cost sensing and control from your PC, usually using the USB port.
IoT: Internet of things, a huge network for connecting people and things. It is becoming the gateway for wirelessly connecting sensors for the purpose of display, monitoring, control, and data logging.