ELECTRONIC SENSORS
Redesigning RTD-Based Temperature Sensors for the Smart Factory Age
Brian Condell, Product Applications Engineer Michael Jackson, End Market Specialist
T
he old saying “if it ain’t broke, don’t fix it” cautions against tampering with
something that performs reliably and regularly proves its worth. Arguably, this advice applies to the circuit design used in many RTD sensors that quietly and efficiently measure temperature in industrial manufacturing facilities worldwide. However, to meet the requirements of Industry 4.0, factories are becoming smarter, and it is becoming clear that many current RTD sensors will not be fit for purpose in these environments. Smaller form factors, flexible communications, and remote configurability are some features that automation engineers now demand from industrial temperature sensors, but incumbent solutions cannot support them. This article revisits the building blocks used in the design of many RTD-based temperature sensors and discusses the limitations that these impose on a sensor’s application. It then shows how to quickly redesign this type of sensor to equip it with the features required in this new industrial age.
Temperature Sensor Building Blocks
The building blocks of an RTD industrial temperature sensor are shown in Figure 2. An RTD translates a physical quantity (temperature) into an electrical signal and is typically used to detect temperatures between –200°C and +850°, having a highly linear response over this temperature
Figure 1. An example of an RTD-based temperature sensor.
range. Metal elements commonly used in RTDs include nickel (Ni), copper (Cu), and platinum (Pt), with Pt100 and Pt1000 platinum RTDs being the most common. An RTD can consist of either two, three, or four wires, but the 3-wire and 4-wire versions are the most popular. Since they are passive devices, RTDs require an excitation current to produce an output voltage. This can be generated using a voltage reference, buffered by an operational amplifier that drives current into the RTD to produce an output voltage signal that varies in response to changes in temperature. This signal varies from tens to hundreds of millivolts depending on the type of RTD used and the measured temperature, as shown in Figure 3.
The AFE amplifies and conditions the low amplitude RTD signal before the analogue- to-digital converter (ADC) digitises it for
the microcontroller to run an algorithm, which compensates for any non-linearity it contains. This sends the digital output to a process controller via a communications interface. The AFE is commonly implemented using a signal chain of components in which each performs a dedicated function, as shown in Figure 4.
Many existing temperature sensor designs use this discrete approach that requires a printed circuit board (PCB) large enough to accommodate the footprint of all the integrated circuits (ICs) and the signal and power routing and sets a de facto minimum size for the sensor enclosure. A superior and more straightforward approach uses an integrated AFE like the AD7124-4 shown in Figure 5. This compact IC is a complete AFE in a single package and includes a multiplexer, voltage reference, programmable gain amplifier, and a sigma-delta ADC. It also provides the excitation currents for the RTD, meaning it can effectively replace five of the signal- chain components from the previous figure, significantly reducing the amount of board space required and enabling a sensor with a much smaller enclosure.
Figure 2. A block diagram of an RTD industrial temperature sensor. 24 JUNE 2024 | ELECTRONICS FOR ENGINEERS
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