Temperature sensors are a critical component in various industrial applications, including Programmable Logic Controller (PLC) systems. These sensors measure the temperature or temperature gradient and typically consist of a measurement element that detects the change in temperature. Understanding the different types of temperature sensors, their technical specifications, and the process of interfacing them with PLCs is essential for effective industrial automation and control.
Types of Temperature Sensors for PLC
Resistance Temperature Detectors (RTDs)
RTDs are contact temperature sensors that use a fine metal wire whose resistance changes with temperature. They are widely used in industrial applications due to their high accuracy, ease of installation, wide operating range, and precise measurement capabilities. RTDs can be made from various metals, such as platinum, nickel, and copper, with platinum being the most common due to its high accuracy and stability over a wide temperature range.
Technical Specifications of RTDs:
– Temperature range: -200 °C to 1000 °C
– Accuracy: ±0.1 °C for high-precision RTDs, ±1 °C for standard RTDs
– Response time: typically in the order of milliseconds
– Environmental conditions: suitable for harsh environments with high temperatures, vibrations, and electromagnetic interference
Thermocouples
Thermocouples are another type of contact temperature sensor that measures the temperature difference between two points. They consist of two dissimilar metal wires joined at one end, creating a voltage proportional to the temperature difference between the two junctions. Thermocouples are known for their fast response time, ruggedness, and wide temperature range, making them suitable for harsh environments.
Technical Specifications of Thermocouples:
– Temperature range: -200 °C to 1800 °C
– Accuracy: ±2.2 °C
– Response time: typically in the order of milliseconds
– Environmental conditions: suitable for harsh environments with high temperatures, vibrations, and electromagnetic interference
Infrared Temperature Sensors
Infrared temperature sensors are non-contact temperature sensors that measure the infrared radiation emitted by an object. They are ideal for measuring the temperature of moving objects or objects that are difficult to access physically. However, they are affected by the emissivity of the object being measured, which can lead to measurement errors if not accounted for.
Technical Specifications of Infrared Temperature Sensors:
– Temperature range: Depends on the specific sensor model
– Accuracy: Depends on the specific sensor model and the emissivity of the target object
– Response time: Typically in the order of milliseconds
– Environmental conditions: Suitable for measuring the temperature of moving objects or objects that are difficult to access physically
Interfacing Temperature Sensors with PLCs
When it comes to interfacing temperature sensors with PLCs, there are several options available. One common method is using 4-20 mA current loops, which are widely supported by PLCs and provide a reliable and accurate method of transmitting temperature data. Another option is using voltage signals, such as 1-5 V DC signals, which can be translated into temperature readings using signal manipulation.
Interfacing RTDs with PLCs
In the case of RTDs, the resistance change can be translated into a 4-20 mA signal using a current-to-resistance converter. This signal can then be wired to a PLC analog input module, which can be programmed to convert the current signal into a temperature reading.
Interfacing Thermocouples with PLCs
Thermocouples require PLC modules (or alternative ways) that can linearize the output and convert it into a temperature reading. This is necessary because the voltage output of a thermocouple is not linearly proportional to the temperature.
Interfacing Infrared Temperature Sensors with PLCs
Infrared temperature sensors typically output a voltage or current signal that can be directly connected to a PLC analog input module. However, it is essential to consider the emissivity of the target object and adjust the sensor settings accordingly to ensure accurate temperature measurements.
DIY Temperature Sensor for PLC
Building a DIY temperature sensor for a PLC can be a cost-effective and customizable solution for industrial applications. Here’s a step-by-step guide on how to build a DIY temperature sensor for a PLC:
- Select the Appropriate Temperature Sensor: Choose the temperature sensor (e.g., RTD or thermocouple) based on the required temperature range, accuracy, and environmental conditions.
- Connect the Temperature Sensor: Connect the temperature sensor to the current-to-resistance converter (for RTDs) or linearization module (for thermocouples) according to the manufacturer’s specifications.
- Design and Build the Signal Conditioning Circuit: Design and build a signal conditioning circuit to amplify and filter the sensor signal. This may include components such as amplifiers, filters, and signal conditioners.
- Connect to the PLC Analog Input Module: Connect the signal conditioning circuit to the analog input module of the PLC.
- Program the PLC: Program the PLC to read the temperature data from the analog input module and convert it into a usable format.
By following these steps, you can create a customized temperature sensor solution that meets the specific requirements of your PLC-based industrial application.
Conclusion
Temperature sensors are a critical component in PLC systems, and understanding the different types of sensors, their technical specifications, and the process of interfacing them with PLCs is essential for effective industrial automation and control. Whether you choose to use commercially available temperature sensors or build a DIY solution, it is crucial to consider factors such as temperature range, accuracy, response time, and environmental conditions to ensure reliable and accurate temperature data transmission.
References:
- Temperature and humidty sensor to PLC | PLCS.net – Interactive Q & A
- Add smart condition monitoring sensors to your PLC control systems | Balluff
- The Complete Guide to Temperature Sensors | SolisPLC
- Data Acquisition (DAQ) | KEYENCE America
- Improving the Accuracy of 1-Wire DS18b20 Temperature Sensor | The Cave Pearl Project
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