How a PLC reads the data from field transmitters?

  • A critical challenge in instrumentation and control is converting physical or process variables into representations that are more actionable for operators. 
  • The process entails complex transformations in which physical changes are translated into various energy forms, ultimately resulting to numeric data suitable for presentation on operator interfaces. 
  • This data is also used to generate output commands, which are then transformed into mechanical actions using a thorough understanding of how different converters interact with one another.
  • In the context of pressure measurement, for example, the process begins with the conversion of pressure within a pipe into mechanical deflection of a diaphragm. 
  • This mechanical deflection is subsequently converted into electrical energy by a strain gauge, resulting in a transducer. 
  • An I/O module then converts this electrical energy into a numeric integer value. 
  • The PLC or HMI subsequently converts this integer value to a floating-point engineering unit value, allowing it to be displayed. 
  • This processed data not only improves operator insights, but it also helps to provide commands that ease electrical-to-mechanical movements.
How a PLC reads the data from field transmitters?
  • The diagram illustrates two standard circuit setups for temperature measurement. 
  • In the top configuration, an external power supply is used to power the temperature transmitter’s signal loop, which is a four-wire loop. 
  • The bottom arrangement, known as a two-wire loop, makes use of an internal power supply (AI card Power) to power the loop. 
  • Note that the following explanation regarding unit conversions is applicable to both types of circuits, with the emphasis being placed on the top arrangement. This is a crucial point to keep in mind.

Thermocouple to Temperature Transmitter Signal Conversion

  • The process of data collecting begins with a thermocouple, a device that uses the concepts of bimetallic contact to produce a millivolt signal in response to temperature changes.
  • Within the desired temperature range, this millivolt signal shows a fairly linear relationship with temperature.
  • A key connection between the physical variable (temperature) and its electrical representation (millivolt signal) is provided by the relationship between temperature and voltage.

Calibration and Scaling

  • The process of data acquisition requires precision calibration. For instance, considering a system with an expected temperature of around 105°C, a temperature transmitter is chosen.
  • The designed system entails an upstream heater that can elevate the temperature to roughly 130°C before triggering an over-temperature interlock.
  • To ensure accurate data representation, the design engineer selects a calibrated span of 15 to 150°C, strategically placing the operational point in the middle of the temperature range.
  • For this specific example, a type K thermocouple is chosen, generating an output of 0.597 to 6.138 mV across the designated temperature interval.

Temperature Transmitter to Analog Input Card

  • The temperature transmitter acts as a pivotal interface between the thermocouple’s millivolt signal and the PLC‘s input processing capabilities.
  • This transmitter undergoes meticulous bench calibration to convert the 0.597 to 6.138 mV input signal into a 4-20 mA output signal.
  • Operating as a current source, the transmitter dynamically adjusts its power output to maintain a consistent milliamp output corresponding to the millivolt input, thereby ensuring proportionality.
  • The 4-20 mA signal, now scaled to represent the 15 to 150°C temperature span, is ready for further processing.

Analog(4-20mA)-to-Voltage(1-5VDC) Conversion

  • The PLC, equipped with an analog input module, interfaces with the temperature transmitter‘s output signal.
  • Although designated as milliamp inputs, these modules translate the current signal into a voltage signal.
  • A 250-ohm resistor, often integrated into the system, facilitates the conversion process, resulting in a voltage range of 1-5 VDC.

Binary Value Conversion

  • The voltage signal, now converted, forms the basis for a binary value within the PLC’s input module.
  • The module’s resolution, often 16 bits, determines the precision of this conversion.
  • In the presented example, this yields a range of count values from 0 to 36727, effectively encoding the voltage signal’s magnitude.
Arithmetical information
UnitsoC (Engineering unit)mV (TC)mADC (TT)VDC (AI)Integer (program)% (Engineering unit)
Zero value004100
Span value7503020532727100

Integration with PLC Program

  • The PLC program interacts with the binary value, enabling various actions based on the temperature data.
  • The PLC program may retrieve this data for usage in control algorithms, display, or data logging purposes.

HMI Interface and Data Presentation

  • Data processing might involve transmitting this information to a Human-Machine Interface (HMI) for operator oversight.
  • Upon receipt, the HMI applies preconfigured formulas stored in its tag-file database to convert the binary value into meaningful engineering units.
  • The resultant value corresponds to the temperature in Celsius (°C), which is then displayed for operator interpretation.
  • Data acquisition process from temperature transmitters by PLCs encompasses a series of intricate stages, from the generation of a millivolt signal by a thermocouple to the ultimate presentation of an accurate temperature reading on the operator interface. 
  • Each step contributes to the seamless translation of physical variables into actionable data, fostering informed decision-making and efficient process management within industrial settings.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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