How a PLC reads the data from field transmitters?

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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.

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.

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.

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.

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.

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.

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.

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.

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