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.

• 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.

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.