The public account has been blocked. In the system, process signals are converted into uniform voltage and current signals via transmitters, and these signals are transmitted to the controller (PLC) in real time. The PLC performs calculations and conversions, transforming these analog signals into internal numerical signals. This enables system monitoring and control. The process from physical signals in the field to numerical signals processed by the PLC involves the following steps:
As can be seen from the above PLC analog signal input process, analog signal input in automated process control systems is very complex. However, in current industrial settings, analog signal processing primarily uses current signal transmission. Compared to voltage signals, current signals offer stronger anti-interference capabilities, longer transmission distances, and greater signal stability. Here, we will provide a simple breakdown of the PLC's analog signal conversion process.
PLC conversion of analog signals
▲ The conversion range of analog signals by the Siemens S7-200 SMART PLC analog module
▲ Delta DVP series analog signal conversion range
From the above, we can see that: 1. After analog signals are input to the PLC, the PLC converts them into integer data, not floating-point numbers (e.g., Siemens -27.648 to 27.648); 2. Different brands of PLCs have different analog conversion ranges (e.g., Siemens -27.648 to 27.648; Delta -32.384 to 32.384); 3. The conversion range of the same PLC module for different types of analog signals is consistent (e.g., Siemens' conversion range for ±10 V, ±5 V, ±2.5 V, or 0 to 20mA analog signals is -27.648 to 27.648). Therefore, from the above points, we know that analog signals input to the PLC need to be further converted to obtain data that matches the actual physical quantity; when performing data conversion processing, it should also correspond to the data processing range of the PLC module used.
PLC data conversion and processing process
1. Conversion between analog signals and PLC-converted data
As we know from the above, the analog signals directly read from the PLC are integer data. Integer data cannot intuitively reflect the actual physical quantity magnitude. Therefore, in order to intuitively reflect the process signal status on site, these integer data should be converted into floating-point signals for more direct and accurate feedback. This will be illustrated using the data processing process of a Delta PLC analog input module as an example.
The above shows the correspondence between the current input signal and the read signal of a Delta PLC. From this correspondence diagram, we can see that for any arbitrary current input signal (X), the correspondence with the read numerical signal (Y) is as follows:
Transforming the above formula, we obtain the following direct conversion formula:
The above formula can be used to convert the integer data read by the PLC into the analog current signal value received by the PLC.
2. Conversion between actual physical values and analog data
The conversion method between actual physical values and analog data is the same as the conversion method described above. Below is the pressure and analog signal range (0.1-10 bar, 4-20 mA) of a pressure transmitter. Similarly, the conversion relationship between pressure and current is as follows:
Transforming the above formula, we can obtain the following direct conversion formula:
The above formula can be used to directly convert analog current signals into pressure data.
3. Direct conversion between actual physical values and PLC internal data
Converting the data read from the PLC into actual physical values can be achieved through the two conversion steps above. Skipping the conversion of the input signal value during processing yields the following relationship:
Transforming the above formula, we can obtain the following direct conversion formula:
This formula allows the data read by the PLC to be directly converted into actual physical quantity values. The above describes the PLC's processing procedure and method for analog signals.
