In the PLC world, analog calculations can often be a headache. You might think, “It’s just some simple voltage and current conversions—how can it be so complicated?” However, when faced with various resolutions and different voltage and current ranges, these simple calculations can suddenly seem overwhelming.
Don’t worry! Today, we will demystify this process so you can easily master PLC analog calculations. No need to sift through complex information—this guide covers everything you need.
What Are Analog Calculations?
Analog calculations in PLCs primarily involve converting voltage or current signals from the physical world into digital signals that the PLC can understand. These digital signals are then processed so the PLC can perform the appropriate control operations. Different PLCs have different resolutions, and the higher the resolution, the greater the accuracy of the signal conversion.
Voltage Signal Calculation
-10 to 10V Voltage Signal:
For a voltage signal ranging from -10V to 10V, a PLC with 12-bit resolution converts the signal to a decimal number ranging from -2048 to 2047. In contrast, a 16-bit resolution PLC converts the same voltage signal to a decimal number ranging from -32768 to 32767.
0 to 10V Voltage Signal:
For a voltage signal from 0 to 10V, a PLC with 12-bit resolution converts it to a decimal number ranging from 0 to 4095. At 16-bit resolution, the same signal converts to a decimal number ranging from 0 to 65535.
Current Signal Calculation
0 to 20mA Current Signal:
For a current signal ranging from 0 to 20mA, a PLC with 12-bit resolution converts it to a decimal number ranging from 0 to 4095. At 16-bit resolution, it converts to a decimal number ranging from 0 to 65535.
4 to 20mA Current Signal:
For a current signal ranging from 4 to 20mA, a PLC with 12-bit resolution converts it to a decimal number ranging from 819 to 4095. At 16-bit resolution, the same signal converts to a decimal number ranging from 13107 to 65535.
Note: These values are not fixed; different PLC manufacturers may use different resolution standards. Always refer to the specific technical manuals for confirmation in your projects.
Analog Input Wiring Considerations
Calculations are just one part of the process; wiring is also crucial. Even with perfect calculations, incorrect wiring can lead to poor results. Here are some common analog input wiring considerations:
Use Shielded Twisted Pair Cable and Connect the Shield: The shield prevents external electromagnetic interference from affecting signal quality and should normally be grounded.
Short Unused Input Terminals: When an input is not in use, short the V IN and COM terminals to prevent interference from floating signals.
Isolate Analog Signal Lines from Power Lines: Keep analog signal lines away from AC power lines, high-voltage lines, etc., to avoid signal interference.
Filter the Power Line: If interference is present on the power line, install a filter between the input section and the power unit to ensure signal stability.
Power-Up Sequence: The correct power-up sequence is to power up the CPU unit first, then the load. In case of a power failure, cut off the power to the load first, then to the CPU.
Although analog calculations may seem complex, they are manageable with the right methods and techniques. By converting signals according to the PLC’s resolution and following proper wiring specifications, you can ensure the reliability and accuracy of your system.
