What are the two most common types of analog PLC signals?

INTRODUCTION

Analog measurements provide information to the PLC with the value of the measured quantity, e.g.

• Pressure [bar, MPa …]
• Flow [m3 / h, hl / h …]
• Level [m]
• Temperature [oC]
• Distance [m]
• Frequency [Hz]
• Current intensity [A]
• etc.

Simple control systems use only digital inputs and outputs of the PLC. The digital inputs are used to collect two-state information from the object and digital outputs to control the on / off type. In more complex control and regulation systems, and where any process begins, it is necessary to use analog inputs and outputs.

1. 0-10V Analog Signal:

• This type of analog signal operates within a voltage range from 0 to 10 volts.
• The signal is generated by varying the voltage level proportionally to the measured parameter (e.g., pressure, temperature, flow rate, etc.).
• 0V typically represents the lowest value or minimum reading, while 10V represents the highest value or maximum reading.
• The signal is easy to understand and troubleshoot since it directly represents the measured parameter in voltage form.
• Widely used for controlling variable speed drives (VSDs), controlling valve positions, and other applications where voltage-based control is preferred.
• May be susceptible to noise and voltage drops over long cable runs.

1. 4-20mA Analog Signal:

• This type of analog signal operates within a current range from 4mA to 20mA.
• The signal is generated by varying the current level proportionally to the measured parameter.
• 4mA typically represents the lowest value or minimum reading, while 20mA represents the highest value or maximum reading.
• The signal is less susceptible to noise and can handle long cable runs better than voltage-based signals.
• Often used in applications where the distance between the sensor and the PLC is significant, as the current signal remains more stable over long distances.
• It is commonly used in industrial process control, such as measuring pressure, level, temperature, and flow in various industries.

Choosing between 0-10V and 4-20mA analog signals depends on the specific application, the distance between the sensor and the PLC, noise considerations, and the preference of the system integrator or engineer. Many modern PLCs have analog input modules that can accept both types of signals, making it easier to adapt to the requirements of different sensors and applications.

In summary, the 0-10V analog signal is voltage-based and often used for simpler control applications, while the 4-20mA analog signal is current-based and preferred for more critical process control applications and situations where long cable runs are involved.

Mostafa Sayed

19 July 2023