Electric Control Valve: Active vs. Passive Signals
Electric control valves are essential components in modern industrial control systems. They are widely used in fluid control, automation, and process control. To achieve precise regulation, electric control valves rely on switch signals that control their opening and closing. These signals are generally divided into two types: active and passive. Understanding the differences between these signals is important for selecting and configuring electric control valve systems correctly. This article will explain active and passive signals, their applications, and the differences between them.
What is a Contact Signal?
A contact signal, also called a switch signal or binary signal, indicates the on/off status of a device. It is a binary signal where a value of "1" means "on" (open), and a value of "0" means "off" (closed). This signal conveys information through the status of the electric control valve's switch. Contact signals typically do not supply power; they only show whether the switch is open or closed.
In industrial automation, contact signals are commonly used in systems like PLCs (Programmable Logic Controllers) as digital inputs or outputs. These signals are widely used in applications such as equipment start/stop control and status monitoring.
Active vs. Passive Signals
Active and passive signals are two types of contact signals. The main difference between them is whether the signal source includes a power supply. Understanding these differences is important for selecting the correct signal type for controlling a valve.
1. Active Signal
An active signal is a signal where the source provides power output. The switch or sensor not only detects status changes but also provides power or signal output. For example, proximity switches, photoelectric switches, and other devices that are powered internally can directly output signals. When activated, these switches send a signal indicating the status and also provide voltage or current to drive other devices.
Common Features
Includes a power supply and can output a signal.
The output voltage range is typically DC 0–30V, with DC 24V being the standard voltage.
Active signals may also use AC 110V–220V voltage outputs, although this is less common.
Application Example
Photoelectric switches, proximity switches, and similar sensors that have internal power. These sensors directly output active signals when activated, used to control electric control valves.
In industrial automation, sensors provide output signals that control electric control valves, enabling precise regulation.
2. Passive Signal
A passive signal is a signal where the source does not output power and only indicates the on/off status of the switch. Passive signals require an external power supply to provide voltage or current. These signals are typically used in switches or sensors that do not have built-in power sources. When activated, the switch's signal output can only be triggered by an external power source.
Common Features
No power output; only indicates the open/closed status of the switch.
The output signal requires external power to be activated.
Common passive signal devices include relay contacts and dry contacts.
Application Example
Relay contact signals, where the contact closes to output a passive signal. These devices do not have a built-in power source and require external power to activate.
In Distributed Control Systems (DCS), passive signals are used to provide feedback on the status of electric control valves. The system controls and monitors these valves via external power sources.
Applications of Electric Control Valves
Electric control valves are categorized into three types based on signal type and functionality: standard electric control valves, passive electric control valves, and intelligent electric control valves. Each type has different requirements for signal input, output, and control methods.
1. Standard Electric Control Valve
Standard electric control valves are typically of the active type. These valves are controlled directly by a 220V power switch to open or close the valve. The valve's movement is powered by an external switch or control system, so the signal source includes power output.
Application Example: Traditional electric control valves are controlled by a 220V power source, with a simple power switch directly controlling the valve's opening and closing.
2. Passive Electric Control Valve
Passive electric control valves rely on external switch signals to control the valve's opening and closing. In a 220V power environment, passive signal devices, such as dry contacts or relay contacts, control the valve's action.
Application Example: External signal devices like relays or dry contacts provide switch signals to control the valve's open or closed state.
3. Intelligent Electric Control Valve
Intelligent electric control valves offer higher levels of automation. These valves typically use a DC 12V voltage and microcontrollers (such as microchips) for precise control. They not only accept input signals but also provide feedback signals, enabling bidirectional communication. Intelligent electric control valves are widely used in systems requiring real-time regulation and feedback.
Application Example: Intelligent electric control valves receive remote control signals and provide valve status feedback, suitable for industrial systems that require precise control and real-time monitoring.
Working Examples of Active and Passive Signals
To better understand the working principles of active and passive signals, consider the following example.
Active Signal: If a switch (S) controls the coil of a 12VDC relay (K), the upper end of switch S is connected to a 12VDC power supply. The lower end outputs an active signal. When relay K is activated, contacts 1 and 2 will provide a passive signal. These contact signals do not need an additional power source to be transmitted to other devices.
Passive Signal: In a passive signal case, switch S itself does not output power. The contacts of relay K (1 and 2) will only function as switch signals. The output signal requires external power to be activated.
Conclusion
Active and passive signals are both crucial in the application of electric control valves. The main difference lies in whether the signal source includes a power supply. Active signals come with their own power source and can directly output control signals, while passive signals need an external power supply to be activated. By choosing the correct signal type and electric control valve based on the control system's needs, more precise valve control and system management can be achieved. Understanding these basic concepts helps optimize system design, improving control accuracy and system stability.
