A positioner is a device that is used in combination with a control valve to adapt the exact position of the valve to the requirements of a control signal. The positioner ensures that the valve reaches the desired position precisely and reliably, regardless of external influences such as pressure fluctuations, friction or vibrations.

In industrial water and wastewater technology, the positioner is an indispensable instrument for the efficient and stable control of processes such as dosing, pressure control, flow control and chemical precipitation.

Structure of a positioner

A positioner typically consists of the following components:

1. Input signal receiver:

   • Accepts control signals (e.g. 4-20 mA, 0-10 V or pneumatic signals) from a process control system or a control unit.

2. Measuring system:

   • Measures the actual position of the valve using potentiometers, sensors or position measuring systems.

3. Comparison unit:

   • Compares the input signal with the current valve position and determines the required correction.

4. Final control element (actuator):

   • Influences the pneumatic or electric control of the valve to achieve the desired position.

5. Feedback system:

   • Transmits the current valve position back to the control unit to ensure precise control and monitoring.

6. Housing:

   • Made of robust materials such as stainless steel or aluminum to protect the positioner from environmental influences such as moisture, dust or chemicals.

Functionality of a positioner

The operation of a positioner can be divided into several steps:

1. Signal reception:

   • A control system sends a setpoint signal (e.g. 12 mA for a 4-20 mA signal) that represents the desired valve position (e.g. 50 %).

2. Position detection:

   • The positioner measures the current valve position via a position sensor or a potentiometer.

3. Variance analysis:

   • The controller compares the setpoint with the actual valve position and calculates a deviation.

4. Correction:

   • In the event of a deviation, the positioner activates the actuator (e.g. a pneumatic or electric actuator) to adjust the valve position accordingly.

5. Feedback:

   • Once the desired position has been reached, the positioner provides feedback to the control system, creating a closed control loop.

Types of positioners

Pneumatic positioners:

• Operate with compressed air and are particularly common in potentially explosive atmospheres (ATEX zones).
• Advantages:
  • Robust construction.
  • Insensitive to electrical interference.
• Typical use:
  • Control of control valves in chemical or petrochemical plants.

Electronic positioners:

• Work with electrical signals (e.g. 4-20 mA) and are compatible with digital control systems.
• Advantages:
  • Precise regulation.
  • Integration into process control systems (e.g. via HART, Profibus or Modbus).
• Typical use:
  • In highly automated water and wastewater systems.

Electro-pneumatic positioners:

• Combine electrical signal inputs with pneumatic control.
• Advantages:
  • High precision and compatibility with pneumatic valves.
• Typical use:
  • Applications with pneumatic control valves and digital control.