Positioner
A positioner (engl. Positioner ) is replaced (for. Example, from a higher-level controller), the control signal in the form of a unit signal (z. B. 4-20 mA), and sets this to site. By mechanically returning the current position of the actuating stroke, disruptive influences such as fluctuations in the operating pressure or actuating energy can be regulated independently. The actuator is often a control valve ( control valve ). The auxiliary energy can also be provided via compressed air or hydraulic fluid .
construction
The (remote) control of a valve is usually done by an actuator ( actuator ). In process automation , pneumatic or hydraulic drives are often used for this . The positioner converts the input signal (electrical or pneumatic) into a corresponding output signal that determines the target position . A characteristic of a positioner is the position feedback, which is used to determine the actual position and to readjust a possible difference.
Types of positioners
Depending on the type of valve, the actuator executes a linear or rotary movement to which the positioner is adapted. Positioners that can be used universally can be adapted to both types of drive using a (lever) mechanism.
Single and double acting positioners
Depending on the type of actuator, single or double-acting positioners are required.
A single-acting drive has only one input for the actuating energy . Often a spring then provides the counterforce that moves the valve back to its starting position in the absence of the actuating energy .
Double-acting drives have two inputs, so control energy is required for every drive and valve movement.
Some positioners are referred to as triple or quadruple acting. These are made up of several positioners that are housed in a housing.
Pneumatic positioners
This type uses the pneumatic standard signal 0.2–1.0 bar. Compressed air (typically 6 bar) is used as auxiliary energy. This type is rarely used today. Current process control systems can generate intrinsically safe control signals at low cost. This eliminates the advantage of this type with regard to potentially explosive areas.
Electric positioners
With this type, the input and output signals are in electrical form. The drive must therefore also be electric. Single and three-phase direct or alternating voltages can be used as voltage sources. Electrical positioners are usually built in digital form, for example, to also be able to evaluate digital input signals from fieldbus systems ( e.g. PROFIBUS ) and to be able to e.g. B. to be able to implement more complex functions for determining the target position. The advantage of this type lies in the minimal energy consumption when the target position is reached, since only the positioner has to be supplied with energy, but not the drive. However, these devices can seldom be used in Ex areas (potentially explosive areas), since enormous amounts of energy are often required to move the drive.
Electropneumatic and electrohydraulic positioners
These positioners combine the advantages of the first two types: They offer explosion protection (with a sufficiently low voltage), can be operated with compressed air or (possibly flame-retardant) hydraulic fluid and can process digital signals. The input signal is converted into the required pressure using a built-in circuit. Then the valve of the positioner is opened by the electronics so that the actuator moves to the set position. They receive signals in analog form (4–20 mA signal) or in digital form (PROFIBUS, Foundation Fieldbus, etc.) and offer the corresponding functions (e.g. non-linear conversion of the input signal into control values). This type is the most common because it offers the benefits of both worlds.
Continuous controller with integrated positioner
In industrial compact controllers (PID controllers), the position of motor-driven actuators is adjusted by the integrated positioner. The actuator is controlled by the controller via binary control z. B. Relay, opened or closed. The actuator must have position feedback. In the simplest case, this feedback is represented by a resistance wiper or a potentiometer and fed to the controller. The subordinate positioner now also regulates the required degree of opening (output level) of the continuous controller.
Many (electro) pneumatic positioners can be built-on in an integrated manner, i.e. the pneumatic connections are made via openings in the base plate of the positioner. This type of attachment is usually based on industrial standards (mainly NAMUR or VDI / VDE 3847) so that the drives and positioners can be used regardless of the manufacturer. If this is not possible, the positioner is attached to the actuator with retaining clips or clamps to provide position feedback. For this, however, pneumatic lines must be laid. If this type of attachment cannot be implemented either (if, for example, the positioner may not be installed in the area of the valve for reasons of explosion protection), the position feedback is implemented via so-called NCS ( non-contact sensors), which then, in addition to the pneumatic connection, the single elements of the controller are mounted on the drive.
Areas of application
Positioners are often used in automation technology and the process industry to control actuators .
Individual evidence
- ↑ JUMO: JUMO Campus - Control Technology - Basics and Tips for Practitioners. Retrieved December 6, 2017 .