Proportional valve
A proportional valve is a continuous valve which, with the help of a proportional magnet, not only allows discrete switching positions, but also a constant transition of the valve opening. Proportional valves are used in hydraulics and pneumatics particularly where variable volume flows are required. They normally have a non-linear volume flow characteristic and are therefore less suitable than servo valves for control tasks .
There are proportional valves as adjustable flow control valves with one input and one output and as directional control valves with more than two working connections.
Designs
Proportional valves are electromagnetic or medium-controlled valves that can assume any intermediate position between open and closed. Electromagnetically controlled valves have an electromagnet that usually presses against a spring. If he presses against the fluid pressure itself, the valves are open in the de-energized state, which can be advantageous as part of other designs. If the valve z. B. directly to the motion control via a cylinder, on the other hand, a valve that blocks when de-energized is recommended. A spring then acts as a passive force against the active electromagnet. Electromagnetic proportional valves are available for pneumatic and hydraulic applications.
Medium-controlled proportional valves are controlled with hydraulic fluid. It comes from other proportional valves. Medium-controlled proportional valves thus serve as the 'main' stage (s) in multi-stage systems. Since pistons and other cylinders cannot be easily positioned pneumatically without control, there are no medium-controlled pneumatic proportional valves.
Proportional valves are usually designed as piston valves . This means that the shut-off body is usually an axially displaceable piston that connects or closes laterally arranged inlet and outlet openings. There are also cone valve and poppet valve shapes, e.g. B. as a pilot control.
If a valve has 2 working connections, e.g. inlet and outlet, it is a flow control valve, also known as a throttle valve, but it can also be viewed as a special form of the directional valve, namely a 2/2-way valve, regardless of its internal structure . Depending on the structure, they can be piston, cone or poppet valves at the same time. Higher-order directional control valves are only available as piston valves; a 5/3-way valve with a disc design would be too complicated.
In a proportional piston valve, the piston is not unlike that of a switching directional control valve. Because the intermediate positions between open and closed are used, it is given a slightly different shape. It results from the required flow characteristic, which describes the relationship between control current and flow.
Proportional valves, especially single-stage hydraulic versions, can require significant electromagnetic force.
Examples
Single-stage pneumatic proportional throttle valve
A simple pneumatic proportional valve controls the air passage opening of a plate shut-off body with the help of an electromagnet and a counteracting spring. Once the coil of the electromagnet has overcome the restoring force of the spring, the valve opens approximately to the extent that the current in the electromagnet increases.
Single-stage pneumatic proportional directional valve
The drive of a single-stage, direct operated proportional directional valve does not have to differ much from that of a switching directional valve. A linearly movable piston that connects or closes the openings to the inlets and outlets is moved in one direction or the other by an electromagnet and a spring. In this example, however, a blocking position in which there is no air flow is desired in the de-energized state. With a 5/3 way valve, this is unfortunately in the middle position, which requires additional changes. The coil must be able to actively move the piston in 2 directions. This happens here z. B. with 2 coils and 2 springs. The coils are arranged to pull the plunger in opposite directions. Usually only one of them is in operation. The springs are used to reset the device to the middle position without current.
Pilot operated proportional throttle valve for low pressure water
A two-way proportional valve or proportional throttle valve for water can be designed as a pilot-operated and thus two-stage diaphragm valve. The movable membrane is the mechanical connection between the main stage and the pilot pre-stage. It is used for pressures around 10 bar. In the de-energized state, the spring presses the piston with the plate-shaped shut-off body downwards. This in turn presses the closure part with the conical shut-off body, which is attached in the center of the circular membrane, downwards and thus closes the valve.
When the coil pulls the piston up, the outlet nozzle in the closure part opens first. The fluid flows from the pressure source inlet (P) through 2 nozzles to the outlet. The pressure above the membrane decreases and pulls it upwards, until the outlet nozzle is closed again and the pressure above and below the membrane is balanced. This also opens the main stage, but only as far as the piston and thus the control current in the coil allow. In this way, the electromagnetic coil force is amplified by the fluid pressure.
After the coil current has dropped, the spring force takes over the closing of the valve again.
Directly operated three-way proportional valve for hydraulic power transmission
Proportional valves can often be found in oil hydraulics. In the 3-way valve shown in the picture, the electromagnet presses on the left of the center against the spring on the right. There is no coil current Ip, but there is still 100% flow from A to T (tank) and from P (pressure source) to B. If the coil current is switched on and has, for example, moved half of the maximum required for full deflection, as in the next picture the piston moves to the right and closes the openings.
The differential transformer (LVDT, left) shown is an optional component for measuring the position of the piston. It is used for electronic control. The valve can for example be brought exactly into the blocking position, with about 50% coil current, and this blocking position can be checked before a cylinder or something similar actually moves.
Directly operated three-way proportional valve with 2 coils
The hydraulic variant of the pneumatic proportional directional valve from the section above uses 2 coils and 2 springs again to reach the blocking position without current. Coils of directly controlled hydraulic proportional valves, however, require more power and also more space than the pneumatic version, which is why they are not arranged next to one another, but on 2 different sides. Each of them pulls in a different direction, so in the rest position the valve is closed.
Two-stage three-way proportional valve
If larger hydraulic volume flows are to be controlled, proportional valves can be connected in series (cascaded) to form two-stage valves. The preliminary stage then moves the piston of the main stage instead of a cylinder, and the main stage then moves the cylinder or whatever. The main stage is then a medium-controlled proportional valve. The space behind the respective end of the main valve piston is connected to the 2 working connections of the preliminary stage like a cylinder.
Overlap
The control edges are the parts on the valve piston that close or open the openings to the working line connections A and B. You are ultimately responsible for the flow through the valve. With proportional valves, they are slightly above the opening of the working line on both sides, here it is above A. This overlap is referred to as positive overlap with directional control valves . It separates the different pressure areas and thus seals. When opening, the dead zone must first be overcome, then the proportional valve opens slowly. This is why standard proportional valves usually have a non-linear volume flow characteristic that is only suitable to a limited extent for control tasks. For example, the dead zone of the overlap must be compensated for by the control system.
Sources, literature
- Hydraulic Proportional and Closed Loop System Design (PDF; 4.7 MB), by Neal Hanson .., Bosch Rexroth AG, 2011. Product overview of proportional and servo valves
- CHAPTER 12: Infinitely Variable Directional Valves , 2007, Website Hydraulics & Pneumatics
- Proportional hydraulics - Learning System for Automation and Communications (PDF; 1.3 MB), Festo Didactic KG, Chapter 2: Proportional valves - Design and mode of operation
Individual evidence
- ↑ Proportional hydraulics - Learning System for Automation and Communications (PDF; 1.3 MB), Festo Didactic KG, part of Chapter 2: Actuation of pressure, flow control and directional control valves, Figure 2.3a to Figure 2.6. The electromagnet presses against the fluid pressure using a spring, e.g. B. as a preliminary stage in an electrically adjustable pressure regulator.
- ↑ Product group data sheet PVQ (PDF; 530 kB), company SMC. See also section #Single-stage pneumatic proportional throttle valve in the text and reference there
- ↑ Data sheet EV260B (PDF; 453 kB), Danfoss. See also section #Pilot-controlled proportional throttle valve for low-pressure water in the text and reference there
- ↑ Product group data sheet Compact Proportional Solenoid Valve - Series PVQ (PDF; 530 kB), company SMC. Data sheet for single-stage pneumatic proportional throttle valves for air and vacuum. There is a sectional view for 2 of them, each with a coil.
- ↑ Data sheet 5/3-way proportional valve 152914 (PDF; 118 kB) and [1] (PDF; 328 kB) for MPYE-5-1 / 8, Festo Didactic or Festo. Data sheet for a single-stage pneumatic proportional directional valve with 2 adjacent coils and position transducer.
- ↑ Data sheet EV260B (PDF; 453 kB), Danfoss: a 2/2-way proportional solenoid valve with diaphragm for water and oil up to 10 bar with removable solenoid.
- ↑ Oil hydraulics, by Gerhard Bauer, 2011, chapter control valves, Figure 8.21 Direct operated control valve with symbol .., page 179. ISBN 9783834814647
- ↑ Product group data sheet Proportional directional control valves and throttle valves without feedback - KBTG-4V-5, KBDG-4V-5 , Eaton / Vickers, 2005, translation. Data sheet for single-stage hydraulic proportional throttle valves and proportional directional control valves. There is a partial sectional view of one of the latter with 2 coils.
- ↑ Product group data sheet Proportional directional control valves with position transducer - KFTG4V-5, KBFTG4V-5, KFDG4V-5, KBFDG4V-5 , company Eaton / Vickers, translation. Data sheet for single-stage hydraulic proportional throttle valves and proportional directional control valves. There is a sectional view of one of the latter with 2 coils and displacement transducer.
- ↑ Product group data sheet Proportional Directional Valves, Two-Stage - K (A) FDG5V-5/7/8, K (A) HDG5V-5/7/8 (PDF; 277 kB), Eaton / Vickers, 1998. Data sheet for 2-stage hydraulic proportional directional control valves. There is a partial sectional view of one of them with a coil and 2 displacement sensors. Off-center zero position is visible in the KHDG5V detail symbol.
- ↑ Dieter Will: Hydraulics - Basics, Components, Circuits , e- ISBN 978-3-642-17243-4 , p. 253.
- ↑ Dieter Scholz: Proportional hydraulics (PDF; 283 kB). Festo Didactic, Denkendorf 2002, comparison of proportional and servo hydraulics (German reading sample).
- ^ Karl-Erik Rydberg: Hydraulic servo systems. ( Memento of the original from May 20, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.6 MB) Linköpings universitet, 2008, Chapter 3.2 Types of valve center. With a schematic valve cross-section and the associated volume flow characteristic, which shows the dependence of the piston position xv on the flow rate (and flow direction) Ql.
