Proportional valves

Continuously adjustable pressure, flow and directional valveswhich convert an adjustable input signal into a proportional hydraulic or pneumatic output signal and are generally actuated by a proportional solenoid .In terms of distinguishing these from other types of continuously adjustable valves such as servo valves, there are no clear specifications or binding standards. Proportional valves are suitable for use in an open loop control system as an actuator.The feedback regarding the position of the control element is provided either by a spring (barometric feedback) or electrically via an inductive displacement transducer (Figure P 24). This electric feedback improves the behaviour of proportional valves significantly in terms of better static behaviour, smaller hysteresis and greater stiffness, response sensitivity and reproducibility.Proportional valves — unlike servo valves — are not particularly sensitive to contamination (e.g. recommended filter unit 25 μm compared with 3...5 μm) and therefore more durable from an operational perspective. The various groups are distinguishable in terms of their stationary behaviour:Proportional pressure control valves,proportional flow valves,proportional directional valves,proportional insert valves.Proportional pressure control valves:Manufactured as pilot valves whereby, compared with normal pressure valves the setting screw is replaced with a distance-controlled proportional solenoid (Figure P 25). With force-controlled solenoids, this acts on the cone directly (Figure P 26). This means the response pressure is specified by a proportional electric current. If more than one pressure level is required, a number of potentiometers will be used accordingly. Proportional valves are increasingly fitted with electrical feedback.Proportional flow valves:As in conventional hydraulic systems , there are throttle valves (Figure P 27) and 2-way and 3-way flow control valves. Here, the throttle (measuring throttle) in the form of a 2/2-way or 4/2-way valve is actuated by a proportional solenoid. Proportional valves are only used relatively infrequently, as their function is mostly performed by proportional directional valves.Proportional direction valves:Generally designed as 4/3-way valves and directly controlled in NG 6 and NG 10 (Figure P 28). In addition, NG 6 valves are used as pilot valves. Continuous adjustment allows the proportional valve to perform a throttle function as well. This is why its control pistons feature fine control grooves . If required, a pressure compensator can be located downstream so that a flow control function is achieved in addition to the directional function.Proportional valves are manufactured with positive overlap (if they have zero cut, they are called high response proportional valves ). As such, they feature a dead zone — in the range where the control current is around ±200 mA — where the control piston is already moving without any flow rate taking place. As well as permitting this overlap, the relatively large stroke of a proportional solenoid also ensures the control piston still has sufficient distance for the proportional valves to be operated with a pressure drop of 5 bar for each control edge at the nominal flow . Nevertheless, natural frequencies of 20 to 50 Hz occur at 90° phase lag are the result. If this proves to be insufficient, high response proportional valves can be used.For pilot actuation purposes, proportional valves with larger nominal sizes use either two proportional pressure valves or one proportionaldirectional valve NG 6 and a normal directional valve as a main stage (Figure P 29).In addition, proportional valves that use a servo valve for pilot actuation are also offered. This combination is also called a high response proportional valve.For electric feedback, it has proven to be advantageous when both valves contain a displacement transducer (see Figure P 27).Proportional insert valves:Control based on proportional valves also increases the range of possible applications for 2-way and 3-way insert valves. Here too, a great deal of importance is attached to the fine control range (Figure P 30).Figure P 24: Electric position feedback in a proportional valve Figure P 25: Proportional pressure valve with electric feedback Figure P 26: Pilot-controlled proportional pressure-limiting valve Figure P 27: Proportional throttle valve Figure P 28: Directly controlled proportional directional valve Figure P 29: Proportional directional valve, pilot-controlled with directional valveFigure P 30: Proportionally controlled 2-way insert valve