In connection with this valve, the points applicable to the following proportional directional valves will also be discussed by way of example such as hysteresis, repetition accuracy, control spool, basic principles tor characteristic curves and the time characteristics of the control spool.
The proportional solenoid acts directly on the control spool in the same way as a conventional directional control valve.
In the case ol the stroke-controlled solenoid (Fig 10). the position of the armature is controlled by a ciosed-loop control circuit and maintained irrespective of the counter-pressure. provided it is within the rated working range of the solenoid.
With the stroke-controlled solenoid, the spools of proportional directional, flow as well as pressure control valves can be directly operated, and be controlled in any stroke position The stroke ot the solenoid is between 3 and 5 mm depending on the size.
As already mentioned, the stroke-controlled solenoid is primarily used for directly operated 4-way proportional valves.
In conjunction with the electncal feedback, the hysteresis and the repetition error of the solenoid are maintained with very tight tolerances. In addition, any flow forces. which occur at the valve spool are compensated (or {relatively small solenoid force in relation to the interfering forces).
In the case of pilot operated valves, the controlled hydraulic pressure is applied to a relatively large control area. The available positioning forces are therefore considerably greater and the percentage effect of interfering forces is not so marked. For this reason, pilot operated proportional valves can be implemented without electrical feedback.
Proportional solenoids represent the linking element between electronic and hydraulic. The proportional solenoids are a form of DC linear solenoids. Proportional to the electrical current as the input variable, the produce force and travel as the output variable.
Corresponding to the practical application, a differentiation is made between:
- Solenoids with comparatively linear stroke/current relationship over a reasonably long stroke length, the so-called “stroke controlled solenoid”.
- Solenoid with particularly defined force/current relationship over a very short stroke, the so-called ” force controlled solenoid”.
Only DC linear solenoid can be used for the current-proportional changes in the output variable force and stroke. Due to their stroke dependent current consumption, AC solenoid must assume their final stroke position as soon as possible.
This combination unit, can be used as a throttle (orifice) or in conjunction with a pressure compensator for controlling high flows The applications include, for example. control systems for presses and plastic processing machines Despite the high flow rates, the unit has a fast response time.
The 2-way throttle valve is an orifice with its opening stroke determined by an electrical signal The throttle valve is supplied as a unit ready for installation with installation dimensions to DIN 24 342 The bushing (2) is screwed into the cover (t) together with the orifice spool (3) as well as the positional transducer (4) and the pilot control (5). including proportional solenoid (6).
The direction of flow is from A to B. The pilot oil port X is linked to the port A The pilot oil outlet Y should be routed to the tank at zero pressure.
At zero signal (no current applied to proportional solenoid (6)) the pressure in port A acts via the pilot line X and the control spool (10) in addition to the spring in chamber (8). The orifice spool (3) is held closed If a signal is led to the amplifier card, the command signal (external signal) is compared In the amplifier (7) with the actual signal (feedback of the transducer signal) The proportional solenoid (6) is energized with a current corresponding to the differential value.
The solenoid shifts the spool (10) against the spring (11). As the result of interaction between the throttling points (13) and (14), the pressure in the spring chamber (8) is set such that the spring-loaded orifice spool (3) assumes a position corresponding to the preset signal and therefore determines the flow.
The orifice spool closes automatically (for safety) in the case of power failure or cable breakage The components of the closed loop position control are designed such that the command signal and the stroke of the orifice spool (3) are directly proportional with respect to each other Consequently,for constant pressure differences the orifice, the flow from A to B is only dependent on the stroke of the orifice spool and the window geometry (9)
Direct proportionality between the signal and flow is applicable to the system with linear opening law (FE..C1X/L). The quadratic opening law (version FE..C1X/0) signifies a volumetric flow increasing qua-dratically with the command signal
The 2-way proportional flow control valve can control, independent o’ pressure and temperature, oil flow specified by the electrical signal. The most important components are the housing (t). the proportional solenoid with inductive positional transducer (2), the measuring orifice (3). the pressure compensator (4) as well as the optionally installed non-return valve (5).
The oil flow setting is determined by an electrical signal (command signal) set at a potentiometer. In conjunction with the electronic control (e.g. amplifier, type VT 5010). this set signal results in a corresponding current and therefore a proportional stroke of the proportional solenoid (stroke-controlled solenoid) Correspondingly, the measunng orifice (3) is shifted downwards, thereby releasing an opening to flow The position of the measuring orifice is fed back by the inductive positional transducer. Any deviations from the command signal are corrected by the closed loop control. The pressure compensator maintains the pressure drop at the measuring orifice at a constant value The oil How is therefore independent of load Good design of the measuring orifice ensures a low temperature drift
The measuring orifice is closed when the command signal is 0 % The measuring orifice closes in the case of power failure or cable breakage at the electrical positional transducer.
Starting without jump is possible from zero signal. The measuring orifice can be opened and closed with a delay via two ramps in the electrical amplifier.
Free return flow from B to A is possible via the non-return valve (5).
Pilot controlled valves are used for applications involving higher How rates.
The valve consists of the pilot valve (1) with proportional solenoid (2), optionally with integrated max. pressure safety control (3) and main valve (4) with main spool (5).
The basic function corresponds to that of the “normal” pilot controlled pressure relief valve. The difference lies in the pilot control section. In this case, the compression spring has been replaced by the proportional solenoid. II is a “force-controlled” proportional solenoid A proportional lorce acting on the pilot poppet (6) therefore corresponds to a certain current, specified by the electronic control. A higher input current signifies a greater solenoid force and therefore higher pressure setting; a lower input current signifies a lower pressure setting. The pressure applied by the system (port A) acts on the main spool (5). At the same time, the system pressure is applied to the spnng-loaded side of the main spool (11) via the pilot line (10) equipped with the orifices (7,8,9). This system pressure acts on the pilot poppet (6) via the orifice (12) against the force of the proportional solenoid (2). The pilot poppet (6) opens when the system pressure increases above the set value corresponding to the solenoid force. The pilot oil can flow to the tank via the port Y (13). It should be noted that flow from port Y is always at zero pressure.
Due to the orifice combination in the pilot control line, a pressure drop now occurs at the main spool (5) such that it is raised from the seat and opens the connection from A to 0 (pump – tank).
To safeguard the system against impermissibly high currents at the proportional solenoid (2) which would inevitably result in impermissibly high pressures, a spnng-loaded pressure relief valve can be installed as an additional option in the form of a max. pressure safety control (3). This valve can also be used to safeguard the pump.
When setting the pressure for the max. pressure safety control, a certain setting clearance with respect to the max pressure setting must be maintained at the proportional solenoid to ensure that it really only responds at pressure peaks.
As a reference value, this “safety range” should be approx. 10 % of the max. operating pressure.
The proportional pressure relief valve is designed as a poppet valve It basically consists of the housing (1), proportional solenoid (2) with inductive positional transducer (3), valve seat (4), valve poppet (5) as well as compression spring (6)
The proportional solenoid is a position controlled solenoid. In this case, it replaces to a certain extent the manual setting by means of the adjusting spindle.
A signal provided via the amplifier results in a stroke at the solenoid proportional to the signal. This preloads the compression spring (6) via the spring plate (7) and presses the poppet against the seat. The position of the thrust pad (i.e. solenoid armature) and therefore indirectly the pressure setting is recorded by the inductive positional transducer and monitored by the electronic control system in a closed loop position control Any control deviations from the signal are corrected by the closed loop control system. Solenoid friction is thus compensated resulting in a high precision, repeatable pretensioning force of the spring: hysteresis 1 % referred to max. setting pressure, repetition accuracy 0.5 % referred to max setting pressure.
The max setting pressure depends on the pressure rating (25 bar, 1S0 bar, 315 bar). The various pressure ratings are achieved by different valve seats, i.e. with different seat diameters Since the solenoid force remains constant, the highest pressure rating has the smallest diameter.
By way of example for the pressure rating 25 bar (Dtag 12), it can be seen from the characteristic curves that the maximum setting pressure also depends on flow.
At command signal 0 – power failure to the proportional solenoid or cable breakage at positional transducer – the lowest setting pressure is assumed. (Dependent on the pressure rating and flow).
The spring (8) must also be mentioned in this connection It ensures, at signal 0. that the moving parts, such as armature, are shifted back in order to always achieve the minimum value p^. It also serves the purpose of compensating the weight of the armature when the valve is installed vertically
The solenoid force is controlled by the change in current I in the lorce-controlled solenoid without the armature of the solenoid performing a measurable stroke.
Due to current feedback in the electrical amplifier, the solenoid current and therefore the solenoid force are kept constant even if resistance changes.
The main feature of the force-controlled proportional solenoids is the characteristic force-stroke curve.
The solenoid force remains constant over a defined stroke range at constant current.
The stroke for the solenoid shown in this example is approx. 1 5 mm. The solenoid is used in this range.
The force-controlled solenoid is of compact design due to the short stroke. In view of this short stroke, the force-controlled solenoid is used particularly for pilot-operated proportional directional and pressure control valves with the solenoid force being converted Into hydraulic force. The proportional solenoid is a controllable “wet pin” DC linear solenoid contained in an oil bath.
