HYDRAULIC PROPORTIONAL PRESSURE MAGNET VALVE

Abstract: An hydraulic proportional pressure magnet valve comprises a control piston (2) which can reciprocate in a boring in a valve housing (1) having an inlet port (9), an outlet port (10) and a load port (11). A solenoid magnet (3) having a declining characteristic curve for the force acts on the piston in a direction corresponding to an opening of the inlet port, and the pressure in the load port (11) provides a counter-force in the opposite direction. The piston is movable without being influenced by mechanical springs, and the control edges (4) of the control piston (2) have a positive overlapping with the inlet port (9) and the outlet port (10).

Description: Hydraulic proportional pressure magnet valve The invention relates to a directly controlled, self- adjusting, three-way proportional pressure magnet valve comprising a valve housing having a boring, an inlet port, an outlet port and a load port, a sliding piston having control edges and a solenoid magnet providing a force on the piston in a direction corresponding to an opening of the inlet port and a closing of the outlet port, the piston being biased in the opposite direction by the pressure in the load port. The valve can be used in systems involving relatively high pressures of 30 to 150 bar or even higher for applications where high accuracy, small hysteresis, quick response and small leakage are required. Hydraulic proportional pressure magnet valves are valves in which the hydraulic pressure in the load port can be adjusted approximately proportional to a control value, such as the power of an electric current to a solenoid magnet. In hydraulic systems operating at high pressure it is diffi¬ cult with the known proportional pressure valves to control a single-acting piston against a spring in such a manner as to obtain high accuracy, small hysteresis, quick response and small leakage, all at the same time. As a classic solution to such a task there has been used a pilot valve which controls the piston of the main proportional valve. All these pilot controlled valves, which exist in various embodiments in the market, have disadvantages in that they have either poor accuracy, large hysteresis (in a graph of magnetic current against pressure) , slow response or too high leakage. Additionally, many embodiments are vulnerable with respect to dirt. Another possibility of obtaining the same result is to use a directly controlled proportional valve having a piston. In such valves one end of the piston is biased by an armature and the other end by a mechanical spring. Through a passage in the valve housing or the piston the pressure in the load port may be transmitted to the end of the piston in order to act thereon in the same direction as the spring, i.e. in the direction corresponding to a closing of the inlet port. This hydraulic pressure and the spring force counteracts the force from the armature on the other side, so that the piston is balanced and the valve becomes self-adjusting. All previously known solutions employ at least one spring stabilizing the main piston. The advantage of a spring is that already prior to an increase in the pressure a certain current in the solenoid windings is required, whereby a stable pressure characteristic from zero is obtained. By replacing the spring with another spring having a different characte¬ ristic curve it is furthermore possible to obtain an adap¬ tation to various ranges of pressure control. On the other side either a larger hysteresis (in the graph for the magnet current versus pressure) or a larger leakage is obtained. The heat development in the magnet is also increased, which in turn adversely influences the hysteresis. Additionally, the valve must be tuned in accordance with the actual charac¬ teristic curve of the spring, which increases the production costs. Further, the characteristic curve of the spring varies with the temperature. Other disadvantages are based on the fact that a spring involves a slower response and instability problems (mass oscillations) and consumes part of the magnet force. Another reason that a spring has been regarded as indispensable, may be that the hydraulic counter-force co¬ operating with the spring can only be obtained with a certain delay which depends on the volume of the hydraulic fluid on the load side. Without a spring too large amplitudes of the control piston may therefore easily occur. The object of the invention is to provide a proportional pressure magnet valve which is devoid of the disadvantages referred to above, for use in an operational range which makes the characteristic curve of the valve in the lower part of the pressure range of no concern. According to the invention it has surprisingly been estab¬ lished that it is possible to do without mechanical springs by incorporating the counter-force which is otherwise provided by a spring, into the characteristic curve (force versus the armature position at constant current) of the solenoid magnet. Thus, the proportional pressure magnet valve according to the invention is characterized in that the characteristic curve of the solenoid magnet, i.e. the force exerted versus the position of the armature at constant current, is declining in the direction of positive force over the entire effective operational area in order to provide a stable balance with the counter-force from the load pressure and thus a self- adjusting valve without the assistance of mechanical springs. Thereby, the movements of the control piston may be kept within reasonable limits in spite of the fact that the counter-pressure requires some time to develop. To obtain the desired automatic adjustment of the piston in a well defined, stable position the characteristic curve must be distinctly declining. A preferred minimum value for the decline over the operational area used is 1 N/mm. On the other hand the decline should not be too steep, since the return forces can then be too strong and cause over- steering and oscillation phenomena. A suitable maximum value for the decline in the force is 5 N/mm. Another important feature of the invention is the use of a positive overlapping of the outlet port (reservoir port) and the inlet port (pump port) by the control edges of the piston. This involves that the distance between the control edges of the piston is smaller than the distance between the inlet port and the outlet port. The positive overlapping reduces leakage through the valve, which allows use of smaller pressure pumps. Previously known proportional pressure magnet valves have a distance between the control edges corresponding with very close tolerances to the distance between the ports. However, it must be added that the positive overlap is rather small, preferably smaller than 1 mm, which is still suffi¬ cient to make it unnecessary " for the tolerance of the distance between the control edges of the piston to be kept so small as to create problems in the machining of the piston or the ports. The advantages achieved according to the invention specifi¬ cally consist in that the valve reacts with a minimum time lag and has a great accuracy and small leakage or consumption, respectively. The production costs of the valve are reduced because the positive overlapping of the control edges allows the manu¬ facture to take place with larger tolerances without the hysteresis becoming unduly large. Since no mechanical spring forces are required, mass pro¬ duction of the valve is simplified. Also, a time consuming adaptation and adjustment of tolerance sensitive springs are avoided. An adjustment mechanism will also be redundant. It is of special interest that the valve according to the invention has good stability margins at pressures up to 200 bar. An adaptation of the valve to different pressure ranges is effected in a simple manner, in fact merely by changing the piston diameter on which the load port pressure acts. Practical embodiments involve the use of a piston having an extension of a smaller diameter at the end. This extension operates within a boring of corresponding diameter in the valve housing. Alternatively, a boring can be provided in the piston, a plunger engaging the valve housing extending into the boring. In such a case, the load port must communi¬ cate with the boring in the piston. According to still another alternative embodiment the diameter of the piston at the control edge for the outlet port (reservoir port) can be larger than the diameter at the control edge for the inlet port (pump port) . An embodiment of the invention is illustrated in the drawing and will now be described in more detail. Fig. 1 illustrates an axial section through a proportional valve according to the invention. Fig. 2 illustrates magnet force characteristic curves, showing the magnet force versus the armature position with the current as a parameter. A piston 2 which has two control edges 4 and an extension 5, can reciprocate in a boring in a valve housing 1 and is activated by a solenoid magnet 3 through an armature rod 6. The valve housing has an inlet port 9 (pump port) and an outlet port 10 (reservoir port) as well as a load port 11 which leads to the device (not shown) the pressure load of which is to be adjusted. In the valve housing 1 there is provided a passage 7 from the load port 11 to the end of the boring in the valve housing into which the extension 5 extends. Alternatively, this connection may be provided by a passage 8 through the piston 2. The pressure in the load port will thereby provide a counter-force to the force from the solenoid magnet 3. The solenoid magnet 3, which operates in conjunction with the counter-pressure on the piston extension 5, has a charac¬ teristic force curve as shown in Fig. 2. The proportional valve is completely without mechanical springs, which means that disturbing mass oscillations are largely avoided. This is possible because the characteristic curve is declining in the direction of a positive force. Pressure medium is supplied through the inlet port 9 when the magnet current and thereby the magnet force increases, so that the piston is shifted to the right in the Figure. The pressure in the load port 11 will then increase. When the magnet forces are reduced the piston will be shifted to the left, and pressure medium will then be discharged through the outlet port 10, thus reducing the pressure in the load port 11. When the force on the piston extension 5 due to the pressure in the load port 11 supplied through the passage 7 is equal to the force exerted by the solenoid magnet 3, the piston will be balanced to a centre position, and the pressure in the load port 11 will be proportional to the magnet force. Stabilization of the piston 2 in this position is obtained because of the declining magnet force characte¬ ristic curve in the direction of positive force. This charac¬ teristic curve makes the valve rather unsensitive to vari¬ ations with respect to overlapping of the control edges 4 of the piston with the inlet port 9 and the outlet port 10, respectively. Such overlapping involves reduced leakage or consumption, respectively, and the tolerance of the distance between the control edges 4 on the piston 2 can be larger. The operational area of the piston will only be 1-2 mm, and the overlapping must be even smaller. The overlapping illu¬ strated in Fig. 1 is therefore somewhat exaggerated for sake of clarity, and the scale in the direction of movement is of course many times larger in Fig. 2 than in Fig. 1. The use of a solenoid magnet having a declining characteristic curve as a function of the armature position and the elimi¬ nation of mechanical springs result in fewer parts and thereby a simpler and cheaper valve. Whereas a mechanical spring changes its spring characteristics when the temperature changes, the valve according to the invention is rather insensitive to temperature changes. The piston diameter can be chosen without having to provide room for a spring. Pro¬ blems with mass oscillations are reduced, and the hysteresis is improved, which provides a quick response. It must be regarded as very surprising that these advantages can be obtained in such a simple manner. The only explanation is that it cannot have been near at hand to incorporate the effect of the spring into the characteristic curve of the solenoid magnet in order to obtain the advantages of the spring without its disadvantages. The valve according to the invention solves tasks which no prior valve has solved, and opens for completely new applications of such valves for precision control.

Claims: PATENT CLAIMS 1. Directly controlled, self-adjusting, three-way proportional pressure magnet valve comprising a valve housing (1) having a boring, an inlet port (9) , an outlet port (10) and a load port (11) , a sliding piston (2) having control edges (4) and a solenoid magnet (3) providing a force on the piston (2) in a direction corresponding to an opening of the inlet port and a closing of the outlet port, the piston being biased in the opposite direction by the pressure in the load port (11), c h a r a c t e r i z e d i n that the characteristic curve of the solenoid magnet, i.e. the force exerted versus the position of the armature at constant current, is declining in the direction of positive force over the entire effective operational range in order to provide a stable balance with the counter-force from the load pressure and thus a self-adjusting valve without the assistance of mechanical springs. 2. Proportional valve according to claim 1, c h a r a c ¬ t e r i z e d i n that the characteristic curve is de¬ clining at 1-5 N/mm over the effective operational range. 3. Proportional valve according to claim 1 or 2, c h a r a c t e r i z e d i n that the distance between the control edges (4) of the piston (2) are smaller than the distance between the inlet port (9) and the outlet port (10), thus providing a positive overlap at both ports. 4. Proportional valve according to any of the claims 1 to 3, c h a r a c t e r i z e d i n that the piston has a extension (5) of smaller diameter which is movable in a corresponding boring in the valve housing (1) , or a boring into which a plunger engaging the valve housing (1) extends, the load port communicating with the said corresponding boring in the valve housing (1) or the boring in the piston (2) , respectively. 5. Proportional valve according to any of the claims 1 to 3, c h a r a c t e r i z e d i n that the diameter of the control edge (4) cooperating with the outlet port (10) is larger than that co-operating with the inlet port (9) , and that these control edges operate in portions of the boring in the valve housing having corresponding diameters.

Inventors: ERVIK KJELL STEINAR L, ERVIK, KJELL, STEINAR, L.
Applicants: KONGSBERG AUTOMOTIVE, KONGSBERG AUTOMOTIVE A/S
Application year: 1988
Main classification: G05D16
IPC codes: F16K31, G05D16