GB2103390A

GB2103390A - Hydraulic system with proportional electrical control

Info

Publication number: GB2103390A
Application number: GB08218210A
Authority: GB
Inventors: Giulio Dotti; Ugo Costelli; Luciano Moretti
Original assignee: Centro Ricerche Fiat SCpA
Current assignee: Centro Ricerche Fiat SCpA
Priority date: 1981-07-01
Filing date: 1982-06-23
Publication date: 1983-02-16
Also published as: FR2509009B1; JPS5821074A; DE3224189C2; FR2509009A1; IT1144260B; DE3224189A1; JPH0369379U; GB2103390B; IT8167914A0

Abstract

A hydraulic system with proportional electrical control is described comprising a hydraulic servo-valve including a pilot chamber 14 and a pilot valve 25 in which a duct (18) communicates with the pilot chamber (14) and is connected to a source of fluid under pressure through an orifice (23) of narrow cross-section, an aperture (15) is connected to a discharge tank, and a passage (19,20) puts the duct (18) in communication with the discharge aperture (15) and the pilot valve (29) cooperates with a seat 28 is characterised in that the seat cooperating with the shutter (29) of the pilot valve is located in a zone of the body (3) of the pilot valve which is within an armature (7) of the electromagnet (4) and the movable core (12) controls the shutter (19) directly without the aid of elements interposed axially between the movable core and the shutter. <IMAGE>

Description

SPECIFICATION Hydraulic system with proportional electric control The present invention relates to a hydraulic system with proportional electrical control of the type comprising a hydraulic servo-valve including a pilot chamber and a hydraulic pilot valve for modulating the pressure in the pilot chamber of the hydraulic servo-valve.

More particularly, the invention relates to a hydraulic system of the type specified above in which the pilot valve is electromagnetically operated and comprises: a body having a duct communicating with the pilot chamber and connected to a source of fluid under pressure through an orifice of narrow cross-section, an aperture intended for connection to a discharge tank, and a passage for putting the duct in communication with the said discharge aperture, a shutter cooperating with a seat formed in the body in correspondence with the said passage, and an electromagnet, including an armature connected to the pilot valve body, a winding, and a movable core urging the shutter into its closed position against the said seat with a force proportional to the strength of the current supplied to the winding.

Pilot valves of the type indicated above have found various applications in different types of apparatus such as, for example, presses, machine tools, machines for working rubber, iron industry plants, machines for working plastics materials, and generally in all applications in which it is desired to modulate the pressure in a hydraulic circuit.

The valves allow the pressure in the pilot chamber of the servo-valve to be varied proportionally by varying the current supplied to the winding of the electromagnet. Thus the current variation causes a proportional variation in the force biassing the shutter against its seat and controlling the communication between the said duct and the discharge tank.

Valves of this type which have been made up to now, however, have a relatively complicated structure, and are expensive, heavy and bulky, disadvantages which make them poorly suited for use in the motor vehicle industry and in particular, for example, in braking systems, transmission systems, and in engine supply systems.

An object of the present invention is the provision of a hydraulic system with proportional electrical control which enables the aforesaid disadvantages to be overcome.

With this object in view the present invention provides a hydraulic system of the type referred to characterised in that the seat cooperating with the said shutter is located in a zone of the pilot valve body which is within the armature of the electromagnet and in that the shutter is acted upon directly by the movable core of the electromagnet.

The invention allows a valve to be made with a simpler structure from a smaller number of parts which is more economical, less bulky and lighter than valves of this type which have been made up to now. In such known valves the seat cooperating with the shutter which controls the communication with the discharge tank is loated in a zone of the body of the pilot valve outside the armature of the electromagnet and the shutter cooperating with the seat is controlled indirectly by the movable core of the electromagnet through elements interposed axially between the movable core and the shutter.

In a first embodiment of the hydraulic system according to the invention, the narrow-section orifice which connects the duct to the source of fluid under pressure is formed in the body of the pilot valve. In a second embodiment, this orifice is, however, formed in the hydraulic servo-valve in a zone such that the pilot chamber is interposed between the said orifice and the duct.

A preferred embodiment of the hydraulic system according to the invention has the following further characteristics: the movable core of the electromagnet is slidable within a support bobbin for the winding; the pilot valve body is essentially in the form of a cylindrical element and has one end inserted within one end of the support bobbin for the winding; this end of the said valve body, together with the end of the movable core facing it, defines a main chamber within the support bobbin; the said duct formed in the pilot valve body opens into the said main chamber; the said passage connecting the duct with the discharge aperture is constituted by the said main chamber to the discharge aperture, and the seat for the shutter is constituted by the end of the duct which opens into the main chamber.

Preferably the portion of the valve body which projects out of the electromagnet is inserted in a seat in the body of the hydraulic servo-valve to achieve a coupling between the servo-valve and the pilot valve. This permits the pilot valve body to perform, simultaneously, different functions: it in fact acts both as the body of the valve itself (in which the said duct and the seat for the shutter are formed) and as the element for coupling with the servo-valve. Moreover, the pilot valve body is preferably of ferromagnetic material, it therefore also assumes the function of attracting the movable core of the electromagnet towards itself upon excitation of the winding.

Further characteristics and advantages of the valve according to the present invention will emerge from the following description with reference to the appended drawings, provided purely byway of non-limiting example, in which: Figure 1 is a partially-sectioned view of a first embodiment of a hydraulic system according to the invention, Figure 2 is a partially-sectioned view of a second embodiment of the hydraulic system according to the present invention, and Figure 3 is a diagram iliustrating the pressure/current characteristic of a practical embodiment of a pilot valve forming part of the system according to the invention.

In Figure 1, reference numeral 1 indicates the body of a hydraulic servo-valve having a cylindrical bore 2 within which a portion of the body 3 of a pilot valve is slidable. The body 3 is cylindrical and is made of ferromagnetic material. The pilot valve further includes an electromagnet 4 having a winding 5 supported by a tubular bobbin 6. The bobbin 6, which is of non-ferromagnetic material, is located within an armature 7 of ferromagnetic material comprising a cup-shaped element 8 having a base wall 8a and provided at its open end with a plate 9 having a central aperture. Between the ends of the bobbin 6 and the bottom 8a on the one hand and the plate 9 on the other hand there are interposed annular plates 10. Both the cup-shaped element 8 and the plates 9, 10 are of ferromagnetic material.

Within the tubular winding support bobbin 6 a ferromagnetic core 12 is slidably mounted, with the interposition of a sleeve 11 of non-ferromagnetic material.

The end of the body 3 of the pilot valve which is opposite that inserted in the bore 2 in the valve body projects through and is located within the plate 9, the plate 10 and the adjacent end of the sleeve 11.

The body 3 ofthe pilot valve has an axial through duct 18 communicating at one end through an orifice 13 with a pilot chamber 14the pressure in which isto be modulated by means ofthe pilot valve. The bore 18 communicates through a restrictor orifice 23 of limited axial extend with a radial hole 21 formed in the body.

This hole 21 in its turn communicates with a port 22 formed in the valve body 1 and intended to be connected to a source of fluid under pressure.

The body 3 has a peripheral groove which defines within the bore 2 an annular chamber 16. The body 3 also has a discharge aperture 15 which communicates through the annular chamber 16 with a port 17 in the body 1, connected to a discharge tank (not shown).

The body 3 further includes a flow passage interconnecting the duct 18 and the discharge aperture 15. This passage is formed by a main chamber 19 (defined within the sleeve 11 by the facing ends of the body 3 and the movable core 12) into which the end of the duct 18 opposite the pilot chamber 14 opens, and at least one auxiliary duct 20 connecting the main chamber 19 to the discharge aperture 15.

The body 3 is mounted within the bore 2 with the interposition of a series of annular sealing rings 24 housed in respective peripheral grooves formed in the body 3. The body 3 further has a peripheral groove 25 outside the body 1 for the location of a bracket 26 fixed by screws 27 to the body 1 and arranged to lock the body 3 of the pilot valve relative to the body of the servo-valve 1.

At that end of the duct 18 which opens into the main chamber 19 there is mounted a sealing bush 28 the internal edge of which facing the chamber 19 defines a seat for a ball shutter 29 which upon excitation of the winding 5 is urged against the seat of the movable core 12 of the electromagnet.

At the end of the sleeve 11 opposite the body 3 a plug 30 is force-fitted. The plug 30 is provided with a peripheral groove in which a retaining ring 31 is located, the retaining ring 31 being in contact with a spacer ring 32.

The plug 30 defines, with the end of the movable core 12 facing it, an auxiliary chamber 33 within the sleeve 11. The auxiliary chamber 33 communicates with the main chamber 19 through a series of axial peripheral grooves 34 (one only of which is shown) formed in the movable core 12. The core 12 also has an intermediate portion of smaller diameter which defines an annular chamber 35. By virtue of the structure described above, in operation, a film of fluid is formed in the chamber 35 and acts as a bearing, facilitating the sliding of the core 12 within the sleeve 11.

The body of the movable core 12 has an axial through bore in which a pin 36 is force-fitted. The pin 36 has two ends which project from the body of the core 12. The portion of the pin 36 which projects from the end of the core 12 facing the chamber 33 prevents the facing surfaces of the core 12 and the plug 30 from coming into contact with each other with the risk of hydraulic "sticking" of these surfaces. The pin 36 is of non-ferromagnetic material, of a hardness much greater than that of the movable core 12.

At that end of the pin 36 which projects from the body of the movable core 12 towards the main chamber 19 the core 12 has a cavity 37 which houses the ball shutter 29.

Between the facing ends of the plug 30 and the movable core 12 there is located a helical spring 38 the function of which will be made clear below.

The orifice 13 is formed if a restrictor plate 39 located at the end of the duct 18 which opens into the pilot chamber 14. The orifice 13 prevents the transmission pressure oscillations between the duct 18 and the chamber 14.

An element fixed to the movable member (not shown) of the servo-valve is indicated by reference numeral 40. The element 40 is subject to the pressure in the chamber 14, so that a variation of this pressure causes a variation in the position of the element 40. In Figure 1 there is also shown an inlet aperture port 41 of the servo-valve which communicates with an outlet port (not illustrated) through a flow passage controlled by the said movable member of the servo-valve.

The operation of the hydraulic system illustrated in Figure 1 is as follows.

The position of the element 40 fixed to the movable member of the servo-valve depends on the pressure in the pilot chamber 14. This pressure is transmitted to the pilot chamber 14 through the port 22 in the body 1 of the servo-valve, the restrictor orifice 23 of the hole 21 formed in the body 3 of the pilot valve, and the duct 18.

The ball shutter 29 is pressed against its seat by the movable core 12 of the electromagnet 4 upon excitation of the winding 5. If the pressure in the duct 18 increases sufficiently to overcome the force exerted by the movable core 12 on the shutter 29 the latter is lifted from its seat together with the movable core 12 and puts the duct 18 into communication with the aperture 15 connected to the discharge tank through the main chamber 19 and the auxiliary duct 20. The pressure in the duct 18 (and consequently in the pilot chamber 14) thus depends on the value of the force exerted by the movable core 12 on the shutter 29.

It follows that modulation of the pressure in the pilot chamber 14 (which causes a variation in the position of the movable member of the servo-valve) may be effected in a proportional manner by varying the intensity of the current supplied to the winding 5.

The arrangement described above, and in particular the fact that the seat for the ball shutter 29 is formed in a zone of the body of the pilot valve located within the winding 5 of the electromagnet 4, and the fact that this shutter is acted upon directly by the movable core 12 of the electromagnet, without any interposed elements, results in a simpler, more economical and less bulky valve than the proportional electrically controlled pilot valves which have been made previously.

Figure 2 (in which the parts common to Figure 1 are indicated by the same reference numerals) illustrates a second embodiment of the invention which differs from that shown in Figure 1 solely in that in this case the orifice of narrow cross section through which the duct 18 communicates with the source of fluid under pressure is formed in the movable member 40 of the servo-valve instead of in the body 3 of the pilot valve. In this case, moreover, the restrictor orifice 13 is omitted.

More particularly, the orifice of narrow cross section (indicated by the reference numeral 42) is formed in the wall of the movable member 40, in a zone such that the pilot chamber 14 is interposed between the orifice 42 and the duct 18. The orifice 42 communicates with a port 43 in the body 1 of the servo-valve intended to be connected to the source of fluid under pressure.

The operation of the valve illustrated in Figure 2 is entirely similar to that described above with reference to the valve of Figure 1.

A theoretical analysis of the operation of the hydraulic system according to the present invention, in the embodiment illustrated in Figure 1, permits the identification of the criteria for dimensioning the pilot valve which ensures a univocal relation between the intensity of the current supplied to the winding 5 and the pressure in the duct 18 (for each value of the current there must correspond one unique value of the pressure) and stability of the system.

The following parameters may be identified: P = supply pressure of the pilot valve (at the inlet port 22).

Q = ideal maximum pilot valve flow rate (ideal conditions occur when the pressure in the bore 18 is nil), p = fluid density Emax = P'max/P,where P'maX is the maximum design pressure in correspondence with the duct 18, Emin = P'mjn/P, where P'mjn is the minimum design pressure in correspondence with the duct 18, = = current value corresponding to Pmax (the action of the spring 38 is neglected), R internal radius of the sleeve 11, t = wall thickness ofthe sleeve 11, w = thickness of the armature facing the core (thickness of plate 10 + thickness of base wall 8a)

where d3 and Dare the diameter of the aperture of the bush 28 and the diameter of the ball shutter 29 respectively.

The following coefficient values may be found from experiment: V = coefficeint of dispersion of the electromagnet, = = permeability in air Cg = flow coefficient through the narrow section 23, C3 = flow coefficeint through the passage controlled by the shutter 29, and C's = flow coefficient through the passage within the bush 28.

Having fixed the values of the parameters specified above, the following characteristic dimensions of the value may be defined:

4 a = 8Q p; (a = diameter of the orifice 23) g fl 7 a5 -Emi = Cg2 ( 1 Emin ) g C'2E.

S c min 2 D = a5 /1- 2 = cr a X = Cs d5 (X = maximum distance of the 4 6 C5 core 12 from the body 3)

where N is the number of turns of the winding of the electromagnet.

The last relation lays down the value of a product of three sizes, whereby it is always possible to choose any two values of these sizes and to determine the third.

where h is the distance between the movable core of the electromagnet and the body 3 of the valve when the shutter 29 is pressed against its seat.

Naturally with reference to the last relation, it is necessary to choose values oft and w which allow values of h to be obtained which are not negative.

Finally:

where 1min is the value of the current corresponding to P'mjn.

In practice, it is convenient to use a value of the maximum strokeXof the movable core of the electromagnet which is greater that that determined by the relationship given above, in order to take account of working tolerances and to allow a greater flexibility of use of the pilot value when supply pressures greater than the design pressure occur.

The adoption of a value of a maximum strokeXg reater than its theoretical value could result on the other hand in a reduction in the electromagnetic force for a given current which would entail the risk, because of the friction in the system, that the movable core would not be able to initiate its displacement.

To solve this problem by increasing the intensity of the current supplied to the winding would be an error in that this would lead to a sharp increase accompanied by oscillations in the pressure in the duct 18.

The function of the helical spring 38 interposed between the plug 30 and the movable core 12 is precisely that of solving the said problem. This spring, when the distance between the movable core 12 and the body 3 of the valve is a maximum, has a load sufficient to balance the minimum pressure existing in the duct 18.

Hence, when there is the minimum design pressure in the duct 18, this pressure is balanced entirely by the spring and the intensity of current supplied to the winding of the electromagnet is substantially nil.

The use of the spring 38 furthermore ensures shorter response times and increases the stability of the system. The body 3 may even be made from non-ferromagnetic material. In this case it is indispensible, however, for the movable core 12 not to be located in the central position within the winding 5 when it presses the shutter 29 against its seat.

Example One example of the dimensions of a pilot valve according to the embodiment illustrated in Figure 1 is given below: Design data: P = 200 Nw/cm2 Q = 20cm3/sec; p = 9.10-6 Nw sec 2/cm4; Emin = 0.02; Emax = 0.99; R = 0.7 cm; t = 0.1 cm; w = 0.3; Imax = 1 Amp.; 6 = 0.8.

Values deduced from experiment: IL = 1.26.106 Nw/(Amp. x turn)2 y = 1.3; Cg = 0.78; C3 = 0.55; C3 = 0.66.

The following characteristic sizes are obtained: dg = 0.07 cm; d3 = 0.2 cm; D = 0.33 cm; X = 0.075 cm; N = 554 turns; h = 0.063 cm; Imin = 0.094 Amp.; P'max = 198 Nw/cm2; = 4Nw/cm2.

Figure 9 of the appended drawings illustrates the pressure/current characteristic of the said practical embodiment of the value illustrated in Figure 1.

Naturally, the constructional details of practical embodiments of the invention may be varied widely with respect to that described and illustrated purely by way of example, without thereby departing from the scope of the present invention.

Claims

1. A hydraulic system with proportional electrical control, comprising a hydraulic servo-valve including a pilot chamber and a pilot hydraulic valve for modulating the pressure in the pilot chamber of the hydraulic servo-valve, in which the pilot valve comprises: a body having a duct communicating with the pilot chamber and connected to a source of fluid under pressure through an orifice of narrow cross section, an aperture for connection to a discharge tank, and a passage for putting the duct into communication with the said discharge aperture; a shutter cooperating with a seat formed in the body in correspondence with the said passage, and an electromagnet, including an armature attached to the pilot valve body, a winding and a movable core biassing the shutter into its closed position against the said seat with a force proportional to the strength of the current supplied to the winding, wherein the seat cooperating with the said shutter is located in a zone of the pilot valve body which is within the armature of the electromagnet the shutter being acted upon directly by the movable core of the electromagnet.

2. Hydraulic system according to Claim 1, in which the orifice of narrow cross-section which connects the duct to the source of fluid under pressure is formed in the body of the pilot valve.

3. Hydraulic system according to Claim 1, in which the orifice of narrow cross-section which connects the duct to the source of fluid under pressure is formed in the hydraulic servo-valve and in that the pilot chamber is interposed between the said duct and the orifice of narrow cross-section.

4. Hydraulic system according to Claim 1, in which the orifice of narrow cross-section has a relatively small axial length.

5. Hydraulic system according to Claim 1, in which: the movable core of the electromagnet is slidably mounted with a tubular support bobbin for the winding; the pilot valve body is substantially in the form of a cylindrical element and has one end inserted within one end of the support bobbin for the winding of the electromagnet; the said one end of the body of the valve, together with the end of the movable core facing it, defines a main chamber within the support bobbin; the said duct formed in the pilot valve body opens into the said main chamber; the said passage connecting the duct to the discharge aperture is constituted by the said main chamber to the discharge aperture, and the seat for the shutter is constituted by the end of the duct which opens into the said main chamber.

6. Hydraulic system according to Claim 5, in which the body of the pilot valve is of ferromagnetic material.

7. Hydraulic system according to Claim 5, in which the movable core is slidably mounted within the support bobbin of the winding with the interposition of a sleeve.

8. Hydraulic system according to Claim 7, in which the valve includes a closure element closing the end of the sleeve opposite the body of the valve, this closure element, together with the end of the movable core facing it, defining an auxiliary chamber within the sleeve and in that the movable core includes a series of peripheral axial grooves which put the said main chamber in communication with the auxiliary chamber.

9. Hydraulic system according to claim 8, in which the movable core has an intermediate portion of smaller diameter which defines an annular peripheral chamber within the sleeve.

10. Hydraulic system according to Claim 8, in which resilient means are interposed between the closure element and the adjacent end of the movable core.

11. Hydraulic system according to Claim 8, which further includes stroke limiting means for defining the limit of the stroke of the movable core in the direction of the closure element and in which in this stroke limit position, the facing ends of the movable core and the closure element are spaced from each other.

12. Hydraulic system according to Claim 11 in which the said stroke limiting means are constituted by an axial pin force-fitted within the movable core.

13. Hydraulic system according to Claim 5, in which the seatforthe shutter is defined by the internal edge of a bush mounted on the body of the valve at that end which opens into the main chamber of the said duct.

14. Hydraulic system according to Claim 5, in which the portion of the body of the valve which projects outwardly of the electromagnet is inserted in a seat of the body of the hydraulic servo-valve, to achieve coupling of the servo-valve with the pilot valve.

15. Hydraulic system according to Claim 5, in which the said duct is constituted by a bore extending axially through the body.

16. Hydraulic system according to Claim 5, in which the end of the duct communicating with the pilot chamber is provided with an apertured restrictor plate for preventing the transmission of oscillations of pressure between the duct and the pilot chamber.

17. A hydraulic system with proportional electrical control substantially as herein described with reference to and as shown in the accompanying drawings.