A self-compensating piezoelectric actuator for a control valve

Abstract: An actuator device for a control valve is described and comprises a piezoelectric actuating member (12) for bringing about the movement of a closure member (30) of the valve between an open position and a closure position; the member (12) has a reference portion (20) for connection to a fixed portion (26) of the device and a free portion (24) for connection to the closure member (30), the free portion (24) being movable relative to the reference portion (20) as a result of mechanical deformations produced in the actuating member (12) by the piezoelectric effect. The reference portion (20) of the actuating member (12) is controllable selectively between a condition in which it is locked in a reference position and is fixed firmly to the fixed portion (26) of the device, and a free condition in which it is movable relative to the fixed portion (26).

Description: The present invention relates to an actuator device for a control valve and, more particularly, to a piezoelectric actuator device of the type referred to in the preamble to Claim 1. The solution of operating a control valve, for example, a fuel-injection control valve, by means of piezoelectric actuators is known. These devices utilize the mechanical deformation of a piezoelectric element, and preferably of a more complex member such as a bar formed by a series of such elements, for bringing about the operation of a valve-closure member. Conventionally, the mechanical deformation of the piezoelectric bar consists of a "positive" change in its length along its longitudinal axis, that is, of an elongation of the bar relative to its rest dimension (in order not otherwise to cause mechanical detachment of the elements "in series"). This elongation leads to linear translation of a free end of the bar, which end is connected to the closure member, relative to the fixed valve body on which the opposite end of the bar is restrained, and consequently causes the closure member to travel between an open position and a closure position, relative to the valve seat. The mechanical deformation due to the piezoelectric effect is normally very small, of the order of a few tens of microns, and is thus comparable to the dimensional variations produced by the range of temperature variation which is established during the operation of the devices with which the control valve is associated, and to their manufacturing tolerances. A further problem is that of ensuring efficient operation of the actuator upon the occurrence of long-term intrinsic drift, due particularly to the mechanical components associated with the actuator, for example, owing to wear of the valve seat and the closure member, which is also of a magnitude comparable to the deformations due to the piezoelectric effect. US 4,750,706 discloses a control valve, particularly an injection valve for fuel injection systems of internal combustion engines, comprising a piezoelectric actuating member having the end opposed to the valve needle connected with an armature displaceable in the axial direction, which armature is arranged within a magnetic yoke so as to form an electromagnetic arresting arrangement. The arresting arrangement is actively switched on by an associated exiting coil at least during a longitudinal change of the piezo stack for valve opening so that the armature is spatially fixed relative to an housing of the valve. In the off condition the arresting device does not operate and the armature can axially displace into the yoke thereby compensating long-term changes of the piezo stack. The aim of the invention is to provide a piezoelectric actuator for a control valve which does not have the disadvantages of the devices of the prior art but which enables the travel of the valve closure member to be controlled effectively, irrespective of the manufacturing tolerances involved and/or of spurious dimensional alterations of the piezoelectric actuating member which may occur during the operation of the device. According to the present invention, this aim is achieved by means of an actuator having the characteristics recited in Claim 1. In summary, the present invention is based on the principle of restraining one end of the piezoelectric actuating member adjustably on the fixed reference body of the valve relative to which the mechanical deformations of the member are defined, and hence with the ability to compensate for the initial tolerances and for all of the possible spurious dimensional variations which may occur (thermal deformations, long-term dimensional alterations). In other words, the coupling between the piezoelectric actuating member and the fixed reference body of the valve is formed in a manner such as to be variable and adjustable in operation, so that the system can be said to be self-compensating. The adjustable connection is achieved by piezoelectric means, either directly or by means of an intermediate member, as will be described in detail below. The invention provides for an electronic system for adjusting the connection between the piezoelectric actuating member and the fixed reference body to be integrated with the electronic control system of the actuator device, directly in the valve body; temperature control (basically cooling) thereof may be achieved by means of the fluid controlled by the valve, in accordance with a known solution. Further characteristics and advantages of the invention will be explained in greater detail in the following detailed description of preferred embodiments thereof, given by way of non-limiting example with reference to the appended drawings, in which: Figure 1 is a schematic illustration, in longitudinal section, of a first embodiment of an actuator device for a control valve, in which the coupling between the piezoelectric actuating member and the fixed reference body of the valve is achieved by electromagnetic means, Figure 2 is a schematic illustration, in longitudinal section, of a second embodiment of an actuator device, in which the coupling between the piezoelectric actuating member and the fixed reference body of the valve is achieved by electromagnetic means, Figure 3 shows, in longitudinal section, a detail of the embodiment of the device according to the invention, in which the coupling between the piezoelectric actuating member and the fixed reference body of the valve is achieved by piezoelectric means, Figure 3a is a cross-section taken on the line IIIa-IIIa of the device of Figure 3, Figure 4 is a block circuit diagram of a control circuit for an actuator device according to the invention, and Figure 5 shows a series of time graphs which represent the operation of the actuator device according to the invention. A piezoelectric actuator device is shown partially and indicated 10 in Figure 1. In this example, a possible application to an on-off control valve for the injection of fuel or diesel fuel is described but this possible use, which is adopted herein for simplicity of discussion, should be understood as purely indicative. The actuator device 10 comprises an elongate piezoelectric actuating member 12 oriented along a longitudinal axis of the device and composed of a plurality (a stack) of elements arranged in succession between a pair of end electrodes 14, 16. A first end 20 of the piezoelectric member is fixed mechanically on a movable, cup-shaped support member 22 and its second end 24 is free relative to the support member 22 and, in particular, can translate longitudinally as a result of variations in the length of the actuating member 12 due to its piezoelectric properties. The support member 22, which is made of ferromagnetic material, is housed and can slide axially with slight friction in a coaxial, fixed reference body 26 which is also cup-shaped and made of ferromagnetic material. The possible travel is small and is of the order of the tolerances and of the dimensional variations to be corrected (a few tenths of a millimetre). The sliding is advantageously arranged to take place after lubrication of the parts by means of the fluid controlled, which is caused to enter this region in a "discharge" stage. The actuating member 12 carries, at its free end 24, a closure member 30 of the valve which can engage in a corresponding seat 32 in order to regulate the flow of the fluid under control. The closure member 30 is normally kept in the closure position by the action, in compression, of a spring 34 arranged between the free end 24 of the actuating member 12 and a wall 36 fixed firmly to the fixed body of the valve and having an axial guide hole for the sliding of the stem of the closure member. The closure member is also kept closed by the pressure of the fluid to be controlled, indicated by the arrows in the drawing. , The fixed reference body 26 also has, on at least a longitudinal portion of its side wall, an electrical winding arranged in a manner such as to form a coaxial locking electromagnet 40 for achieving the adjustable connection between one end of the piezoelectric actuating member 12 and the fixed reference body 26 by electromagnetic means. A second embodiment of the actuator device with adjustable connection achieved by electromagnetic means is shown in Figure 2 in which identical or functionally equivalent elements or components are indicated by the same reference numerals. As will be clear to a person skilled in the art, the solution is achieved by a simple reversal of the embodiment of Figure 1. In particular, a simpler closure-member unit is shown, in which the stem does not occupy part of the valve outlet opening. However, in addition to the compression spring 34 which acts on the free end 24 of the piezoelectric actuating member 12 to keep the closure member 30 in a rest, closure position, this embodiment provides for the use of a second, contact spring 42 for controlling the travel of the support member 22 in the fixed reference body 26, keeping the piezoelectric discs of the "stack" always "in contact". In contrast to the electromagnetic locking known in the prior art, the connection between the actuating member 12 and the fixed reference body 26 of the valve is achieved according to the invention by piezoelectric means and is shown in Figures 3 and 3a, with particular reference to the differences with respect to the solutions described above. The intermediate movable support member 22 in the connection between the piezoelectric actuating member 12 and the fixed reference body 26 of the valve comprises a pair of opposed sliding blocks 50, 51 arranged longitudinally in at least a portion of the cup-shaped body 26. The support member also comprises a connecting element 52 which connects the sliding blocks 50, 51 to one another and to a first end 20 of the piezoelectric member 12. According to this embodiment, the adjustable connection is achieved by means of a piezoelectric locking member 54 arranged diametrally between the sliding blocks 50, 51. Naturally, the mechanical deformation (the "travel") of the locking member 54 owing to the piezoelectric effect must advantageously be greater than the distance which separates the sliding blocks from the side wall of the fixed body 26, taking account of the mechanical tolerances, of any thermal expansion, and of the long-term dimensional stability of the mechanical components involved. As will be understood from the drawings, the piezoelectric locking member 54 always acts on the blocks 50, 51 in compression, bringing them into contact with the inner wall of the fixed body 26 and stopping any possible sliding of the support member 22 by friction, as in a drum braking system. To facilitate the travel of the sliding blocks during locking operations, the connecting element 52 advantageously has a structure with radial flexibility achieved, for example, by the provision of longitudinal notches. In the rest condition, the closure member 30 is in the closure position. The spring 34 can keep the closure member in this position even in the absence of fluid pressure, overcoming the frictional forces between the support member 22 and the fixed reference body 26 so as to ensure reasonable safety margins. The support member 22 is freely slidable in the fixed reference body 26. The sliding is advantageously arranged to take place after lubrication of the parts by means of the fluid controlled, which is caused to enter this region in a "discharge" stage. The closure position defined by the action of the spring 34 and by the fluid pressure takes account of the actual dimensions of the piezoelectric member and of the mechanical components of the valve at rest, as well as of the manufacturing tolerances involved. In dependence on this position, the support member 22 reaches a first reference position in the fixed body 26. When the actuator device is activated, the locking means acts to lock the support member 22 in its current position relative to the body 26 by radial attraction and friction. The support member 22 is thus hold firmly to the fixed reference body 26. The valve is opened by exciting the piezoelectric member 12 electrically by means of the end electrodes 14, 16 in a manner such as to bring about axial extension thereof, that is, to cause its free end 24 to translate relative to the end 20 which is fixed to the body 26, overcoming the opposing force of the spring 34 and hence - to sum up - moving the closure member 30 away from the seat 32 and opening the outlet opening of the valve. The operation of the actuator according to the invention is characterized by a high degree of precision since the deformations in length of the piezoelectric actuating member 12 are transmitted to the valve closure member 30 as a control useful for varying its position relative to its seat 32 and not for also taking up the spurious dimensional variations arising in the device. The closure member can be returned rapidly to the closure position by cancelling the control of the actuating member 12 or even by providing a suitable control of opposite sign, without, however, inducing substantial contraction deformations since they are not tolerable, for example, by the piezoelectric material conventionally used (a series of "stacked" discs). This latter solution may, however, be adopted effectively in the device according to the invention since this device ensures practically zero tolerances at any moment of the operative life of the device The locking means advantageously does not require fast response rates and therefore has a low power consumption. During continuous operation of the actuator device, progressive heating normally takes place, possibly causing dimensional variations of the components. The locking means is therefore de-energized periodically so as to permit relative (micrometric) movements between the support member 22 of the piezoelectric member 12 and the fixed reference body 26 such as to take up the spurious dimensional variations occurring in the device. An example of the controls for the operation and adjustment of the device is shown in the graphs of Figure 5. The upper graph shows, on a time scale, a portion of the control signals for the actuating member 12 in which two successive stages of actuation (ON) of the valve can be seen. The corresponding lower graph shows a portion of the control signals (energization and de-energization) for the locking means, for the fine adjustment of the reference position. De-energization (OFF) is brought about in dependence on the dynamics of the system, for example, every 100 valve control pulses. The ability to adjust the reference position of the actuating member 12 relative to the fixed body of the valve periodically, even at frequencies greater than that indicated, leads to a further advantage: that is, the fact that the member 12 can be made of materials which have a high degree of mechanical deformation for small control voltages, but which have poor characteristics of accuracy, mechanical stability, and thermal expansion. In particular, this solution opens the way to the use of actuator devices of the type described, with relatively large travel, for controlling the lifting of engine valves. In the above-described embodiment, the integration of the electronic control for the adjustment of the connection between the piezoelectric actuating member 12 and the fixed reference body 26 with the electronic control of the actuator device is also important. Figure 4 is a functional block diagram of a preferred control circuit. The electrodes 14, 16 of the piezoelectric actuating member 12 are indicated in the single block 60. The valve is actuated by means of electrical excitations induced in the electrodes in the form of a pulse-width-modulated pulse-train produced by a generator 62 supplied by an electrical supply via the connection l 1 . The pulse generator 62 is connected to the electrode block 60 by means of a switch 64 which is controlled at a driving input by an injection-control circuit (not shown), via the connection l 2 . The block 66 indicates the circuit module for locking the reference for the actuating member 12 according to the solution described (the piezoelectric locking member 54), this module also being supplied by the electrical supply, via the connection l 1 . The input of the circuit module 66 is connected to a looking logic circuit 68 which receives its supply via the connection l 1 and is driven by the injection-control circuit (not shown) via the connection l 2 , in common with the switch 64. According to the preferred solution described, the length of the electrical connections, particularly of the high-voltage connections, is advantageously reduced and logic circuits for controlling the activation of the valve and for the adjustment of the reference system for the actuating member may be formed on a single substrate. Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated purely by way of non-limiting example, without thereby departing from the scope of protection of the present invention defined by the appended claims.

Claims: An actuator device for a control valve, comprising a piezoelectric actuating member (12) for bringing about the movement of a closure member (30) of the valve between an open position and a closure position, in which the actuating member (12) has a reference portion (20) for connection to a fixed portion (26) of the device and a free portion (24) for connection to the closure member (30), the free portion (24) being movable relative to the reference portion (20) as a result of mechanical deformations produced in the actuating member (12) by the piezoelectric effect, where the reference portion (20) of the actuating member (12) is controllable selectively between a condition in which it is locked in a reference position and is fixed firmly to the fixed portion (26) of the device, and a free condition in which it is movable relative to the fixed portion (26), and where the actuator device comprises means (34) for bringing the actuating member (12) to a reference position at rest which corresponds to a rest condition of the valve and relative to which the open and closure positions are defined, and means (50, 51, 54) for locking the reference portion (20) of the actuating member (12) in the reference position thus reached, characterized in that the reference portion (20) can be locked relative to the fixed portion (26) of the device by friction. An actuator device according to Claim 1, characterized in that the reference portion (20) of the actuating member (12) is hold on a support member (22) movable relative to the fixed portion (26) of the device. An actuator device according to Claim 2, characterized in that the fixed portion (26) has a cup-shaped portion in the cavity of which the actuating member (12) is arranged axially, and in that the support member (22) can slide axially in the cavity. An actuator device according to Claim 3, characterized in that the locking means comprise at least one pair of opposed sliding blocks (50, 51) associated with the support member (22) and a corresponding piezoelectric locking member (54) arranged diametrally between the blocks (50, 51) for moving the sliding blocks (50, 51) apart to bring them into contact with the fixed portion (26) of the device as a result of mechanical deformations induced in the locking member (54) by the piezoelectric effect. An actuator device according to any one of the preceding claims, characterized in that the means for bringing the actuating member (12) to the reference positions comprise a reaction spring (34) associated with the fixed portion (36, 26) of the device and acting on the free portion of the member (12) when the member (12) is not electrically excited. An actuator device according to Claim 3, characterized in that it is arranged to direct some of the fluid controlled by the valve into a space provided between the support member (22) and the fixed portion (26) of the device, so as to lubricate the portions (22, 26). An actuator device according to any one of the preceding claims, which can be controlled by means of an electronic driver circuit (60-64), characterized in that it also comprises an electronic control circuit (66, 68) for the locking means (50, 51, 54), integrated with the driver circuit (60-64) of the actuator. An actuator device according to Claim 7, characterized in that the electronic control circuit (66, 68) is arranged for periodic activation of the locking means (50, 51, 54). An actuator device according to Claim 7 or Claim 8, characterized in that it is arranged to direct some of the fluid controlled by the valve towards the electronic control circuits (66, 68) of the locking means (50, 51, 54) and the driver circuit (60-64) of the actuator, so as to enable the circuits (60-64; 66, 68) to be cooled.

Inventors: FAGGIOLI EUGENIO, MONTUSCHI MARIO, FAGGIOLI, EUGENIO, MONTUSCHI, MARIO
Applicants: FIAT RICERCHE, C.R.F. SOCIETA CONSORTILE PER AZIONI
Application year: 2001
Main classification: F02M59
IPC codes: F02D41, F02M63, F02M59, H01L41, F02M47, F02M61, F02M51, F16K31