US20160222963A1

US20160222963A1 - Variable displacement pump with electric control of displacement regulation and method of regulating pump displacement

Info

Publication number: US20160222963A1
Application number: US15/021,164
Authority: US
Inventors: Leonardo Cadeddu; Alessandro Fauda
Original assignee: VHIT SpA
Current assignee: VHIT SpA
Priority date: 2013-09-11
Filing date: 2014-09-09
Publication date: 2016-08-04
Also published as: ITTO20130735A1; WO2015036913A2; CN105593468A; WO2015036913A3; EP3044414A2

Abstract

A rotary positive displacement pump for fluids, in particular, for the lubrication of a motor vehicle engine (61), has a displacement that can be regulated through the rotation of a stator ring (42) having an eccentric cavity (43) in which the rotor (15) of the pump (1) rotates. The stator ring (42) is housed within an in eccentric cavity (13) of an external ring (12). A rotor actuator (50), controlled by the electronic control unit of the motor vehicle, causes a synchronous rotation by an equal amount in opposite directions of both rings. A method of regulating the displacement of the pump (1) and lubrication system for a motor vehicle engine in which the pump is used are also provided.

Description

TECHNICAL FIELD

The present invention relates to variable displacement pumps, and more particularly it concerns a rotary positive displacement pump of the kind in which the displacement variation is obtained by means of the rotation of an eccentric ring (stator ring).
The invention also concerns a method of regulating the displacement of such a pump.
Preferably, but not exclusively, the present invention is employed in a pump for the lubrication oil of a motor vehicle engine.

PRIOR ART

It is known that, in pumps for making lubricating oil under pressure circulate in motor vehicle engines, the capacity, and hence the oil delivery rate, depends on the rotation speed of the engine. Hence, the pumps are designed so as to provide a sufficient delivery rate at low speeds, in order to ensure lubrication also under such conditions. If the pump has a fixed geometry, at high rotation speed the delivery rate exceeds the necessary rate, so that part of the delivered flow is to be discharged in order to limit the delivery rate and the pressure. Of course, the discharged oil volume has already been compressed, whereby high power absorption occurs, with a consequent higher fuel consumption and a greater stress of the components due to the high pressures constantly generated in the pump.
In order to obviate this drawback, it is known to equip the pumps with systems allowing a delivery rate regulation at the different operating conditions of the vehicle, in particular through displacement regulation. Different solutions are known to this aim, which are specific for the particular kind of pumping elements (external or internal gears, vanes . . . ).
A system often used in rotary pumps employs a stator ring with an internal cavity, eccentric relative to the external surface, inside which the rotor, in particular a vane rotor, rotates, the rotor being eccentric with respect to the cavity under operating conditions of the pump. By making the stator ring rotate by a given angle, the relative eccentricity between the rotor and the cavity, and hence the displacement, is made to vary between a maximum value and a minimum value, substantially tending to zero (stall operating condition). A suitably calibrated opposing resilient member allows the rotation when a predetermined delivery rate is attained and makes the pump substantially deliver such a predetermined delivery rate under steady state conditions. An example is disclosed in U.S. Pat. No. 2,685,842.
Pumps with a pair of stator rings are also known, where displacement is varied by rotating the rings relative to each other in opposite directions. An example is disclosed in U.S. Pat. No. 4,406,599.
The evolution of such pumps and the diffusion of electronics in motor vehicle engines have lead to displacement regulation systems controlled by the electronic control unit of the vehicle depending on the oil pressure, preferably detected downstream the filter, and possibly on other operating parameters of the engine. Generally, such systems are electro-hydraulic systems, where the control unit controls electrically operated valves that, in turn, control hydraulic actuators acting on the stator ring. For instance, US 2011/0209682 discloses a system in which a control module of the pump, being part of the electronic control unit, controls through an electrically operated valve the flow of pressurised oil towards either of two chambers, which apply the oil pressure to the stator ring. Application of the pressure of either chamber corresponds to two different pressure/delivery rate conditions of the pump.
Generally, the provision of the hydraulic actuators makes electro-hydraulic systems complex and expensive. Moreover, when the engine is off, it is impossible to modify the displacement presetting, since no control pressure is available.
It is an object of the present invention to provide a variable displacement pump, and a method of regulating the displacement of such a pump, which obviate the drawbacks of the prior art.

DESCRIPTION OF THE INVENTION

According to the invention, this is obtained in that the pump includes an electromagnetic rotary actuator, integrated into or coupled with the pump, which is driven by an electronic system detecting operating conditions of the pump and is arranged to transmit the rotary motion to the stator ring.
Advantageously, the stator ring is housed within an eccentric cavity of an external ring, and the rotary actuator is arranged to simultaneously transmit the rotary motion to both rings, in such a way as to cause a synchronous rotation thereof by an equal amount in opposite directions.
The invention also implements a method of regulating the displacement of a rotary positive displacement pump by means of the rotation of an eccentric stator ring inside which the rotor rotates, the method comprising the steps of:

- providing an electromagnetic rotary actuator integrated into or coupled with the pump;
- supplying the actuator with rotation commands corresponding to a desired rotation of the stator ring.

Preferably, the method further comprises the steps of:

- providing an external ring having an eccentric cavity within which the stator ring is housed; and
- making both rings rotate by a same angle at the same time and in opposite directions.

According to a further aspect of the invention, a lubrication system for a motor vehicle engine is also provided, in which the adjustable displacement pump and the method of regulating the displacement set forth above are employed.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the invention will become apparent from the following description of preferred embodiments, given by way of non limiting examples with reference to the accompanying drawings, in which:

FIG. 1 is a plan view of a first embodiment of the pump according to the invention, from which the cover and the regulation actuator have been removed, in the minimum displacement position;

FIG. 2 is a view similar to FIG. 1, in the maximum displacement position;

FIG. 3 is a plan view similar to FIG. 2, showing the delivery rate regulation mechanism integrated in the cover;

FIG. 4 is a cross-sectional view of the pump taken according to a plane passing through line Y-Y in FIG. 3;

FIGS. 5 and 6 are views similar to FIGS. 1 and 2, relating to a second embodiment of the pump according to the invention; and

FIG. 7 is a principle block diagram of the displacement regulating circuit.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1 to 4, a pump 1 according to the invention, more particularly a vane pump, includes a body 10 having a cavity 11 with substantially circular cross-section in which a first movable ring 12 (external ring) is located. The ring in turn has an axial cavity 13, also with substantially circular cross-section, eccentrically arranged relative to cavity 11. A second movable ring 42 (stator ring) is located in cavity 13, which ring in turn has an axial cavity 43, also with substantially circular cross-section, eccentrically arranged relative to cavity 13 and having a centre O′. Rings 12 and 42 are arranged to rotate in mutually opposite directions by a certain angle in order to vary the pump displacement, as it will be disclosed below. Advantageously, cavities 13, 43 have the same eccentricities. In the example illustrated, cavity 11 is blind and is closed at one end by a cover 14 (FIG. 4), also closing the corresponding ends of cavities 13, 43.
Cavity 43 in turn houses a rotor 15, rigidly connected to a driving shaft 15 a making it rotate about a centre O, for instance in clockwise direction, as shown by arrow F. Cavity 43 thus forms the pumping chamber. In a minimum displacement position (shown in FIG. 1), rotor 15 and cavity 43 are coaxial or substantially coaxial, whereas, in a maximum displacement position (shown in FIG. 2), centres O and O′ are located on the same axis X-X and are mutually spaced apart, and rotor 15 is substantially tangent to side surface 43 a of cavity 43. In the present description, the term “coaxial or substantially coaxial” is used to denote a minimum distance, tending to 0, between centres O and O′.
Advantageously, eccentric rings 12 and 42 are mounted in such a manner that, in the minimum displacement position, external ring 12 is oriented so that its minimum radial thickness is located at the top in the Figure and internal ring 42 is oriented so that its minimum radial thickness is located at the bottom in the Figure. Otherwise stated, the eccentricities of the respective cavities 13, 43 are offset by 180°. Preferably, cavities 13, 43 have the same eccentricity relative to the external surface of the respective ring.
Rotor 15 has a set of vanes 16, radially slidable in respective radial slots. At an outer end, vanes 16 are at a minimum distance from side surface 43 a of cavity 43, whereas at their inner end they rest on guiding or centring rings 17, mounted at the axial ends of rotor 15 and arranged to maintain the minimum distance between vanes 16 and surface 43 a under any condition of eccentricity. Also centring rings 17 will be coaxial or substantially coaxial with rotor 15 in the minimum displacement position.
A suction chamber 18, communicating with a suction duct 20, and a delivery chamber 19, communicating with a delivery duct 21, are defined at the bottom of body 10 between rotor 15 and surface 43 a. Such chambers are substantially symmetrical with respect to a plane passing through axis X-X and have phasings that are ideal for the maximum volumetric efficiency, as it is clearly apparent for the skilled in the art. It is to be appreciated that, should the rotor rotate in counterclockwise direction, the functions of such chambers, and hence of the respective ducts, would be mutually exchanged
In order to control the rotation of rings 12, 42, toothed sectors 51, 52 are formed on their facing surfaces and are preferably positioned at the base of suitable stator cavities 11 a, 11 b formed in rings 12, 42. A toothed wheel 53 having a shaft 54 rigidly connected to an actuator 50 (FIG. 4) driving it into rotation is interposed between toothed sectors 51, 52 located in said stator cavities 11 a, 11 b. Thus, rings 12, 42 rotate in opposite directions and are synchronous with each other.
Preferably, actuator 50 is an electromagnetic actuator. It may be a rotary actuator, e.g. a step-by-step micromotor integrated into pump 1 or coupled therewith (e.g. interfaced through the partition wall separating the inside from the outside of the engine sump), or a linearly moving actuator coupled with a suitable escapement ratchet gear in order to convert the actuator motion into a rotary motion.
Actuator 50 is controlled by the electronic control unit of the vehicle, which manages the displacement variation in closed loop (e.g. with feedback), by increasing or reducing the displacement depending on the requirements of the thermal engine and the accessories thereof. The variation is independent of the pressures upstream and downstream the oil filter.
Shaft 54 is guided within a support 40 formed in cover 14 or in body 10. Toothed sectors 51, 52, while rotating, develop according to a profile defined by the involute of the teeth of wheel 53, which, on the contrary, rotates about its stationary axis. If the eccentricities are the same, the relative rotation of the rings causes a translation of centre O′ of pumping chamber 43 along axis X-X. This makes the geometry of pumping chamber 43 perfectly symmetric in all displacement conditions, and makes the ratio between the rotation of toothed wheel 53 and the displacement variation because of the translation of axis of chamber 43 constant.
In the illustrated embodiment, wheel 53 cooperates with a member 34 opposing the rotation of rings 12, 42, in particular a flat spiral spring, preloaded so as to prevent the rotation of the rings as long as the torque applied by actuator 50 is lower than a predetermined threshold. Spiral spring 34 is located in a casing 33 that, in the illustrated exemplary embodiment, is fastened to cover 14. The inner end portion of spring 34 is so shaped as to be coupled with the end portion of shaft 54 of wheel 53, whereas the outer end portion is locked to the internal wall of casing 33. The latter may be rotated, for instance by using a dynamometric key, in order to adjust the preloading of spring 34. A ring nut 55 allows blocking casing 33 in the desired calibration position, independently of the constructional tolerances of the whole mechanism. A sealing gasket 56 is moreover provided between casing 33 and cover 14 in order to isolate the internal chamber of the same casing from the outside. A drain puts such a chamber in communication with suction chamber 18, for the aims that will be disclosed below.
It is to be appreciated that, during the regulation rotation, spiral spring 34, thanks to the negligible variation of the twisting torque and to the transmission ratio of the gear mechanism, will undergo negligible variations of its torque opposing the hydraulic torque. In the preferred embodiment in which actuator 50 is a step-by-step motor, spring 34 may contribute to make the magnetic resistance torque between subsequent steps sufficient to maintain the position of rings 12, 42 when the motor in not excited (energy saving). Moreover, due to a diametrically opposite effect, spring 34 could contribute to maintaining a maximum displacement upon the occurrence of an electric failure.
Rings 12 and 42, as well as centring rings 17, rotor 15 and wheel 53, are preferably formed by moulding and/or metal powder sintering, with possible finishing operations on some limited areas, according to the dictates of the art. More particularly, axial thicknesses will undergo finishing. Body 10 and cover 14 can be formed by moulding either an aluminium alloy or a thermoplastic and/or thermosetting resin. Advantageously, spring 34 may be made of a bimetallic material, so that its characteristic may change depending on the operation temperature.
A second embodiment of the pump according to the invention, denoted 101, is shown in FIGS. 5 and 6. Elements that are functionally identical to those already disclosed with reference to FIGS. 1 to 4 are preferably denoted by the same reference numerals, increased by 100. Pump 101 differs from pump 1 in that external ring 12 is lacking and therefore actuator 150 acts through wheel 153 onto stator ring 142 alone, which is formed internally of body 110 with substantially circular cross-section.
In accordance with such an embodiment, as shown in FIGS. 5 and 6, stator ring 142 preferably comprises a stator cavity 111 in which both toothed sector 152 and toothed wheel 153 are arranged.
More particularly, toothed sector 152 is located at the base of stator cavity 111 and the toothed wheel is preferably wholly included between toothed sector 152 and body 110 with substantially circular cross-section.
Thanks to such a structure, in accordance with the second embodiment, the arrangement of toothed sector 152 and toothed wheel 153 allows minimising the size of pump 101.
Moreover, rotor 115 rotates in counterclockwise direction (arrow F′). With such an arrangement, the translation of centre O′ of chamber 143 takes place along a non-rectilinear trajectory. Apart from those aspects, the structure is identical to that of pump 1 ad it is not necessary to describe it again.
FIG. 7 shows a principle block diagram of the regulation of the displacement of pumps 1, 101. Dashed line denotes the mechanical drive of the pump by actuator 50 and hence corresponds to toothed wheels 53, 153 of the previous Figures. Dotted and dashed line 60 denotes the lubrication circuit which conveys oil from pump 1 to the engine and the various accessories, denoted in the whole 61. Reference numeral 62 denotes the electronic control unit of the vehicle, which receives signals from detectors denoted in the whole 63 and controls actuator 50, possibly through a digital-to-analogue converter, not shown. Solid lines denote the paths of the electric signals incoming into/outgoing from control unit 62, and dotted lines denote the detection of the operating parameters of engine 61, pump 1, lubrication circuit 60 and possibly actuator 50 by detectors 63. The parameters on which regulation of the delivery rate of the pump for lubrication of a motor vehicle engine may depend are well known to the skilled in the art and are not of interest for the invention. A more detailed description can be found in US 2011/0209682.
The operation of the pump described is as follows.
Considering first pump 1, under rest conditions, the pump is in the maximum displacement condition shown in FIG. 2. As said, centre of rotation O of rotor 15 is offset relative to centre O′ of cavity 43 of eccentric ring 42 and rotor 15 is located close to wall 43 a of the cavity. When pump 1 is started, the clockwise rotation of rotor 15 will give rise to an oil flow through chamber 19 and the associated delivery duct 21 and, at the same time, an equal volume of oil will be sucked from chamber 18 and the associated suction duct 20. As the rotation speed and the flow rate increase, the lubrication system of the engine, by opposing an increasing resistance to the flow, will make pressure increase.
The delivery pressure or the pressure downstream the oil filter are detected by the suitable detectors 63 and communicated to control unit 62, which will make actuator 50 rotate. The actuator will in turn generate a rotation torque that, through wheel 53 and once the calibration value of counteracting spring 34 has been attained, will make rings 12, 42 rotate by the same angle in opposite directions. If, as it has been assumed, the eccentricities of cavities 13, 43 relative to the external surfaces of the respective rings are the same, the rotation of ring 42 will cause a rectilinear translation of centre O′ towards the right, proportional to the amount of the rotation, thereby proportionally reducing the eccentricity between rotor 15 and cavity 43, and consequently the pump displacement, and stabilising the pressure at the calibration value. As parameters such as the speed, the fluidity/temperature of the fluid, the engine “permeability” (intended as the amount of oil used by the engine) and so on, detected by detectors 63, change, such a pressure will be maintained and controlled through the variation of the eccentricity and hence of the displacement.
When, as a function of the different operating parameters of the engine, it is desired to operate at a lower pressure value, with a consequent reduction in the absorbed power, control unit 62 will generate a suitable command for actuator 50, so as to further reduce the displacement.
The rotation of the rings may continue until the position shown in FIG. 1 is attained, where centres O and O′ coincide and vanes 16 and centring rings 17 rotate with the rotor without changes in their radial relative position. Consequently, the displacement is null and the pump is in stall condition. It is to be pointed out that this position may be taken when a hydraulic lock of the delivery pressure is approaching. In the constructional practice, a minimum displacement is preferably maintained by protecting the pump with a maximum pressure valve.
The operation of pump 101 is wholly similar, with the changes due to the provision of stator ring 42 alone.
An important parameter in managing the delivery rate/pressure of an oil pump for thermal engines is temperature, the increase of which makes oil become more fluid and the engine permeability increase. Consequently, the pump displacement should proportionally increase. This may be favoured if the opposing load of the counteracting spring increases. In order to obtain this, flat spiral spring 34 may be made of a bimetallic material such that temperature causes an increase in the rigidity and hence in the counteracting torque. In order to obtain the change in the rigidity, the small oil flow rate for the lubrication of shaft 54 of wheel 53 may be exploited: the oil, after having licked casing 33 of spring 34 and having transmitted its temperature to the same spring, freely discharges towards the suction chamber through the drain provided in chamber 57.
The invention actually attains the desired aims.
Use of an electromagnetic actuator, directly controlled by the electronic control unit of the vehicle, allows eliminating the hydraulic actuators of the prior art and the necessary connections to the lubrication circuit, and makes the system less cumbersome, simpler and more reliable as well as less expensive. Moreover, elimination of the hydraulic actuators enables modifying the displacement presetting even when the thermal engine is off, since no control pressure is required. This is advantageous in particular for vehicles provided with the “stop and go” function, because it allows for instance increasing the displacement between the stop of the thermal engine and its start in order to start again the engine with a good lubrication.
Moreover in both embodiments, given the respective rotation direction of the rotor, in case of an electric failure causing deactivation of actuator 50 the hydraulic torque of the pump causes the rotation of the stator ring, or of the stator ring and the external ring, towards the maximum displacement condition. As said, this action can be favoured by spring 34. In case of other failures, the minimum displacement is ensured and the electronic control, by detecting low lubrication pressures, will bring control unit 62 to the vehicle “recovery” function,
It is clear that the above description has been given only by way of non-limiting example and that changes and modifications are possible without departing from the scope of the invention.
For instance, even if in the illustrated embodiment shaft 15 a of rotor 15 is guided by body 10 whereas spiral spring 34 and the calibration means consisting of casing 33 and ring nut 55 are housed within cover 14, the arrangement could be reversed, or also the spring and the calibration means could be housed within body 10.
Moreover, spring 34 could not be a bimetallic spring and, at least in the embodiments where actuator 50 is a step-by-step motor, the spring could be dispensed with, the only magnetic resistance torque between subsequent steps maintaining the position of rings 12, 42 when the motor is not excited.
Lastly, even if the invention has been disclosed in detail with reference to a pump for the lubrication oil of a motor vehicle engine, it may be applied to any positive displacement pump for conveying a fluid from a first to a second working environment, in which a delivery rate reduction as the pump speed increases is convenient.

Claims

1. A variable displacement rotary positive displacement pump for fluids, comprising a rotor (15; 115) arranged to rotate within an eccentric cavity (43; 143) of a stator ring (42; 142) in turn arranged to be rotated within a predetermined angular interval, as operating conditions of the pump (1; 101) vary and upon command of a system (62, 63) detecting such conditions, in order to vary a relative eccentricity between the eccentric cavity (43; 143) and the rotor (15; 115) and hence the pump displacement, the pump (1; 101) further including an electromagnetic actuator (50), integrated into or coupled with the pump, which is driven by said detecting system (62, 63) and is arranged to generate a rotary motion and to transmit it to said stator ring (42; 142) through a toothed wheel (53; 153);

the pump being characterised in that:

said toothed wheel (53; 153) is located at least in part in a stator cavity (11 a, 11 b; 111) located in a peripheral position relative to said stator ring (42; 142);

and in that

a toothed sector (52; 152) is located at the base of said peripheral stator cavity (11 a, 11 b; 111).

2. The pump as claimed in claim 1, wherein said toothed sector (52; 152) located at the base of said peripheral stator cavity (11 a, 11 b; 111) meshes with said toothed wheel (53; 153) driven by the actuator (50) and develops according to a profile defined by an involute of the teeth of the wheel (53; 153).

3. The pump as claimed in claim 2, wherein the stator ring (42) is housed within an eccentric cavity (13) of an external ring (12), and said actuator (50) is arranged to transmit the rotary motion to both rings (12; 42) in such a way as to cause a synchronous rotation thereof by an equal amount in opposite directions.

4. The pump as claimed in claim 3, wherein the eccentric cavities (13, 43) have the same eccentricity and, in a minimum displacement condition, are arranged so that their eccentricities are offset by 180°.

5. The pump as claimed in claim 3, wherein facing surfaces of the external ring (12) and the stator ring (42) have formed thereon respective toothed sectors (51, 52) with which a toothed wheel (53) driven by the actuator (50) meshes and which develop according to a profile defined by an involute of the teeth of the wheel (53), so that, during the rotation of the rings (12, 42), a centre (O′) of the cavity of the stator ring moves along a rectilinear path.

6. The pump as claimed in claim 2, wherein the toothed wheel (53; 153) is arranged to cooperate with a member (34) opposing the rotation of the stator ring (42; 142), which member consists of a flat spiral spring secured at one end to a shaft (54) of the toothed wheel and at the other end to an element (33) rigidly connected to the pump body, the spring being associated with setting means (33, 55) arranged to set a desired steady state value for the displacement of the pump (1; 101), and wherein the flat spiral spring (34) is made of a bimetallic material and has a temperature-depending characteristic.

7. The pump as claimed in claim 2, wherein the actuator (50) is a step-by-step micromotor or is a linearly moving actuator equipped with an escapement ratchet gear arranged to convert the actuator motion into a rotary motion.

8. The pump as claimed in claim 1, wherein the pump (1; 101) is a pump for a lubrication circuit (60) of a motor vehicle engine (61).

9. A method of regulating the displacement of a rotary positive displacement pump (1; 101) of a kind comprising a rotor (15; 115) arranged to rotate within an eccentric cavity (43; 143) of a stator ring (42; 142), the method comprising the step of making the stator ring (42; 142) rotate within a predetermined angular interval in order to vary the eccentricity between the cavity (43; 143) and the rotor (15; 115) as operating conditions of the pump (1; 101) vary, and being characterised in that it further comprises the steps of:

providing an electromagnetic actuator (50) integrated into or coupled with the pump and arranged to transmit a rotary motion to the stator ring (42) through a toothed wheel (53; 153) located at least in part in a stator cavity (11 a, 11 b; 111) located in a peripheral position relative to said stator ring (42; 142);

supplying the actuator (50) with commands corresponding to a desired rotation of the stator ring (42; 142).

10. The method as claimed in claim 9, further comprising the steps of:

providing an external ring (12) having an eccentric cavity (13) within which the stator ring (42) is housed; and

making the stator ring (42) and the external ring (12) rotate by a same angle at the same time and in opposite directions.

11. The method as claimed in claim 9 or 10, arranged for regulating the displacement of a pump for the lubrication oil of a motor vehicle engine.

12. A lubrication system for a motor vehicle engine (61), comprising a pump (1; 101) as claimed in claim 1.

13. The pump as claimed in claim 1, wherein the stator ring (42) is housed within an eccentric cavity (13) of an external ring (12), and said actuator (50) is arranged to transmit the rotary motion to both rings (12; 42) in such a way as to cause a synchronous rotation thereof by an equal amount in opposite directions.

14. The pump as claimed in claim 13, wherein the eccentric cavities (13, 43) have the same eccentricity and, in a minimum displacement condition, are arranged so that their eccentricities are offset by 180°.

15. The pump as claimed in claim 14, wherein facing surfaces of the external ring (12) and the stator ring (42) have formed thereon respective toothed sectors (51, 52) with which a toothed wheel (53) driven by the actuator (50) meshes and which develop according to a profile defined by an involute of the teeth of the wheel (53), so that, during the rotation of the rings (12, 42), a centre (O′) of the cavity of the stator ring moves along a rectilinear path.

16. The pump as claimed in claim 13, wherein facing surfaces of the external ring (12) and the stator ring (42) have formed thereon respective toothed sectors (51, 52) with which a toothed wheel (53) driven by the actuator (50) meshes and which develop according to a profile defined by an involute of the teeth of the wheel (53), so that, during the rotation of the rings (12, 42), a centre (O′) of the cavity of the stator ring moves along a rectilinear path.

17. The pump as claimed in claim 13, wherein the toothed wheel (53; 153) is arranged to cooperate with a member (34) opposing the rotation of the stator ring (42; 142), which member consists of a flat spiral spring secured at one end to a shaft (54) of the toothed wheel and at the other end to an element (33) rigidly connected to the pump body, the spring being associated with setting means (33, 55) arranged to set a desired steady state value for the displacement of the pump (1; 101), and wherein the flat spiral spring (34) is made of a bimetallic material and has a temperature-depending characteristic.

18. The pump as claimed in claim 13, wherein the actuator (50) is a step-by-step micromotor or is a linearly moving actuator equipped with an escapement ratchet gear arranged to convert the actuator motion into a rotary motion.

19. The pump as claimed in claim 1, wherein the toothed wheel (53; 153) is arranged to cooperate with a member (34) opposing the rotation of the stator ring (42; 142), which member consists of a flat spiral spring secured at one end to a shaft (54) of the toothed wheel and at the other end to an element (33) rigidly connected to the pump body, the spring being associated with setting means (33, 55) arranged to set a desired steady state value for the displacement of the pump (1; 101), and wherein the flat spiral spring (34) is made of a bimetallic material and has a temperature-depending characteristic.

20. The pump as claimed in claim 1, wherein the actuator (50) is a step-by-step micromotor or is a linearly moving actuator equipped with an escapement ratchet gear arranged to convert the actuator motion into a rotary motion.