GB2057128A

GB2057128A - Improvements in or relating to servo drive arrangements

Info

Publication number: GB2057128A
Application number: GB8025738A
Authority: GB
Original assignee: Elektrowatt AG
Current assignee: Siemens Building Technologies AG
Priority date: 1979-08-09
Filing date: 1980-08-07
Publication date: 1981-03-25
Also published as: DE3028532A1; NL8004515A; JPS6358780B2; NL189423C; DE3028532C2; FR2463297A1; SE441788B; JPS5654271A; NL189423B; CH638873A5; FR2463297B1; GB2057128B; SE8005212L

Abstract

A compartment 17 exposed to temperature fluctuations contains a fluid 18 vapourising under the action of a heater 19. The compartment 17 has a piston 15 displaceable by means of the volumetric change of the fluid. Displacement of the piston 15 is transmitted to an actuating member 25 in an inertia-reduced manner by means of flow of the fluid 18 through a pipe 22 into a bellows 24 and a spacer 30. The compartment 17 comprises a thermally-conductive portion 12 with cooling fins 20 and an insulating portion 13; the piston 15 is of insulating material. <IMAGE>

Description

SPECIFICATION Improvements in or relating to servo drive arrangements The present invention relates to thermodynamic servo drive arrangements of the kind comprising an encapsulated compartment which is exposed to temperature fluctuations, for containing a fluid that will evaporate under the action of heat and has a partition displaceable in operation by means of the volumetric variation of the fluid, and further comprising means for transmitting the displacement of the movable partition to an actuating member.

Known servo drive arrangements of this nature have a very sluggish reaction to temperature changes and this inertia renders it doubtful whether the known servo drives may be applied to actuate control valves, in view of the long adjusting periods.

It can be envisaged to reduce the sluggishness of the known servo drives by co-ordinating a heating or cooling system reacting particularly rapidly and requiring an increased expenditure of energy, with the compartment in which the fluid changes in volume by transition from the liquid into the vapour phase or vice versa.

On the other hand, it is a known fact that particularly long setting paths may be obtained with servo drive arrangements of the kind referred to above, considering that volumetric ratios between the liquid and vapour phases of say 1 :20 to say 1 :40 result within the compartment, depending on the fluid utilised and on pressure conditions.

It is an object of the invention to provide a servo drive arrangement of the kind referred to above, whose response inertia is particularly low.

Accordingly the invention consists in a servo drive arrangement of the kind comprising an encapsulated compartment, which is exposed to temperature fludtuations, and for containing a fluid that will evaporate under the action of heat and has a partition displaceable in operation by means of the volumetric variation of the fluid, and further comprising means for transmitting such displacement of the movable partition to an actuating member, wherein the compartment is a first partial volume of a capsule subdivided by the movable partition, whose second partial volume, thermally insulated from the first partial volume, is also filled with the fluid in operation, the volumetric capacity of the first partial volume, whilst the fluid is in the liquid condition, corresponding to a fraction of the volumetric capacity of the second partial volume, and wherein a hydraulic transmission system is located between the latter and the actuating member.

In order that the invention may be more clearly understood reference will now be made to the accompanying drawing, which shows a diagrammatical axial section through a servo drive arrangement.

Referring to this drawing, the servo drive 10 illustrated comprises a housing 11, a cyiindrical capsule 14 assembled from two bowls 12, 13 being situated in its upper portion. A piston 1 5 produced from a material which is thermally insulating or has poor thermal conductivity is displaceably installed in the capsule 14 and sealed by a piston ring 16 with respect to the inner surface of the capsule 14. The piston 1 5 thus separates the internal space of the capsule 14 into two partial volumes 1 7, 18. Both partial volumes are charged with a fluid, e.g.

trichlorofluoromethane, which is liquid at normal ambient temperature and gradually changes completely into the vapour phase with rising temperature. The fluid is liquid in the position illustrated, so that the volumetric capacity of the partial volume 17 corresponds to no more than a small fraction of the volumetric capacity of the partial volume 1 8.

The bowl 12 enclosing the partial volume 17, which has satisfactory thermal conductivity, has co-ordinated with it an electric heating coil 19 on the one hand, and a set of cooling plates 20 on the other hand. Slots 21 in the housing 11 make provision for ventilation of the cooling plates 20 by the ambient atmosphere.

The bowl 13 surrounding the partial volume 18 has connected to it the one extremity of a connecting pipe 22 whose other extremity is tightly anchored within a partition 23 which is fixedly anchored in the housing. Radially spaced from the connecting pipe 22, the partition 23 has secured to it one end of a bellows 24 whose other end has the top extremity of an actuating member 25 joined to it via a disc 26. The actuating member 25 is in the form of a plunger and passes through a boss-like sleeve 27 in the centre of the base 28 delimiting the housing 11, and is acted upon by a return spring 29 which tends to impel the actuating member 25 into the housing 11 and thereby to compress the bellows 24.

The bellows 24 is also filled with the fluid and is in communication with the partial volume 18 via the connecting pipe 22.

If - as in the present case - the crosssectional area of the piston 1 5 acting on the partial volume 1 8 is equal to the effective crosssectional area of the bellows 24, the transmission ratio between the stroke displacements of the piston 15 and of the actuating member 25 is 1 :1.

In order that the equidirectional displacement of the piston 1 5 and of the actuating member 25 may be kept as free of hysteresis as possible in this case, a spacer bar 30 -- e.g. of glass, a stainless steel or of any other material having a low thermal conductivity - is situated between the displaceable end of the bellows 24 and the side of the piston 1 5 facing towards the partial volume 18, this bar 30 extending with clearance through the connecting pipe 22.

If a particular quantity of heating energy - the adjusting signal - is fed to the heating coil 19, the temperature of the bowl 12 and thus the partial volume 1 7 therewithin is raised until the heat dissipated by the cooling plates 20 corresponds to the heating power supplied. This temperature rise causes vapourisation of a part of the fluid in the partial volume 17, and the piston 15 is thereby pushed downwards against the action of the spring 29. Due to its small volumetric capacity, the vapourisation of the fluid in the partial volume 17 occurs very rapidly and continues until a balance is established with the pressure rise in the partial volume 1 7 caused by the downward displacement of the piston 1 5.

By its downward displacement, the piston 1 5 displaces fluid from the partial volume 1 8 via the connecting pipe 22 and past the bar 30 into the bellows 24, which expands correlatively and as stated transmits its displacement against the action of the spring 29 to the actuating member which, for its part, may be coupled to a servo element which is not illustrated, e.g. a control valve.

If a change is made in the heating power supply, the temperature in the partial volume 17 alters accordingly, and more vapour is generated or a part of the vapour is recondensed until the pressure/temperature balance is restored.

It should be observed that the volumetric capacity of the unheated partial volume 17 should be selected so that the quantity of fluid present in the same is sufficient to yield the required stroke under complete vapourisation. This stroke may however be limited by stops on the actuating member 25 and/or on the piston 1 5.

The space between the bowl 13 and the partition 23 may be filled with a thermally insulating material 31 so that the parts of the servo drive exposed to the temperature fluctuations are thermally insulated in optimum manner from its other parts.

The servo drive operates with particularly low inertia if the range within which the temperature fluctuate within the partial volume 17 lies substantially above the normal ambient temperature. As a matter of fact, the cooling action within the partial volume 1 7 then occurs considerably faster upon reducing the heating power. The normal temperature range in the partial volume 17 may thus extend between 100 and approx. 1 800 C. A comparatively low heating power only is needed to maintain such temperatures within the partial volume 17, thanks to the low volumetric capacity of the latter.

The servo drive illustrated may be installed in an optional position. It is evident that a diaphragm or a bellows may also be utilised instead of the piston 15.

Claims

1. A servo drive arrangement of the kind comprising an encapsulated compartment, which is exposed to temperature fluctuations, and for containing a fluid that will evaporate under the action of heat and has a partition displaceable in operation by means of the volumetric variation of the fluid, and further comprising means for transmitting such displacement of the movable partition to an actuating member, wherein the compartment is a first partial volume of a capsule subdivided by the movable partition, whose second partial volume, thermally insulated from the first partial volume is also filled with the fluid in operation, the volumetric capacity of the first partial volume, whilst the fluid is in the liquid condition, corresponding to a fraction of the volumetric capacity of the second partial volume, and wherein a hydraulic transmission system is located between the latter and the actuating member.

2. A servo drive arrangement as claimed in claim 1, wherein the capsule is cylindrical and the movable subdividing partition is in the form of a displaceable piston, bellows or diaphragm dividing the volume into the two said partial volumes, the second partial volume being joined via a connecting pipe to a second bellows filled with the fluid, which is stationarily supported at one extremity and is coupled at the other extremity to the actuating member acted upon by a return spring.

3. A servo drive arrangement as claimed in claim 2, wherein the capsule, the connecting pipe and the second bellows are arranged mutually coaxially.

4. A servo drive arrangement as claimed in claim 2, wherein the cross-sectional area of the piston, of the bellows or of the diaphragm which acts on the second partial volume is equal to the cross-sectional area of the second bellows.

5. A servo drive arrangement as claimed in claim 4, wherein a spacer bar passes with clearance through the connecting pipe and is supported at one extremity on the piston, bellows or diaphragm, and is supported at its other end on the other extremity of the second bellows.

6. A servo drive arrangement as claimed in any of the preceding claims, wherein the movable partition is formed from a material having low thermal conductivity.

7. A servo drive arrangement as claimed in claim 5 or 6, wherein the connecting pipe and the spacer bar are formed from a material of low thermal conductivity.

8. A servo drive arrangement as claimed in any of claims 2 to 7, wherein the wall of the first partial volume situated opposite the piston, bellows or diaphragm, is effectively coupled to a heating system and a cooling system.

9. A servo drive arrangement as claimed in claim 8, wherein the heating system comprises an electric heating coil.

10. A servo drive arrangement substantially as hereinbefore described with reference to the accompanying drawings.