Actuation device

An actuation device for a friction clutch arranged in the drive train of a motor vehicle includes an engagement/disengagement arrangement having a pressure fluid cylinder and a pressure fluid piston axially displaceably arranged therein. The engagement/disengagement arrangement is operatively connected to a pressure fluid source via a control valve which is actuatable as a function of a first control parameter representing a required value and a second control parameter representing an actual value of the axial position of the pressure fluid piston. The control valve being actuated by an electromagnetic positioning device.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to an actuation device for a friction clutch arranged
 in the drive train of a motor vehicle including an
 engagement/disengagement arrangement having a pressure fluid cylinder and
 a pressure fluid piston axially displaceable therein, the
 engagement/disengagement arrangement being effectively connected to a
 pressure fluid source via a control valve actuatable as a function of a
 first control parameter representing a required axial position and a
 second control parameter representing the actual axial position of the
 engagement/disengagement arrangement.
 2. Description of the Related Art
 A known actuation device is known, for example, from German reference DE 33
 21 578 C2 which discloses a positioning arrangement comprising a vacuum
 servo-force amplifier. The vacuum servo-force amplifier is constructed as
 a vacuum braking force amplifier arranged outside of a housing bell and is
 integrally configured with a pneumatic power cylinder and a control valve.
 A piston is axially guidably arranged with an elastic diaphragm in the
 pneumatic power cylinder and separates the pneumatic power cylinder into
 two working chambers. A first working chamber is configured as vacuum
 chamber and connected to an induction line system of an internal
 combustion engine. The second working chamber is used as control chamber
 and is selectively connectable by the control valve either to the vacuum
 chamber, i.e., the first working chamber, or to the atmosphere via a
 pressure-balance opening. The vacuum servo-force amplifier is driven by a
 control rod which is axially displaceable by a motor-driven cam. The
 position of the control valve is switched by the axial displacement of the
 control rod. The piston follows the motion of the control rod with an
 amplified force. The motion of the piston acts directly on a hydraulic
 master cylinder which in turn acts on a slave cylinder which is arranged
 outside the housing bell and acts, in turn, on an engagement/disengagement
 fork associated with the engagement/disengagement bearing arrangement. The
 control valve has a valve body which interacts with a flexible valve seat.
 A connecting duct is provided in the valve seat and the connection between
 the control chamber and the ambient air takes place via this connecting
 duct, provided the valve body is not pressed against the elastic valve
 seat in order to close the connecting duct by means of the valve body. A
 further connecting duct connects the control chamber with the vacuum
 chamber.
 In addition, another known actuation device has a pneumatic power cylinder
 comprising a pressure fluid power cylinder arrangement. This known
 actuation device is fastened on the outside of a housing bell as an
 integral unit which comprises a pneumatic power cylinder, a hydraulic
 slave cylinder and the control valve. The piston of the pneumatic power
 cylinder is installed on a rod element forming the piston of the hydraulic
 slave cylinder. The rod element is connected to a push-rod which extends
 inside the housing bell and acts on an engagement/disengagement fork
 associated with the engagement/disengagement bearing arrangement. A master
 cylinder which is actuatable by a clutch pedal and a control input for the
 control valve are connected to the slave hydraulic cylinder. The control
 valve controls the supply of compressed air to the pneumatic power
 cylinder and the release of air from the pneumatic power cylinder as a
 function of the hydraulic pressure present at the control input so that a
 specified hydraulic pressure, determined by a compression spring
 arrangement, is set at the control input. In this arrangement, the slave
 hydraulic cylinder is used as a measurement cylinder which records the
 position of the rod element. Since the rod element is also connected to
 the push-rod which extends inside the housing bell and acts on an
 engagement/disengagement fork, the slave hydraulic cylinder indirectly
 records the position of the engagement/disengagement bearing arrangement.
 When the master cylinder is actuated, forces are exerted directly on the
 rod element, and therefore on the engagement/disengagement bearing
 arrangement, via the hydraulic slave cylinder used as the measurement
 cylinder. These forces are additional to the actuation forces of the
 pneumatic power cylinder due to the supply of compressed air to the
 latter.
 It has been deemed advantageous to arrange the pressure fluid power
 cylinder of these known actuation devices within the housing bell.
 However, it is to be expected that the compact construction and known
 hydraulic control of these actuation devices will involve relatively high
 manufacturing costs with respect to the sealing requirements. Because, on
 the other hand, the engagement/disengagement function is to be achieved by
 means of an intrinsic engagement/disengagement arrangement so that the
 control line will not have to transmit any high forces, an economic and
 reliable alternative is desirable.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide an actuation device of
 the type previously mentioned which is simple and functionally reliable
 and which does not comprise an external hydraulic control line.
 To achieve this object, the actuation device for a friction clutch arranged
 in the drive train of a motor vehicle according to the present invention
 comprises an engagement/disengagement arrangement with a pressure fluid
 cylinder and a pressure fluid piston axially displaceable within it. The
 arrangement is operatively connectable to a pressure fluid source via a
 control valve that is actuatable as a function of a first control
 parameter representing a required position value and a second control
 parameter representing the actual axial position. Furthermore, the control
 valve is actuatable by an electromagnetic positioning device. According to
 the above-described embodiment of the present invention, hydraulic lines
 are not required between the clutch pedal and the control device. Rather,
 conventional cables, which are cheaper--and, in addition, simpler to lay
 and maintain--may be used. Furthermore, the conventional cables are less
 sensitive to aging and mechanical effects. Instead of direct actuation by
 the driver via the clutch pedal, a completely automatic solution can be
 readily effected by providing an appropriate control unit instead of the
 clutch pedal. A further advantage of the design is the complete decoupling
 of the engagement/disengagement device from the clutch pedal. Oscillating
 conditions occurring in the control circuit are accepted not by the pedal
 but by the electromagnetic positioning device and may be hydraulically or
 electrically damped in the control circuit.
 The actuation device according to the invention is preferably arranged
 within a fixed, stationary clutch housing such, for example, as a clutch
 bell. The friction clutch may therefore be constructed in a very compact
 manner and additionally protect the actuation device from mechanical
 damage and dirt. In an optional embodiment, an arrangement that
 facilitates maintenance is ensured by a plug-in connection between the
 engagement/disengagement arrangement, which is coaxial with the gearbox
 shaft, and the positioning device and the control valve.
 In a preferred embodiment, the engagement/disengagement arrangement acts on
 the friction clutch via an axially displaceable engagement/disengagement
 bearing element for engaging and disengaging the clutch. An arrangement is
 then particularly preferred in which the engagement/disengagement
 arrangement and the engagement/disengagement bearing element are arranged
 on the same center line as the friction clutch.
 A rotating or linearly effective electric motor is preferably employed as
 the electromagnetic positioning device. When an electric motor that
 generates a rotational motion is used, axial positioning may be effected
 by a threaded spindle and a spindle acceptance device. However, other gear
 stages which convert a rotational motion into an axial motion may also be
 employed equally satisfactorily. For example, the same results may be
 achieved, for example, by a rack or by an eccentric cam with rocker arm.
 Step motors may also be used as the electromagnetic positioning device to
 select the desired position. General purpose electric motors may also,
 however, be employed equally satisfactorily with the positional
 information extracted from the electric current characteristic.
 In an alternative embodiment, the electromagnetic positioning device
 actuates the control valve via a fluid. The hydraulic transmission allows
 for a choice of any given transmission paths and the actuation device can
 be very precisely matched to the installation space. In particular, this
 embodiment allows for actuation by a displacer which protrudes into a
 hydraulic space effectively connected to the control valve. For example,
 the displacer may be axially moved by via a connection to a linear motor
 or by a threaded spindle to displace a specified fluid quantity in the
 hydraulic space so that the increasing pressure actuates the control
 valve. In addition, a displacement sensing device may be arranged with the
 pressure fluid piston so that it moves axially with the pressure fluid
 piston and protrudes into the hydraulic pressure space. Therefore,
 movement of the pressure fluid piston changes the volume of the hydraulic
 space via the displacement sensing device as a function of the position of
 the pressure fluid piston. Accordingly, the control valve is actuated as a
 function of the positions of both the displacer and the displacement
 sensing device. In consequence, the control valve may be actuated so that
 any given position of the pressure fluid piston may be maintained.
 To avoid fluid losses from the hydraulic system, sealing devices may be
 provided. Sealing devices pointing toward the hydraulic space are
 preferably fastened to the displacement sensing device and the seals
 pointing away from the hydraulic space are preferably fastened to the
 housing. The advantage of this arrangement is that the corresponding
 recesses in the housing are easily applied at the position pointing toward
 the outside.
 In addition, the above objects may be achieved if a signal processing
 device (with an amplification device) which compares the actual value of
 the position of the actuator and the required value of the position of the
 actuator is furthermore used for controlling the electromagnetic
 positioning device. In this embodiment, the entire control process
 operates without a hydraulic device, thus substantially simplifying the
 construction of the actuation device. This lowers the manufacturing costs
 and reduces the maintenance and repair outlay. The recording of the actual
 value of the position of the actuator is preferable effected by an
 electrical displacement-sensing device. This sensing device is directly
 actuated by the position of the pressure fluid piston.
 Furthermore, interpretations of the amount of pedal displacement may be
 selected substantially independently of mechanical and hydraulic
 considerations of the clutch itself. A preferred characteristic of pedal
 movement versus clutch bearing movement, for example, exhibits
 comparatively short displacements of the clutch pedal in the region of
 complete engagement or complete disengagement of the clutch. In
 consequence, long actuation paths may be required in the metering region,
 i.e., the region between the fully engaged and fully disengaged regions.
 This characteristic allows for precise operation of the slip of the clutch
 such, for example, as during a starting procedure, thereby substantially
 reducing the likelihood of abrupt engagement. Since only very small forces
 are necessary on the clutch pedal for driving the signal generator for the
 required value of the actuator position, any given pedal force
 characteristics may, for example, be generated by springs arranged on the
 clutch pedal.
 The various features of novelty which characterize the invention are
 pointed out with particularity in the claims annexed to and forming a part
 of the disclosure. For a better understanding of the invention, its
 operating advantages, and specific objects attained by its use, reference
 should be had to the drawing and descriptive matter in which there are
 illustrated and described preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
 FIG. 1 is a schematic diagram of an actuation device 100 according to an
 embodiment of the present invention having a pressure fluid cylinder 1 and
 an axially movable annular pressure fluid piston 2 inserted therein. The
 annular pressure fluid piston 2 is operatively connected for axially
 displacing an engagement/disengagement bearing 3 axially by thrust and/or
 traction. The pressure fluid cylinder 1, pressure fluid piston 2, and
 engagement/disengagement bearing 3 comprise an engagement/disengagement
 arrangement 110. Sealing devices 11 are arranged between the pressure
 fluid cylinder 1 and the pressure fluid piston 2. An electromagnetic
 positioning device 24 in this embodiment comprises an electric motor. A
 control valve device 25 driven by the electromagnetic positioning device
 24 is connected via pressure fluid lines 6 to the inside of the pressure
 fluid cylinder 1 so that a volume enclosed by the pressure fluid piston 2
 in the pressure fluid cylinder 1 may be modified by the control valve
 device 25. A pressure fluid source 28 supplies fluid to the control valve
 device 25 which has, in addition, a relief device 27.
 A signal processing device 21 is arranged with a control and amplification
 device 22 for controlling the actuation device 100. The signal processing
 device 21 is connected via signal supply lines 26 to a required-value
 signal generator 10 and an actual-value signal generator 4, which records
 the position of the piston 2. In the embodiment of FIG. 1, the
 required-value signal generator 10 comprises a clutch pedal 1Oa actuating
 a potentiometer 12. The control and amplification device 22 is connected
 to the electromagnetic positioning device 24 via control lines 242.
 Instead of a motor, the electromagnetic positioning device 24 may comprise
 any device which converts electrical current into motion.
 During operation, the control valve device 25 is adjusted as a function of
 the position of the electromagnetic positioning device 24 so that pressure
 fluid is supplied to the cylinder 1 via the pressure fluid source 8,
 pressure fluid supplied to the cylinder 1 is relieved via the relief
 opening 27, or the volume enclosed by the pressure fluid piston 2 in the
 pressure fluid cylinder 1 is maintained. Accordingly, the
 engagement/disengagement arrangement 110 may force the
 engagement/disengagement bearing 3 away from the pressure fluid cylinder
 1, maintain a position of the engagement/disengagement bearing 3 in a
 condition without force, thereby allowing the engagement/disengagement
 bearing 3 to be moved back, for example, by spring force, or maintain any
 given position by an equilibrium of forces being generated, for example,
 by the above-mentioned spring force. This spring force is preferably
 generated by a pressure spring device of the clutch such, for example, as
 a plate spring which acts on the pressure fluid piston 2 via the
 engagement/disengagement bearing 3.
 The position of the electromagnetic positioning device 24 is controlled as
 a function of required-value signals and actual-value signals. For this
 purpose, a voltage controlled by the potentiometer 12 is interpreted as a
 required-value signal by the signal processing device 21 and is compared
 with the actual-value signal which is directly related to the piston
 position supplied by the actual-value signal generator 4. Until the
 actual-value parameter reaches the required-value signal, a voltage is
 applied to the electromagnetic positioning device 24 via the control and
 amplification device 22, which moves the control valve device 25 to the
 desired position. The actual-value signal generator 4 preferably comprises
 an electrical displacement sensing device including a coil connected to
 the housing and a permanent magnet which is moved with the piston. This
 electrical displacement sensing device therefore determines the current
 position of the piston by the deformation of the common magnetic field.
 Alternatively, any other electrical actual-value displacement sensing
 device could be equally satisfactorily employed for this purpose--such as
 a potentiometer circuit. When the electromagnetic positioning device 24
 comprises an electric motor, the control and amplification device 22 is
 provided with the pulses of current shown in FIG. 3, wherein each of the
 periodic pulses corresponds to a particular position of the motor and a
 constant high current corresponds to the stop location. During operation,
 the control device 22 obtains periodic information on the current position
 of the electromagnetic positioning device 24 and, in consequence, on the
 setting of the control valve 25, so that a position of the
 engagement/disengagement arrangement 110 and the engagement/disengagement
 bearing 3 corresponding to the required-value signal may be achieved.
 Instead of a general purpose electric motor for the registration of the
 current pulses, the electromagnetic positioning device 24 may comprise a
 step motor whose control signal already contains the information on its
 required position.
 A linear dependence between the force of pedal 10a and the displacement of
 the engagement/disengagement bearing is shown in FIG. 2. However, any
 other given transmission relationship could also be generated. A preferred
 characteristic is shown, for example, in FIG. 5 in which the slopes at the
 ends of the movement of pedal 10a are noticeably steeper than the slopes
 of movement in the central region of the movement. This characteristic is
 preferred because the clutch does not have to be exactly controlled when
 the clutch is completely engaged or disengaged whereas, in the
 intermediate position the exact control of the desired slip of the clutch
 ensures a smooth pull-away. FIG. 4 shows an example of torque transmission
 as a function of pedal position.
 Because the actuation of the clutch pedal 10a requires a very small force
 to produce the required-value signal, any given actuation characteristics
 can be generated by different spring elements. Instead of direct actuation
 by the driver via the clutch pedal 10a, a completely automatic actuation
 may be effected by an appropriate control unit 23.
 A further preferred embodiment is shown in FIGS. 6 and 7. FIG. 6 shows an
 actuation device 101 with a pressure fluid cylinder 1 and an axially
 movable annular pressure fluid piston 2 inserted therein to which an
 engagement/disengagement bearing 3 is fastened so that it can be axially
 displaced by thrust and traction. Annular sealing elements 11 fitted at
 the radial inner side and radial outer side of the pressure fluid piston
 2. The pressure fluid cylinder 1 is fastened within a concentric circular
 opening in a housing 20.
 An electromagnetic positioning device 24 is here configured as a rotating
 electric motor that drives, via a gear 243, a threaded spindle 241. A
 displacer 221 is connected to the spindle 241 via a spindle acceptance
 device 240. The motor is torsionally fastened in the housing 20 by a
 torque support 7 and the displacer 221 is fastened in the housing via an
 axial guide 9 so that the displacer is torsionally fixed but axially
 displaceable.
 A lower end of the displacer 221 protrudes into a hydraulic pressure space
 201 configured in the housing 20 and is operatively connected via the
 hydraulic pressure space 201 to a control valve 25. The displacer 221
 frees, in its rest position on the motor-end stop, a snifter hole 202 to
 the balance container 203 which, as shown, preferably surrounds the motor
 and is sealed radially toward the outside by means of a cap 204 with a
 sealing ring 205. This preferred arrangement of the balance container 203
 produces heating of the hydraulic fluid during operation. Due to the
 corresponding effect on the viscosity, mineral oils may also be employed
 here.
 An actual-value signal generator 4 is axially movably supported in the
 housing 20 and includes a recess 41 which engages the pressure fluid
 piston 2 so that the actual-value signal generator 4 always moves with the
 pressure fluid piston 2. An end of the actual-value signal generator 4
 opposite to the recess 41 protrudes into the hydraulic space 201 and there
 displaces a certain volume as a function of the position of the pressure
 fluid piston 2. During this procedure, seals 42 and 43 arranged on the
 actual-value signal generator 4 prevent the escape of any hydraulic fluid.
 As shown in FIG. 6, the longitudinal center line of the control valve
 device 25 is parallel to that of the electromagnetic positioning device 24
 and the control valve device 25 is approximately the same distance as the
 electromagnetic positioning device 24 from the rotational center line A of
 the clutch. The control valve device 25 comprises an axially movable valve
 plunger 251 which protrudes into the hydraulic space 201 and is pressed by
 a helical spring 252 in the direction toward the hydraulic space 201
 against a stop 253. Sealing elements 250 are arranged at the lower end of
 the valve plunger 251 for sealing the valve plunger 251 relative to the
 hydraulic space 201. The valve plunger 251 also includes a relief opening
 27 associated with a longitudinal hole 271. The valve plunger 251 further
 comprises seals 273 arranged about the relief openings 27 so that the
 relief openings 27 are sealed relative to the hydraulic pressure space 201
 and a pneumatic pressure space 254 arranged at the upper end of the valve
 plunger 251. A valve seat 255 is attached to the housing 20 in the region
 of the radial outer end of the valve plunger 251 above the pneumatic
 pressure space 254. An axially movable valve head 256 is pressed by a
 spring element 257 against the valve seat 255 and provides gas-tight
 closure of the valve seat 255 in the rest position of the valve plunger.
 The other end of the spring element 257 is supported on a plug cap 258
 which additionally contains the pressure fluid connection 259. The radial
 outer circumference of the plug cap 258 is closed by seals 260.
 During operation, the volume in the hydraulic pressure space 201 is
 substantially constant. If the clutch pedal is actuated starting from the
 rest position, for example, the control device 22 applies a voltage to the
 electromagnetic positioning device 24 until the position or rotational
 speed of the electromagnetic positioning device 24 positions the
 engagement/disengagement bearing until the actual-value signal reaches the
 required-value signal generated by the pedal position. The rotation of the
 threaded spindle 241 in the spindle acceptance device 240 produces a
 movement of the displacer 221 by a distance corresponding to the pedal
 position and, in the process, displaces a corresponding volume in the
 hydraulic pressure space 201. Because of the very much higher reaction
 force, particularly in the installed condition, the actual-value signal
 generator 4 initially remains in its position and the pressure in the
 hydraulic pressure space 201, which rises in consequence, moves the valve
 plunger 251 onto the valve head 256.
 As may be seen from FIGS. 6 and 7, there is initially a clearance D between
 the valve plunger 251 and the valve head 256 so that as the valve plunger
 251 is moved within the clearance D, the relief hole 271 is free. Until
 this clearance is taken up, pressure can not be transmitted from the
 pneumatic cylinder because the pressure has an escape through the relief
 hole 271. It is only when the clearance D has been taken up, therefore,
 that pressure can be transmitted from the pneumatic pressure space 254 via
 the pressure line 6 into the pressure fluid cylinder 1. When this occurs,
 the pressure fluid piston 2 moves together with the
 engagement/disengagement bearing 3 toward the open end of the cylinder 1
 with the actual-value signal generator 4 being taken along with the piston
 2. This moves the actual-value signal generator 4 out of the hydraulic
 pressure space 201 and therefore decreases the pressure in the hydraulic
 pressure space 201. Accordingly, the force on the valve plunger 251 falls
 and the helical spring 252 urges the valve plunger 251 to move away from
 the valve head 256 until the valve head 256 is again located on the valve
 seat 255. Because the valve plunger 251 is above the clearance D and the
 relief hole 271 is initially still closed, the pressure in the pressure
 line 6 and in the pressure fluid cylinder 1, and therefore also the
 position of the pressure fluid piston 2 and the engagement/disengagement
 bearing 3, are kept constant. If, as a consequence of a corresponding
 signal, the displacer 221 is now moved further into the hydraulic space by
 the positioning device 24, the procedure can be repeated until the
 displacer 221 reaches its maximum stop. If, on the other hand, the
 displacer 221 is pushed out of the hydraulic space 201, the pressure on
 the valve plunger 251 falls. In this configuration, the plunger 251 is
 moved further from the valve head 256 and frees the relief hole 271. The
 pressure in the pressure fluid cylinder 1 then escapes via the pressure
 line 6 through the relief hole 271 and the relief opening 27.
 Accordingly, there are three operatively relevant positions of the control
 valve device 25. In a rest position, the valve head 256 closes the valve
 seat 255 and the relief hole 271 is open. In this position, atmospheric
 pressure is present in the pressure fluid cylinder 1 in the equilibrium
 condition and the clutch is engaged. In an actuation position, the plunger
 251 is pushed fully upward and the valve head 256 is lifted from the valve
 seat 255 and the relief hole 271 is closed. In this position, the pressure
 generated by the pressure fluid source 28 is present as a quasi-steady
 state in the pressure fluid cylinder 1 and the clutch is disengaged. In a
 middle position of the control valve device 25, the valve head 256 is
 located on the valve seat 255 and the relief hole 271 closed. In this
 position, the pressure in the pressure fluid cylinder 1 and the position
 of the engagement/disengagement bearing 3 are kept constant.
 Therefore, in the embodiment of FIGS. 6 and 7, the control of the control
 valve device 25 is effected by pressure and volume in the hydraulic space
 201. This additionally provides the possibility of fitting the control
 valve 25 and the electromagnetic positioning device 24 in parallel to one
 another and at right angles to the rotational center line A, as in FIG. 6,
 because the transmission of the positioning device to the control valve
 may likewise take place hydraulically.
 FIGS. 8 and 9 show a further embodiment of an actuation device 102. It
 differs from the embodiment shown in FIGS. 6 and 7 in that the control
 valve device 25 is directly driven by the electromagnetic positioning
 device 24. This embodiment dispenses with the hydraulic transmission and
 hydraulic actual-value recording. In this case, the electric or
 electromagnetic actual-value recording by known methods may be employed.
 The use of a contactless electromagnetic actual-value signal generator is
 preferred, such as shown in FIG. 9. This can be arranged with angular
 offset relative to the electromagnetic positioning device 24, thus
 economizing in space.
 FIGS. 10 and 11 show yet a further embodiment of an actuation device 103.
 In this embodiment, the control valve device 25 is likewise mechanically
 connected in that a conversion of the force generated by the
 electromagnetic positioning device 24 occurs via an eccentric cam and a
 rocker arm. This embodiment allows a construction that is almost as
 compact as the hydraulic conversion embodiment disclosed in FIGS. 6 and 7.
 The electromagnetic positioning device 24 of FIGS. 10 and 11 comprises a
 rotating electric motor on whose output shaft an eccentric cam 241' is
 fastened such that its radial extent engages a recess 244 of a rocker arm
 240'. The rocker arm 240' is pivotally mounted about a pivot axis that is
 perpendicular to the center line of the motor. The rocker arm 240'
 comprises an actuation arm 245 that abuts the valve plunger 251 such that
 pivoting the rocker arm 240' produces an axial motion of the valve plunger
 251. The pivoting of the rocker arm 240' is generated by rotation of the
 motor and the eccentric cam 241' thereby enabling the selective position
 control of the control valve 25. As in the case of the embodiment shown in
 FIGS. 8 and 9, the control may again preferably take place by electric or
 electromagnetic actual-value recording.
 In some cases, it may be more favorable--for maintenance purposes--not to
 arrange the structural unit including the electromagnetic positioning
 device 24 and the control valve 25 so that it points upward, as shown in
 FIGS. 6, 7, 8 and 9, but to arrange them oppositely so that they point
 downward. The configuration shown in FIGS. 8 and 9 may be arranged so that
 it points downward without any further alteration. However, the snifter
 hole 202 has always to be arranged under the balance container 203 in the
 embodiment shown in FIGS. 6 and 7 because, otherwise, the hydraulic space
 201 would empty into the balance container 203 via the snifter hole 202.
 For the same reason, the liquid level in the balance container 203 must
 always be above the hydraulic space 201. In this connection, the balance
 container 203 may be arranged near the electromagnetic positioning device
 24 at the level of the displacer 21.
 Individual features of the embodiment examples can, of course, be exchanged
 relative to one another without departing from the concept of the
 invention.
 The invention is not limited by the embodiments described above which are
 presented as examples only but can be modified in various ways within the
 scope of protection defined by the appended patent claims.