METHOD FOR OPERATING A LINEAR MOTOR COMPRESSOR, AND LINEAR MOTOR COMPRESSOR

Abstract

A method for operating a linear motor compressor an electric linear motor, a cylinder, and a linearly movable free piston assembly having a piston. The cylinder and the piston form a compression chamber, and a free piston assembly is directly driven by the linear motor and is moved back and forth between an upper dead center and a lower dead center along a stroke path. The compression chamber is supplied with a fluid from the outside, and the supplied fluid is compressed or expanded in the compression chamber and subsequently dispensed back outside. At least one state variable is specified for the linear motor compressor, and the linear motor is actuated such that the linear motor compressor exhibits the specified state variable.

Description

The invention relates to a method of operating a linear motor compressor. The invention further relates to a linear motor compressor.

STATE OF THE ART

It is known to compress a gas by means of a linear motor compressor. The document US2018/0051690A1 discloses a free-piston linear motor compressor, in which the compressor is designed as a reciprocating piston compressor, whereby the linear motor is designed with two poles, and whereby the entire free-piston linear motor compressor is operated at a resonant frequency. This linear motor compressor is used for compressing a gaseous process fluid, in particular natural gas. This linear motor compressor is operated continuously and at a sinusoidal resonant frequency during refueling of a natural gas vehicle. The operating possibilities of this linear motor compressor are extremely limited and economically disadvantageous.

Representation of the Invention

It is the task of the invention to operate a linear motor compressor with a more advantageous operating method for compressing and/or expanding a gaseous process fluid. In addition, it is the task of the invention to design an economically more advantageous linear motor compressor for compressing and/or expanding a gaseous process fluid.

This task is solved by a method having the features of claim 1. The dependent claims 2 to 18 concern further advantageous process steps. The task is further solved with a linear motor compressor having the features of claim 19. The dependent claims 20 and 21 concern further, advantageous embodiments.

The task is solved in particular with a method for operating a linear motor compressor comprising an electric linear motor, a cylinder and a linearly movable free piston arrangement with a piston, wherein the cylinder and the piston form a compression chamber, wherein the free piston arrangement is driven directly by the linear motor and is moved back and forth along a stroke path between a top dead center and a bottom dead center, wherein a fluid is supplied to the compression chamber from the outside, wherein the supplied fluid is compressed or expanded in the compression chamber and then discharged to the outside again, wherein at least one state variable is predetermined for the linear motor compressor, and wherein the linear motor is controlled in such a way that the linear motor compressor has the predetermined state variable.

The task is further solved in particular with a linear motor compressor comprising at least one electric linear motor, a cylinder and a linearly movable free piston arrangement with at least one piston, wherein the cylinder and the piston form at least one compression chamber, wherein the free piston arrangement is driven directly by the linear motor, the compression chamber being connected to the outside in a fluid-conducting manner via an outlet valve and an inlet valve, a control device controlling the linear motor in such a way that the free piston arrangement is moved back and forth between a top dead center and a bottom dead center with a predetermined state variable.

Preferably, at least one stroke path point along the stroke or at least one stroke path time and, assigned to this, a setpoint velocity or a setpoint acceleration or a setpoint force is specified as a state variable. Preferably, a relation between the stroke path of the free piston arrangement and its velocity is specified as a predetermined state variable, hereinafter also referred to as the velocity-displacement curve. This velocity-displacement curve may comprise at least one point, a stroke path and a predetermined associated velocity, and preferably comprises a plurality of points, each point comprising a position along the stroke path and a velocity associated with that position.

Advantageously, a travel setpoint curve to be maintained along at least a section of the entire stroke path, and preferably along the entire stroke path, i.e. a setpoint profile concerning stroke path and setpoint velocity, setpoint acceleration and/or setpoint force, is specified as a state variable.

Advantageously, a time setpoint curve, i.e. a setpoint profile relating to setpoint velocity, setpoint acceleration and/or setpoint force as a function of the stroke time, which is to be maintained during a partial duration or a partial section of the entire stroke, and preferably a stroke time required during the duration of the entire stroke, is specified as a state variable.

In order to achieve the predetermined state variable during operation of the linear motor compressor, the linear motor compressor is advantageously operated with a control strategy in which the free piston arrangement can be moved “freely” on the basis of the forces acting in the compression chamber and, if necessary, additionally acting frictional forces, whereby the linear motor can exert a controllable force on the free piston arrangement and thereby influences the free movement of the free piston arrangement from the outside and preferably in a predetermined manner. Preferably, the linear motor is given a velocity profile or force profile in a displacement dependency or in a time dependency, respectively, whereby this force profile can be modified during the operation of the linear motor compressor by a control intervention to ensure that the free piston arrangement has the predetermined state variable, or that the behavior of the free piston arrangement approaches the predetermined state variable due to the control intervention.

In an advantageous control strategy, the path-time dependency of the movement of the free piston arrangement and thus the path-time dependency of the piston movement is not controlled directly, i.e. no predefined path-time curve for the movement of the free piston arrangement is specified, but the movement curve of the free piston arrangement or of the piston results as a consequence of the force profile used or resulting from the forces acting. In this embodiment, the specified state variable is thus ultimately achieved by specifying a force profile. The force profile used is adapted in particular to the corresponding application and the corresponding operating method of the linear motor compressor. As an application, the linear motor compressor can be operated, for example, as a compressor or as an expander of a gas. Preferably, the linear motor compressor is operated to compress a gas. In the application as a compressor, the operating method or the force profile can be optimized, for example, in that the free piston arrangement is moved relatively fast, in particular with a higher mean velocity, during the compression phase of the gas, and that the free piston arrangement is moved with reduced velocity, in particular with a lower mean velocity, during the subsequent expulsion phase of the gas, which reduces the flow resistance when the gas flows out of the compression chamber. Thus, for example, it is possible to keep the time for a complete cycle of compression constant, but by running through the compression phase faster and the ejection phase slower, the flow resistance of the gas during ejection can be reduced, and thus also the energy required to push the gas out of the compression chamber.

Due to the predefined state variable, the free piston arrangement and thus the entire linear motor compressor can be operated in a wide variety of ways, depending on the desired variable to be optimized. In addition to the example already described, the state variable can, for example, be selected in such a way that the maximum force to be delivered by the linear motor or the maximum power to be delivered by the linear motor is limited, or the energy required to operate the linear motor compressor is minimized by also extracting energy from the linear motor in sections during the cycle and supplying it to the linear motor again with a time delay. The method according to the invention therefore has the advantage that the linear motor compressor can be operated with a large number of possible, predetermined state variables. A reciprocating compressor known from the prior art, the piston of which is driven by a piston drive via a crosshead, has the disadvantage that the movement of the piston is rigidly coupled to the velocity of the crankshaft, and that the velocity of the piston is determined as a function of the angle of rotation of the crankshaft, in particular also by the geometrical arrangement of the crankshaft and crosshead. In contrast, the linear motor compressor according to the invention can be operated in a wide variety of ways, and in particular independently of motion sequences determined by a crankshaft, by a corresponding specification of a state variable such as, for example, the setpoint velocity, the setpoint acceleration or the setpoint force as a function of the stroke in accordance with this specification. In addition, the operating method can be optimized, depending on requirements, for a variable such as energy consumption, maximum linear motor power or maximum linear motor force.

The linear motor compressor may comprise a single compression chamber. Particularly advantageously, the linear motor compressor comprises two compression chambers, a first and a second compression chamber. The free piston arrangement preferably has a piston on each of the two end faces spaced apart in the stroke direction, these two pistons being operated by the free piston arrangement in opposite directions or in opposite directions, so that alternately in one compression chamber, for example the first compression chamber, compression and then ejection of a fluid take place, and in the other compression chamber, for example the second compression chamber, simultaneously expansion and then suction of the fluid take place, and vice versa.

The linear motor or the free piston arrangement particularly preferably has a stroke length in the range between 50 mm and 500 mm. The linear motor has at least three actively controllable magnetic poles arranged in succession in the stroke direction, and preferably between 5 and 50 actively controllable magnetic poles, and particularly advantageously between 10 and 20 controllable magnetic poles. Such a number of actively controllable magnetic poles results in the advantage that the force exerted by the linear motor on the free piston arrangement during the movement along the stroke path can be controlled as a function of the stroke path or as a function of time by a corresponding selective excitation of the magnetic poles connected individually or in groups. In an advantageous method of operation, only positive electrical power is supplied to the linear motor to thereby drive the free piston arrangement. In a further advantageous operating method, electrical power is dissipated from the linear motor along at least a partial section of the entire stroke path, so that the linear motor generates a braking effect within this partial section, thereby braking the free piston arrangement by the linear motor. Advantageously, the braking effect or the braking power output can also be controlled as a function of the stroke distance. The linear motor can thus be operated only in a driving manner, or in a driving and braking manner, or in a combination of at least two of the properties driving, braking and neutral, neutral being understood to mean that the linear motor effects neither a driving nor a braking force. In a particularly advantageous operating method, the electrical power dissipated by the linear motor is temporarily stored in an electrical accumulator, and subsequently fed back to the linear motor with a time delay. This enables particularly energy-efficient operation of the linear motor compressor according to the invention. The linear motor compressor is preferably operated at a velocity in the range between 200 and 1000 revolutions per minute or at a stroke frequency of 200 to 1000 periods or reciprocations per minute.

A period of movement of the free piston arrangement is a complete cycle of movement starting from a starting point, passing once through the top dead center and the bottom dead center of the piston movement. A period of a piston movement comprises, for the movement from bottom dead center to top dead center, a compression phase in the compression chamber and then an ejection phase, and then comprises, for the movement from top dead center to bottom dead center, an expansion phase in the compression chamber and then an intake phase for the fluid to be conveyed. The starting point is basically arbitrary. For example, the starting point is the bottom dead center.

The free piston assembly is preferably moved back and forth between top dead center and bottom dead center with a predetermined velocity-displacement curve.

Preferably, the free piston arrangement is moved from a bottom dead center during a compression phase to the opening point of the exhaust valve with a predetermined velocity-displacement curve in such a way that the linear motor has to deliver a constant or essentially constant power. This has the advantage that no high and possibly unpredictable current peaks occur during the electrical supply of the linear motor.

Preferably, the free piston arrangement is driven from bottom dead center during a compression phase to the opening point of the exhaust valve and then during an exhaust phase to the closing point of the exhaust valve with a predetermined velocity-displacement curve such that the mean velocity during the compression phase is higher than during the exhaust phase and/or that the mean velocity during the expansion phase is higher than during the intake phase.

In an advantageous method, the predetermined velocity-displacement curve or the predetermined velocity-time curve of the free piston arrangement has a reduced velocity at least in the region of one of the switching points: Opening of the exhaust valve, Closing of the exhaust valve, Opening of the inlet valve, and Closing of the inlet valve, has a reduced velocity compared to the rest of the velocity-displacement curve, so that the exhaust or inlet valve opening or closing at reduced velocity of the free piston arrangement is moved at reduced velocity. The reduced velocity of the opening or closing valve preferably results in reduced wear of the valve, which advantageously results in an increased service life or service life of the valve.

In an advantageous method, the compression chamber has an expansion phase between a closing point of the exhaust valve and the opening point of the intake valve, with the linear motor being controlled in such a way that it actively drives the free piston arrangement throughout the expansion phase.

In an advantageous method, the volume delivered by the linear motor compressor is changed by changing the maximum stroke of the linear motor or the location of the top dead center and/or the location of the bottom dead center, so that the volume delivered can be changed in the short term or also in the long term, for example by reducing or increasing this.

In an advantageous method, the free piston arrangement is braked at least in sections during the to-and-fro movement between top dead center and bottom dead center by operating the linear motor as a generator. This makes it possible to reduce the velocity of the free piston arrangement particularly quickly. Preferably, the braking energy released is converted into electrical energy and stored temporarily for later use.

In an advantageous method, the linear-movable piston arrangement is operated as an expander for a fluid, and the linear motor is thereby operated as a generator at least during a partial section of a movement in stroke direction X, in that the compression chamber of the linear motor compressor is now used as an expansion chamber, in that a pressurized fluid is supplied to the expansion chamber via the outlet valve, the fluid is expanded in the compression chamber operated as an expansion chamber, and is subsequently expelled via the inlet valve, and in that the free piston arrangement of the linear motor operated as a generator is moved back and forth with a predetermined velocity-distance course or a predetermined velocity-time course. In an advantageous process, the opening and closing of the exhaust valve and/or intake valve is actively controlled as a function of the position of the free piston arrangement.

Advantageously, the linear motor compressor comprises at least one electric linear motor, a cylinder and a linearly movable free piston arrangement with at least one piston, the cylinder and the piston forming at least one compression chamber, the free piston arrangement being driven directly by the linear motor, the compression chamber being connected to the outside in a fluid-conducting manner via an outlet valve and an inlet valve, a control device controlling the linear motor in such a way that the free piston arrangement is moved back and forth preferably with a predetermined motor and/or generator power curve between a top dead center and a bottom dead center.

Advantageously, the linear motor compressor comprises a first and a second compression chamber which are arranged in opposite directions with respect to the free piston arrangement so that they act in opposite directions.

Advantageously, the linear motor of the linear motor compressor is operable as a motor and/or as a generator, wherein the control device controls the linear motor in such a way that the free piston arrangement has a predetermined velocity-displacement curve or a predetermined velocity-time curve during the movement between a top dead center and a bottom dead center.

The linear motor comprises at least three pole pairs, and preferably between 5 and 50 pole pairs distributed or mutually spaced in the longitudinal direction of the linear motor.

The linear motor compressor comprises at least one electric linear motor, a cylinder and a linearly movable free piston arrangement with at least one piston, wherein the cylinder and the piston form at least one compression chamber, wherein the free piston arrangement is directly driven by the linear motor, the compression chamber being connected to the outside in a fluid-conducting manner via an outlet valve and an inlet valve, a control device controlling the linear motor in such a way that the free piston arrangement is moved back and forth between a top dead center and a bottom dead center with a predetermined state variable Z_nominal.

The linear motor compressor according to the invention has the advantage that, in a preferred embodiment, with the exception of the valves and the free piston arrangement, it has no moving parts, which improves the service life and efficiency of the linear motor compressor, and also reduces manufacturing costs, installation and maintenance. In addition, the linear motor compressor is preferably designed to be oil-free, meaning that no oil is required for lubrication purposes. The linear motor compressor according to the invention is particularly suitable for compressing gases such as natural gas, other hydrocarbons, hydrogen or air. However, the linear motor compressor according to the invention is also suitable for expanding pressurized gases, whereby in particular during expansion the linear motor can be operated at least temporarily as a generator. Furthermore, the linear motor compressor according to the invention is also suitable for simultaneously compressing a gas and expanding a gas by expanding a gas in one chamber of the linear motor compressor and simultaneously compressing a gas in the other chamber of the linear motor compressor.

A combination of the linear motor into a free-piston compressor allows the construction of a compact linear motor compressor. Due to the direct mechanical connection of the two systems, the static and dynamic behavior is coupled. Therefore, good performance and high efficiency of the linear motor compressor can preferably be achieved if the compressor and linear motor are designed to work together optimally and are preferably operated in the range of a resonant frequency. Preferably under such operating conditions, the free-piston compressor can fully exploit its advantages. An advantage in addition to the compact design is the fact that the piston can be hermetically sealed against the outside in a relatively simple manner and therefore at low cost, because the two cylinders in which the two pistons are located can be designed to be hermetically sealed against the outside at low cost, which makes it possible to compress gases under high demands on ambient conditions, since there is only extremely low or no leakage of the pumped gas at the linear motor compressor. Advantageously, the two cylinders and the stator of the linear motor form a gas-tight outer shell. In addition, no crank mechanisms are required, as is the case with conventional piston compressors. This eliminates parts requiring lubrication, which have mechanical energy conversion losses. The ability to dispense with lubricants also makes the linear motor compressor according to the invention suitable for applications with high cleanliness requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is explained in more detail below with reference to advantageous embodiments and the accompanying drawings.

FIG. 1 schematically shows a linear motor compressor and the associated pressure-volume diagram;

FIG. 2 schematically shows a longitudinal section of another double-acting linear motor compressor;

FIG. 3 schematically shows a longitudinal section along the line of intersection F-F through the linear motor of the linear motor compressor according to FIG. 2;

FIG. 4 schematically shows a cross-section along the line of intersection E-E through the linear motor of the linear motor compressor according to FIG. 3;

FIG. 5 four diagrams of an operating method of an idling linear motor compressor showing stroke, velocity, acceleration and motor force as a function of time;

FIG. 6 four diagrams of a further operating method of an idling linear motor compressor showing stroke, velocity, acceleration and motor force as a function of time;

FIG. 7a velocity-displacement diagram of the two pistons of the linear motor compressor according to FIG. 2 in accordance with a first operating method;

FIG. 8a velocity-distance diagram according to a second operating method;

FIG. 9a velocity-distance diagram according to a third operating method;

FIG. 9a a detailed aspect of the third operating method;

FIG. 9b another detailed aspect of the third operating method;

FIG. 10 a control device for a linear motor compressor according to FIG. 2.

In principle, the same parts are given the same reference signs in the drawings.

WAYS TO CARRY OUT THE INVENTION

FIG. 1 schematically shows a linear motor compressor 1 comprising a linear motor 14 and a double-acting reciprocating piston compressor 15. The reciprocating piston compressor 15 comprises a cylinder 2 in which a linearly movable piston 3 is arranged, which is directly connected to the linear motor 14 via a piston rod 9 and directly driven by the latter. Directly connected or directly driven is understood herein to mean that no gear is arranged between the piston 3 and the linear motor 14, so that the force is transmitted between the linear motor 14 and the piston 3 directly and thus without any interposed gear. In a possible embodiment, a flexible coupling could also be arranged between the piston 3 and the linear motor 14, which preferably allows independent alignment of the piston 3 and the linear motor 14. The cylinder interior 5 is divided by the piston 3 into a first compression chamber 5a and a second compression chamber 5b, wherein the first and the second compression chambers 5a, 5b, due to the geometrical arrangement, are operated in opposite directions during operation. The first as well as the second compression chamber 5a, 5b are each fluid-conductively connected to an external space outside the cylinder interior 5 via an inlet valve 7a, 7b as well as via an outlet valve 6a, 6b. Usually, fluid lines are arranged downstream of the valves 6a, 6b, 7a, 7b, as indicated in FIG. 1, which pass the fluid on to downstream devices or supply it upstream of upstream devices. From the multitude of possible piston positions that the piston 3 can assume during its operation in the cylinder interior 5, FIG. 1 shows three exemplary positions of the piston 3, a first piston position 3a at the bottom dead center X_UTP, a second piston position 3c at the top dead center XOPT, as well as a third piston position 3b, whereby the third piston position 3b corresponds to the position in which the exhaust valve 6a is ideally opened at the opening point B. The first piston position 3a at the bottom dead center X_UTP, the second piston position 3c at the top dead center XOPT, and the third piston position 3b at the top dead center XOPT. Due to existing friction, the exhaust valve 6a is usually opened slightly later or at a slightly higher pressure than the pressure Pa shown in FIG. 1, i.e. in the area of the opening point B shown.

Above the reciprocating compressor 15, the associated idealized p-V diagram, also referred to as a pressure-volume diagram, is shown, which shows the pressure P of a gas compressed by the reciprocating compressor 15 in the first compression chamber 5a as a function of the volume of the first compression chamber 5a. The first compression chamber 5a has a displacement volume VH, a suction volume VS and a dead space volume Vtot, with the volume V increasing towards the right. The same diagram also shows the pressure P of the gas in the first compression chamber 5a as a function of the stroke X of the piston 3, where the stroke X in the diagram shown increases positively towards the left, so that the positive direction of the stroke X is towards the left. FIG. 1 shows the piston 3 in the first piston position 3a, in which the piston 3 is in the bottom dead center X_UTP, whereby the gas in the first compression chamber 5a has a suction pressure Ps. The course of the idealized p-V diagram is briefly explained below. Starting from the bottom dead center X_UTP, the piston 3 is moved in the positive X direction, whereby the inlet valve 7a is automatically closed at the closing point A due to the increasing pressure in the first compression chamber 5a, idealized, and the gas in the first compression chamber 5a is compressed to an outlet pressure Pa during a compression phase BA, whereby the outlet valve 6a is automatically opened at the outlet pressure Pa, i.e. idealized at the opening point B. During the subsequent ejection or extension phase BC, the piston 3 is moved towards the top dead center X_OTP so that the gas located in the first compression chamber 5a is ejected via the outlet valve 6a until the piston 3 has reached the position of the top dead center X_OTP, and the outlet valve 6a is closed, idealized at the closing point C. During the subsequent expansion phase CD, the piston 3 is moved toward the bottom dead center X_UTP, and the residual gas still in the first compression chamber 5a is expanded to a suction pressure Ps, so that the inlet valve 7a is opened automatically at the opening point D, idealized. During the subsequent intake phase DA, gas is drawn into the first compression chamber 5a via the inlet valve 7a until the piston 3 has reached the bottom dead center X_UTP, and the inlet valve 7a is closed at the closing point A, idealized. The term idealized in connection with the points A, B, C and D expresses that these points in real operation, caused for example by existing friction of the valves, are not located exactly at the positions indicated in FIG. 1 but in a close range around the points.

During the operation of the reciprocating compressor 15, the process shown in FIG. 1, which takes place between points A, B, C and D, is constantly repeated. The same process also runs in opposite directions in the second compression chamber 5b in such a way that the second compression chamber 5b is in the relaxation phase or suction phase while the first compression chamber 5a is in the compression phase or discharge phase, and vice versa.

In a particularly advantageous embodiment, the inlet valves 7a, 7b and the outlet valves 6a, 6b open and close automatically. However, it may also prove advantageous to open and/or close the inlet valves 7a, 7b and/or the outlet valves 6a, 6b in a controlled manner. This is particularly necessary when the linear motor compressor 1 is used to expand a gas under a pressure Pa by reversing the cycle shown in FIG. 1, i.e. along the points A, D, C and B back to A by opening the discharge valve 6a under control, and gas under pressure Pa enters and expands the first compression chamber 5a by moving the piston 3 along the line CB until the exhaust valve 6 at point B is closed, and the gas present in the first compression chamber 5a is expanded along the line BA until the piston 3 reaches the lower low point X_UTP, respectively the inlet valve 7a at point A is opened under control. Then the piston 3 is moved along the line AD, the gas is expelled from the first compression chamber 5a, and the inlet valve 7a is closed at point D under control. The residual gas located in the first compression chamber 5a is compressed along the line DC to a pressure Pa and the outlet valve 6 is opened in a controlled manner at point C, so that gas again flows into the first compression chamber 5a under pressure Pa. Particularly advantageously, the cyclic process described above is operated in the direction of the successive points A, D, C and B with a double-acting reciprocating compressor 15 comprising a first and a second compression chamber 5a, 5b, as shown in FIG. 1. The circular process in the direction of the successive points A, D, C and B has the advantage that the linear motor 14 can be operated as a linear generator, so that mechanical energy can be converted into electrical energy by the described expansion of the gas. In a further advantageous embodiment, the linear motor compressor 1 can thus be operated as required for compressing or for expanding a fluid, or in a mixed mode with intermittent compression and two-way expansion of the fluid, whereby electrical energy is supplied to the linear motor 15 or electrical energy is dissipated, depending on the mode of operation. In a further advantageous embodiment, the linear motor compressor 1 can also be operated in such a way that in each case a fluid is compressed in the first compression chamber 5a and a fluid is expanded in the second compression chamber 5b, so that the expansion energy released in the first compression chamber 5a can be used to compress the fluid located in the second compression chamber 5b.

The linear motor compressor 1 can also be operated in reverse, compressing a fluid in the first compression chamber 5a and expanding it in the second compression chamber 5b, as required for compression or expansion The linear motor 15 can be supplied with electrical energy or discharged with electrical energy, depending on the mode of operation, or can be operated idle without supplying electrical energy.

In another possible embodiment of the reciprocating compressor 15, the second compression chamber 5b could be omitted, so that the reciprocating compressor 15 has only a first compression chamber 5a but no second compression chamber 5b that can be operated in the opposite direction.

FIGS. 2 to 4 show another embodiment of a linear motor compressor 1. This linear motor compressor 1 comprises a free-piston arrangement 16, comprising a linear motor rotor 10 and a first piston 3 and a second piston 4. A stator 8 forms a linear permanent magnet synchronous motor 14 together with the linear motor rotor 10. The linear motor compressor 1 comprises a cylinder 2 having two cylinder interiors 5, wherein a first compression chamber 5a is formed by a first cylinder 2a and the first piston 3, and wherein a second compression chamber 5b is formed by a second cylinder 2b and the second piston 4. The first compression chamber 5a is fluidly connected to the outside via a first exhaust valve 6a and a first inlet valve 7a. The second compression chamber 5b is connected to the outside via a second outlet valve 6b as well as a second inlet valve 7b conducting fluid. The first and second compression chambers 5a, 5b are operated in opposite directions via the free piston arrangement 16. Preferably, sealing rings and/or bearing rings are also arranged on the first and second pistons 3,4 for supporting the free piston arrangement 16 within the linear motor compressor 1, as well as for sealing the pistons 3,4 with respect to the compression chambers 5a, 5b, such rings being generally known and not shown in FIG. 2. The pistons 3,4 could also be designed as labyrinth pistons, so that sealing rings can be dispensed with because the labyrinth structure on the surface of the piston 3, 4 provides the sealing function. FIG. 3 shows a longitudinal section along the line of intersection F-F through the linear permanent magnet synchronous motor 14 comprising a stator 8 with laminated stator segments 8a, comprising a plurality of preferably individually controllable stator windings 12a, 12b, 12c, 12d, 12e, 12f for generating actively controllable magnetic poles 13a-13f resp. magnetic fields, and comprising a linear motor rotor 10 having a plurality of mutually longitudinally spaced permanent magnets 10a, the linear motor rotor 10 forming part of the piston rod 9. The piston rod 9 includes mounting portions 9a to which the first and second pistons 3,4 are connected, respectively. In addition, the linear permanent magnet synchronous motor 14 advantageously comprises two radial bearings 11 for guiding the piston rod 9.

FIG. 4 shows a section along section line E-E through the linear permanent magnet synchronous motor 14, with a section through the stator segment 8 and the second stator winding 12b, and a section through the piston rod 9 and the permanent magnet 10a.

A permanent magnet motor, an asynchronous motor or a reluctance motor is also suitable as a linear motor 14, for example. The linear motor 14 and thus also the driven pistons 3,4 advantageously have a maximum stroke XL in the range between 50 mm and 500 mm. The linear motor 14 thus permits relatively long-stroke movements.

The linear motor 14 shown in FIGS. 2 to 4 comprises six actively controllable magnetic poles 13a-13f, each pole being surrounded by a stator winding 12a-12f, preferably each stator winding 12a-12f being individually controllable. The linear motor 14 preferably has between three and ten actively controllable magnetic poles 13a-13f, or preferably between 10 to 50 actively controllable magnetic poles 13a-13f. The number of actively controllable magnetic poles 13a-13f depends in particular on the length of the maximum stroke XL. The number of actively controllable magnetic poles 13a-13f can also have an influence on how precisely the force applied by the stator 8 to the linear motor rotor 10 can be controlled as a function of time or as a function of the stroke X.

FIG. 5 shows exemplary characteristic curves 30 to 33 of a possible control of a linear motor 14 as shown in FIG. 1 or in FIG. 2, whereby the characteristic curves shown show the case in which the linear motor 14 is not connected to the reciprocating compressor 15, but is only operated alone and independently of the reciprocating compressor 15. The characteristic curve 30 shows the stroke X of the linear motor 14 as a function of time t during a complete cycle from bottom dead center X_UPT to top dead center X_OTP and back. The characteristic curve 31 shows the velocity of the linear motor rotor 10 as a function of time t, wherein the linear motor 14 is controlled such that the velocity of the linear motor rotor 10 as a function of time increases linearly in section 31a, is constant in section 31b, decreases linearly in section 13c, and is zero in section 13d, so that the linear motor rotor 10 is at a standstill. When the linear motor rotor 10 is at a standstill, it has reached the top dead center X_OTP, as can be seen from the characteristic curve 30. During the return movement to bottom dead center X_UTP, the velocity of the linear motor rotor 10 increases linearly in section 31e with a negative sign, remains constant in section 31f, and decreases linearly in section 13g until the linear motor rotor 10 comes to a standstill at bottom dead center X_UTP. The characteristic curve 32 shows the acceleration of the linear motor rotor 10 as a function of time t, where the linear motor rotor 10 is accelerated with constant positive acceleration in sections 32a and 32g, decelerated with constant negative acceleration in sections 32c, 32e, and moved without acceleration in sections 32b, 32d and 32f. The characteristic curve 33 shows the force applied by the linear motor 14 to the linear motor rotor 10 as a function of time t, with a constant acceleration force being applied in section 33a, with a small constant force being applied in section 33b to overcome the frictional forces applied during movement of the linear motor rotor 10, and with a negative force being applied in section 33c to bring the linear motor rotor 10 to a standstill at top dead center X_OTP. During the standstill, i.e. during section 33d, no acceleration is applied. Thereafter, the linear motor rotor 10 is accelerated again by a constant force in section 33e, kept in motion by the small constant force acting in section 33f to overcome the applied frictional forces, and braked to a stop by the constant force acting in section 33g at bottom dead center X_UTP. During sections 33c and 33g, the linear motor 14 brakes the linear motor rotor 10, and the energy released in this process can be converted into heat, wherein the linear motor 14 is preferably operated as a generator during sections 33c and 33g, and the electrical energy released in this process is temporarily stored in an energy store, preferably in the drive device.

The linear motor 14 can be controlled via a corresponding control by a plurality of possibilities in function of the time t or the stroke X, wherein preferably at least one of the characteristic curves of stroke, velocity, acceleration and motor force is predetermined in function of the time t, wherein the control device controls the linear motor 14 in such a way that it moves at least approximately according to the predetermined characteristic curve. FIG. 6 shows another example of a control of the linear motor 14. The characteristic curve 34 shows the stroke X of the linear motor 14 as a function of the time t during a complete cycle. The characteristic curve 35 shows the velocity, the characteristic curve 36 the acceleration and the characteristic curve 37 the motor force of the linear motor 14 as a function of time t. Up to time t1, the characteristic curve according to FIG. 6 shows a similar course as the characteristic curve according to FIG. 5, whereby the time axis in FIG. 6 is considerably shorter, i.e. the movement in FIG. 6 is considerably faster, which can also be seen from the fact that the values for velocity, acceleration and motor force in FIG. 6 are considerably higher compared to FIG. 5.

The characteristic curves shown in FIG. 6 between the time t1 and the total cycle time T, the completion of the complete cycle, show a further control example of the linear motor 14.

This control method has the advantage that all characteristic curves 34, 35, 36, and 37 change continuously between the time t2 and the time t3 and have no kinks, which means that the linear motor 14 is operated more gently, since kinks usually cause an abrupt change in the operating behavior, which results in an increased mechanical load. The control example shown in FIG. 6 is only one example of a multitude of possibilities to control the linear motor 14. The possibility of operating the linear motor 14 with a plurality of different characteristic curves as a function of time results in the advantage that a reciprocating compressor 15 driven by such a linear motor 14 can be operated in a plurality of ways. Preferably, the linear motor 14 is operated in such a way that the linearly movable free piston arrangement 16 is given at least one state variable Z_nominal as a function of the time t or as a function of the stroke X, the linear motor 14 being controlled in a regulated manner in such a way that the free piston arrangement 16 has or at least approximately assumes the given state variable Z_nominal. This control method allows the operation of the linear motor compressor 1 to be optimized, for example, with regard to predefinable key figures, in that, for example, the maximum force to be output by the linear motor 14, the maximum power required to operate the linear compressor 1, the maximum acceleration occurring and/or the maximum velocity occurring is predefined, which may not be exceeded during operation.

As shown in FIGS. 1 and 2, the linear motor compressor 1 comprises an electric linear motor 14 and a reciprocating compressor 15 driven by the linear motor 14. The overall dynamics of the movement of the linear motor compressor 1 is thus essentially determined by the dynamics of the linear motor 14 in combination with the dynamics of the reciprocating piston compressor 15 connected to the linear motor 14, the overall dynamics being essentially determined by the inertial forces acting, by the electromagnetic forces caused by the linear motor 14, by the gas forces caused by the reciprocating piston compressor 15 or acting in the reciprocating piston compressor 15, and by the gas forces acting in the reciprocating piston compressor 15. in the reciprocating compressor 15, and by the frictional forces caused by the movement of the piston and the linear motor 14.

In the following, the dynamics of the movement of the free piston arrangement 16 are described in more detail by establishing an equation of motion.

As shown in FIG. 2, the two pistons 3,4 are driven by the force caused by the linear motor 14 in a reciprocating motion running in stroke direction X. The first piston 3 moves in a positive stroke direction X in the range between bottom dead center X_UTP and top dead center X_OTP. The first piston 3 moves back and forth in a positive stroke direction X in the range between the bottom dead center X_UPT and the top dead center X_OTP. The second piston 4 moves in the opposite direction to the first piston 3 and, as shown in FIG. 2, in a positive stroke direction X, in the range between the top dead center X_OTP and the bottom dead center X_UTP. Since the two pistons 3, 4 are moved in opposite directions, the motion analysis of the free piston arrangement 16 is considered only for the cycle of the first piston 3 moving in a positive X direction from the bottom dead center X_UPT to the top dead center X_OTP and back to the bottom dead center X_UTP. According to this simplified model, the equation for the driving force FLM to be applied to move the free piston assembly 16 by the linear motor 14 is as follows:

$m_g\ddot{}x=F_{\mathrm{LM}}+F_{\mathrm{pr}}-F_{\mathrm{pl}}-F_{\mathrm{fr}}-F_{\mathrm{fl}}$

where mg is the total mass of the free piston assembly 16, x is the displacement and stroke of the free piston assembly, respectively, Fpr and Fpl are the forces acting on the first and second pistons 3,4 due to the gas pressure in the right, second and left, first compression chambers 5b, 5a, respectively, and Ffr and Ffl are the frictional forces of the right, second piston 4 and the left, first piston 3, respectively.

The force caused by the gas pressure on the first and second pistons 3,4 respectively can be calculated according to the following equation:

$F_{\mathrm{pi}}=\frac{\pi }{4}{d}_p^2{P}_i$

where d_p is the diameter of the first and second pistons 3, 4, respectively, and P_i is the gas pressure in the second, right compression chamber 5b (i=1) and in the first, left compression chamber 5a (i=r), respectively.

In view of the known overall dynamics of the linear motor compressor 1, a state variable Z_nominal can be specified to the control device, whereby the control device controls the linear motor 14 in such a way that the linear motor compressor 1 has the specified state variable Z_nominal at least approximately.

In the simplest case, a stroke travel point X₁, i.e. a defined point along the stroke X, and a setpoint velocity v_nominal and/or a setpoint acceleration a_nominal and/or a setpoint force F_nominal of the free piston arrangement 16 can be specified as the state variable Z_nominal. Instead of the stroke travel point X₁, a stroke travel time T_L1 could also be specified, i.e. a defined time within the total cycle time T, with the bottom dead center X_UTP preferably being used as the time measurement reference. Thus, in the simplest case, a stroke time T_L1 and, assigned to it, a setpoint velocity v_nominal and/or a setpoint acceleration a_nominal and/or a setpoint force F_nominal of the free piston arrangement 16 can also be specified as the state variable Z_nominal. If, as shown in FIG. 9, for example, it is a question of ensuring that the velocity of the free piston arrangement 16 is reduced in the region of points B and D, then a state variable Z_nominal would suffice, which specifies the velocity v_nominal at the stroke path point X₁ and the velocity −v_nominal at the stroke path point X₂. In a similar way, a target acceleration a_nominal and/or a target force F_nominal could of course also be specified at a stroke path point.

Advantageously, a course of the state variable Z_nominal to be maintained along at least a partial section of the stroke path X, and preferably along the entire stroke path XL, is specified as the state variable Z_nominal.

In a further advantageous method, a curve of the state variable Z_nominal to be maintained during part of the total cycle time T and preferably during the total cycle time T is specified as the state variable Z_nominal.

In a further advantageous method, a velocity-displacement curve between the bottom dead center X_UPT and the top dead center X_OTP and/or the top dead center X_OTP and the bottom dead center X_UTP, according to which the free piston arrangement 16 is moved back and forth during operation of the linear motor compressor 1 shown in FIG. 2, is specified as the Z_nominal state variable. FIG. 7 shows an example of such an operating method of the linear motor compressor 1 comprising both the linear motor 14 and the reciprocating piston compressor 15. FIG. 7 shows as state variable Z_nominal velocity-displacement curves G₁, G₂, G₃, G₄ according to an operating method according to the invention, which is explained with the aid of FIG. 2. The diagram according to FIG. 7 shows on the left side, to the left of point A, the movement of the first piston 3, which, starting from point A, the bottom dead center X_UTP, moves according to curve G₁ via point B shown in FIG. 1 to point C, the top dead center X_OTP, and which, according to curve G₂, moves via point D back to point A, the bottom dead center X_UTP. The diagram according to FIG. 7 shows on the right side, to the right of point A, as the state variable Z_nominal the velocity-displacement curves G₃, G₄ of the second piston 4. Since the second piston 4 moves in the opposite direction to the first piston 3, the second piston 4 moves in the opposite direction to the first piston 3, in that the second piston 4 moves from point C, the top dead center X_OTP, via point D shown in FIG. 1 to point A, the bottom dead center X_UTP, according to curve G₃, and moves back via point B to point C, the top dead center X_OTP, according to curve G₄. Since the first and second pistons 3,4 are fixed to each other and therefore have the same velocity, except for the different sign of the velocity, the trajectories G₁ and G₃ otherwise have an identical course. For the same reasons, the curves G₂ and G₄ also exhibit an identical course, with the exception of the different sign of the velocity. Preferably, the linear motor compressor 1 is moved between the bottom dead center X_UPT and the top dead center X_OTP as well as on the return path between the top dead center X_OTP and the bottom dead center X_UTP with the same state variable Z_nominal or with the same velocity-displacement curve, so that all curves, with the exception of the different sign concerning velocity, have the same curve G₁, G₂, G₃, G₄. As shown in FIG. 7, the first piston 3 has a first mean velocity V_m1 between points A and B, the compression phase AB, and the piston 3 has a second mean velocity V_m2 between points B and C, the ejection phase BC, the first mean velocity V_m1 being greater than the second mean velocity V_m2. The mean velocity is understood to be the meand velocity value of piston 3 or 4 between two points. Thus, the first mean velocity V_m1 corresponds to the mean velocity between points A and B, respectively, the first mean velocity V_m1 corresponds to the time integral of the velocity V(t) between points A and B divided by the time required to move the piston 3 between points A and B, respectively, the first mean velocity V_m1 corresponds to the integral of the velocity V(X) along the path X between points A and B divided by the path distance A-B required to move the piston 3 between points A and B. Similarly, the second mean velocity V_m2 thus corresponds to the mean velocity between points B and C, respectively, the second mean velocity V_m2 Corresponds to the integral of the velocity V(t) or V(X) between points B and C, divided by the time or the path distance B-C, respectively, required to move the piston 3 between points B and C.

As shown in FIG. 7, during the return movement from point C to point A, the first piston 3 has a third mean velocity V_m3 between points C and D, the relaxation phase CD, and between points D and A, the suction phase DA, the piston 3 has a fourth mean velocity V_m4, the third mean velocity V_m3 being greater than the fourth mean velocity V_m4. Thus, the third mean velocity V_m3 Corresponds to the mean velocity between points C and D, respectively the third mean velocity V_m3 corresponds to the integral of the velocity V(t) or V(X) between points C and D, divided by the time or the distance C-D, respectively, required to move the piston 3 between points C and D. Similarly, the fourth mean velocity V_m4 thus corresponds to the mean velocity between points D and A, or the fourth mean velocity V_m4 corresponds to the integral of the velocity V(t) or V(X) between points D and A, divided by the time or the distance D-A required to move the piston 3 between points D and A, respectively.

The linear motor compressor 1 is preferably operated in such a way that the pistons 3 and 4 have identical velocity-displacement curves G₁ and G₄, or identical velocity-displacement curves G₂ and G₃, on their reciprocating motion, apart from the mirroring required on the axes according to FIG. 7. In another possible mode of operation, it is also possible that the two pistons 3 and 4 each have a different velocity-displacement curve on their reciprocating movement, for example, a different, different velocity-displacement curve on their movement from right to left than on their movement from left to right.

The interaction of linear motor 14 and reciprocating compressor 15, for example, can also be understood from the velocity-displacement curve G1 shown in FIG. 7. Starting from point A up to the stroke point X3, the course G1 shows a relatively rapid increase, which is due in particular to the fact that small forces are still required in the reciprocating compressor 15 at the beginning of the compression phase. In addition, the second compression chamber 5b or the gas therein is in a relaxation phase, so that this gas drives the second piston 4, so that the combination of the driving force of the linear motor 14 and the relaxation force acting on the second piston 4 result in rapid movement, i.e. an increasing velocity V or acceleration of the free piston arrangement 16. In a further advantageous method, the free piston arrangement 16 is driven by the linear motor 14 so quickly that the relaxation force makes a negligible contribution or no contribution at all to the movement of the free piston arrangement 16. In the stroke direction X after the stroke point X3, the compression power absorbed by the reciprocating compressor 15 steadily increases so that the velocity of the free-piston arrangement 16 is reduced, which is especially true if the linear motor 14 is operated at constant power. After point B, the discharge valve 6a is opened, and the velocity of the free piston arrangement 16 is further reduced due to the flow resistance occurring at the discharge valve 6a. Beginning with the stroke point X4, the free piston arrangement 16 must be braked and brought to a standstill up to the top dead center X_OTP, Which is preferably done by the linear motor 14 generating a braking force, and advantageously operated as a generator, the generated electrical energy preferably being temporarily stored in a control device, for example in order to accelerate the free piston arrangement 16 again in the section A-X3.

FIG. 7 shows a velocity-displacement diagram (v-x diagram) as state variable Znom. Instead of a velocity-distance diagram, one of the characteristic curves 30 to 37 shown in FIG. 5 or 6, for example, stroke, velocity, acceleration or force as a function of time, could also be specified as the state variable Znom. In addition, a combination of several of the characteristic curves 30 to 37 shown in FIG. 5 or 6 could also be specified, for example by selecting the state variable Z_nominal in such a way that, for example, a maximum velocity and/or acceleration and/or force to be output by the linear motor 14 and/or electrical energy consumed by the linear motor 14 is not exceeded.

In an advantageous method, the free piston arrangement 16 is moved from the bottom dead center X_UTP during a compression phase AB, up to the opening point B of the exhaust valve 6, with a predetermined velocity-displacement curve G1 in such a way that the linear motor 14 has to output a constant or essentially constant power. The power is calculated from the driving force FLM to be applied by the linear motor 14 multiplied by the velocity V of the free piston arrangement 16. With a predetermined constant power, the predetermined velocity-displacement curve G1 can thus be calculated. This method has the advantage that the linear motor compressor can also be operated safely with lower power.

During continuous operation of the linear motor compressor 1, it has an expansion phase CD between points C and D, during which the gas located in the dead volume Vtot is expanded. In one possible method, the linear motor 14 can be operated as a generator at least along a partial section of the expansion phase CD, in that the linear motor 14 brakes the movement of the piston 3, 4 caused by the expansion forces by means of generator operation, the electrical energy generated in the process preferably being temporarily stored. In a particularly advantageous method, the linear motor 14 is controlled along at least a partial section of the relaxation phase CD and preferably during the entire relaxation phase CD in such a way that the linear motor 14 does not exert an active braking effect on the free piston arrangement 16 during the entire relaxation phase CD, preferably in such a way that the linear motor 14 exerts a positive braking effect on the free piston arrangement 16 during the entire relaxation phase CD and preferably during the points C and A, i. e.i.e. the entire phase CA, exert a positive force on the free piston arrangement 16 acting in the direction towards the bottom dead center X_UTP. This method ensures that the energy released by the gas located in the dead space Vtot during expansion along the expansion phase CD is preferably completely converted into a kinetic energy of the free piston arrangement 16, which supports compression of the gas located in the second compression chamber 5b by transferring the kinetic energy of the free piston arrangement 16 to the gas via the second piston 4.

FIG. 8 shows a further, advantageous operating method of the linear motor compressor 1 shown in FIG. 2. FIG. 8 shows a rather schematic, i.e. slightly idealized velocity-distance diagram of the first piston 3, where the diagram shows the velocity of the first piston 3 as a function of the stroke X during the phase AC, for a simplified illustration of the process taking place, and where the diagram shows subsequently to the right the velocity of the first piston 3 as a function of the stroke X during the phase CA, where for a better illustration the stroke X during the phase CA is shown running to the right, in contrast to FIG. 7. In itself, the first piston 3 is at bottom dead center X_UTP both at the beginning and at the end of the diagram shown in FIG. 8. The predetermined state variable Z_nominal are the curves G1 and G2 for the free piston arrangement 16, namely velocity-displacement curves as shown in FIG. 8. The compression phase AB is run through at a relatively high velocity, in particular at a relatively high first mean velocity V_m1, and thus relatively quickly in terms of time. In the region of the opening point B of the outlet valve 6, the velocity v is reduced, so that the free piston arrangement 16 is moved during the ejection phase BC at reduced velocity, or at a lower second mean velocity V_m2 compared to the compression phase AB. Starting from the bottom dead center X_UTP, the free piston arrangement 16 is moved during a compression phase AB up to the opening point B of the exhaust valve 6 and subsequently during an ejection phase BC up to the closing point C of the exhaust valve 6 in such a way that a predetermined state variable Z_nominal, a predetermined velocity-displacement curve or a predetermined velocity-time curve, in such a way that the first mean velocity V_m1 during the compression phase AB is higher than the second mean velocity V_m2 during the ejection phase BC and/or that the duration of the compression phase AB is shorter than the duration of the ejection phase BC. In particular, this method has the advantage that it is possible to increase the time duration of the ejection phase BC. This method has the advantage that the free piston arrangement 16 can be operated at reduced velocity during the ejection phase BC, i.e. during the outflow of the gas from the outlet valve 6a, which reduces the outflow resistance caused by the outlet valve 6a, and which therefore also reduces the loss energy caused by the outflow. Since no outflow takes place during the compression phase AB, the compression phase AB can be run through at increased velocity or at a higher mean velocity with no or extremely low additional energy, so that the ejection phase BC can preferably be extended in time by running through the ejection phase BC at a lower mean velocity compared to the compression phase AB. This method has the advantage that the energy loss caused by the gas flowing out through the exhaust valve can be reduced. For a given cycle time Tz of a complete reciprocating motion of the linear motor compressor, the latter is advantageously operated in such a way that the first mean velocity V_m1 during the compression phase AB is set high, preferably as high as possible, and that the second mean velocity V_m2 is set lower than the first mean velocity V_m1, preferably as low as possible, but in such a way that the given cycle time Tz of a complete reciprocating motion is maintained. Thus, with a given cycle time Tz, this method makes it possible to extend the duration of the ejection phase BC or to reduce the flow rate of the gas out of the cylinder interior at the exhaust valve 6a, which reduces the energy dissipation occurring at the exhaust valve. This process makes it possible to increase the efficiency of the linear motor compressor. In a particularly advantageous method, the linear motor 8 drives the free piston arrangement 16 by motor at least during a partial section of the compression phase AB, wherein the linear motor 8 brakes the free piston arrangement 16 at least during a partial section of the discharge phase BC, and in doing so is preferably operated as a generator which releases electrical energy which is preferably temporarily stored and preferably reused to supply the linear motor 8 with electrical energy during the compression phase AB. This short-term intermediate storage of electrical energy allows the linear motor compressor to be operated particularly efficiently, ensuring in particular that the first mean velocity V_m1 during the compression phase AB is higher than the second mean velocity V_m2 during the discharge phase BC. The state variable Z_nominal according to the curve G₂ shows the movement of the free piston arrangement 16 during the phase CA, respectively the movement of the first piston 3 from the top dead center X_OTP to the bottom dead center X_UTP. If the linear motor compressor 1 has a first and a second compression chamber 5a, 5b, which are compressed by counter-rotating pistons 3, 4, as shown in FIG. 2, the two curves G1, G2 have the same course, except for the sign of the velocity v, as shown in FIG. 8. If the linear motor compressor 1 has only a single, first compression chamber 5a, the two curves G1 and G2 can also have a different curve. The mean velocity V_m3 of the expansion phase CD is higher than the mean velocity V_m4 of the suction phase DA, as shown in FIG. 8.

FIG. 9 shows a further operating method of the linear motor compressor 1 shown in FIG. 2. FIG. 9 shows a velocity-displacement diagram of the first piston 3. As a predetermined state variable Z_nominal, the progressions G1 and G2 are specified for the free piston arrangement 16. These progressions G1 and G2 are selected in such a way that the first and second pistons 3,4 respectively move in the region of points A, B, C and D at low or reduced velocity v, the velocity at points C and A being reduced to 0 m/s, since the free piston arrangement comes to a standstill for a short time at these reversal points. Preferably, the velocity in the region of points C and A, i.e. in particular immediately before reaching points C and A, is reduced to a particularly large extent, so that this velocity is lower than in points B and D. FIGS. 9a and 9b show in detail the velocity of FIG. 9 in the region of point C and A, respectively. In an advantageous process, the velocity at points C and A is very low, and is, for example, less than 0.1 m/s. The reduced velocity at points C and A has the effect that the inlet valve 7a and the outlet valve 6a are closed at low velocity, respectively, which has the effect that these valves are mechanically only slightly stressed by this gentle closing, so that the valves can be operated reliably and preferably maintenance-free for a long time. As shown in FIG. 9b, the free piston arrangement 16 is first decelerated with a larger negative acceleration towards the end of the stroke, towards the bottom dead center X_UTP, and then decelerated with a reduced negative acceleration, whereby the free piston arrangement 16 is decelerated at the dead center X_UTP with reduced negative acceleration until it comes to a standstill and is then accelerated again in the opposite direction. As shown in FIG. 9a, the free piston arrangement 16 is first decelerated with a larger negative acceleration towards the end of the stroke, towards the top dead center X_OTP, and then decelerated with a reduced negative acceleration, whereby the free piston arrangement 16 is decelerated to a standstill at the top dead center X_OTP with reduced negative acceleration, and is then accelerated again in the opposite direction. In the simplest case, a state variable Z_nominal can consist of only a single point, for example the stroke path point X1 with the value v_nominal as shown in FIG. 9. The control is thus carried out in such a way that the free piston arrangement 16 has the velocity v_nominal at the stroke path point X1. This ensures that the velocity of the free piston arrangement 16 at the stroke path point X1 has the desired, low velocity v_nominal.

FIG. 10 shows a control device 20 for the operation of a linear motor compressor 1. A control device 27 detects with at least one sensor 21 via a signal line an actual state variable 29a at least one actual state variable of the linear motor compressor 1, preferably the stroke X and/or the velocity v and/or the acceleration and/or the applied force F of the free piston arrangement 16. The setpoint value of a state variable Zset is preset via a setpoint presetting device. The control device 27 calculates a control signal 29b from the actual state variable 29a and the setpoint state variable Zsetpoint 29e, which is fed to an inverter control device 26. The inverter drive device 26 drives a power supply 23 and an inverter 22 via control lines 29c, 29d, wherein the inverter 22 comprises a plurality of drives to individually drive a plurality of stator windings 12a, 12b, 12c, 12d via electrical conductors 24a, 24b, 24c, 24d. The power supply 23 is connected to the inverter 22 via a power line 25. In a particularly advantageous embodiment, the energy supply 23 comprises an energy storage device, wherein the inverter 22 is controllable such that electrical energy can be extracted from the linear motor compressor 1 and supplied via the inverter 22 to the energy supply 23, in which the electrical energy is stored, preferably for a short time, during a period of preferably less than one second or less than one minute.

Claims

1-2. (canceled)

3. The method according to claim 21, wherein the velocity of the free piston arrangement in the region of the opening point of the exhaust valve is reduced to a velocity lower than the mean velocity during the compression phase.

4. The method according to claim 21, wherein the velocity of the free piston arrangement in the region of the opening point of the inlet valve is reduced to a velocity lower than the mean velocity during the expansion phase.

5. The method according to claim 3, wherein the free piston arrangement is accelerated again after at least one of the opening point of the exhaust valve and the opening point of the inlet valve.

6. (canceled)

7. The method according to claim 21, wherein the free piston arrangement is braked with a greater negative acceleration towards the end of the stroke, towards the top dead center, and is subsequently braked with a reduced negative acceleration, the free piston arrangement being braked at the top dead center with reduced negative acceleration until it comes to a standstill.

8. The method according to claim 21, wherein a stroke travel point and, assigned to this, a setpoint velocity of the free piston arrangement are specified as the state variable.

9. The method according to claim 21, wherein a nominal profile to be maintained along at least a partial section of the stroke path is specified as a state variable.

10. The method according to claim 9, wherein a setpoint profile to be maintained during a stroke time required for the entire stroke path is specified as the state variable.

11. The method according to claim 21, wherein the linear motor compressor comprises a first and a second compression chamber which are operated in opposite directions by the free piston arrangement.

12. The method according to claim 21, wherein a velocity-displacement curve between at least one of

the bottom dead center and the top dead center and

the top dead center and the bottom dead center,

according to which velocity-displacement curve the free piston arrangement is moved back and forth, is predetermined as a state variable.

13. The method according to claim 21, wherein the free piston arrangement, starting from the bottom dead center, is moved during a compression phase, up to the opening point of the exhaust valve, with a predetermined velocity-displacement curve, in such a way that the linear motor has to deliver a constant or substantially constant power as a function of time.

14. The method according to claim 21, wherein the predetermined state variable, a predetermined velocity-displacement curve at least in the range of one of the following points:

opening point of the exhaust valve, closing point of the exhaust valve, opening point of the inlet valve, closing point of the inlet valve,

has a reduced velocity compared to the rest of the velocity path, so that the outlet or inlet valve is moved at reduced velocity.

15. The method according to claim 21, wherein the linear motor exerts a positive force on the free piston arrangement acting in the direction towards the bottom dead center during the entire expansion phase.

16. The method according to claim 21, wherein the volume delivered by the linear motor compressor is changed by changing the maximum stroke of the linear motor or specifically by changing at least one of the location of the top dead center and the location of the bottom dead center.

17. The method according to claim 21, wherein the free piston arrangement is braked at least in sections during the reciprocating movement between the top dead center and the bottom dead center by operating the linear motor as a generator.

18. The method according to claim 21, wherein the linear motor is operated as a generator by supplying a pressurized fluid to the compression chamber via the outlet valve, expanding the fluid in the compression chamber, and subsequently discharging it via the inlet valve, and in that the free piston arrangement of the linear motor operated as a generator is moved back and forth with a predetermined velocity-displacement curve.

19. A linear motor compressor comprising at least one electric linear motor, a cylinder and a linearly movable free piston arrangement with at least one piston, the cylinder and the piston forming at least one compression chamber, the free piston arrangement being driven directly by the linear motor, the compression chamber being connected to the outside in a fluid-conducting manner via an outlet valve and an inlet valve, a control device controlling the linear motor in such a manner that the free piston arrangement is moved back and forth between a top dead center and a bottom dead center with a predetermined state variable,

wherein the control device controls the free piston arrangement starting from the bottom dead center during a compression phase up to the opening point of the outlet valve and subsequently during an ejection phase up to the closing point of the outlet valve with a predetermined velocity-displacement curve, in such a way that the mean velocity during the compression phase is higher than the mean velocity during the ejection phase, and wherein the control device controls the free piston arrangement, starting from the bottom dead center during a compression phase up to the opening point of the outlet valve and subsequently during an ejection phase up to the closing point of the outlet valve with a predetermined velocity-displacement curve, in such a way that the free piston arrangement is first braked with a greater negative acceleration towards the end of the stroke, towards the bottom dead center, and is subsequently braked with a reduced negative acceleration, the free piston arrangement being braked at the bottom dead center with reduced negative acceleration until it comes to a standstill.

20. The linear motor compressor according to claim 19, wherein said linear motor is operable as a motor and as a generator, and that said drive device drives said linear motor such that said free piston assembly is driven with a predetermined velocity-displacement curve when moving between a top dead center and a bottom dead center.

21. A method for operating a linear motor compressor comprising an electric linear motor, a cylinder, and a linearly movable free piston arrangement with a piston, wherein the cylinder and the piston form a compression chamber, wherein the free piston arrangement is driven directly by the linear motor and is moved back and forth along a stroke path between a top dead center and a bottom dead center, wherein a fluid is supplied to the compression chamber from the outside, wherein the supplied fluid is compressed or expanded in the compression chamber and is subsequently discharged to the outside again, wherein at least one state variable is preset for the linear motor compressor, and wherein the linear motor compressor is controlled in such a way that the linear motor compressor has the at least one preset state variable,

wherein the free piston arrangement, starting from the bottom dead center during a compression phase up to the opening point of the outlet valve and subsequently during an ejection phase up to the closing point of the outlet valve, is driven with a predetermined state variable, a predetermined velocity-displacement curve, in such a way that the mean velocity during the compression phase is higher than the mean velocity during the ejection phase,

wherein the free piston arrangement is first braked with a greater negative acceleration towards the end of the stroke, towards the bottom dead center, and is subsequently braked with a reduced negative acceleration, the free piston arrangement being braked at the bottom dead center with reduced negative acceleration until it comes to a standstill.