WO2019091512A1

WO2019091512A1 - Method and device for determining the position of a piston of a hydraulic cylinder of a machine tool

Info

Publication number: WO2019091512A1
Application number: PCT/DE2018/100875
Authority: WO
Inventors: Herbert Wieser; Matthias Streller; Robert Schmidt
Original assignee: Grob-Werke Gmbh & Co. Kg
Priority date: 2017-11-10
Filing date: 2018-10-26
Publication date: 2019-05-16
Also published as: DE102017129445A1

Abstract

The invention relates to a method and to a device (28) for determining the position of a piston (30) of a hydraulic cylinder (26) of a machine tool (10). In order to enable more accurate position detection, in particular in order to monitor the function of the hydraulic cylinder (26) during operation of the machine tool (10), the method comprises: a) producing a reference signal by sending an acoustic signal through a cylinder wall (32) of the hydraulic cylinder (26) to the piston (30), which is in a reference position, and capturing an echo signal as the reference signal; b) sending the acoustic signal through the cylinder wall (32) to the piston (30) in order to determine the position of the piston and capturing the echo signal as a measurement signal; c) producing a comparison signal from the measurement signal and the reference signal; and d) determining the propagation time from the comparison signal in order to determine the position of the piston (30) from the propagation time. The device (28) is designed accordingly for carrying out the method. The invention further relates to a hydraulic cylinder unit (24) having the device and to a machine tool (10) having said hydraulic cylinder unit.

Description

Method and device for determining the position of a piston of a hydraulic cylinder of a machine tool

The invention relates to a method for determining the position of a piston of a hydraulic cylinder of a machine tool. Furthermore, the invention relates to a device for determining the position of a piston of a

Hydraulic cylinder for a machine tool. Furthermore, the invention relates to a hydraulic cylinder unit for a machine tool with such

Device and a corresponding machine tool. Finally, the invention relates to a computer program product with a computer program for loading into a computer system connected to a hydraulic cylinder power tool to the piston positioning method at the

Execute machine tool.

The invention is in the field of machine tools and in particular in the field of machine tools with hydraulic cylinders. For example, with hydraulic tensioning cylinders of machine tools, in particular workpieces such as e.g. Cylinder blocks or cylinder heads of internal combustion engines,

tightened. In such hydraulic cylinders, particularly high pressures of e.g. 10 bar to 150 bar. The adjustment paths are small and are for example in the range of 0 to 50 mm. Due to the high pressures are high

Requirements for the stability of the cylinder set. Therefore, the should

Cylinder walls should be as stable as possible and weak points such as windows or

Recesses are undesirable. In addition, the operating temperatures vary greatly.

On the other hand, the exact control and / or monitoring of

Clamping information of the position of the piston of the hydraulic cylinder desirable. In the literature

[1] DE 102 46 565 B4

For this purpose, a volume flow of the hydraulic fluid in the inflow or outflow of the clamping cylinder is measured.

For other technical applications, methods and apparatus for determining the position of pistons by means of ultrasound are as follows

References known:

DE 10 2007 035 252 A1,

[4] US 4 543 649 A

[4] JP 59062705 A.

DE 103 30 914 A1,

DE 10 2006 030 761 A1,

[4] US 4,938,054 A,

DE 30 31 980 A,

DE 103 22 718 B4,

[10] DE 601 20 027 T2 and

[1 1] EP 1 079 1 18 A2.

However, the methods and apparatuses shown in [2] to [11] are not for hydraulic cylinders of machine tools, and especially not for

Hydraulic clamping cylinder suitable for machine tools, all of which provide windows or at least recesses on the cylinder wall, not for as high pressures, such as occur in hydraulic cylinders for machine tools, are suitable and at such operating temperature fluctuations and at such small adjustment ranges as hydraulic clamping cylinder of

Machine tools occur to deliver no usable signals.

Starting from the closest prior art according to DE 102 46 565 B4, the invention has for its object to provide a method and an apparatus with which the position of a piston of a hydraulic cylinder of a machine tool can be determined more accurately.

To achieve this object, the invention provides a method with the steps of claim 1 and an apparatus having the features of claim 7. A hydraulic unit for a machine tool, provided therewith

Machine tool and a computer program product or

Computer program medium with control instructions for carrying out the method are the subject of the additional claims.

Advantageous embodiments are the subject of the dependent claims.

The invention provides, according to a first aspect thereof, a method for determining the position of a piston of a hydraulic cylinder

Machine tool comprising:

a) generating a reference signal by sending an acoustic signal through a cylinder wall of the hydraulic cylinder to the in a

Reference position located piston and detecting an echo signal as

Reference signal and

b) sending the acoustic signal through the cylinder wall to the piston to determine its position and detecting the echo signal as a measurement signal and

c) generating a comparison signal from the measurement signal and the

Reference signal and

d) determining the transit time from the comparison signal to determine the position of the piston from the transit time.

It is preferred that the transmission of the acoustic signal and the detection of the echo signal in steps a) and b) by a particular

piezoelectric transducer takes place, which is attached to the outside of the cylinder wall formed entirely of metal. The sound transducer, in particular piezo transducer, can be arranged directly on the cylinder wall. But it can also intermediate layers between the cylinder wall and the

Be provided sound transducer. For example, the transducer can be part of a

be multi-layered transducer.

In an alternative embodiment of the method is provided that on the cylinder wall, in particular on the cylinder base, a first sound transducer for generating the sound waves (transmitter) and a second sound transducer for Converting the received sound waves into electrical signals (ie

Receiver), to separate the transmission function and the reception function.

It is preferred that as the cylinder wall, through which the transmission and reception takes place, a cylinder bottom extending transversely to the direction of movement of the piston is selected.

It is preferred that step c) comprises forming a difference signal from the measurement signal and the reference signal.

It is preferred that step a) comprises:

a1) generating the reference signal several times in the course of operation of the hydraulic cylinder.

It is preferred that step a) comprises:

a2) moving the hydraulic cylinder to the reference position.

It is preferred that step a) comprises:

a3) selecting one of the end positions of the piston as the reference position. It is preferred that step a) comprises:

a4) Selecting that position of the piston that causes the maximum transit time for the echo signal as a reference position.

It is preferred that step a) comprises:

a5) storing the reference signal or several at different times or different operating conditions detected reference signals in a memory.

It is preferred that step a) comprises:

a6) detecting a plurality of reference signals for different ones

Operating temperatures. It is preferred that step a) comprises:

a7) storing the reference signal together with information about at least one operating parameter present during the detection of the reference signal.

It is preferred that step a) comprises:

a8) detecting the reference signal, comparing the detected reference signal with at least one previously stored further reference signal, using the stored or a stored reference signal if same or similar to the detected reference, and using and storing the detected one

Reference signal, if this to the at least one stored

Reference signal is dissimilar.

It is preferred that step a) comprises:

a9) detecting the reference signal before any change in the position of the piston to a position to be measured towards or before or after a predetermined number of position determinations or after expiration of a predetermined period of time since the last reference signal determination or since the last one

Piston position determination or depending on a change of a

Operating parameters of the hydraulic cylinder or the hydraulic oil.

It is preferred that the position of the piston on a hydraulic cylinder at a pressure of above 10 bar, in particular above 50 bar and more in particular in a pressure range of 10 bar to 150 bar, is determined.

It is preferable that the position of the piston on a hydraulic cylinder is determined with a piston stroke in the range of 0 to 100 mm, more preferably 0 to 50.0 mm.

It is preferable that the position of the piston on a hydraulic cylinder is determined with a distance of a sound transmission and reception position to the piston of less than 200 mm, in particular less than 100 mm and in particular in a range of 1 mm to 50 mm. In a preferred embodiment, the method further comprises:

Providing a first reflection area for reflecting the acoustic signal and a second reflection area for reflecting the acoustic signal on the piston such that the reflection areas have a defined distance from one another in the running direction of the acoustic signal, and

Determining a transit time difference between an echo signal from the first reflection area and an echo signal from the second reflection area.

According to a further aspect, the invention provides a device for

Determining the position of a piston of a hydraulic cylinder for a

Machine tool comprising:

an externally attachable to a cylinder wall of the hydraulic cylinder

Sound transmission and reception device for transmitting and receiving acoustic signals through the cylinder wall in the direction of the piston and a control and evaluation device for the sound transmission and reception device,

wherein the control and evaluation device is designed:

for detecting that the piston is in a reference position, and / or for driving the hydraulic cylinder to turn the piston into a position

Move reference position,

for driving the sound transmission and reception means for transmitting an acoustic signal through the cylinder wall of the hydraulic cylinder to the piston located at a reference position and for recording thereupon a sound transmitted and received by the sound transmission and reception means

Echo signal as a reference signal,

for driving the sound transmission and reception device for transmitting the acoustic signal through the cylinder wall to the piston located in the position to be measured and for detecting the subsequently received echo signal as a measurement signal,

for generating a comparison signal from the measurement signal and the reference signal and

for determining the transit time from the comparison signal to determine from the transit time the position of the piston. Of course, more than one piston may be provided, and the device may also be configured to detect the position of more than one piston. For example, a first and a second

Hydraulic cylinder be provided, for which the position of a piston can be determined by a common control and evaluation. However, a hydraulic cylinder unit with several pistons may also be provided, e.g.

arranged concentrically or side by side, whose position can be determined by a common control and evaluation.

It is preferred that the sound transmission and reception device has a particular piezoelectric sound transducer.

It is preferred that the control and evaluation device is designed to be a difference signal from the measurement signal and the

To generate reference signal.

It is preferred that the control and evaluation device is designed to generate the reference signal several times in the course of the operation of the

Hydraulic cylinder.

It is preferred that the control and evaluation device is designed for moving the hydraulic cylinder into the reference position.

It is preferred that the control and evaluation device is designed to select one of the end positions of the piston as the reference position.

It is preferred that the control and evaluation device is designed to select that position of the piston which causes the greatest transit time for the echo signal as the reference position.

It is preferred that the control and evaluation device is designed to store the reference signal or a plurality of reference signals detected at different times or different operating conditions in a memory. It is preferred that the control and evaluation device is designed to detect a plurality of reference signals for different operating temperatures.

It is preferred that the control and evaluation device is designed to store the reference signal together with information about at least one present during the detection of the reference signal

Operating parameters.

It is preferred that the control and evaluation device is designed to detect the reference signal, compare the detected reference signal with at least one previously stored further reference signal, use the stored or a stored reference signal, if this is the same or similar to the detected, and use and store the detected reference signal when it is dissimilar to the at least one stored reference signal.

It is preferred that the control and evaluation device is designed to detect the reference signal before each movement of the piston to a position to be measured towards or before or after a predetermined number of position determinations or after a predetermined period of time since the last reference value determination or since the last Position determination or depending on a change of an operating parameter of

Hydraulic cylinder.

In another aspect, the invention provides a hydraulic cylinder unit for a machine tool comprising:

a hydraulic cylinder, in particular clamping cylinder, with a cylinder whose cylinder walls are formed entirely of a metal, and a piston movable in the cylinder; and

a device according to one or more of the previously described embodiments, wherein the sound transmitting and receiving device is mounted externally on the cylinder wall. It is preferred that the hydraulic cylinder is designed for a pressure range from 10 bar to 100 bar, in particular 10 bar to 150 bar.

It is preferable that the piston has a stroke measured by a

Cylinder bottom, on the outside of the sound transmitting and receiving device is mounted in a range between 0 mm and 100 mm, more preferably between 0 mm and 50.0 mm.

It is preferred that on the piston a first reflection region for reflecting the acoustic signal and a second reflection region for reflecting the acoustic signal is provided such that the reflection regions have a defined distance from each other in the running direction of the acoustic signal, and that the control and evaluation device is formed is for determining a transit time difference between an echo signal from the first one

Reflection area and an echo signal from the second reflection area.

In another aspect, the invention provides a machine tool comprising a hydraulic cylinder unit according to one or more of the previously discussed embodiments.

In another aspect, the invention provides a

A computer program product (algorithms) or computer readable computer program medium comprising control instructions for carrying out the steps of the method for determining the position of a piston of a computer

Machine tool according to one or more of the previously explained

Embodiments when loaded into a computer.

With preferred embodiments of the invention can be monitored during the process of machining a workpiece on a machine tool, such as a machining center, the safe operation of hydraulic cylinders. This can be achieved by determining the piston position in the hydraulic cylinder. Preferably, the current piston position is also reported during a piston movement.

Preferred embodiments of the invention relate to an acoustic

Piston position determination with fingerprint procedure.

Particularly preferred embodiments of the invention relate to a method and a device for determining the position of the piston in hydraulic cylinders made of metal, in particular steel, in machining centers based on acoustic

Delay measurements.

Preferred embodiments of the invention use ultrasound, pulse-echo method, reference formation and / or transit time measurement.

The invention is based on the basic idea of measuring the position of the piston directly acoustically, in particular by means of ultrasound. Thus, measurement errors arising in indirect measurements due to only indirect measurement across other elements, e.g. incurred by material expansion, play, or the like, avoided. However, results in hydraulic cylinders of

Machine tools the problem that as possible no interference with the cylinder and in particular no interference with the formation of the cylinder wall should be made and also the cylinder wall is formed of solid metallic materials. In particular, windows or breakthroughs - as provided in the method according to [2] to [1 1] - are avoided.

Preferred embodiments of the invention therefore go the way to perform the measurement through a cylinder wall.

It comes at the interface between the cylinder wall and

Hydraulic fluid to an impedance jump leads to reflections. With only very small adjustment can be at the transition between

Cylinder wall and hydraulic fluid generated echo signals very difficult by runtime fades from the measured echo signals from the piston. Therefore, preferred embodiments of the invention provide a Generation of a reference signal, which is a kind of fingerprint for the hydraulic cylinder to be measured and can be used in a comparative measurement to obtain the actual measurement signal.

Advantageous embodiments of the invention have the following technical data:

• Pressure range of the cylinder from 10 bar to 150 bar

• stroke of the piston 0 to 50.0 mm (with conventional ultrasonic sensors the measuring range does not start until approx. 200 mm)

• Temperature range -10 ° C to 80 ° C, especially 0 to 60 ° C

• Temperature fluctuations can be eliminated with preferred embodiments of the fingerprint method

• The sensor is referenced via the fingerprint procedure; e.g. in which

clear state in the position "piston fully extended"

• Measurement accuracy e.g. 0, 1 mm

Advantageous embodiments of the invention provide some, several or all of the following advantages:

• No interference with the mechanics of the cylinder:

o The sensor element may e.g. be attached externally to the cylinder wall, in particular to the cylinder bottom, e.g.

simply glued - this results in a simple installation

o The sensor can be connected directly to an actuator for actuating the cylinder

(e.g., a valve); it results in a direct

measure up

• Applicable to very different cylinder types

• Compared to previously known methods for determining the position of

Pistons of hydraulic cylinders of machine tools (see, for example, [1]), the position value of the piston is detected much faster and reported to the controller - thus can achieve a cycle time reduction for the machine tool.

• There is no caster as with previous position sensors, eg turbine sensors, which measure the hydraulic volume flow • The high measuring accuracy opens up the possibility of a workpiece type detection; ie the type of clamped by the cylinder

Workpiece can be detected via the position value

In an advantageous embodiment of the position sensor can also be

Detecting air bubbles in the piston chamber

• Turbine sensors are expendable; the mounting on the hydraulics will be

simplified and space in the hydraulic system is saved

• Via central electronics, several position measuring devices (sensors), e.g. evaluated by a multiplex method - e.g. 2 to 8 sensors can be operated simultaneously

Embodiments of the invention are explained in more detail below with reference to the accompanying drawings. It shows:

1 is a highly schematic representation of a machine tool with a clamping device, the hydraulic cylinder units for clamping a workpiece with hydraulic cylinders and a device for determining the position of the piston of the hydraulic cylinder has;

Fig. 2 shows another embodiment of the hydraulic cylinder unit for the

Machine tool;

Fig. 3 is a schematic representation of a hydraulic cylinder for a

Hydraulic cylinder unit together with a sensor element of a device for determining the position of the piston;

4 shows an enlarged representation of a part from FIG. 3 for explaining a method for determining the position of the piston by means of acoustic signals;

5 is a flowchart showing an embodiment of the present invention

Method for determining the position of the piston used fingerprint method for Referenzsignalbildung explained; and Fig. 6 is a representation comparable to FIG. 4 in another

Embodiment of the hydraulic cylinder unit for explaining a further embodiment of the method for determining the position of the piston.

In Fig. 1, an embodiment of a machine tool 10 is shown very schematically. The machine tool 10 is designed, for example, for machining a workpiece 12 and has a tool spindle 14, which is shown in a highly schematized manner, with a tool carrier on which a plurality of e.g. from a tool magazine 16 selectable and automatically replaceable tools 18 are attached, and a clamping device 20 for clamping the workpiece 12. The tensioning device 20 and the rotatable

Tool spindle 14 are movable relative to each other to perform the machining. Furthermore, a control device 22 for controlling the machine tool 10 is provided.

The machine tool 10 can be designed, for example, as a machining center or as part of a machining center, as described and shown, for example, in one of the following references [12] to [19], to which reference is expressly made for further details of the machine tool 10:

[12] Company brochure "MODULAR SPECIAL MACHINE" of GROBWERKE GmbH & Co. KG with the imprint 07/2013 / D

[14] Company brochure "G-SERIES - THE FLEXIBLE

MANUFACTURING SYSTEM "of GROB-WERKE GmbH & Co. KG with the imprint 09/2017 / DE

[15] Company brochure "MACHINING CENTERS FOR

FRAME STRUCTURE COMPONENTS "of GROB-WERKE GmbH & Co. KG with the imprint 09/2017 / DE

[16] Company brochure "G-SERIE -

MACHINING CENTER - G800 "of GROB-WERKE GmbH & Co. KG with the imprint 09/2016 / DE [17] Company brochure "G-SERIE - 5-ACHS-UNIVERSAL-

MACHINING CENTERS - G352 - G552 - G751 "of GROBWERKE GmbH & Co. KG with the imprint 09/2017 / DE

[18] Company brochure "G-SERIE - 5-ACHS-UNIVERSAL-

MACHINING CENTERS - G350T - G550T - G750T "of GROB-WERKE GmbH & Co. KG with the imprint 09/2017 / DE

[19] Company brochure "G-SERIES -

MACHINING CENTER - G1050 "of GROBWERKE GmbH & Co. KG with the imprint 09/2017 / DE

Accordingly, the clamping device 20 on a movable

To be arranged machine tool table. The workpiece 12 may be, for example, an engine block of an internal combustion engine whose cylinder is machined, e.g. are produced by milling or to edit. According to the forces occurring during such machining and the desired precision, the workpiece 12 must be clamped very firmly and very precisely positioned.

For this purpose, as shown in FIG. 1, the clamping device 12 has at least one hydraulic cylinder unit 24, which has at least one hydraulic cylinder 26 and a device 28 for determining the position of a piston 30 of the piston

Hydraulic cylinder 26 has.

In the tensioning device 12 shown in Fig. 1, a plurality of hydraulic cylinders 26 are provided, each bounded by a cylinder walls 32

Cylinder interior 34 and a movable in the cylinder interior 34 piston 30, so that on both sides of the piston 30th

Hydraulic chambers 36, 38 are formed. With valves 40, 42 provided

Hydraulic lines 44, 46 are from a hydraulic system 48, the

Hydraulic fluid 50 at high pressure delivers, led to the hydraulic chambers 36, 38. Thus, the piston 30 by the control device 22 by means of

Actuation of the valves 40, 42 are moved and set for clamping under pressure. For sensory monitoring of the safe operation of the hydraulic cylinders 26 during the machining operation, the hydraulic cylinder unit 24 is provided with the device 28 for determining the position of the piston. The

Device 28 for determining the position of the piston has, as a sensor element 52 an externally attachable to a cylinder wall 32 of the hydraulic cylinder 26 sound transmitting and receiving device 54 for transmitting and receiving acoustic signals through the cylinder wall 32 in the direction of the piston 30 and a control and Evaluation device 56 for the sound transmission and reception device 54 on. The latter is shown in Fig. 1 as part of the control device 22 and for simultaneously controlling and evaluating a plurality of sound transmission and

Receiving devices 54 designed different hydraulic cylinders 26, may be formed in other embodiments, as indicated in Fig. 2, but also separately and for example on each hydraulic cylinder 26, e.g. be arranged on the valves 40, 42.

In the embodiment of the clamping device 20 of Fig. 1 are a plurality

Hydraulic cylinder 26 is formed such that by extending the piston 30, the workpiece 12 is clamped against abutment devices 58. Presently more preferred is the embodiment shown in FIG

Clamping device 20, in which the workpiece 12 is tensioned by retraction of the piston 30 against an abutment device 58.

Hereinafter, details of the hydraulic cylinder units 24,

Hydraulic cylinder 26, the device 28 for determining the position of

Piston and a feasible method for determining the position of the piston in more detail with reference to the in Figs. 3 and 4 shown

Schematic diagrams explained.

3 is a schematic representation of the hydraulic cylinder 26 with piston 30, a piezo element, hydraulic fluid 50 and cylinder walls 32. FIG. 4 is a detailed illustration of acoustic paths and the relevant interfaces 68, 76 between cylinder wall 32 and hydraulic fluid 50 and between pistons 30, in particular Piston head 72, and hydraulic fluid 50. The hydraulic cylinder 26 has cylindrical walls 32 made of metal and more

especially made of steel. As the cylinder walls 32, at least one cylinder wall extending in the direction of movement of the piston 30, e.g. in the case of a circular-cylindrical design of the hydraulic cylinder 26, it may be formed by a cylinder jacket wall 60 which is annular in cross-section and may be formed by four cylinder side walls 60 'in the case of a rectangular hydraulic cylinder, and a cylinder bottom 62 may be provided.

The pressure range of the hydraulic cylinder 26 is 10 to 150 bar. The stroke of the piston 30 is e.g. 0 to 50.0 mm. There are 26 large temperature fluctuations during operation of the hydraulic cylinder. For example, is the temperature in the hydraulic cylinder 26 between 0 to 60 ° C. The wall thickness of the cylinder walls 32 is e.g. 3 mm to 20 mm. With the device 28, the position of the

Piston 30 without modification of the hydraulic cylinder 26 and without intervention in the same very accurately.

For this purpose, a non-invasive method for piston position determination by means of acoustic signals or waves 64 in metallic hydraulic cylinders 26 is used.

For this purpose, the sensor element 52 is externally applied to the cylinder bottom 62 in the illustrated embodiments, in particular adhesively bonded. The sensor element 52 includes the sound transmitting and receiving device 54, the one or more sound transducer 66, in particular designed as

piezoelectric element. In the illustrated embodiments, a sound transducer 66 is provided, which is designed both for transmitting sound, insbesondre a sound pulse, more particularly an ultrasonic pulse, as well as for receiving an echo, which is generated by the sound at interfaces. For others, not shown here

Embodiments may separate transducers for transmission and

Receiving be provided. The mechanical structure of the system includes the components cylinder housing 65 - with the cylinder walls 32 - piston 30 and a sound-generating piezoelectric element, short sound transducer 66th

The cylinder housing 65 is made of a metallic material such as

For example, steel and has a wall thickness of several millimeters. Inside the cylinder housing 65 is a hydraulic fluid 50.

For the use of the method are no structural changes to the

Hydraulic cylinder 26, in particular not on the cylinder housing 65, necessary; Also, this procedure is non-invasive. As a result, the method is suitable for use in machining centers, where increased hydraulic pressures can be expected.

The generation of an acoustic signal in the form of a compression wave 64 is effected by a sound transducer 66. The acoustic coupling of the

Sound transducer 66 via a permanent non-positive connection, such as adhesive bonding, with the cylinder bottom 62nd

Stimulating the sound transducer 66 with a harmonic alternating voltage, so it comes due to the piezoelectric effect for Materialverdichtung- and thinning (thickness vibration), ie for the deflection of the material. These

Deflection may propagate as a compression shaft 64 in the cylinder bottom 62 at a typical material speed.

Does this compression wave 64 on the first interface 68 between

Cylinder bottom 62 and hydraulic fluid 50, due to the large acoustic impedance differences at this point, an almost complete reflection 70 of the compression shaft 64. Due to the low sound attenuation in metals a multiple passage through the compression shaft 64 is possible.

On the reception side, this shows up in multiple reflections of the cylinder bottom 62 with high amplitudes. Few percent of the introduced sound energy will be at the first

Interface 68 cylinder bottom / hydraulic fluid in the hydraulic fluid 50 is transmitted, in which also a sound wave or compression wave 64 can propagate. This is after passage of the hydraulic fluid 50 am

Piston floor 72 reflects and reaches again after passing through the first interface 68 cylinder bottom / hydraulic fluid and the cylinder bottom 62 also the transducer 66. This represents the echo signal 74 of the piston 30, which is used for position determination.

For the detection of the compression wave 64 is in the transducer 66, which now serves as a receiver, by means of the inverse piezoelectric effect again an electrical

AC voltage generated, which then the other electronic

Signal processing is supplied.

At now used as a receiving transducer transducer 66 results in a

Superposition of the metal echoes - reflection 70 - from the first high amplitude interface 68 and the weaker echo signal 74 from the piston bottom 72.

To separate the superimposed echoes, a referencing method is used, which is referred to below as the fingerprint method.

A fingerprint sets the unique for the used cylinder configuration acoustic echo signal of the entire cylinder, i. in particular all immobile parts in the cylinder, which also includes the multiple reflections 70 of the sound wave in the cylinder bottom 62. This echo signal is from the

Independent piston position, provided no superimposition of echoes in the time domain occurs.

For this reason, the fingerprint of the hydraulic cylinder 26 is received in the fully extended piston 30 state. After changing the position, the fingerprint is subtracted from the current measurement signal. While this subtraction reduces the echoes 70 of the cylinder bottom 62 to an amplitude value of zero, the echo signals 74 are retained by the piston crown 72 since these are dependent on the position of the piston 30. Since the method is referencing, it is susceptible to changes in the

Sound transit times, which are caused by a change in temperature. This is avoided in preferred embodiments of the method by the fact that in use a continuous re-referencing before each clamping of the

Hydraulic cylinder 26 is performed, as shown in Fig. 5.

5 shows a flow chart of the fingerprint method with continuous re-referencing and describes the following sequence of steps S1 to S7 using the example of a hydraulic cylinder 26 of the configuration of a clamping device 22 shown in FIG. 2:

51 Storing the fingerprint (piston completely released)

52 position change (piston is cocked)

53 Difference between fingerprint and current sound signal

54 Determination of the transit time from the difference signal

55 Position change (piston is completely released)

56 pistons completely released

57 Neureferencing during initialization Piston movement

General is used to form a fingerprint or in other words

Reference signal, which takes into account the configuration and parameters of the hydraulic cylinder, the piston 30 moved to a reference position. The method for determining the position of a piston 30 of a hydraulic cylinder 26 of a machine tool 10 accordingly comprises the steps:

a) generating a reference signal by sending an acoustic signal 64 through a cylinder wall 32 of the hydraulic cylinder 26 through the located in a reference position piston 30 and detecting an echo signal 70 as a reference signal and

b) transmitting the acoustic signal 64 through the cylinder wall 32 to the piston 30 to determine its position and detecting the echo signal 70, 74 as a measurement signal and

c) generating a comparison signal 74 from the measurement signal 70, 74 and the reference signal 70 and d) determining the transit time from the comparison signal 74 to determine the position of the piston 30 from the transit time.

The steps for sending and evaluating are controlled in particular by the control and evaluation device 56. A preferred embodiment of the method including control and evaluation and in particular of the algorithms used therein will be explained in more detail below.

In the following, an embodiment for the already mentioned fingerprint method will be described in detail, central processing unit in the preferred

Embodiment is an FPGA 78 (Field Programmable Gate Array), which is an embodiment of a central processing unit of the control and

Evaluation device 56 represents.

At the beginning of each measurement, a two-fold square burst signal with a frequency of 2 MHz is generated in the FPGA 78. This is pre-amplified, and the

Sound transducer 66 is excited with it. Due to the inverse piezoelectric effect, the sound transducer 66 is set in oscillation at the excited frequency. Since the transducer 66 is frictionally coupled to the cylinder bottom 62, so a sound wave - compression wave 64 - is generated in this, which passes through the cylinder bottom 62 and coupled into the hydraulic fluid 50 of the hydraulic cylinder 26.

The sound wave - compression wave 64 - passes through the hydraulic fluid 50, is reflected at the piston head 72 of the corresponding piston 30 and finally converted in the transducer 66 back into an electrical signal.

The actual position of the piston 30 corresponds to the transit time of the signal through the hydraulic fluid 50. The electrical signal thus generated is processed on the central electronics by means of various filter and amplifier stages, digitized in an analog-to-digital converter and the FPGA 78 for further processing made available. The sampling of the signal happens eg at 50 MHz. Depending on the mechanical structure of the hydraulic cylinder 26, the signal is stored in the FPGA 78 as an array of several thousand samples, eg with a width of 12 bits each.

The fingerprint method is preferably a

difference-forming method. In order to deliver its best performance, the actual fingerprint is measured when the piston is fully extended, e.g. in the embodiment of Fig. 2 in the relaxed state, recorded and stored.

In one embodiment, the samples of the

Fingerprints permanently from the corresponding samples of the current

Subtracted present signal. The resulting difference signal has a unique maximum, which corresponds to the position of the piston 30 in

Hydraulic cylinder 26 corresponds.

In a preferred embodiment, a regular update of the fingerprint is provided to compensate for any existing temperature drift of the mechanical total system. As the temperature of the hydraulic fluid changes, so does the speed of sound in it. If the temperature change is too great, the difference formation leads to defects in the

Differential signal. The fingerprint used would then be invalid or at least less well suited. Therefore, it is in a preferred embodiment

provided multiple fingerprints in a memory 80, which were recorded at different temperatures. In one embodiment, the storage of the fingerprint together with information about an operating parameter, e.g. the time of the recording or one of them

measured operating temperature.

In a preferred embodiment, during operation, the currently used fingerprint is filled with the currently available signal

extended piston 30 compared. If it turns out that the resulting difference is too large, the current fingerprint is classified as invalid, but remains in memory 80. The present signal when fully extended piston 30 is compared with other fingerprints that may already be present in the memory 80. If a fingerprint matching the current signal is found, it will be used. If this is not the case, the current signal becomes the new current fingerprint. In this way arises in the course of the operating time of the system a number of

Fingerprints that have been recorded at different temperatures and can compensate for a possible temperature drift of the system.

In Fig. 5, as a preferred embodiment, a constant re-referencing prior to each clamping operation, i. explained before each movement of the piston 30. As explained above, but also previously stored reference signals can be used. The recording of new reference signals can be done as needed. It is also conceivable to perform a new referencing after a certain number of piston movements or after a certain time has elapsed since the last reference signal formation. Also conceivable is parameter-dependent re-referencing, e.g. making a new one

Reference signal when the operating temperature exceeds a permissible limit

Tolerance value range changes out.

In the embodiments shown in FIGS. 1 to 4, the piston 30 is shown with a flat piston crown 72, wherein the second interface 76 formed thereon constitutes a uniform reflection region for forming the echo signal 74.

FIG. 6 shows a further embodiment of the hydraulic cylinder unit 22 with hydraulic cylinder 26, piston 30 and sensor element 52-externally mounted sound transducer 66-in which defined, different transit time paths are provided. For this purpose, the piston 30, for example on the piston head 72, has a first reflection region 82 and a second reflection region 84. The second reflection region 84 can be formed, for example, by a groove or a bore 86 in the piston bottom. In the direction of movement of the piston and / or in the running direction of the compression shaft 64, the first reflection region 82 and the second reflection region 84 have a defined distance A from each other. Thereby, a first echo signal 74 'at the first reflection region 82 and a second echo signal 74 "generated at the second reflection region 84, which due to the defined running distance (2 times A) have a defined transit time difference and which can be used to determine further parameters, in particular for determining whether inconsistencies, eg caused by

Air bubbles in the hydraulic fluid 50 occur.

The above is a fingerprint or subtraction method for

Piston position determination in cylinders of machine tools 10 explained. The method can be carried out in a correspondingly programmed computer system, which is connected to the hydraulic cylinder 26 and a sensor element 52 attached thereto. For this purpose, an electronics made of adapted standard elements can be used. The in the tax and

The evaluation device and / or the control device 22 for carrying out the piston position determination method control instructions may be present as software that may be stored in a computer program product or a computer program medium.

As a sound transducer 66 one or more piezo elements can be used, as they are already known in principle. It can to that extent on the

References [2] to [1 1] are referenced. Especially with respect to the known solutions, the type of mounting and application is particularly important: the sound transducer 66 is attached and applied to the outside of the otherwise unchanged cylinder wall 32, in particular the cylinder bottom 62. in the

In contrast to the methods known from [2] to [11], the method described here is non-invasive. The hydraulic cylinder 26 is unchanged, it finds only a frictional coupling between the transducer 66 and

Cylinder bottom 62 instead.

In addition to the already mentioned FPGA 78 adapted standard electronic elements can be used.

The sound transducer 66 may be soldered or glued on a circuit board in some embodiments. In some embodiments, parts of analog electronics may already be present in the immediate vicinity of the sound transducer 66, eg a preamplifier. As mentioned above, separate transducers 66 can be used for transmission and reception. Preferably, the sound transmitting and receiving device 54, which has the one or more of the sound transducer 66 and electronic parts and possibly a circuit board, in a housing (not shown) and screwed to the cylinder base 62.

A particular advantage of preferred embodiments of the piston position determination method described here is that only one

Sensor element 52 / transducer 66 is required, and the emission takes place only on one side from the bottom to the piston.

As mentioned above, it is quite conceivable that two sound transducers 66 are mounted on the cylinder wall 32, in particular on the cylinder bottom 65, namely a sound transducer for generating the sound waves (transmitter) and a second transducer for converting the received sound waves into electrical signals (ie Receiver), instead of doing so in just one transducer 66, which is the transmitter and receiver at the same time.

The embodiment shown in FIG. 6 and summarized here again can bring greater advantages in connection with the method according to the invention: An additional reference running path with a defined length A can be attached to the piston head 72, e.g. by introducing a groove or bore 86. Then the acoustic signal - e.g. the compression shaft 64, in particular formed as an ultrasonic signal - from the piston head 72 due to the additional transit time through the reference running distance A at different times reflected (i.e., once from the unchanged piston crown 72 - first

Reflection area 82 -, and another reflection from the groove o. second reflection area 84). This creates an additional sound running distance with a defined length (2 times A). By a determination of

Duration difference between the two positions (ie running time at reflection from the normal piston crown and running time at reflection from the groove, etc.) can be changed properties of the hydraulic fluid (esp. Aging, temperature, air bubbles) recognize. Air bubbles can be a major problem with hydraulic cylinders used as tension cylinders; therefore, a detection is eg by means of the additional reference run for the planned

Monitoring purposes particularly advantageous. Detecting air bubbles in the piston chamber is also possible with the other illustrated embodiments; in the embodiment of Fig. 6 is the possibility of recognition of

Air bubbles, however, significantly improved.

LIST OF REFERENCE NUMBERS

10 machine tool

12 workpiece

14 tool spindle

16 Tool magazine

18 tool

20 clamping device

22 control device

24 hydraulic cylinder unit

26 hydraulic cylinders

28 Device for determining the position of a piston

30 pistons

32 cylinder wall

34 cylinder interior

36 first hydraulic chamber

38 second hydraulic chamber

40 first valve

42 second valve

44 first hydraulic line

46 second hydraulic line

48 hydraulic system

50 hydraulic fluid

52 sensor element

54 sound transmission and reception device

56 control and evaluation device

58 counter bearing device

60 cylinder jacket wall

60 'cylinder side wall

62 cylinder bottom

64 compression wave (acoustic signal / sound wave)

65 cylinder housing

66 sound transducers

68 first interface cylinder bottom / hydraulic fluid Reflection at first interface (echo or echo signal initiated by first interface)

piston crown

Echo signal from the piston crown

'Echo signal from the first reflection area

"Echo signal from the second reflection area

second interface hydraulic fluid / piston crown

FPGA

Storage

first reflection area

second reflection area

Bore (alternatively groove)

Defined distance between the first and second reflection area Saving the fingerprint (piston completely released)

Position change (piston is cocked)

Difference formation from fingerprint and current sound signal

Determining the transit time from the difference signal

Position change (piston is completely released)

Piston completely released

Neureferencing during initialization piston movement

Claims

Claims:

1 . Method for determining the position of a piston (30) of a

Hydraulic cylinder (26) of a machine tool (10), comprising:

a) generating a reference signal by sending an acoustic signal through a cylinder wall (32) of the hydraulic cylinder (26) through the located in a reference position piston (30) and detecting an echo signal as a reference signal and

b) sending the acoustic signal through the cylinder wall (32) on the piston (30) for determining its position and detecting the echo signal as a measurement signal and

c) generating a comparison signal from the measurement signal and the

Reference signal and

d) determining the transit time from the comparison signal in order to determine from the transit time the position of the piston (30).

2. The method according to claim 1,

characterized,

in that the transmission of the acoustic signal and the detection of the echo signal in steps a) and b) take place by means of a particularly piezoelectric sound transducer (66) which is externally attached to the cylinder wall (32) formed entirely of metal and / or

in that a cylinder bottom (62) extending transversely to the direction of movement of the piston (30) is selected as the cylinder wall (32), by which the transmission and reception takes place.

3. Method according to one of the preceding claims,

characterized,

in that step c) comprises forming a difference signal from the measurement signal and the reference signal.

4. Method according to one of the preceding claims,

characterized,

in that step a) comprises at least one, several or all of the following steps:

a1) generating the reference signal several times in the course of operation of the hydraulic cylinder (26);

a2) moving the hydraulic cylinder (26) to the reference position;

a3) selecting one of the end positions of the piston (30) as a reference position; a4) selecting that position of the piston (30) which causes the greatest transit time for the echo signal as the reference position;

a5) storing the reference signal or a plurality of reference signals detected at different times or different operating conditions in a memory (80);

a6) detecting a plurality of reference signals for different ones

Operating temperatures;

a7) storing the reference signal together with information about at least one operating parameter present during the detection of the reference signal;

Reference signal, if this to the at least one stored

Reference signal is dissimilar; and or

a9) detecting the reference signal before any change in the position of the piston to a position to be measured towards or before or after a predetermined number of position determinations or after a predetermined time has elapsed

Duration since the last reference signal determination or since the last one

Piston position determination or depending on a change of a

Operating parameters of the hydraulic cylinder (26) or the hydraulic fluid (50).

5. The method according to any one of the preceding claims, characterized,

in that the position of the piston (30) on a hydraulic cylinder (26)

5.1 at a pressure of above 10 bar, in particular above 50 bar and more especially in a pressure range of 10 bar to 150 bar and / or

5.2 with a piston stroke in the range of 0 to 100 mm, more preferably 0 to 50.0 mm, and / or

5.3 with a distance of a sound transmission and reception position to the piston (30) of less than 200 mm, in particular less than 100 mm and in particular in a range of 1 mm to 50 mm,

is determined.

6. The method according to any one of the preceding claims,

marked by

Providing a first reflection region (82) for reflecting the acoustic signal and a second reflection region (84) for reflecting the acoustic signal on the piston (30) such that the reflection regions (82, 84) are at a defined distance (A) from one another in the direction of the acoustic Have signals, and

Determining a transit time difference between an echo signal (74 ') from the first reflection region (82) and an echo signal (74 ") from the second reflection region (84).

Device (28) for determining the position of a piston (30) of a hydraulic cylinder (26) for a machine tool (10), comprising:

an externally on a cylinder wall (32) of the hydraulic cylinder (26) attachable sound transmitting and receiving means (54) for transmitting and receiving acoustic signals through the cylinder wall (32) in the direction of the piston (30) and

a control and evaluation device (56) for the sound transmission and reception device (54),

wherein the control and evaluation device (56) is formed:

for detecting that the piston (30) is in a reference position and / or for driving the hydraulic cylinder (26) to move the piston (30) to a reference position, for driving the sound sending and receiving device (54) to send an acoustic signal through the cylinder wall (32) of the

Hydraulic cylinder (26) through the piston located in a reference position (30) and for recording an echo signal then received by the sound transmission and reception means (54) as a reference signal,

for driving the sound transmitting and receiving device (54) for transmitting the acoustic signal through the cylinder wall (32) to the piston (30) located in the position to be measured and for detecting the subsequently received echo signal as a measuring signal,

for determining the transit time from the comparison signal in order to determine from the transit time the position of the piston (30).

8. Device (28) according to claim 7,

characterized,

the sound transmission and reception device (54) has a particularly piezoelectric sound transducer (66).

9. Device (28) according to claim 7 or 8,

characterized,

in that the control and evaluation device (54) is designed as

Comparison signal to generate a difference signal from the measurement signal and the reference signal.

10. Device (28) according to one of claims 7 to 9,

characterized,

the control and evaluation device (54) is designed to:

a1) generating the reference signal several times in the course of the operation of the

Hydraulic cylinder (26); and or

a2) moving the hydraulic cylinder (26) to the reference position; and / or a3) selecting one of the end positions of the piston (30) as a reference position; and or a4) selecting that position of the piston (30) which causes the greatest transit time for the echo signal as the reference position; and or

a5) storing the reference signal or a plurality of reference signals detected at different times or different operating conditions in a memory (80); and or

a6) detecting a plurality of reference signals for different ones

Operating temperatures; and or

a7) storing the reference signal together with information about at least one operating parameter present during the detection of the reference signal; and or

Reference signal, if this to the at least one stored

Reference signal is dissimilar; and or

a9) detecting the reference signal before each movement of the piston (30) towards a position to be measured towards or after a predetermined number of position determinations or after expiration of a predetermined period of time since the last reference value determination or since the last position determination or depending on a change of an operating parameter of

Hydraulic cylinder (26) or the hydraulic fluid (50).

1 1. Hydraulic cylinder unit (24) for a machine tool (10), comprising: a hydraulic cylinder (26), in particular clamping cylinder, with a

Cylinder housing (65) whose cylinder walls (32) are formed entirely of a metal, and a in the cylinder housing (65) movable piston (30); and

a device (28) according to any one of claims 7 to 10,

wherein the sound transmitting and receiving device (54) on the outside of the

Cylinder wall (32) is mounted.

12. hydraulic cylinder unit (24) according to claim 1 1, characterized,

in that the hydraulic cylinder (26) is designed for a pressure range from 10 bar to 100 bar, in particular 10 bar to 150 bar, and / or

in that the piston (30) has a stroke, measured from a cylinder base (62) on whose outside the sound transmitting and receiving device (54) is mounted, in a range between 0 mm and 100 mm, more particularly between 0 mm and 50, 0 mm has.

13. hydraulic cylinder unit (24) according to any one of claims 1 1 or 12, characterized

in that a first reflection region (82) for reflecting the acoustic signal and a second reflection region (84) for reflecting the acoustic signal are provided on the piston (30) such that the reflection regions (82, 84) are at a defined distance from one another in the direction of travel of the acoustic signal have and

in that the control and evaluation device (54) is designed to determine a transit time difference between an echo signal (74 ') from the first one

Reflection region (82) and an echo signal (74 ") from the second

Reflection area (84).

14. A machine tool (10), comprising a hydraulic cylinder unit (24) according to any one of claims 1 1 to 13.

A computer program product or computer readable computer program medium comprising control instructions for carrying out the steps of the method of any one of claims 1 to 6 when loaded into a computer (78, 22).