阅读说明：本技术 在机床主轴上的支承监测 (Bearing monitoring on machine tool spindle ) 是由 丹尼尔·科列斯京·迪米特里·拉巴季耶夫 托马斯·凯勒 于 2019-02-05 设计创作，主要内容包括：本发明涉及一种用于在机床主轴(2)上对工件(1)或刀具进行支承监测的装置,所述装置具有用于工件(1)或刀具的支承面(3)。在所述支承面的区域中设置有至少一个测量喷嘴(4),以便产生背离所述支承面(3)指向的流体流。所述流体流在测量喷嘴上游引导穿过真空喷嘴,所述真空喷嘴能够包括喷射喷嘴(7c)和收集喷嘴(7b)。所述真空喷嘴在被所述流体介质穿流时在负压空间(9c)中产生负压。压力传感器(6)或压力开关检测所述负压空间中的测量压力(p3)。(The invention relates to a device for monitoring the mounting of a workpiece (1) or a tool on a machine tool spindle (2), comprising a mounting surface (3) for the workpiece (1) or the tool. At least one measuring nozzle (4) is arranged in the region of the bearing surface in order to generate a fluid flow directed away from the bearing surface (3). The fluid flow is directed through a vacuum nozzle upstream of the measuring nozzle, which can comprise a spray nozzle (7c) and a collection nozzle (7 b). The vacuum nozzle generates a negative pressure in a negative pressure space (9c) when being traversed by the fluid medium. A pressure sensor (6) or a pressure switch detects the measured pressure (p3) in the negative pressure space.)

1. A device for supporting and monitoring a workpiece (1) or a tool (11) on a spindle (2; 16) of a machine tool (20) by means of a fluid medium, comprising:

a bearing surface (3) for the workpiece (1) or tool (11);

at least one measuring nozzle (4) which is arranged in the region of the bearing surface (3) in order to generate a fluid flow directed away from the bearing surface (3); and

at least one first pressure sensor (6) or pressure switch for detecting a measurement pressure upstream of the measurement nozzle (4),

it is characterized in that the preparation method is characterized in that,

the device has a vacuum nozzle (7) upstream of the measuring nozzle (4), which is designed to generate a negative pressure in a negative pressure space (9c) when traversed by the fluid medium, and

the first pressure sensor (6) or pressure switch is designed to detect a measured pressure (p3) in the vacuum space (9 c).

2. The device according to claim 1, wherein the first pressure sensor (6) is configured for detecting positive and negative values of the measured pressure (p3) in the negative pressure space (9 c).

3. The device according to claim 1 or 2, wherein the device is configured such that in the absence of a workpiece (1) or a tool (11) on the bearing surface (3), the measured pressure (p3) in the negative pressure space (9c) has a negative value.

4. A device according to any one of claims 1 to 3, having a pressure regulator (5) for generating a predetermined supply pressure (p1) upstream of the vacuum nozzle (7).

5. The device according to claim 4, wherein the pressure regulator (5) is designed to generate a supply pressure (p1) of at most 2 bar.

6. The device according to any one of the preceding claims, having at least one second pressure sensor in order to determine the supply pressure (p1) upstream of the vacuum nozzle (7).

7. The device according to one of the preceding claims, having a control device (17), the control device (17) being designed to receive a signal from the first pressure sensor (6) or pressure switch, which signal is dependent on the measured pressure (p3), and to determine the position of the workpiece (1) or tool (11) taking into account the received signal.

8. The device according to claim 7, wherein the control device (17) is designed to take into account a supply pressure (p1) upstream of the vacuum nozzle (7) when determining the workpiece (1) or tool (11).

9. The device according to one of the preceding claims, wherein the measuring nozzle (4) is arranged in the region of the bearing surface (3) in such a way that the measuring nozzle (4) is completely closed when a workpiece (1) or a tool (11) is supported in a precise plane parallel on the bearing surface (3).

10. The device according to one of claims 1 to 8, wherein the measuring nozzle (4) is arranged in the region of the bearing surface (3) such that a workpiece or a tool is supported parallel to the bearing surface (3) in a precise plane at a defined distance from the measuring nozzle (4) such that the workpiece (1) or the tool (11) does not completely enclose the measuring nozzle (4).

11. The device according to any of the preceding claims, wherein the vacuum nozzle (7) and the measuring nozzle (4) are arranged on a rotatable part of the spindle (2; 16).

12. The device according to any one of the preceding claims, wherein the vacuum nozzle (7) comprises a spray nozzle (7c) and a collection nozzle (7b) arranged upstream of the spray nozzle (7c), and

wherein the negative pressure space (9c) communicates with a region provided between the spray nozzle (7c) and the collection nozzle (7 b).

13. Device according to any one of the preceding claims, comprising a functional unit (14b) comprising a housing in which at least the vacuum nozzle (7) is arranged.

14. The device according to claim 13, wherein the functional unit (14b) further comprises the measuring nozzle (4).

15. Device according to claim 13 or 14, wherein the functional unit (14b) is arranged as a sleeve in a bore hole in the rotatable part of the spindle (2; 16).

16. The device according to any of the preceding claims, wherein the first pressure sensor (6) or pressure switch comprises means for wireless signal transmission.

17. The device of claim 15, wherein the means for wireless signal transmission is configured as a passive RFID transponder.

18. The device according to any one of the preceding claims, having an additional compressed air connection (9d) for evacuating at least the vacuum nozzle (7) and the measuring nozzle (4).

19. The device according to any of the preceding claims, having at least one further vacuum nozzle, and wherein the device is configured for, in the case of the use of the further vacuum nozzle, achieving one of the following:

measuring the spacing between two machine elements;

measuring dynamic pressure;

measuring the flux;

regulating the through flow and the through flow;

the pressure is adjusted.

20. Method for support monitoring of a workpiece (1) or a tool (11) by means of a fluid medium on a spindle (2; 16) of a machine tool (20), wherein the spindle has a support surface (3) for the workpiece (1) or the tool (11), and wherein the method has:

generating a fluid flow directed away from the bearing surface (3) by means of at least one measuring nozzle (4);

-detecting a measurement pressure upstream of the measurement nozzle (4); and

determining the position of the workpiece (1) or tool (11) relative to the bearing surface (3) taking into account the measured pressure,

it is characterized in that the preparation method is characterized in that,

the fluid flow is guided through a vacuum nozzle (7) upstream of the measuring nozzle (4), which vacuum nozzle is designed to generate a negative pressure in a negative pressure space (9c) when traversed by the fluid medium, and

-detecting a measured pressure (p3) in the underpressure space (9 c).

21. Method according to claim 20, wherein the measured pressure (p3) in the negative pressure space (9c) has a negative value in the absence of a workpiece (1) or tool (11) on the bearing surface (3).

22. Method according to claim 20 or 21, wherein the method is performed by means of a control device (17), wherein the control device (17) receives a signal which is influenced by the measured pressure (p3), and wherein the control device determines the position of the workpiece (1) or tool (11) taking into account the received signal.

23. Method according to claim 22, wherein the control device (17) also detects a supply pressure (p1) upstream of the vacuum nozzle (17), and wherein the control device determines the position of the workpiece (1) or tool (11) taking into account the supply pressure (p 1).

24. Method according to claim 22 or 23, wherein a signal influenced by the measured pressure (p3) is transmitted wirelessly to the control device (17).

25. The method of claim 24, wherein the signal is transmitted with a passive RFID transponder.

26. The method according to any one of claims 20 to 25, further comprising evacuating the vacuum nozzle (7) and the measurement nozzle (4) by means of an additional compressed air interface (9 d).

Technical Field

The invention relates to a device for monitoring the mounting of a machine tool spindle and to a corresponding method for determining the presence and position of a workpiece or a tool.

Background

Support monitoring devices are used on machine tools to detect the presence and position of a workpiece on a clamping device.

In particular, bearing monitoring devices are known from the prior art, which are based on the pneumatic length measuring principle. The measuring principle is described in detail in the standard DIN 2271 (12.2016). An important class of pneumatic bearing monitoring devices is based on the dynamic pressure measurement principle. The fluid is delivered to a measuring nozzle. When the workpiece covers the outlet of the measuring nozzle, the dynamic pressure in the measuring nozzle changes. The change is measured. Pneumatic bearing monitoring devices based on this measurement principle are commercially available from different manufacturers. Typical query distances are 0.02 to 0.2 mm.

Examples of pneumatic bearing monitoring devices can also be found in the patent literature. Thus, DE 102005002448 a1 discloses a monitoring device for monitoring the position of a workpiece in front of an outflow opening of the monitoring device, wherein a fluid source delivers compressed air under preload to the outflow opening via a throttle element. The differential pressure sensor measures the pressure drop into the throttle element, i.e. the pressure difference between the pre-pressure provided by the source and the dynamic pressure occurring at the outflow. The change in differential pressure is used to determine position.

Further examples of pneumatic bearing monitoring devices are disclosed in DE 10239079 a1, DE 10155135 a1, EP 1537946 a1, EP 0794035 a1, EP5,540,082, DE 10012216073 a1 and WO 2012/160204 a 1.

Other measurement principles for support monitoring have already been proposed in the prior art. Thus, EP 3085490 a2 discloses a support monitoring device using ultrasonic measurements. DE 102014112116 a1 discloses a bearing monitoring device which uses a microwave resonator as a sensor.

Accurate bearing monitoring is very important in particular in gear-making machines. The workpiece is usually moved in the gear-making machine by frictional engagement with the clamping device, while the workpiece rests with a flat surface on a flat mating surface of the clamping device. Small chips, grinding sludge and other contaminants can prevent plane-parallel support of the workpiece on the mating surfaces, so that the workpiece is clamped in a skewed manner. On the one hand, this deteriorates the machining accuracy and causes more defective products. On the other hand, the skewed clamping of the workpiece can also lead to a worsening of the friction fit, so that in some cases a reliable entrainment of the workpiece is no longer ensured. It is also important to identify workpieces whose flat faces have been misworked or damaged. This leads to very high requirements on the accuracy of the bearing monitoring. Even a few micrometers of oblique placement should be detectable. For this reason, existing bearing monitoring devices are often not accurate enough. High accuracy is also important in the support monitoring of the tool.

In gear making machines, the workpiece change is usually automated within a few seconds. Therefore, bearing monitoring must be performed very quickly within a fraction of the replacement time. In order to achieve shorter measuring times, it is desirable to arrange the bearing monitoring device directly on the rotatable spindle. However, existing support monitoring devices are generally not suitable for this purpose.

DE 10017556 a1 discloses a device for setting the negative pressure generated in a venturi nozzle. For this purpose, a baffle is provided in the outflow region of the fluid behind the venturi nozzle. The baffle is axially movable by the adjustment unit. By linearly displacing the flap by means of the adjusting unit, the negative pressure generated by the venturi nozzle can be varied in a targeted manner. This document does not relate to bearing monitoring, but rather to the targeted generation of a settable negative pressure.

Disclosure of Invention

The object of the present invention is to provide a device for supporting and monitoring a workpiece or a tool in a machine tool, which device can achieve high accuracy, but can nevertheless be implemented in a compact and cost-effective manner.

The object is achieved by a bearing monitoring device according to claim 1.

Therefore, a device for supporting and monitoring a workpiece or a tool by means of a fluid medium on a spindle of a machine tool, in particular on a spindle of a gear machine, is proposed. The device has:

a bearing surface for a workpiece or tool;

at least one measuring nozzle, which is arranged in the region of the bearing surface in order to generate a fluid flow directed away from the bearing surface;

a vacuum nozzle arranged upstream of the measuring nozzle, which vacuum nozzle is designed to generate a negative pressure in the negative pressure space when the fluid medium flows through it; and

at least one first pressure sensor or pressure switch, which is designed to detect the measured pressure in the vacuum space.

The pressure sensor or the pressure switch is therefore designed and arranged such that it detects the measured pressure in the vacuum space. In the proposed device, the measurement window is greatly increased compared to known devices, i.e. the time period before and after which the measurement pressure can be changed maximally due to the presence or absence of a workpiece or tool. When the measuring nozzle is completely closed, the measuring pressure substantially corresponds to the (positive) supply pressure supplied to the vacuum nozzle. Whereas when the measuring nozzle is fully released, the measuring pressure may have a significant negative value. Thus, the maximum change in the measured pressure is greater in magnitude than the supply pressure. Whereas in the known device only pressure values with the same sign are always generated. The maximum pressure change that can be detected in this way is always only a small fraction of the supply pressure.

Due to the significantly larger measuring window of the device according to the invention, small changes in the distance between the bearing surface and the tool or workpiece can lead to large changes in the measuring pressure. Thereby, also very small distances can be detected accurately and reproducibly. The invention thus provides a pneumatic signal gain to improve the accuracy and reproducibility of bearing monitoring or spacing measurements.

The term "support monitoring" is understood in this context to mean a method for determining the position of a workpiece or a tool relative to a support surface. The result of the method can be, for example, a continuous measured value (for example, a pitch value) or a binary value (for example, "sufficient support" versus "insufficient support").

The term "vacuum nozzle" is understood herein to mean a device having a fluid inlet and a fluid outlet, wherein a fluid flow entering the device through the fluid inlet is guided through a constriction on its way to the fluid outlet, so that the fluid flow is accelerated. A negative pressure is thus generated in the region of the constriction and/or downstream of the constriction. The negative pressure is measured. For this purpose, the vacuum nozzle can have a negative pressure connection, and a separate negative pressure space can be connected to the negative pressure connection, where in turn a first pressure sensor can be provided. However, the vacuum space can also be an integral part of the vacuum nozzle, and the first pressure sensor can also be arranged directly on the vacuum nozzle for this purpose, without a separate vacuum connection.

The negative pressure space preferably has no further communication to the exterior space. Therefore, the negative pressure space does not suck in the outside air having impurities. The device can therefore be operated in particular with compressed air which is oil-free or oil-containing.

The vacuum nozzle can be designed in particular as a simple venturi nozzle or as a laval nozzle. However, more complex designs are also possible, in particular two-stage or multi-stage designs. In particular, the vacuum nozzle can have a spray nozzle with a constriction which accelerates the fluid flow. Downstream of the spray nozzle, a separate collecting nozzle can be provided, which again widens gradually in order to decelerate the fluid flow again. The vacuum space can then be formed as a chamber between the spray nozzle and the collecting nozzle or can communicate with the region between the spray nozzle and the collecting nozzle. By means of the vacuum nozzle, the fluid flow can be accelerated up to supersonic speeds. Thus, a negative pressure of up to, for example, about-0.9 bar can be generated relative to the ambient pressure.

The vacuum nozzle is sometimes also referred to as a vacuum suction nozzle or a vacuum ejector. Vacuum nozzles can be obtained in different embodiments at low cost.

The term "pressure sensor" is understood herein to mean any device that converts a measured variable, i.e. pressure, into an analog or digital electrical signal. Different measurement principles exist for pressure sensors. Known measurement principles use, for example, strain gauges which are arranged on a deformable membrane. The diaphragm is deformed by the pressure change. The deformation is recorded by a strain gauge. Other measurement principles utilize the piezoelectric effect. A variety of different pressure sensors are known to those skilled in the art, and the present invention is not limited to a particular type of pressure sensor. The pressure sensor can be, in particular, an absolute pressure sensor or a differential pressure sensor, which determines a measured pressure relative to an arbitrary reference pressure. The reference pressure can be, for example, ambient pressure or the supply pressure at the inlet of the vacuum nozzle.

In this context, a "pressure switch" is understood to mean a device which opens or closes a contact as a function of pressure.

In this context, all pressure values relate to the ambient pressure, i.e. pressure values which are less than the ambient pressure have a negative sign. This definition is used herein independently of the manner and method of determining the pressure value.

In this context, the term "measuring nozzle" is to be understood in a broad sense. The measuring nozzle can have any shape. The measuring nozzle has an opening that causes the fluid stream to flow away from the bearing surface as the fluid stream exits the measuring nozzle. Preferably, the fluid flow exits the measuring nozzle perpendicularly to the bearing surface.

In order to be able to determine the pressure value over the entire measuring window, the first pressure sensor is advantageously designed to detect positive and negative values of the measured pressure in the vacuum space. In order to also make sure that the entire measuring window is used, the device is preferably dimensioned such that the measuring pressure in the underpressure space has a negative value in practice if the device is operated according to the regulations, i.e. if a workpiece or a tool is present on the support surface if a fluid is supplied to the vacuum nozzle at a predetermined supply pressure. In particular, the dimensions of the measuring nozzle and of the lines between the vacuum nozzle and the measuring nozzle and the design of the vacuum nozzle are matched to one another in a suitable manner. The predetermined supply pressure is preferably less than 2bar, particularly preferably in the range from 0.8bar to 1.6 bar.

The support monitoring device can have a pressure regulator to produce a predetermined supply pressure upstream of the vacuum nozzle. Alternatively or additionally, the bearing monitoring device can have a second pressure sensor in order to determine the supply pressure upstream of the vacuum nozzle.

The device can also have a control device. The control device is then designed to receive signals from the first pressure sensor or pressure switch, which signals are correlated with the measured pressure, and to determine the position of the workpiece or tool taking into account the received signals. The signal can in particular be a quasi-continuous signal which is correlated with the measured pressure; however, the signal can also be a simple binary signal which indicates that a certain pressure threshold is undershot or exceeded.

The control device can furthermore be designed to take into account the supply pressure upstream of the vacuum nozzle when determining the position of the workpiece or the tool. For this purpose, the control device can receive a signal from the second pressure sensor. However, it is also conceivable to fix the supply pressure by means of a pressure regulator and to feed the corresponding pressure into the control device in a different manner than by means of a pressure sensor.

The measuring nozzle can be arranged in the region of the bearing surface in such a way that, with exactly plane-parallel bearing on the bearing surface, the workpiece or the tool completely closes the measuring nozzle. In the case of plane-parallel mounting, the measurement pressure therefore corresponds substantially to the supply pressure. In this way the entire measurement window is utilized.

Alternatively or additionally, however, the measuring nozzle can also be arranged in the region of the bearing surface, so that, with a precisely plane-parallel bearing on the bearing surface, the workpiece or the tool is arranged at a defined distance from the measuring nozzle, so that the workpiece or the tool does not completely enclose the measuring nozzle. Then, even in the case of exact plane parallel support, a certain amount of fluid will flow through the vacuum nozzle, whereby the measurement pressure is smaller than the supply pressure even in the case of exact plane parallel support. Although the entire measurement window is not utilized in this manner. For this purpose, however, the device can be operated, for example, in the range in which it is most sensitive to changes in the distance, i.e., in the range in which the characteristic curve describing the dependence of the pressure on the distance is the steepest.

The vacuum nozzle and the measuring nozzle can be constructed very compactly. The vacuum nozzle and the measuring nozzle can thus be arranged on the rotatable part of the spindle. The measuring line between the vacuum nozzle and the measuring nozzle can thus be of very short construction. This improves the response behavior of the device and enables shorter measurement times.

The device can in particular have a compact functional unit which comprises a housing in which at least a vacuum nozzle is arranged. Alternatively, the measuring nozzle can also be a component of the functional unit. The functional unit can form a sleeve which is arranged in a bore on the rotatable part of the spindle and is inserted, for example pushed or screwed, into such a bore, in particular counter to the flow direction. The bore hole is in this case preferably parallel to the spindle axis.

In order to facilitate the signal transmission to the control device, the first pressure sensor or the pressure switch and/or the second pressure sensor can comprise a device for wireless signal transmission, in particular a passive RFID transponder here.

In order to facilitate cleaning of the device, the device can comprise additional pressure interfaces for evacuating at least the vacuum nozzle and the measurement nozzle. In order to prevent compressed air from exiting again through the fluid inlet of the device, a check valve can be provided at the fluid inlet of the device, which closes if the pressure at the additional compressed air interface exceeds the supply pressure at the fluid inlet. Likewise, a check valve is provided at the additional compressed air connection, which prevents fluid from escaping through the additional compressed air connection during normal operation.

The device can have at least one further vacuum nozzle, wherein the device is designed to achieve one of the following objectives when using the further vacuum nozzle:

measuring the spacing between two machine elements;

measuring dynamic pressure;

measuring the flux;

regulating the flow rate;

the pressure is adjusted.

The invention also relates to a method for monitoring the support of a workpiece or a tool by means of a fluid medium on a machine tool spindle, in particular on a spindle of a tooth-making machine. The spindle has a bearing surface for a workpiece or a tool. The method comprises the following steps:

generating a fluid flow directed away from the bearing surface by means of at least one measuring nozzle, wherein the fluid flow is guided through the vacuum nozzle upstream of the measuring nozzle, the vacuum nozzle being designed to generate a negative pressure in a negative pressure space during flow through by the fluid medium;

detecting a measured pressure in the negative pressure space; and

the position of the workpiece or tool relative to the support surface is determined taking into account the measured pressure.

The method is preferably carried out such that the measured pressure in the underpressure space has a negative value in the absence of a workpiece or tool on the bearing surface.

As already explained, the method can be carried out by means of a control device, wherein the control device receives a signal influenced by the measured pressure, and wherein the control device determines the position of the workpiece or the tool taking into account the received signal. The control device can optionally also detect the supply pressure upstream of the vacuum nozzle and determine the position of the workpiece or tool taking into account the supply pressure additionally.

The signal influenced by the measured pressure can advantageously be transmitted to the control device in a wireless manner. The same applies to the signal for the supply pressure. In particular, the corresponding signal can be transmitted by means of a passive RFID transponder.

The method can also comprise evacuating the vacuum nozzle and the measurement nozzle by means of a fluid, which is conveyed through an additional compressed air interface.

Drawings

Preferred embodiments of the invention are described below with the aid of the accompanying drawings, which are intended to be illustrative only and are not to be construed as limiting. Shown in the drawings are:

fig. 1 shows a clamping device with a gear clamped thereon and a device for supporting monitoring in a front view; the clamping device and the gear wheel are shown in a central longitudinal section in detail a;

FIG. 1a shows an enlarged view of detail A;

FIG. 2 shows an exemplary view of the correlation of pressure measured in bearing monitoring with query distance b according to the prior art;

FIG. 3 shows an exemplary view of the correlation of the pressure measured in the bearing monitoring according to the invention with the query distance b;

fig. 4 shows a symbolic view of the bearing monitoring according to the invention according to a first embodiment;

fig. 5 shows a symbolic view of a bearing monitoring according to the invention according to a second embodiment;

fig. 6 shows a symbolic view of a bearing monitoring according to the invention according to a third embodiment;

fig. 7 shows a symbolic view of a bearing monitoring according to the invention according to a fourth embodiment;

FIG. 8 shows a central longitudinal cross-sectional view through a bearing monitor according to the present invention, which operates according to the principles of FIG. 5;

fig. 9 shows a schematic view for the support monitoring of a workpiece and a tool on a gear machine according to the invention.

Figure 10 shows a detailed view of the workpiece spindle with cut-out B;

figure 10a shows an enlarged detail view of cut-away portion B in figure 10;

figure 11 shows a detailed view of the workpiece spindle with a cut-out C;

figure 11a shows an enlarged detail view of the cut-away portion C in figure 11;

fig. 12 shows a detailed view of the grinding spindle with a cut-out D; and

fig. 12a shows an enlarged detail view of the cut-out D in fig. 12.

Detailed Description

In the figures, identical reference numerals are used for identical or similar faces, pressures, functional elements or other elements.

Principle of function

Fig. 1 and 1a show a workpiece 1 in the form of a gear wheel, which has been placed on an automatic workpiece clamping device 2. The gear wheel 1 has a flat end face on its lower end face in fig. 1. With this end face, the gear wheel 1 rests on a bearing surface 3 of the clamping device 2. The bearing surface 3 serves as a reference surface for the position of the workpiece 1. A measuring gap b is formed between the bearing surface 3 and the end face of the workpiece. One or more, preferably three measuring nozzles 4 are formed in the bearing surface 3. The measuring nozzle 4 is acted upon with compressed air at a supply pressure p1 via a schematically illustrated compressed air connection 10 a.

For such pneumatic devices, in the prior art, it is customary to determine directly or indirectly the dynamic pressure in the measuring nozzle, which is generated as follows: the workpiece obstructs the flow of compressed air from the measuring nozzle. Fig. 2 schematically illustrates the dependence of the measured pressure p2 in the measuring nozzle on the size of the measuring gap b. The measured pressure is a function of the supply pressure and the size of the measurement gap: p2 ═ f (p1, b). If the measuring gap is very large (in the present example, for example b >1mm), the outflow of compressed air from the measuring nozzle through the workpiece is not significantly impeded. The dynamic pressure caused by the workpiece is negligibly small and the measured pressure p2 in the measuring nozzle corresponds to the limit value pmin. If, on the other hand, the workpiece completely closes the measuring nozzle (b ═ 0), the dynamic pressure reaches a maximum value and the measured pressure p2 in the measuring nozzle corresponds to the supply pressure p 1.

Instead of measuring the absolute pressure p2, it is also possible to measure the differential pressure Δ p — p1-p2, which therefore likewise represents a function of the supply pressure p1 and the measurement gap b. If the workpiece does not obstruct the flow of compressed air from the measuring nozzle, the pressure difference Δ p reaches a maximum value: Δ pmax is p 1-pmin. If the workpiece completely closes the measuring nozzle, the pressure difference becomes zero. The maximum pressure difference Δ pmax corresponds to the maximum pressure change due to the presence of the workpiece. Which defines the size of the available measurement window. In the example of fig. 2, the maximum pressure difference is Δ pmax ≈ 0.5bar at a supply pressure p2 of 1.6 bar.

In practice, it has been shown that the maximum differential pressure Δ pmax is usually only weakly dependent on the supply pressure. Thus showing: for example, in the example of fig. 2, the maximum pressure difference is likewise about 0.5bar at a supply pressure p1 of 0.8 bar. In the measuring principle according to the prior art, the size of the measuring window is always about 0.5bar in the present example, substantially independent of the size of the supply pressure.

If the measurement window is to be enlarged, this can only be achieved by means of a greatly increased supply pressure and/or a higher throughflow. However, smaller and lighter workpieces are pushed apart as a result of the higher supply pressure and reliable support is no longer possible. For economic reasons, an increase in the throughflow should be dispensed with.

In contrast, in the present invention, the (absolute or relative) measurement of the pressure p3 is performed in the negative pressure space of the vacuum nozzle. Fig. 3 schematically shows a curve of the pressure as a function of the size of the measurement gap b. When the measurement gap is completely closed (b ═ 0), no air flows through the vacuum nozzle. Accordingly, the pressure measured in the vacuum nozzle corresponds to the supply pressure p 1. When the measurement gap is fully open (in this example, b >1mm), the air can flow unimpeded through the vacuum nozzle. By means of the bernoulli principle, a negative pressure is generated in the vacuum nozzle, which in the present example is at most pmin ≈ 0.9 bar. In this example, the maximum differential pressure Δ pmax is Δ pmax ≈ p1-pmin ≈ 1.6bar- (-0.9bar) ≈ 2.5 bar. The size of the measuring window is therefore approximately 2.5bar, approximately five times as large as in the measuring principle according to the prior art. However, the measurement window is now strongly correlated with the supply pressure p 1. The larger the supply pressure, the larger the measurement window. Therefore, accurate monitoring or at least determination of the supply pressure is particularly important here.