Monitoring device of a machine tool

A monitoring device of a machine tool, in particular a circular saw bench, includes a safety unit having at least one first sensor. The monitoring device further includes at least one reference sensor, which is provided to convert a characterizing physical measured variable of the workpiece into an electric characteristic variable during machining of a work piece using the machine tool. The characteristic variable is provided to establish a reference value for a signal of the first sensor.

This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2010/062523, filed on Aug. 27, 2010, which claims the benefit of priority to Serial No. DE 10 2009 046 040.3, filed on Oct. 27, 2009 in Germany, the disclosures of which are incorporated herein by reference in their entirety.

BACKGROUND

The disclosure is based on a monitoring apparatus for a machine tool.

The document DE 10 2007 032 221 A1 has already proposed a machine tool having a safety unit which comprises a sensor unit and an actuator system unit. The actuator system unit is intended to trigger a safety operation on the basis of a parameter sensed by the sensor unit. The machine tool has a calibration unit for calibrating the safety unit in order to prevent the actuator system unit from being triggered incorrectly on account of an altered work environment.

SUMMARY

The disclosure is based on a monitoring apparatus for a machine tool, particularly for a circular table saw, having a safety unit having at least one first sensor.

It is proposed that the monitoring apparatus have at least one reference sensor which is intended, during machining of a workpiece using the machine tool, to convert a characterizing physical measured variable from the workpiece into an electrical parameter which is intended to stipulate a reference value for a signal from the first sensor. In this connection, a “safety unit” is intended to be understood to mean particularly a unit which is intended to take a signal from the at least one sensor, in situations which present a potential hazard for an operator of the machine tool, as a basis for outputting a signal and/or particularly intervening in operation of the machine tool. In this connection, a “characterizing physical measured variable” is intended to be understood to mean particularly a quantitative description of a specific physical property of the workpiece, particularly an electrical permittivity or an electrical conductivity. In this connection, “during machining of the workpiece” is particularly intended to be understood to mean that conversions of the characterizing physical measured variable from a portion of the workpiece into electrical parameters by the reference sensor take place essentially continually and particularly from startup of the machine tool, with an interval of time of less than ten seconds between successive measurements, preferably with an interval of time of less than one second and particularly preferably with an interval of time of less than 0.2 second. In this connection, a “reference value for a signal from the first sensor” is intended to be understood to mean particularly a value which is intended to be used as a reference variable during evaluation of the signal from the first sensor in at least one operating state. As a result, in the course of operation of the machine tool, it is a simple matter to essentially compensate for any influence on the first sensor in the safety unit by the workpiece, which means that a function of the safety unit can be provided, with constant reliability, even in the case of varying physical properties of the workpiece which influence a signal from the first sensor.

If the reference sensor is intended to sense in a signal a physical measured variable which is determined essentially by a moisture content of the workpiece, it is advantageously possible to compensate for any influence on the first sensor by a varying moisture content of the workpiece, so that it is possible to allow constant reliability for the function of the safety unit. Preferably, the physical measured variable used which is determined essentially by the moisture content of the workpiece may be an electrical conductivity or an electrical permittivity.

In one advantageous refinement, the monitoring apparatus has a bearing unit which is used to support the reference sensor in a region which is arranged in front of the tool in relation to a prescribed direction of advance in which the workpiece is moved against a tool in the machine tool during machining. This arrangement allows an evaluation of the reference value and a corresponding matching operation relating to the first sensor, specifically particularly compensation for any influence on the first sensor by the workpiece, to take place effectively in a machining phase in which a body part of an operator approaches the tool, in which case the safety of the operator can advantageously be increased. A region which is arranged “in front of” the tool is intended to be understood to mean particularly a subregion of a working surface of the machine tool which is provided for laying and guiding the workpiece, which subregion is used to guide that portion of the workpiece which is intended to be machined by the tool, and/or to mean a region of the machine tool which has a projection—produced at right angles to the working surface—in this subregion of the working surface. Preferably, this subregion of the working surface is arranged directly in front of the tool. If the tool is in the form of a saw, this subregion of the working surface is advantageously crossed by a sectional plane which corresponds to the imaginary continuation of the saw blade contrary to the direction of advance and defines what is known as a saw line in the working surface.

It is also proposed that the reference sensor be in the form of a UWB sensor unit. In this connection, a “UWB sensor unit” (UWB=ultra wide band) is intended to be understood to mean particularly a sensor unit which is intended to emit and/or receive electromagnetic signals having a center frequency between 30 MHz and 15 GHz and a bandwidth of at least 500 MHz, typically a few GHz. By virtue of the reference sensor being in the form of a UWB sensor unit, it is advantageously possible to obtain a particularly high information content in relation to the characterizing physical measured variable from the workpiece. Preferably, a spectral power density of the UWB sensor unit is no more than −41.3 dBm/Hz EIRP (Equivalent Isotropic Radiated Power), which means that interference with other appliances which are operated at frequencies between 30 MHz and 15 GHz can be advantageously avoided.

In this connection, it is proposed that the UWB sensor unit be intended to be operated at a center frequency which varies discretely in a prescribed frequency range. In contrast to wideband excitation of the frequency range, this involves the UWB sensor unit being excited discretely, preferably in the form of pulsed excitation of said UWB sensor unit, with the center frequency successively assuming different discrete values in the prescribed frequency range. Preferably, frequency intervals between two successive excitation frequencies assume different values in order to take particularly effective account of regulations from regulatory authorities. In this connection, a “center frequency” for the UWB sensor unit is intended to be understood to mean particularly a frequency which is an arithmetic mean of two frequencies for which a spectral power density has fallen by 10 dB relative to a maximum spectral power density of the UWB sensor system. By varying the center frequency of the UWB sensor unit, it is possible to increase the information content in relation to the characterizing physical variable from the workpiece by means of measurements in different frequency ranges.

In one advantageous refinement, the UWB sensor unit is intended to be operated at a center frequency which varies periodically in a prescribed frequency range. In this case, the frequency range can be scanned by continuously varying the center frequency. However, it is advantageous if the UWB sensor unit is excited in an excitation sequence starting from a lower limit of the frequency range with discrete values for the center frequency, the center frequency being increased in steps until an upper limit of the frequency range has been reached, and this excitation sequence being repeated periodically.

If the monitoring apparatus comprises at least one third sensor, in the form of a UWB sensor unit, which is intended to be operated at a center frequency which is at an interval of at least 500 MHz from a center frequency for the reference sensor, it is a particularly simple manner to achieve an increase in the information content in relation to the characterizing physical variable of the workpiece by means of measurement in different frequency ranges.

It is also proposed that, of the reference sensor in the form of a UWB sensor unit and the sensor in the form of a UWB sensor unit, at least one sensor, and particularly advantageously precisely one sensor, has a coplanar antenna. In this connection, a “coplanar antenna” is intended to be understood to mean particularly an antenna which is formed by inherently known stripline elements which are arranged in one plane on a dielectric support. Preferably, the coplanar antenna may be in the form of a patch antenna or spiral antenna. In principle, however, use of other designs which appear to be appropriate to a person skilled in the art is also conceivable. By equipping one of the sensors in the form of a UWB sensor unit with a coplanar antenna, it is advantageously possible to achieve a concentration for a sensitivity range of the one sensor onto a surface of the workpiece. By equipping the other of the sensors in the form of a UWB sensor unit with an antenna which differs from a coplanar design, for example a free space antenna, it is possible to achieve averaging of the characterizing physical variable over an extensive volume range of the workpiece. Advantageously, by combining the signals from the two sensors, it is possible to distinguish a surface film of moisture, which influences the mechanical properties of the workpiece only insignificantly, from increased moisture in the interior of the workpiece, which greatly influences the mechanical properties of the workpiece.

It is also proposed that the monitoring apparatus have a bearing unit which is used to support the reference sensor at a greater depth than a working surface of the machine tool which is provided for the purpose of laying the workpiece. In this context, the term “at a greater depth” relates to a direction which is oriented at right angles to the working surface. When the machine tool is in the form of a fixed machine, this direction may correspond to the vertical direction. Such an arrangement of the reference sensor makes it possible to avoid such proximity of a workpiece material, particularly direct contact between the reference sensor and the workpiece material, as is disadvantageous for effective operation of the reference sensor.

By virtue of the bearing unit, the reference sensor is spaced apart from the working surface by an interval in a direction perpendicular to the working surface. A particularly effective sensing operation by the reference sensor can also take place when the reference sensor is spaced apart from the working surface by an air interval by means of the bearing unit. In this space that is designed to be free of material, electromagnetic waves can be produced and propagated particularly efficiently. Preferably, this is accomplished by cutting out a cavity from a material which forms the working surface. In order to provide an uninterrupted working surface for the operator, the working surface may also contain a cover means which covers said cavity from the top and serves as a subregion of the working surface.

Furthermore, it is proposed that the first sensor be in the form of a capacitive sensor and the reference sensor provide a piece of information which is a measure of a physical parameter from the workpiece and which is intended to be used to stipulate a reference value for the signal from the first sensor. Particularly advantageously, the physical parameter may be in the form of an electrical permittivity. In this connection, a “capacitive sensor” is intended to be understood to mean particularly a sensor for which the signal level is dependent on a capacitance value of a capacitor, a capacitor arrangement or a system of capacitors. In this connection, a “reference value for the signal from the first sensor” is intended to be understood to mean particularly a value which is intended to be used as a reference variable during evaluation of the signal from the capacitive sensor in at least one operating state. This makes it a simple and effective matter to provide a basis for the evaluation of the signal from the capacitive sensor which compensates for changes to the capacitance value by the workpiece, as a result of which the capacitive sensor can, with constant reliability, sense changes in the capacitance value which are caused by an operator. Preferably, the capacitive sensor is in the form of a contactlessly operating proximity sensor. In principle, however, it may also be in a form such that a signal change is triggered essentially by contact by the operator. On account of the comparatively high electrical permittivity of water in the frequency range between 30 MHz and 15 GHz, it is advantageously possible to avoid particularly any influence on the capacitive sensor in the safety unit by a varying moisture content of the workpiece.

In addition, it is proposed that the monitoring apparatus comprise an evaluation unit which is intended to ascertain from a signal from the reference sensor a measure of a physical parameter from the workpiece and to provide this measure as a signal. Advantageously, this makes it possible to provide a piece of information about material properties of the workpiece, such as the moisture content, and particularly about externally invisible materials which the workpiece contains. Particularly advantageously, the physical parameter may be in the form of an electrical permittivity. Advantageously, it is possible to detect metal inclusions, or inclusions which have a specific electrical conductivity of more than 5×105S/m in at least one spatial direction, which the workpiece contains, so that potentially increased wear can be avoided for the machine tool or portions thereof and/or a potential hazard can be avoided for the operator. An advantage in respect of saving material is that the evaluation unit may be in a form integral with the safety unit.

Advantageously, the monitoring apparatus is equipped with a display unit which is intended to display a parameter produced by the evaluation unit on the basis of a signal from the reference sensor in order to inform an operator about a level of the characterizing physical measured variable from the workpiece by means of a visual and/or audible presentation.

In one advantageous refinement, the evaluation unit is intended to provide a specific signal in the event of a value which has been stipulated in advance for the physical parameter, for example the electrical permittivity, being exceeded. As a result, it is advantageously possible to provide a means for monitoring the safety of a machining operation and for avoiding increased wear on portions of the machine tool and/or a risk to the operator.

If the safety unit is intended to request an acknowledgment by the operator to continue operation of the machine tool in the event of a value which has been stipulated in advance for the electrical parameter from the reference sensor being exceeded, it is possible to render machining of a workpiece, which potentially involves a risk in respect of the safety of the operator and/or in respect of the increased wear on portions of the machine tool, advantageously dependent on a check and to avoid uncontrolled machining.

DETAILED DESCRIPTION

FIGS. 1 and 2show alternative exemplary embodiments. Essentially constant components, features and functions are labeled with the same reference symbols in principle. To distinguish between the exemplary embodiments, however, the reference symbols for the exemplary embodiments have the letters a and b added, with reference being able to be made to the description of the exemplary embodiment inFIG. 1for constant components, features and functions.

FIG. 1shows a machine tool10aformed by a circular table saw. The machine tool10ahas a tool12awhich is formed by a circular saw blade and, in the assembled state, projects partly upwards out of a recess in a working surface14aof the machine tool10a. The machine tool10acomprises a motor unit16awhich is intended to produce a drive torque for driving the tool12aduring operation. While the machine tool10ais operating, an operator18aimparts on a workpiece20ain the form of a wooden beam, in a known manner, a mechanical advance along the working surface14ain a direction of advance22awhich is directed at the tool12a. In addition, the machine tool10ahas an input unit24afor inputting operating parameters and a display unit26afor inspection purposes. The machine tool10aalso comprises a safety unit30ahaving a first sensor, in the form of a capacitive sensor32a, which comprises a plurality of capacitance elements34awhich are arranged in proximity to the tool12aand stipulate a protection region36awhich is associated with the tool12ain the machine tool10a. If an operator18aof the machine tool10ahas a body part enter the protection region36aduring operation, a signal level from the capacitive sensor32aincreases. The safety unit30ais intended, in the event of a threshold value—which is stipulated in advance and is stored in the safety unit30a—for the capacitive sensor32abeing exceeded, to produce a signal which acts on the motor unit16ain a known manner and switches off the drive torque for the tool12ain order to protect the operator18a.

In addition, the machine tool10acomprises a monitoring apparatus28ahaving a reference sensor38awhich is in the form of a UWB sensor unit and which is arranged within the protection region36aassociated with the tool12a. The excitation of the UWB sensor unit is described in more detail with reference toFIG. 5.

The monitoring apparatus28ais intended to start up the reference sensor38aat the same time as the machine tool10aand the safety unit30awith the capacitive sensor32aare switched on. The reference sensor38ais intended, during machining of a workpiece20ausing the machine tool10a, to convert a characterizing physical measured variable from the workpiece20ainto an electrical parameter which is intended to stipulate a reference value for a signal from the capacitive sensor32a. To this end, the reference sensor38aprovides a piece of information which is a measure of an electrical permittivity of the workpiece20a, and thereby senses in its signal a physical measured variable which is determined essentially by a moisture content of the workpiece20a. The information provided by the reference sensor38ais intended to be used to stipulate a reference value for the signal from the capacitive sensor32a. In the position of the workpiece20awhich is shown inFIG. 1, there is a wooden beam portion being machined by the tool12asimultaneously above the reference sensor38aand in the protection region36aassociated with the tool12a, which portion is sensed by the capacitive sensor32a, as a result of which the moisture content of the wooden beam influences the signal from the capacitive sensor32aand from the reference sensor38a. The monitoring apparatus28aalso comprises an evaluation unit40awhich is intended to use a test frequency of 20 Hz to ascertain a measure of an electrical permittivity of the workpiece20afrom a signal from the reference sensor38aand to provide said measure for the display unit26aas a signal. The display unit26ais intended to display a parameter produced by the evaluation unit40aon the basis of a signal from the reference sensor38ain order to inform an operator18aabout a level of the characterizing physical measured variable from the workpiece20aby means of visual and audible presentation. The evaluation unit40ais also intended to take into account the signal from the reference sensor38aas a correction when evaluating the signal from the capacitive sensor32a. In this way, an operating situation shown inFIG. 1becomes an initial situation for an assessment of a potential hazard for an operator18aof the machine tool10a, despite the workpiece20awhich is being machined with its moisture content for the capacitive sensor32aof the safety unit30a. Should the moisture content of the workpiece20achange with a machining length, the test frequency of 20 Hz, which the evaluation unit40auses to evaluate the signal from the reference sensor38a, ensures sufficiently fast adjustment of the signal from the capacitive sensor32a.

FIG. 2likewise shows a machine tool formed by a circular table saw, said machine tool differing from that shown inFIG. 1only in a few equipment features, which are described fully below.

A monitoring apparatus28bas shown inFIG. 2comprises a third sensor42b, in the form of a UWB sensor unit, which is arranged within a protection region36bwhich is associated with a tool12bformed by a circular saw blade. The third sensor42bis intended to be operated at a center frequency of 10 GHz. A reference sensor38bin the form of a UWB sensor unit is intended to be operated at a center frequency of 10 GHz, which means that the center frequency of the third sensor42bis at an interval of 2 GHz from the center frequency of the reference sensor38b. An evaluation unit40bin the monitoring apparatus28bis intended to use a signal from the third sensor42bto increase an accuracy for a signal from the reference sensor38b.

Furthermore, the evaluation unit40bis intended to provide a specific signal, which is formed by a signal tone and a flashing lamp44b, in the event of a value—which is stipulated in advance and stored in the evaluation unit40b—of the electrical permittivity being exceeded. Hence, the monitoring apparatus28bis able to warn an operator18babout inclusions made of iron which are hidden in a workpiece20bin the form of a wooden beam and which are large enough for there to be a risk of increased wear for a tool12bformed by a circular saw blade.

In addition, a safety unit30bis intended to request acknowledgement by the operator to continue operating a machine tool10bin the event of a value stipulated in advance for the electrical parameter from the reference sensor38bbeing exceeded. The value stipulated in advance is oriented to a situation that occurs routinely during operation of the machine tool10band in which the machine tool10bis intended to be used to saw a workpiece20bin the form of a composite material with inserted, electrically conductive carbon fibers, which requires the use of a special circular saw blade, since a standard circular saw blade would be subjected to excessive wear in this situation. The acknowledgement requested by the safety unit30bto continue operating the machine tool10bproduces a checking situation and avoids uncontrolled machining of the workpiece20b.

FIG. 3shows a detail view of the machine tool10bfromFIG. 2. The description which follows, which is admittedly based on the exemplary embodiment in thisFIG. 2with a third sensor42b, can likewise be applied accordingly to the exemplary embodiment inFIG. 1, the monitoring apparatus in this embodiment having no third sensor42b. A material46bwhich forms the working surface14bis shown in a sectional illustration. From this material46b, a cavity48bis cut out which is associated with at least the reference sensor38b. This cutout produces an interruption in the surface of the material46b, which interruption contains a plastic window50bas a cover portion for covering the cavity48band for forming a portion of the working surface14b. This cavity48bis adjoined by the reference sensor38band the third sensor42b, which are therefore arranged at a greater depth than the working surface14brelative to the vertical direction52at right angles to the working surface14b. A bearing unit54bsupports the reference sensor38band the third sensor42bin this position below the working surface14b, specifically below the cavity48bor, in an alternative embodiment, which is not shown, in the cavity48b. The bearing unit54bsupports the reference sensor38band the third sensor42bin region55bof the machine tool10bwhich is arranged in front of the tool12brelative to the direction of advance22b. In particular, the vertical projection of the reference sensor38bor of the third sensor42bonto the working surface14bis arranged in a subregion of the working surface14bwhich is situated directly in front of a working edge of the tool12b. This can be seen particularly from the view inFIG. 4.

The bearing unit54balso produces an interval between the reference sensor38bor the third sensor42band the working surface14bin the vertical direction52, as a result of which it is possible to prevent the reference sensor38bor the third sensor42bfrom being in disadvantageous proximity to the workpiece32bto be machined (seeFIG. 1), and particularly in direct contact with the material of the workpiece32b. A space which is left free of material between the plastic window50band the reference sensor38bor the third sensor42bforms an air interval56bin the form of an air gap between the reference sensor38bor the third sensor42band the plastic window50b, with the reference sensor38bor the third sensor42bbeing spaced apart from the working surface14band from the workpiece32bby the air interval56b. Preferably, the air interval56bhas a vertical extent of approximately 20 mm.

FIG. 4shows a subsection of the working surface14bfrom above (see arrows IV inFIG. 3). It is possible to see the cavity48bwhich is cut out from the material46b, which cavity is associated with the reference sensor38bor the third sensor42b. For the purpose of describing the reference sensor38bor the third sensor42b, the plastic window50bhas not been shown, as a result of which it is possible to see the reference sensor38band the third sensor42bin a view from above. The reference sensor38bhas a coplanar antenna58bwhich has three stripline elements60bwhich are arranged in one plane62bof a dielectric support. In this configuration, the third sensor42bis in the form of a free space antenna. In an alternative configuration, the reference sensor38bmay be in the form of a free space antenna, with the third sensor42bhaving a coplanar antenna.

FIG. 5shows an X-Y graph in which a frequency f is plotted on the X axis and a signal intensity I is plotted on the Y axis. In particular, a frequency range64is shown in which the UWB sensor unit produced by the reference sensor38a(FIG. 1) or by the reference sensor38bor the third sensor42b(FIG. 2) is operated. This frequency range64corresponds to an interval between a lower frequency f1and an upper frequency f2in which the relevant UWB sensor unit is excited discretely. In this context, the UWB sensor unit is excited at successive times tiat a center frequency fMi, where i is an index which assumes values between 1 and the number of discrete excitation frequencies in the frequency range64. By way of example, the UWB sensor unit is operated at a center frequency fM1at time t1, whereas it is operated at further center frequencies fM2, fM3, fM4, etc., at later times t2, t3, t4, etc., as indicated by means of dashed lines. The center frequency at which the UWB sensor unit is operated is gradually increased in steps by an amount66.1,66.2,66.3, etc., the amounts66.1,66.2,66.3preferably assuming different values. The frequency range64can be scanned periodically or aperiodically. In one preferred exemplary embodiment, the frequency range64corresponds to the interval between f1=2.7 GHz and f2=7.5 GHz, the number of discrete center frequencies fMiadvantageously being above 50. The frequency range64is preferably scanned in approximately 2.5 ms.