Machine Tool and Detection System

Abstract

The disclosure relates to a machine tool having a driveable tool and having a detection system for detecting contact between the tool and a user. The detection system has an excitation device for applying an excitation voltage signal to the tool, a current sensor for generating a current sensor signal that represents an electric current consumed by the tool, and a processing device for processing the current sensor signal in order to detect contact between the tool and the user. The processing device has an analog-to-digital converter and further devices arranged downstream of the analog-to-digital converter, comprising an IQ demodulator, a filter device and an evaluation device. The current sensor signal is able to be digitized using the analog-to-digital converter and processed in digital form using the further devices. The disclosure furthermore relates to a detection system for a machine tool.

Claims

1. A machine tool comprising: a driveable tool; and a detection system configured to detect contact between the driveable tool and a user, the detection system comprising: an excitation device configured to apply an excitation voltage signal to the driveable tool, a current sensor configured to generate a current sensor signal that represents an electric current consumed by the driveable tool; and a processing device configured to process the current sensor signal to detect the contact between the driveable tool and the user, the processing device having an analog-to-digital converter and further devices arranged downstream of the analog-to-digital converter, the further devices including an IQ demodulator, a filter device, and an evaluation device, the current sensor signal being digitized using the analog-to-digital converter and being processed in digital form using the further devices.

2. The machine tool according to claim 1, wherein at least one of: the machine tool is a sawing machine; the excitation device is configured to apply the excitation voltage signal to the driveable tool by capacitive coupling; the machine tool comprises a reaction system configured to deactivate operation of the driveable tool; and the IQ demodulator is arranged downstream of the analog-to-digital converter, the filter device is arranged downstream of the IQ demodulator, and the evaluation device is arranged downstream of the filter device;

3. The machine tool according to claim 1, wherein: the IQ demodulator is configured to convert the digitized current sensor signal into a digital I signal and a digital Q signal; the filter device is configured to filter the digital I signal and Q signal; and the evaluation device is configured to process the filtered digital I signal and Q signal to detect the contact between the driveable tool and the user and to generate a trigger signal configured to deactivate operation of the driveable tool based on the processing by the processing device.

4. The machine tool according to claim 1, wherein the filter device comprises a CIC filter and a compensation filter arranged downstream of the CIC filter.

5. The machine tool according to claim 1, wherein one of: the evaluation device is configured to (i) calculate an instantaneous energy value of a power consumption of the driveable tool and an adaptive threshold value and (ii) generate a trigger signal configured to deactivate operation of the driveable tool based on a comparison between the instantaneous energy value and the adaptive threshold value; and the evaluation device is configured to (i) calculate an instantaneous energy value of a current consumption of the driveable tool and an adaptive threshold value, (ii) calculate a signal-to-noise ratio, and (iii) generate a trigger signal configured to deactivate operation of the driveable tool based on a comparison between the instantaneous energy value and the adaptive threshold value and based on a comparison of the calculated signal-to-noise ratio with a predetermined limit value.

6. The machine tool according to claim 1, wherein the evaluation device comprises an artificial neural network.

7. The machine tool according to claim 1, wherein the processing device comprises at least one of: an FPGA; a microcontroller; and a CPU.

8. The machine tool according to claim 1, wherein: the processing device is configured to process the digitized current sensor signal via a first processing channel and a second processing channel; the processing device has a respective IQ demodulator, a respective filter device and a respective evaluation device with respect to each of the first processing channel and second processing channel; the processing device comprises at least one comparison device to which processing data accumulated in the first processing channel and second processing channel is transmitted; and the at least one comparison device is configured to compare the processing data of the first processing channel and second processing channel and generate, based on the comparison, at least one of: a switch-off signal configured to switch off operation of the driveable tool; and a warning signal.

9. The machine tool according to claim 8, wherein: the respective IQ demodulator of each of the first processing channel and second processing channel is configured to convert the digitized current sensor signal into a digital I signal and a digital Q signal; the respective filter device of each of the first processing channel and second processing channel is configured to filter the digital I signal and Q signal; and the respective evaluation device of each of the first processing channel and second processing channel is configured to process the filtered digital I signal and Q signal in order to detect the contact between the driveable tool and the user and generate a trigger signal configured to deactivate operation of the driveable tool based on the processing by the processing device.

10. The machine tool according to claim 9, wherein: the trigger signals generated by the respective evaluation devices of the first processing channel and second processing channel are transmitted to the at least one comparison device; and the at least one comparison device is configured to generate its own trigger signal configured to deactivate operation of the driveable tool, provided that at least one of the trigger signals is transmitted to the at least one comparison device by at least one of the respective evaluation devices of the first processing channel and second processing channel.

11. A detection system for a machine tool having a driveable tool for detecting contact between the driveable tool and a user, the detection system comprising: an excitation device configured to apply an excitation voltage signal to the driveable tool, a current sensor configured to generate a current sensor signal that represents an electric current consumed by the driveable tool; and a processing device configured to process the current sensor signal to detect the contact between the driveable tool and the user, the processing device having an analog-to-digital converter and further devices arranged downstream of the analog-to-digital converter, the further devices including an IQ demodulator, a filter device and an evaluation device, the current sensor signal being digitized using the analog-to-digital converter and being processed in digital form using the further devices.

Description

[0055] The invention is explained in more detail below with reference to the schematic figures. In the drawings:

[0056] FIG. 1 shows a machine tool designed as a table saw with a detection system for detecting contact between a saw blade and a user;

[0057] FIG. 2 shows a printed circuit board with components of the detection system;

[0058] FIG. 3 shows a processing device of the detection system with an analog-to-digital converter, an IQ demodulator, a filter device and an evaluation device;

[0059] FIGS. 4 and 5 show block diagrams of the IQ demodulator and a mixer;

[0060] FIGS. 6 and 7 show diagrams illustrating a processing performed by the evaluation device using a predetermined algorithm;

[0061] FIG. 8 shows an evaluation device having an artificial neural network;

[0062] FIG. 9 shows a two-channel design of the processing device;

[0063] FIG. 10 shows isolated regions of the processing device of FIG. 9, in the form of an FPGA; and

[0064] FIG. 11 shows a two-channel design of the processing device with a plurality of semiconductor chips.

[0065] Based on the following figures, possible embodiments of a machine tool having a detection system are described. The figures are merely schematic and are not true to scale. Therefore, components and structures shown in the figures may be exaggeratedly large or reduced in size for better understanding.

[0066] FIG. 1 shows a machine tool 100, in the present case in the form of a table circular saw. The machine tool 100 has a driveable tool in the form of a rotatable saw blade 103, with which a workpiece made of wood (not shown) can be machined. The saw blade 103 has an electrically conductive or metallic material. The saw blade 103 is connected to a drive shaft 104, which in turn is operatively connected to a motor 105 of the machine tool 100. The motor 105 can be an electric motor. The saw blade 103 can be driven, i.e. set into a rotational movement, via the motor 105 and the drive shaft 104. The machine tool 100 further includes a table 106 having a through-opening through which the saw blade 103 protrudes, thereby standing out relative to an upper surface of the table 106, as shown in FIG. 1.

[0067] The machine tool 100 is designed to minimize, to the greatest possible extent, the risk of (serious) injury to a human user during operation of the machine tool 100. For this purpose, the machine tool 100 comprises a detection system 101, using which contact between the saw blade 103 and the user, or a body part of the user such as a hand 500 indicated in FIG. 1, can be detected, and a reaction system 400 coupled to the detection system 101 for deactivating operation of the saw blade 103.

[0068] As shown in FIG. 1, the detection system 101 comprises an excitation device 110 for applying an excitation voltage signal to the saw blade 103 by capacitive coupling, a current sensor 120 for generating an analog current sensor signal 200 and a processing device 130 for processing the current sensor signal 200. The application of the excitation voltage signal to the saw blade 103 and the generation and processing of the current sensor signal 200 can take place continuously during the operation of the machine tool 100 and the detection system 101. An analog pre-filter 126 (cf. FIG. 2) can be arranged upstream of the processing device 130.

[0069] The excitation device 110 comprises an oscillator 111, a first voltage amplifier 112 connected to the oscillator 111 and driven by it, and an excitation electrode 117 arranged in the region of or near the saw blade 103. The excitation electrode 117, which, like the saw blade 103, is plate-shaped and can also be referred to as an excitation plate, has an electrically conductive or metallic material. The excitation electrode 117 and the first voltage amplifier 112 are electrically connected via a first line 113. Using the oscillator 111, the excitation voltage signal can be generated, amplified via the first voltage amplifier 112 and coupled capacitively into the saw blade 103 via the excitation electrode 117. As indicated in FIG. 1, the table 106 can serve as a (local) ground to which the oscillator 111 can be referenced.

[0070] The excitation voltage signal, which is used as carrier signal, is a periodic or sinusoidal voltage signal. In FIG. 1, the excitation voltage signal is indicated by an oscillation wave at oscillator 111. The (amplified) excitation voltage signal can have a peak-to-peak voltage of 12 V and a frequency, or carrier frequency, of 1.25 MHz. In this way, contact between the saw blade 103 and the user can be reliably detected and distinguished from another contact, for example between the saw blade 103 and a workpiece.

[0071] The current sensor 120 is arranged on the line 113 connecting the excitation electrode 117 and the voltage amplifier 112. Using the current sensor 120, the electric current 190 flowing in the line 113 and thus consumed by the saw blade 103 (also designated Iblade in FIG. 1) can be acquired. Here, the analog current sensor signal 200 generated by the current sensor 120 represents the current 190 drawn by the saw blade 103. The current sensor 120 can be in the form of a current transformer.

[0072] The saw blade 103 and the excitation electrode 117 together form a plate capacitor. For reliable detection, the capacitor can have a capacitance in the range of 30 pF. For this purpose, the distance between the saw blade 103 and the excitation electrode 117 can be in the range of 1 mm. The detection of contact between the saw blade 103 and the user can be based on the capacitance between the saw blade 103 and the excitation electrode 117 is greater than the capacitance between the body of the user and the saw blade 103. In the event that there is no contact between the saw blade 103 and the user, the electric current 190 consumed by the saw blade 103, and thus the current sensor signal 200, can be relatively small or near zero. On the other hand, if the user or a body part thereof contacts the saw blade 103, this can appear as a low resistance connected in series with a larger capacitance to ground. Here, there can be a significant increase in the current draw of the saw blade 103 and thus in the current sensor signal 200, whereby contact between the saw blade 103 and the user can be detected.

[0073] As shown in FIG. 1, the machine tool 100 further comprises a shield 108 arranged in the region of the saw blade 103 and the excitation electrode 117. During operation of the machine tool 100, the shield 108 is subjected to the excitation voltage signal corresponding to the excitation electrode 117. For this purpose, the oscillator 111 is connected to a second voltage amplifier 114, which is connected to the shield 108 via a second line 115. Due to the shield 108 surrounding the saw blade 103 and the excitation electrode 117, the static capacitance between the saw blade 103 and the table 106 can be reduced. Further, the shielding can act as a Faraday cage, which can reduce the influence of interference effects from the environment on the detection system 101.

[0074] The processing device 130 of the detection system 101 is used to process the analog current sensor signal 200 generated by the current sensor 120 in order to detect contact between the saw blade 103 and the user. As shown in FIG. 1, the current sensor signal 200 coming from the current sensor 120 can be transmitted to the processing device 130 via an optional third voltage amplifier 124, the aforementioned pre-filter 126 and further lines. The sensor signal 200 can be amplified via the voltage amplifier 124, and pre-filtered via the pre-filter 126.

[0075] The reaction system 400, which can be activated by the detection system 101, is used to deactivate operation of the saw blade 103 to prevent the occurrence of (serious) injury to the user. The reaction system 400 can include means for rapidly moving the saw blade 103 away from the user, so that the saw blade is no longer protruding relative to the top of the table 106, as shown in FIG. 1, but is below the top of the table. A pyrotechnic charge can be used here. The reaction system 400 can additionally or alternatively have a braking device for braking and thereby stopping a rotational movement of the saw blade 103 (each not shown). Activation of the reaction system 400 for deactivating the operation of the saw blade 103 is controlled via a trigger signal 210, 211, which can be generated by the processing device 130 on the basis of the processing of the current sensor signal 200, and can be transmitted to the reaction system 400.

[0076] In the following, further possible embodiments are described that may be considered for the machine tool 100 and its detection system 101. In addition, reference is made to the possibility of combining several of the described embodiments with one another. Accordingly, features and details mentioned with respect to one embodiment may also be applied in another embodiment.

[0077] According to the design shown in FIG. 2, the processing device 130 and the current sensor 120 are arranged on a printed circuit board (PCB) 127 of the machine tool 100. In this respect, the arrangement shown in FIG. 2 can be referred to as a PCBA (printed circuit board assembly). The processing device 130 is preceded by the analog pre-filter 126, which can be used to pre-filter the analog current sensor signal 200 generated by the current sensor 120 (and possibly amplified by the optional voltage amplifier 124 according to FIG. 1). In this way, frequency components of the current sensor signal 200 that are not required for the signal processing or that interfere with it can be removed. The pre-filter 126 can be a low-pass filter. For differentiation, the pre-filtered current sensor signal is marked (only) in FIG. 2 with the reference sign 200′. Further components shown in FIG. 1, such as the oscillator 111 and the voltage amplifiers 112, 114, 124, can also be arranged on the circuit board 127. In an embodiment including the optional voltage amplifier 124, this voltage amplifier can be situated between the current sensor 120 and the pre-filter 126.

[0078] FIG. 3 shows a possible embodiment of the processing device 130. The processing device 130 has an analog-to-digital converter (ADC) 131 on the input side, which can be used to sample the analog pre-filtered current sensor signal 200 and convert it into a digital current sensor signal 202. A further component of the processing device 130 is an IQ demodulator 133 arranged downstream of the analog-to-digital converter 131, using which demodulator the digital current sensor signal 202 can be demodulated and converted into a digital I signal 205 (in-phase signal) and a digital Q signal 207 (quadrature signal). The processing device 130 further comprises a filter device 134 arranged downstream of the IQ demodulator 133 for filtering the digital I signal 205 and Q signal 207, and thereby providing a filtered digital I signal 206 and a filtered digital Q signal 208. The filtering of the I signal 205 and of the Q signal 207 is performed separately. An evaluation device 137 of the processing device 130 is arranged downstream of the filter device 134. The evaluation device 137 is designed to further process the filtered digital I signal 206 and Q signal 208 in order to detect contact between the saw blade 103 and the user, and to generate a trigger signal 210 for deactivating operation of the saw blade 103 on the basis of the processing. As explained above, the trigger signal 210 can be transmitted to the reaction system 400 (cf. FIG. 1) for the activation thereof.

[0079] The IQ demodulator 133, the filter device 134 and the evaluation device 137 of the processing device 130 are digital or digitally operating components. In this way, a relatively short processing or detection time can be achieved for detecting contact between the saw blade 103 and the user. Furthermore, the signal processing can be subject to low external interferences, and consequently can be relatively reliable. In this way, the machine tool 100 equipped with the detection system 101 can have a high degree of functional safety. The processing device 130 and thus the detection system 101 can furthermore be relatively compact and have a space-saving design. This therefore applies in a corresponding manner to the PCBA design shown in FIG. 2.

[0080] As indicated in FIG. 3, the processing device 130 can be in the form of a single semiconductor chip or signal processing chip 160. Depending on the embodiment of the semiconductor chip 160, the devices 133, 134, 137 arranged downstream of the analog-to-digital converter 131 and components thereof may be in the form of hardware or software components or modules. Here the analog-to-digital converter 131 and the further devices 133, 134, 137 are in suitable effective data-communicating connection with each other.

[0081] The processing device 130, which is designed as a semiconductor chip 160, can be, for example, in the form of a microcontroller or in the form of a CPU (central processing unit). Here, the devices 133, 134, 137 and the components thereof can be in the form of software modules.

[0082] Alternatively, the processing device 130 designed as a semiconductor chip 160 can be in the form of an FPGA (field programmable gate array). Here, the devices 133, 134, 137 and their components may be in the form of programmed hardware or logic blocks, also referred to as intellectual property (IP) blocks or functional blocks. An embodiment of the processing device 130 as an FPGA makes it possible to subsequently reconfigure the operation of the processing device 130 by reprogramming. Reprogramming is conceivable, for example, with respect to the filter device 134 in order to adapt the processing device 130 with respect to changed environmental conditions and disturbing influences. The processing device 130, in the form of an FPGA, can further be designed such that parallel signal processing is possible. This can be considered, for example, with respect to the filter device 134. In this way, a short detection time for detecting contact between the tool and the user can be further promoted.

[0083] The analog-to-digital converter 131 of the processing device 130 can be designed to carry out digitization of the current sensor signal 200 (cf. FIG. 3) by oversampling. In this embodiment, the sampling frequency used for sampling can be freely selected. In this way, low demands can also be made on the analog pre-filter 126 (cf. FIG. 2). As a result, the pre-filter 126 can be implemented with a relatively simple design, for example in the form of an anti-aliasing filter. It can be considered for the sampling frequency of the analog-to-digital converter 131 to be four times the carrier frequency of the excitation voltage signal applied to the saw blade 103. Given a carrier frequency of 1.25 MHz, the sampling frequency can thus be 5 MHz. In this way, the demodulation of digital current sensor signal 202 carried out using the IQ demodulator 133 can be simplified. The analog pre-filter 126 can have a cutoff frequency equal to half the sampling frequency of the analog-to-digital converter 131. At a sampling frequency of 5 MHz, the cutoff frequency can thus be 2.5 MHz.

[0084] The IQ demodulator 133 of the processing device 130 is used to demodulate the digital current sensor signal 202, converting it into a digital I signal 205 and a digital Q signal 206, as indicated above (see FIG. 3). The I signal 205 can agree with the phase of the carrier frequency or excitation voltage signal, whereas the Q signal 207 is ninety degrees out of phase with the carrier frequency or excitation voltage signal.

[0085] FIG. 4 shows a block diagram according to which the IQ demodulator 133 can be designed. The IQ demodulator 133 includes a numerically controlled oscillator 140 (NCO), a first mixer 141 and a second mixer 142. The two mixers 141, 142 are used for signal mixing and frequency conversion. Using the numerically controlled oscillator 140, a digital periodic first oscillator signal 241 and a digital periodic second oscillator signal 242 can be generated, wherein the first oscillator signal 241 is fed to the first mixer 141 and the second oscillator signal 242 is fed to the second mixer 142. Furthermore, the digital current sensor signal 202, also designated s(z) in FIG. 4, coming from the analog-to-digital converter 131 is transmitted to each of the two mixers 141, 142. Using the mixers 141, 142, the current sensor signal 202 is multiplied by the relevant first or second oscillator signal 241, 242. In this way, the I signal 205, also designated I(z) in FIG. 4, is output by the first mixer 141, and the Q signal 207, also designated Q(z) in FIG. 4, is output by the second mixer 142.

[0086] Oscillator signals 241, 242 may have the same frequency as the excitation voltage signal, i.e. the carrier frequency. The first oscillator signal 241 used to generate the I signal 205 can further agree with the phase of the excitation voltage signal. The first oscillator signal 241 can therefore be a sinusoidal signal, corresponding to the excitation voltage signal, as is also indicated in FIG. 4 by the term sin(2*TT*f.sub.c*z). Here, f.sub.c is the carrier frequency of the excitation voltage signal. The second oscillator signal 242 used to generate the Q signal 207, which can be a cosine signal as indicated in FIG. 4 by the term −cos(2*π*f.sub.c*z), is, in contrast, ninety degrees out of phase therewith.

[0087] The two mixers 141, 142 of the IQ demodulator 133 may be designed to perform down conversion during frequency conversion. By way of explanation, FIG. 5 shows a circuit diagram that can be used for the two mixers 141, 142. Here, the signals present at the two inputs and at the output of the relevant mixer 141, 142, i.e. the current sensor signal 202, the first and second oscillator signals 241, 242, and the I signal 205 and Q signal 207, respectively, are assigned corresponding frequencies, i.e. an input frequency f.sub.RF (radio frequency), an oscillator frequency f.sub.LO (local oscillator), and an output or intermediate frequency f.sub.IF (intermediate frequency). For the downmixing, the following applies:

f.sub.IF=f.sub.LO−f.sub.RF  (1)

[0088] As indicated above, oscillator signals 241, 242 may have the same frequency as the excitation voltage signal acting as carrier signal, i.e. the carrier frequency. The carrier signal can be contained in the current sensor signal 200 generated by the current sensor 120 (cf. FIG. 1), and thus also in the digital current sensor signal 202. The carrier signal can be removed by the downmixing. An item of useful information of interest, or a useful signal which can be used to detect the contact between the saw blade 103 and the user, can in contrast be retained, and can thereby be contained in the digital I signal 205 and Q signal 207.

[0089] The filter device 134 (cf. FIG. 3) used after demodulation of the current sensor signal 202 is used to remove unneeded or interfering frequency components of the digital I signal 205 and Q signal 207. The useful signal contained in the I signal 205 and Q signal 207 can, on the other hand, pass through the filter device 134, and can thereby continue to be contained in the filtered digital I signal 206 and Q signal 208 output by the filter device 134. The filter device 134 can be in the form of a low-pass filter in order to remove high-frequency components of the I signal 205 and Q signal 207. Furthermore, the filter device 134 can be designed to reduce the sampling rate (downsampling) and thus the data rate. This can promote fast performance of further signal processing using the evaluation device 137 arranged downstream of the filter device 134.

[0090] As shown in FIG. 3, the filtering of the digital I signal 205 and of the Q signal 207 is performed separately. For reliable effective filtering, the filter device 134 according to the embodiment shown in FIG. 3 has a CIC filter 135 (cascaded integrator comb filter) and a compensation filter 136 arranged downstream of the CIC filter 135 for both the I signal 205 and the Q signal 207. The compensation filters 136 can be FIR filters (finite impulse response filters). The aforementioned sample rate reduction can be implemented by means of the CIC filters 135. The associated compensation filters 136 are used to compensate for a sharp drop in the passband of the CIC filters 135 associated with the sampling rate reduction.

[0091] For example, the following parameters may be considered for the filter device 134 and its filters 135, 136. The CIC filters 135 may have a decimation factor of 48 reflecting the amount of sample rate reduction, a differential delay of 2 and five filter stages. The compensation filters 136 may have a passband cutoff frequency of 20 kHz, a stopband cutoff frequency of 30 kHz, a filter order of 36, a passband attenuation of 0.01 dB and a stopband attenuation of −40 dB.

[0092] The above-mentioned cutoff frequency of the filter device 134 of 20 kHz can be based on the following criteria. During operation of the machine tool 100, the saw blade 103 (cf. FIG. 1) can be rotated at a speed of, for example, 3650 rpm, and thus approximately 60 revolutions per second. The saw blade 103 can have a maximum number of saw teeth of, for example, 200. To calculate a maximum achievable frequency of the useful signal, the speed of the saw blade 103 (60 revolutions per second) can be multiplied by the maximum sawtooth number (200 saw teeth). This results in a maximum frequency of 12 kHz for the useful signal. With a cutoff frequency of 20 kHz for the filter device 134 or the compensation filter 136, a safety margin can be provided. As a result, contact between the saw blade 103 and the user can be reliably detected even if the operation of the machine tool 100 is at a greater speed of the saw blade 103, differing from the aforementioned speed, and/or the saw blade 103 has a greater number of saw teeth, differing from the aforementioned number of saw teeth.

[0093] The evaluation device 137 arranged downstream of the filter device 134 (cf. FIG. 3) is used for further processing of the filtered digital I signal 206 and Q signal 208 in order to detect contact between the saw blade 103 and the user and to generate a trigger signal 210 for deactivating operation of the saw blade 103 on the basis of the processing. The evaluation device 137 can be designed to carry out a predetermined calculation algorithm for signal processing using the I signal 206 and Q signal 208, or sampled values of the I signal 206 and Q signal 208.

[0094] FIG. 6 shows a diagram with a possible embodiment of the algorithm that can be carried out by the evaluation device 137. Here, there is a calculation of an instantaneous energy value of the current consumption of the saw blade 103 (calculation block 301), a calculation of an adaptive threshold value (calculation blocks 302, 303 and summation nodes 304), and a comparison of the instantaneous energy value with the adaptive threshold value (comparison unit 305). The trigger signal 210 can be generated on the basis of the comparison.

[0095] The instantaneous energy value (instant energy) can be calculated in the calculation block 301 from the sum of squared samples of the I and Q signals 206, 208 as follows:

Instant energy(n)=I(n).sup.2Q(n).sup.2  (2)

[0096] Here, n refers to the relevant sample value of the I and Q signal 206, 208, and I(n).sup.2, q(n).sup.2 are the squared values. During operation, a separate instantaneous energy value and an associated adaptive threshold value can be calculated for temporally successive sampling values n. The adaptive threshold (adaptive energy) can be calculated as follows in each case:

Adaptive energy(n)=TotalBiasBlock(n)+PeakDetectorOutput(n)  (3)

[0097] A calculation of the first term in formula (3) can be performed in the calculation block 302 (cf. FIG. 6) according to

[00001]$\begin{array}{cc}\mathrm{TotalBiasBlock}\left(n\right)=\frac{G}{N}\underset{N=16}{\overset{1}{.Math.}}\mathrm{Instant}\mathrm{Energy}\left(n\right)+\mathrm{Fixed}\mathrm{Bias}& \left(4\right)\end{array}$

The sum term is used to calculate an average value of previous instantaneous energy values, which relates to a relevant previous or last block of samples of the I and Q signals 206, 208 with respect to the instantaneous energy value under consideration. The number N of samples of the block can be sixteen, as indicated in equation (4), thereby allowing the calculation to be efficiently and quickly performed. G is a predetermined normalization factor. Furthermore, a predetermined bias value (fixed bias) is added to the sum term.

[0098] A calculation of the second term in formula (3) can be performed in the calculation block 303 (cf. FIG. 6) according to

[00002]$\begin{array}{cc}\mathrm{LastPeak}=\mathrm{PeakDetectorOutput}\left(n-1\right)& \left(5\right)\end{array}$$\mathrm{PeakDetectorOutput}\left(n\right)=\{\begin{array}{cc}\mathrm{LastPeak}+A,& \mathrm{Instant}\mathrm{Energy}\left(n\right)>\mathrm{LastPeak}\\ \mathrm{LastPeak}-D,& \mathrm{Instant}\mathrm{Energy}\left(n\right)\le \mathrm{Last}\mathrm{Peak}\end{array}$

Here, an output value (PeakDetectorOutput) taking into account the occurrence of peak values of the instantaneous energy can be calculated. With the term LoadPeak, referring to a peak value, the last value of the term PeakDetectorOutput, i.e. from a previous sample of the I and Q signals 206, 208, is taken into account. If the (relevant) instantaneous energy considered (instant energy) exceeds the term LoadPeak, a value A is added to the term LoadPeak; otherwise a value D is subtracted from it to obtain the term PeakDetectorOutput(n) for the (relevant) adaptive threshold to be calculated.

[0099] A summation according to formula (3) is performed in summation node 304 according to FIG. 6 to form the adaptive threshold. The adaptive threshold value is further compared with the associated instantaneous energy value, as is indicated in FIG. 6 on the basis of the comparison unit 305. The presence of contact between the saw blade 103 and the user can be assumed when the relevant instantaneous energy value considered exceeds the associated adaptive threshold value, i.e. if the following applies:

Instant energy(n)>adaptive energy(n)  (6)

[0100] If the condition indicated in formula (6) is fulfilled or the presence of this condition as a result of a corresponding evaluation by the evaluation device 137 is determined, the trigger signal 210 can be generated by the evaluation device 137 (cf. FIG. 3).

[0101] Using the adaptive threshold value, a temporal course or a temporal development of the I and Q signal 206, 208 preceding the relevant instantaneous energy value considered can be taken into account. In this way, the influence of individual sampled values of the I and Q signals 206, 208, which may deviate (considerably) from other or neighboring sampled values, for example due to interference from the environment, can be suppressed. As a result, the detection system 101 can have a high degree of robustness, and the detection of contact between the saw blade 103 and the user can take place with a high degree of reliability. Faulty generation of the trigger signal 210, even though there is no contact between the tool 103 and the user (false positive event), can be suppressed here.

[0102] The evaluation device 137 can optionally be designed such that the condition indicated in formula (6) is applied as a primary condition, and that the generation of the trigger signal 210 by the evaluation device 137 is subject to an additional secondary condition. For this purpose, the evaluation device 137 can carry out an additional evaluation according to the diagram shown in FIG. 7. Here, a signal-to-noise ratio present in the relevant case is calculated (calculation block 307), which is compared (comparator 309) with a predetermined fixed limit value (preset block 308).

[0103] The signal-to-noise ratio can be calculated in the calculation block 307 from a mean value and a standard deviation of sampled values of the filtered I and Q signal 206, 208. To minimize computation time, the computation can be performed in a distributed manner with local mean values and local variances related to multiple blocks of samples from the I and Q signals 206, 208. With regard to an efficient calculation, the number of samples of a block can here as well also be sixteen. Further, a current block of samples comprising samples causing the primary condition to be satisfied, multiple previous blocks of temporally preceding samples, and a subsequent block of temporally succeeding samples may also be taken into account. The number of the previous blocks can be six, so that a total of eight blocks are taken into account.

[0104] The local mean value LM of a block of sampled values can each be calculated as follows:

[00003]$\begin{array}{cc}\mathrm{LM}=\frac{1}{n}\underset{i=1}{\overset{n}{.Math.}}{x}_i& \left(7\right)\end{array}$

Here, Xj denotes a sample value. With reference to the above-mentioned number of sixteen samples per Block, n=16. The global mean value GM from all local mean values LM can be calculated as follows:

[00004]$\begin{array}{cc}\mathrm{GM}=\frac{1}{N_{i,M}}\underset{i=1}{\overset{n}{.Math.}}{\mathrm{LM}}_i& \left(8\right)\end{array}$

Here, NLM denotes the number of local mean values LM, and thus blocks. With regard to the above-mentioned number of eight blocks, NLM=8 and n=8. The local variance LV of a block can be calculated in each case as follows:

[00005]$\begin{array}{cc}\mathrm{LV}=\frac{1}{n}\underset{i=1}{\overset{n}{.Math.}}{\left(x_i-{\mathrm{LM}}_1\right)}^2& \left(9\right)\end{array}$

The calculation is done with the local average value LM and the sampled values Xj of the block in question. With reference to sixteen sampling values per block, n=16 again applies.

[0105] Calculating the global variance is not the same thing as the formation of the mean from all local variances of the blocks; rather, it requires a correction factor to be added to the local variances. This can be done as follows:

CLV=(GM−LM).sup.2+LV  (10)

Here, CLV denotes the corresponding corrected local variance of a block. The correction factor is calculated from the global mean value GM and the local mean value LM of the block in question.

[0106] The global variance GV can be calculated based on the corrected local variances CLVj of the blocks as follows:

[00006]$\begin{array}{cc}\mathrm{GV}=\frac{1}{n}\underset{i=1}{\overset{n}{.Math.}}{\mathrm{CLV}}_i& \left(11\right)\end{array}$

[0107] With regard to the above-mentioned number of eight blocks, n=8. To form the standard deviation, the square root of the global variance GV is calculated, i.e.

Standard Deviation=√{square root over (GV)}  (12)

[0108] The signal-to-noise ratio can be calculated in the calculation block 307 (cf. FIG. 7) by dividing the global mean value GM by the standard deviation. Furthermore, the signal-to-noise ratio is compared with the predetermined limit value provided by the preset block 308, hereinafter referred to as SNR, as indicated in FIG. 7 by the comparator 309. The secondary condition is met when the calculated signal-to-noise ratio exceeds the limit value SNR, i.e. if the following applies:

[00007]$\begin{array}{cc}\frac{\mathrm{GM}}{\mathrm{Standard}\mathrm{Deviation}}>\mathrm{SNR}& \left(13\right)\end{array}$

[0109] If the primary condition indicated in formula (6) and the secondary condition indicated in formula (13) are met or the presence of these conditions is determined as a result of a corresponding evaluation by the evaluation device 137, the trigger signal 210 can be generated by the evaluation device 137 (cf. FIG. 3). By taking into account the secondary condition, faulty generation of the trigger signal 210 as a result of a strong noise signal can be suppressed.

[0110] Furthermore, an embodiment in the form of a self-learning machine which has an artificial neural network can be considered for the evaluation device 137 of the processing device 130 (cf. FIG. 3). FIG. 8 shows a possible embodiment of the evaluation device 137 with such an artificial neural network 150. The neural network 150 is used to further process the filtered digital I signal 206 and Q signal 208, or samples of the I and Q signal 206, 208, in order to generate, based thereon, a trigger signal 210 for deactivating operation of the saw blade 103.

[0111] The artificial neural network 150 is constructed from a plurality of networked nodes. As shown in FIG. 8, the artificial neural network 150 can include an input layer 151, a hidden layer 152 and an output layer 153. The input layer 151 has a plurality of input nodes 155 to which the samples of the I and Q signal 206, 208 can be supplied. Differing from the illustration in FIG. 8, the input layer 151 can comprise, for example, thirty-two input nodes 155. The hidden layer 152 can comprise three nodes 156, as shown in FIG. 8. The output layer 153 can have an output node 157, via which the trigger signal 210 (cf. FIG. 3) can be output.

[0112] The mode of functioning of the evaluation device 137 with the artificial neural network 150 during operation of the machine tool 100 and of the detection system 101 can be based on a previously performed training of the neural network 150. During training, supervised learning, reinforcement learning or unsupervised learning can be used. The training can be performed using samples of the I and Q signal 206, 208 with correct trigger signals (true positive events) and incorrect trigger signals (false positive events). The correct trigger signals and the corresponding samples of the I and Q signals 206, 208 may relate to the presence of contact to be detected between the saw blade 103 and the user. In contrast, in the case of the wrong trigger signals and the associated sampled values there is no such contact. As a result of the training, the trigger signal can therefore be generated by the evaluation device 137 only in the event of contact actually taking place between the saw blade 103 and the user, and to this extent a distinction can be made between a true positive and a false positive event.

[0113] During operation, the nodes 155, 156, 157 of the artificial neural network 150 may process data or data values in the usual manner. Weight factors and activation functions can be used. In order to enable efficient and fast processing, an embodiment of the artificial neural network 150 in the form of a binary neural network can be considered. In this embodiment, it is possible for the weight factors and activation to have binary values only.

[0114] The artificial neural network 150 can be designed with a larger network size, differing from the above description and from FIG. 8. For example, the input layer 151 and the hidden layer 152 may have a greater number of nodes 155, 156. Further, instead of one hidden layer 152, the artificial neural network 150 can have multiple successive hidden layers 152.

[0115] In the following, further embodiments are described which can be considered for the detection system 101 or the processing device 130. Matching features along with components that are the same and have the same effect are not described in detail again in the following. For details thereof, reference is made instead to the above description.

[0116] FIG. 9 shows a further embodiment of the processing device 130. The processing device 130 has an analog-to-digital converter 131 at the input side in order to sample the analog pre-filtered current sensor signal 200 and convert it into a digital current sensor signal 202. Here as well, the analog-to-digital converter 131 and the further devices of the processing device 130 are also suitable operatively connected to one another in terms of data communication. The processing device 130 is designed to process the digital current sensor signal 202 via a first digital processing channel 171 and a second digital processing channel 172. For this purpose, the processing device 130 has a data distributor 132 (data splitter) arranged downstream of the analog-to-digital converter 131, which distributor is used to split the digital current sensor signal 202 and to distribute it to the two processing channels 171, 172.

[0117] The processing device 130 has an IQ demodulator 133, a filter device 134 and an evaluation device 137 with respect to each of the two processing channels 171, 172. The devices 133, 134, 137, along with the analog-to-digital converter 131, can be designed as described above. The affiliation to the processing channels 171, 172 is indicated in FIG. 9 by the subscripts 1 and 2. Devices 133.sub.1, 134.sub.1, 137.sub.1 are associated with first processing channel 171, and devices 133.sub.2, 134.sub.2, 137.sub.2 are associated with second processing channel 172. IQ demodulators 133.sub.1, 133.sub.2, arranged downstream of the data distributor 132, receive the digital current sensor signal 202 coming from the data distributor 132, allowing it to be converted into a digital I signal 205 and a digital Q signal 207, respectively. The filter devices 134.sub.1, 134.sub.2 arranged downstream of the IQ demodulators 133.sub.1, 133.sub.2 are used to filter the digital I signal 205 and Q signal 207, and thereby to provide a filtered digital I signal 206 and a filtered digital Q signal 208. The filter devices 134.sub.1, 134.sub.2 each include a CIC filter 135 and a downstream compensation filter 136 for the I signal 205 and the Q signal 207, respectively. The evaluation devices 137.sub.1, 137.sub.2, which are arranged downstream of the filter devices 134.sub.1, 134.sub.2, are designed for further processing of the filtered digital I signal 206 and Q signal 208.

[0118] The processing device 130 shown in FIG. 9 further comprises a comparison device 138 arranged downstream of the evaluation devices 137.sub.1, 137.sub.2, to which digital processing data 220 accumulated in the first and second processing channels 171, 172 can be transmitted. The comparison device 138 is, like the other devices 132, 133.sub.1, 133.sub.2, 134.sub.1, 134.sub.2, 137.sub.1, 137.sub.2 arranged downstream of the analog-to-digital converter 131, a digitally operating component. The comparison device 138 is designed to compare the processing data 220 of the first and second processing channels 171, 172 and, on the basis of the comparison, to generate a switch-off signal 230 for switching off operation of the saw blade 103 and/or a warning signal 231. The transmission of the processing data 220 to the comparison device 138 and the comparison thereof using the comparison device 138 can be carried out continuously during the operation of the machine tool 100 or of the detection system 101. The switch-off signal 230 and/or the warning signal 231 can be generated by the comparison device 138, provided there is a difference between the processing data 220 of the two processing channels 171, 172 or such a difference is determined by the comparison device 138. The occurrence of a difference in the processing data 220 of the processing channels 171, 172 is a sign of faulty functioning of at least one of the two processing channels 171, 172, which can be identified on the basis of the comparison by the comparison device 138. In this way, a self-test or continuous self-test of the detection system 101 can take place. As a result, the two-channel processing device 130 enables a high degree of functional safety and reliability of the detection system 101 and of the machine tool 100.

[0119] The shutdown signal 230 used for self-shutdown can be transmitted, for example, to the motor 105 (cf. FIG. 1) of the machine tool 100 or to a control device (not shown) controlling the motor 105 to shut down the operation of the saw blade 105 and thereby place the machine tool 100 in a safe condition. The warning signal 231 can, for example, be reproduced acoustically or visually or used to control devices such as a loudspeaker or a display device (not shown) of the machine tool 100.

[0120] As indicated in FIG. 9, the processing data 220 used for the comparison may be provided by the evaluation devices 137.sub.1, 137.sub.2 of the two processing channels 171, 172 and transmitted to the comparison device 138. The evaluation devices 137.sub.1, 137.sub.2 can be designed, for example, as explained above with reference to FIGS. 6 and 7, i.e. to carry out a detection algorithm for signal processing and to calculate in each case an instantaneous energy value of the current consumption of the saw blade 103 and an adaptive threshold value. In such an embodiment, the processing data 220 transmitted to the comparison device 138 for the purpose of the comparison can be instantaneous energy values calculated by the evaluation devices 137.sub.1, 137.sub.2.

[0121] The evaluation devices 137.sub.1, 137.sub.2 of the two processing channels 171, 172 may further be designed according to FIG. 8 and may each comprise an artificial neural network 150. In such an embodiment, the processing data 220 transmitted to the comparison device 138 for comparison can be data which are output by one or more nodes of the relevant network 150, for example from a hidden layer 152. With reference to the artificial neural network 150 shown in FIG. 8, an output of processing data 220 by a node 156 of the hidden layer 152 is indicated by way of example.

[0122] In the comparison of the processing data 220, in each case it is possible for only a portion of the bit values of the corresponding data to be compared. It is possible, for example, for the processing data 220 to be in the form of 32-bit numbers, and for the sixteen bits having the lowest bit value (LSB, least significance bit) to be used for the comparison. In this way, the comparison can be performed by the comparison device 138 with a high speed.

[0123] In the two-channel processing device 130 of FIG. 9, the evaluation devices 137.sub.1, 137.sub.2 are designed as described above to generate a trigger signal 210 in response to the processing of the I and Q signals 206, 208. As shown in FIG. 9, the trigger signals 210 coming from the evaluation devices 137.sub.1, 137.sub.2 can be transmitted to the comparison device 138. The comparison device 138 can be designed to generate a separate trigger signal 211 for deactivating operation of the saw blade 103, if the comparison device 138 receives a trigger signal 210 from at least one of the two evaluation devices 137.sub.1, 137.sub.2. As explained above, the trigger signal 211 can be transmitted to the reaction system 400 (cf. FIG. 1) for the activation thereof.

[0124] In FIG. 9 it is further indicated that the two-channel processing device 130 can also be in the form of a single semiconductor chip or signal processing chip 160. Depending on the embodiment, the devices 132, 133.sub.1, 133.sub.2, 134.sub.1, 134.sub.2, 137.sub.1, 137.sub.2, 138 arranged downstream of the analog-to-digital converter 131 may be in the form of hardware or software components. The processing device 130 can be, for example, in the form of a microcontroller or a CPU. Alternatively, the processing device 130 can be in the form of an FPGA.

[0125] Further, with reference to an FPGA embodiment, the design shown in FIG. 10 can also be considered for the dual-channel processing device 130. Here, the processing device 130 is designed according to the so-called IDF methodology (isolation design flow) and has several, i.e. in the present case four, isolated regions 180, 181, 182, 183 on the physical level. The regions 180, 181, 182, 183 may be isolated from each other by an isolation structure (not shown) also referred to as a “fence.” The first isolated region 180 comprises the data distributor 132 shown in FIG. 9. The region 180 can further comprise a data interface for the digital current sensor signal 202 coming from the analog-to-digital converter 131, along with further data processing elements. The second isolated region 181, which relates to the first processing channel 171, includes the devices 133.sub.1, 134.sub.1, 137.sub.1 shown in FIG. 9. The third isolated region 182, which relates to the second processing channel 172, includes the devices 133.sub.2, 134.sub.2, 137.sub.2. The third insulated region 183 comprises the comparison device 138 shown in FIG. 9. As a result of the design according to the IDF methodology, malfunctions can be avoided with a high degree of reliability. In a corresponding manner, a high degree of reliability and safety of the detection system 101 and of the machine tool 100 can be further promoted.

[0126] In order to achieve a high degree of reliability and safety, it is also conceivable to construct a two-channel processing device 130 from a plurality of semiconductor chips, as is shown in FIG. 11. In the embodiment shown in FIG. 11, the processing device 130 comprises an analog-to-digital converter 131, which is in the form of a separate first semiconductor chip or ADC chip 165. The analog-to-digital converter 131 can be used to convert the pre-filtered analog current sensor signal 200 into a digital current sensor signal 202 and to distribute it to digital first and second processing channels 171, 172. Apart from this, the analog-to-digital converter 131 can be designed as described above. The processing device 130 has a separate second semiconductor chip 161 with respect to the first processing channel 171 and a separate third semiconductor chip 162 with respect to the second processing channel 172. The two semiconductor chips 161, 162 can each be in the form of a microcontroller, CPU or FPGA, and each comprise an IQ demodulator 133.sub.1 or 133.sub.2, a filter device 134.sub.1 or 134.sub.2 arranged downstream of the IQ demodulator 133.sub.1, 133.sub.2, an evaluation device 137.sub.1 or 137.sub.2 arranged downstream of the filter device 134.sub.1, 134.sub.2 and a comparison device 138.sub.1 or 138.sub.2 arranged downstream of the evaluation device 137.sub.1, 137.sub.2. These devices can each be designed as described above. The IQ demodulators 133.sub.1, 133.sub.2 are used to convert the digital current sensor signal 202 into a digital I and Q signal 205, 206, which can be filtered by the filter devices 134.sub.1, 134.sub.2 and further processed by the evaluation devices 137.sub.1, 137.sub.2. The evaluation devices 137.sub.1, 137.sub.2 are designed to generate a trigger signal 210 on the basis of the signal processing. As explained above, the trigger signal can be transmitted to the reaction system 400 (cf. FIG. 1).

[0127] The comparison devices 138.sub.1, 138.sub.2 of the two semiconductor chips 161, 162 are used to compare digital processing data 220, which can be provided by the evaluation devices 137.sub.1, 137.sub.2, accumulated in the two processing channels 171, 172, and to generate a switch-off signal 230 for switching off operation of the saw blade 103 and/or a warning signal 231 on the basis of the comparison, or if there is a difference. According to the above description, the processing data 220 can be instantaneous energy values calculated by the evaluation devices 137.sub.1, 137.sub.2, or data from an artificial neural network 150 of the evaluation devices 137.sub.1, 137.sub.2, depending on the design of the evaluation devices 137.sub.1, 137.sub.2. The transmission of the processing data 220 to the comparison devices 138.sub.1, 138.sub.2 and the comparison thereof can take place continuously during operation of the machine tool 100.

[0128] In addition to the embodiments described above and depicted in the figures, further embodiments which can include further modifications and/or combinations of features are conceivable. In this regard, numerical values given above, such as those given for carrier frequency and sampling frequency and for parameters of filter device 134, may be replaced by other numerical values.

[0129] With regard to the execution of a detection algorithm, it is possible to apply only the primary condition explained in FIG. 6 and to omit the secondary condition explained in FIG. 7. This is because, due to the construction of the processing device 130 according to which the signal processing takes place in the digital domain, interfering influences such as noise can be largely avoided. Taking into account only the primary condition favors rapid execution of the signal processing.

[0130] The machine tool 100 can be not only in the form of a stationary machine tool or table circular saw, but also in the form of a compact hand-held machine tool such as, for example, a hand circular saw (not shown). This is made possible by the above-described aspects, such as a compact and space-saving design of the processing device 130 and thus of the detection system 101, along with a short detection or processing time for detecting contact. In one embodiment as a hand-guided sawing machine, the reaction system 400 can comprise only one braking device. Furthermore, the machine tool can be designed not only in the form of a sawing machine, but also in the form of some other type of machine tool with a driveable tool.

[0131] An excitation voltage signal can be applied to a tool or saw blade 103 not only via a capacitively coupled excitation electrode 105, but also in other ways. With reference to FIG. 1, this can be done, for example, via the drive shaft 104 and a sliding contact.

[0132] Although the invention has been further illustrated and described in detail by preferred embodiments, the invention is not limited by the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of protection of the invention.