Patent Publication Number: US-7712386-B2

Title: Method and device for the contactless detection of flat objects

Description:
The invention relates to methods and devices for the contactless detection of flat objects. 
   Methods and devices of this type are used e.g. in the printing industry to establish in the case of paper, foils, films or similar flat materials in printing and production processes whether a single or multiple sheet or alternatively a missing sheet exists. In the printing process it is normally necessary to have a single sheet and if a multiple sheet, e.g. a double sheet is detected it is necessary to eliminate such a double sheet in order to protect the printing press. Analogously when it is found that instead of a single sheet a “missing sheet” is present, the normal printing press must be modified or interrupted until once again a single sheet is detected. 
   In a comparable manner such methods and devices are also used in the packaging industry, in which labels e.g. applied to the base or support material are counted or monitored for presence or absence. Another field of use is the detection of tear-off threads or break points, particularly in the case of thin foils used for enveloping purposes, such as e.g. cigarette packs. However, also metal-laminated papers, flat plastic sheets or foils and plates can be detected in contactless manner in production processes using such methods and devices. 
   The measuring principle used in such methods and devices when e.g. employing ultrasonics and detecting papers in flat sheet form is based on the fact that the ultrasonic wave emitted by the transmitter penetrates the paper and the transmitted fraction of the ultrasonic wave is received as a measuring signal by the receiver and evaluated with respect to its amplitude. If a multiple or double sheet is present, a much smaller amplitude is set in the receiver than when a single sheet is present. 
   The following evaluation of the measuring signal received has consequently hitherto taken place with approximately linearly operating amplifiers or similarly designed amplifying circuits and downstream filters. As a result of the relatively limited dynamic range present, particularly of linear amplifiers, it was often difficult or impossible to detect thick papers, cardboard box materials or even corrugated boards. In addition the fluttering behaviour which often occurs more particularly with very thin papers or foils and which is in fact a movement of a thin, flexible sheet during detection between transmitter and receiver in the direction of the sheet normal line, could only be inadequately controlled using such amplifiers. A comparable behaviour is exhibited by highly inhomogeneous materials. 
   With a view to a better control of the aforementioned problems, specifically in the case of widely differing material-specific attenuation of the transmitted signal and in connection with which hereinafter reference will be made solely to weights per unit area and gram weights, a learning step was performed. Before the start of the actual detection process the flat object to be detected, such as e.g. a paper sheet, is detected in connection with its gram weight or its sound absorption characteristics and inputted into the evaluating device in the sense of a learning step. 
   A significant disadvantage is that in the case of other flat objects with a different gram weight it is once again necessary to perform a corresponding learning step, which is on the one hand complicated and on the other normally leads to considerable disuse periods for the corresponding plants. 
   In connection with the material specifications for papers reference is made to the relevant standards, e.g. DIN pocketbook 118 (2003-06 edition), DIN pocketbook 213 (2002-12 edition), DIN pocketbook 274 (2003-06 edition), DIN pocketbook 275 (1996-08 edition) or to DIN 55468-1 relative to corrugated board. 
   DE 200 18 193 U1/EP 1 201 582 A discloses a device for the detection of single or multiple sheets. For detecting such sheets the known device has at least one capacitive sensor and at least one ultrasonic sensor. An evaluating unit is provided for deriving a signal for detecting the single or multiple sheet. Said signal is derived from a logical interconnection of the output signals of the sensors, the detection signal being established in a balancing phase. 
   Another device in the form of a capacitive sensor is known from DE 195 21 129 C1. This device primarily directed at the contactless detection of labels on a base material works with two capacitor elements and an oscillator influencing the same. The dielectric characteristics of the paper or of other flat objects consequently influence the resonant circuit of the oscillator with regards to the frequency, which is evaluated for detection purposes. 
   However, it is disadvantageous that it is difficult or even impossible to detect relatively thin papers, as well as metal-laminated papers. Due to their limited thickness and in part the fact that their dielectric constant only differs slightly from one, very thin foils are also difficult to detect. 
   Further detection methods using ultrasonic proximity switches are e.g. described in EP 997 747 A2/EP 981 202 B1. In the case of these keying sensors there is an automatic frequency adjustment in which following the emission of an ultrasonic pulse and subsequent reflection on the object to be detected, the optimum transmitting frequency is evaluated as a function of the level of the ultrasonic echo amplitude received. 
   Another device of the aforementioned type is known from DE 203 12 388 U1. This ultrasonically operating device establishes the presence and thickness of the corresponding objects via the transmission and reflection of radiation. However, this device also uses reference reflectors, so that the device has a relatively complicated construction. 
   DE 297 22 715 U1 discloses an inductively operating device for measuring the thickness of plates, which can be made from ferrous or nonferrous metals. The measurement of the plate thickness takes place through the evaluation of the operating frequency of a frequency generator or through evaluating its amplitude. For setting this device it is firstly necessary to perform a learning step, in which a calibration plate is introduced into the measurement zone and the operating frequency or amplitude of the frequency generator is set in accordance with a standard thickness curve. 
   Admittedly such a device makes it possible to distinguish between single, missing and multiple plates, but for this purpose different standard thickness curves must be stored and evaluated for making the decision in question. In addition, this device is suitable for detecting plate thicknesses up to approximately 6 mm. Due to the limited attenuation change the detection of thin plates or foils is not very reliable. 
   DE 44 03 011 C1 describes a device for separating nonmagnetic plates. For this purpose a travelling field inductor exerts a force opposing the plate set conveying direction when a double plate is present, so that the said double plate is separated into two plates. This device is completely unsuitable for nonmetallic, flat objects or foils. 
   DE 42 33 855 C2 describes a method for the control and detection of inhomogeneities in sheets. This method operates optically and is based on a transmission measurement. However, particularly when controlling paper sheets with respect to the presence of single and multiple sheets, the problem arises that as a result of the material characteristics of the sheets there can be very considerable fluctuations as a result of inhomogeneities or the reflection behaviour and fluttering of the sheets. To overcome this problem this document provides a measuring value evaluation using fuzzy logic rules. 
   U.S. Pat. No. 6,511,064 B1 and in comparable manner DE 36 20 042 A1 disclose methods and devices for the detection of multiple sheets, said methods and devices being based on ultrasonics and take account of both an amplitude and a phase difference evaluation for the detection signal. 
   However, they suffer from the disadvantage of a relatively complicated, digital processing of the measuring signals and there is a need to improve uncertainties and inaccuracies in the amplitude evaluation in connection with interfering variables and amplification problems. 
   In addition, in U.S. Pat. No. 6,511,064 B1 the problem arises in connection with amplitude evaluation that in said system the spacing between transmitter and receiver must be fixedly predetermined in order to be able to perform amplitude evaluation. 
   In DE 36 20 042 A1 it is also necessary to have two sensor devices as transducer pairs and the device is relatively expensive. It is normally also necessary to have a learning step with adjustment with respect to the paper weight of the material to be detected. In addition, during phase evaluation only relatively thin papers, such as are more particularly used in printers, can be detected with respect to multiple sheets. The wavelength of the ultrasonic signal must be relatively long compared with the paper thickness and normally of low frequency, e.g. approximately 40 kHz, in order to bring about a good sheet penetration. 
   US 2003/0006550 discloses a method performing a digital evaluation based on ultrasonic waves and the phase difference between a reference phase and the phase received and on this basis a signal is determined for the detection of missing, single or multiple sheets. However, solely evaluating the phase difference can be inadequate in the case of special papers or foils and lead to incorrect information, which is to be avoided for bringing about a reliable detection. 
   DE 30 48 710 C2 discloses a method more particularly usable for counting banknotes, but also for other papers and foils. This method based on determining the weight per unit area or thickness of the materials to be detected, operates with pulse-shaped ultrasonic waves and for detecting a double sheet, i.e. the presence of two mutually covering or overlapping banknotes, use is more particularly made of the evaluation of the integration of the phase shift. Thus, the main use of this method is the counting of banknotes or comparable papers and foils, whilst taking account of the weights per unit area of such materials. Therefore this method would appear to be unsuitable for use with packaging materials or for counting labels. 
   DE 40 22 325 C2 discloses another acoustically or ultrasonically based method. This method, which is based on controlling missing or multiple sheets in the case of sheet or foil-like objects, requires a first pass of the corresponding flat object with a calibration and setting process, which is automatically performed in microprocessor-controlled manner. Thus, with this method a learning step is initially required concerning the thickness of the object relative to an optimum measuring and frequency range and during such a first pass a corresponding threshold value must be detected and stored. 
   Comparable methods and devices are known in connection with the detection or counting of labels. Firstly the difference relative to a label must be considered, because it is provided as an applied material coating to a base or support material. This laminated material behaves to the outside with regards to opacity, dielectric, electromagnetic conductivity or sound travel time in the manner of a composite material piece, so that there is a comparatively limited, but still evaluatable attenuation in the case of such detection possibilities. 
   DE 199 21 217 A1, together with DE 199 27 865 A1 and EP 1 067 053 B1 discloses a device for detecting labels or flat objects. This device uses ultrasonic waves with a modulation frequency and for distinguishing single and multiple sheets a threshold value is determined during a balancing process or a learning step. By means of the learning step it is possible to adjust the detection to a specific flat object in the sense of a label. However, this learning step makes the device more complex and requires longer setting times when changing to a different flat object. This shows that a broader material spectrum cannot be detected per se, but only matched to a specific, individual material. 
   Bearing in mind this prior art, the object of the invention is to design a method and a device for the contactless detection of flat objects, permitting in a very flexible manner over a wide material spectrum a reliable detection of single, missing or multiple sheets with different flat materials on the one hand, particularly papers, foils, films, plates, etc., and on the other in the case of labels and similar laminated materials, without requiring a learning step and using different beams or waves such as those of an optical, acoustic, inductive or similar nature. 
   A fundamental idea of the invention is to provide for the evaluation of the measuring signal over a gram weight and weight per unit area range a correction characteristic, so that over the material range provided it is possible to achieve a target characteristic with a substantially or virtually linear course or for papers and similar materials a characteristic approaching the ideal characteristic for single sheet detection and permitting in the case of an amplitude evaluation of the amplified measuring signal a clear distinction, particularly compared with a corresponding threshold value for air, as a threshold for a missing sheet, or compared with a threshold value for double sheets. 
   To achieve this, it is a further essential idea of the invention that in the case of a signal amplification of the measuring signal received, the correction characteristic of the corresponding signal amplification is given statically or dynamically in order to obtain a readily evaluatable target characteristic. 
   However, the invention also takes account of the fact that a direct conversion of the measuring signal can be performed within the framework of an A/D conversion and the digital values of the measuring signal characteristic obtained are subject to the corresponding, purely digital correction characteristic, so as to directly obtain the evaluatable target characteristic. 
   This principle of using a correction characteristic also has the major advantage that it is possible to use different sensor devices, particularly as a barrier or barrier arrangement, e.g. with a forked shape and advantageously use is made of ultrasonics, optical, capacitive or inductive sensors and the same method can be used for each of them. 
   The corresponding correction characteristic for papers and similar materials is more particularly obtained by mirroring the measuring value characteristic on the ideal target characteristic for single sheet detection, optionally using a special transformation of the Cartesian coordinate system. 
   The correction characteristic can also be chosen inversely or virtually inversely to the characteristic of the input voltage UE of the measuring signal. It is possible in this way and in a good approximation to obtain an ideal target characteristic for single sheet detection over a relatively wide gram weight or weight per unit area range of the objects to be detected, particularly between 8 and 4000 g/m2. Inverse is considered to be an inverse function. 
   Thus, the inventive method is not only suitable for detecting single, multiple or missing sheets of thin to thick papers, which are in the aforementioned gram weight range. It is also possible to detect stackable, box-like packs of paper or plastic or labels applied to base material, or splice, tear-off or break points of paper or foils. 
   If, from the method standpoint, the measuring signal obtained at the output of the receiver or measuring signal converter is subject to a signal amplification for further evaluation purposes, preferably the corresponding amplifier device impresses the corresponding correction characteristic, which can also comprise a combination of several correction lines, so as at the output side to obtain for further evaluation purposes a readily evaluatable target characteristic over the entire weight per unit area range. Using this target characteristic it is possible in a downstream method step which can e.g. be implemented in a microprocessor, to detect the corresponding flat object with regards to specific threshold values, so as to obtain a clear detection signal regarding single, missing or multiple sheets. 
   As an alternative the method also provides that the measuring signal or its measuring signal characteristic obtained in the receiver is directly subject to an analog-digital conversion and, taking account of a corresponding purely digital correction characteristic, said digital values are processed to a target characteristic for producing a corresponding detection signal. 
   According to the invention these measures lead to the advantage that a reliable detection is obtained of the corresponding flat objects over a very wide gram weight and weight per unit area range without the need for a learning process, which would lead to plant disuse times. In addition, the dynamic range of the evaluating device is significantly extended, so that it is reliably possible to detect very thin or very inhomogeneous materials having a fluttering tendency. Therefore the method according to the invention makes it possible on the basis of the amplitude evaluation of the measuring signal received in the receiver and by using a correction characteristic and target characteristic to make a reliable distinction between single, missing and multiple/double sheets and this applies also for very thin or very sound-transmissive objects, e.g. with a weight per unit area from 8 g/m2 or a thickness of approximately 10 μm to relatively thick and highly sound-transmissive objects up to 4000 g/m2 and e.g. a thickness of 4 mm, without any prior learning process being required to enable a reliable distinction to be made. 
   In order to make possible a better distinction between single, missing and multiple sheets, according to the invention, in addition to the evaluation of the measuring signals by correction characteristic as amplitude evaluation the measuring signal is also supplied to a phase evaluation. By linking these two evaluations a decision is made regarding single, missing or multiple sheets. Through additionally using information obtainable through the phase a better and more accurate detection result can be obtained. Also through the use of an additional phase evaluation the gram weight range in which the inventive method can be used is extended. 
   If an ultrasonic signal is transmitted through a single sheet and detected again on the receiver side, it can be established that the phase of the received signal differs from the transmitted signal by approximately 90°. If the ultrasonic signal is transmitted through two sheets, then the phase of the signal received differs by approximately 180° compared with the transmitted signal. Thus, in the present invention determination takes place of the difference between the transmitted ultrasonic signal and the received ultrasonic signal in the phase. The evaluation of this phase difference has the further advantage that it is hardly dependent on the gram weight of the penetrated sheet, but decisively depends on the material transitions, i.e. from the transition from air to sheet material and sheet material to air. 
   To obtain an adequate, analog output signal for the phase difference, it is possible to determine the often very noisy signal, e.g. on inserting a multiple sheet, by means of a synchronous rectifier or correlator or lock-in amplifier. It is also possible to generate a digital output signal of the phase difference by analog multipliers as synchronous rectifiers. The use of an analog or digital output signal is dependent on the choice of the further signal processing. However, other factors can also play a part when making a choice decision, such as e.g. the fault sensitivity or durability of the components required for the evaluation. 
   To make a decision as to whether there is a missing, single or multiple sheet, the two output signals of the phase and amplitude evaluation must be combined with one another. Thus, if it is impossible to overlook missing or double sheets, both signals can be linked with a logical OR. Therefore the detection of a multiple sheet via phase or amplitude evaluation leads to the sensor outputting a corresponding signal for a multiple sheet detection. It is particularly preferable to link the two signals with a logical AND, in order to again compare the results of the individual evaluations. 
   Another possibility for combining the two signals from the phase and amplitude evaluation is a weighted comparison of both signals. A weighted comparison e.g. offers the advantage that if a double sheet was detected in the phase evaluation, whereas in the amplitude evaluation the “single sheet” result only just differs from a “double sheet”, a conformation of a double sheet is obtained in the combination of both signals. An advantage of this method is that decisions at the boundary of the decision range can be “overdetermined” or verified by the other evaluation method if there a clear result exists. Thus, in general a correct detection result can be obtained with a higher degree of reliability. 
   In connection with high flexibility, not only relative to the most varied papers such as corrugated board or plastic packs, the invention also provides the taking into account of correction characteristics, which represent a combination of different correction characteristics, said combined correction characteristics also being applicable solely in a zonal manner over parts of the overall gram weight range. As a result the target characteristics can have an improved approximation to the ideal characteristic for detecting single sheets. 
   Corresponding to the circumstances of the circuitry design of the evaluating device, the sensor device used and/or the sought material spectrum, the correction characteristic can also be designed zonally as a linear or nonlinear characteristic, as a single or multiple logarithmic characteristic, as an exponential characteristic, as a hyperbolic characteristic, as a polygonal line, as a random degree function or empirically determined or calculated characteristic or as a combination of several of these characteristics. 
   With a view to the combined detection of labels and single, missing and multiple sheets, preferably the correction characteristic is designed as an approximately linearly rising and weighted or exponentially or similarly rising characteristic or as a logarithmic, multiple logarithmic or similar nonlinear characteristic, also in combination with the first-mentioned correction characteristics. 
   Thus, according to the invention, both in a method and by means of a device it is possible to detect labels, splice, tear-off or break points and similarly built up materials without a learning step. It must be borne in mind that the weight per unit area range for labels and similar materials can be from approximately 40 to approximately 300 g/m2, i.e. is relatively narrow. 
   It is also to be borne in mind that with labels, in certain circumstances with only minor gram weight differences between the base or support material and the adhesively applied, multilaminated materials, such as e.g. labels, there is a relatively small difference in the attenuation, e.g. of ultrasonic waves, so that the aim is to obtain in the target characteristic a maximum voltage swing of target characteristic ZK in the case of a small voltage swing of the measuring value characteristic MK. 
   The correction characteristic for detecting labels is therefore preferably at least linear and said linear correction characteristic KK has a weighting function, or is chosen in exponentially rising manner. 
   As a substantially ideal target characteristic for labels and similar materials in optimum manner the function of the output voltage UA or UZ as a function of the gram weight g/m2 is sought in the form of a curve or straight line, namely with a maximum, constant negative gradient (ΔUZ=maximum and constant) and therefore maximum voltage difference. Therefore there is a maximum voltage swing (ΔUZ=max.) with respect to the base or support material and the adhesively applied, multilaminated materials, such as e.g. labels, even in the case of minor gram weight variations as a function of the total gram weight or weight per unit area range. 
   Therefore such an ideal target characteristic for the detection of labels, even in the case of small to very small gram weight differences makes it possible to generate a clearly defined detection signal for detecting labels and similar materials. In the case of labels and similar materials evaluation primarily takes place regarding the presence or absence or a multiple layer reduced by at least one layer. 
   The invention also makes it possible to implement such a combination of correction lines, e.g. also in separate paths or channels. The logarithmic and/or double logarithmic correction line can e.g. be impressed in the first channel, so as to consequently primarily permit reliable double sheet detection. The second channel can e.g. be subject to an exponentially or linearly rising correction characteristic, so as to be able to implement in optimum manner in said path the detection of labels, splices or threads. 
   This combination of the two methods with logarithmic correction characteristic combined with exponentially rising correction characteristic, consequently permits an optimum detection possibility for labels and similar materials, such as tear-off or break points and/or tear-off threads and single, missing and multiple sheets. 
   Thus, for label detection the aim is to permit a maximum constant signal swing over the entire material range in the case of the aforementioned design of the correction characteristic as a result of the target characteristic, i.e. ΔUZ should be at a maximum/constant. 
   As opposed to this, the correction characteristic method for detecting single, missing and multiple sheets is based on a design of the target characteristic in which, over the entire gram weight range, for single sheet detection purposes there is a minimum change to the amplitude values, i.e. ΔUZ=0 and ideally there is a constant magnitude or target characteristic with a gradient of approximately 0. 
   For practical purposes importance is attached to the combination of a logarithmic and a linear correction characteristic. The advantage of a signal amplifier with impressed logarithmic correction characteristic or a similar correction characteristic is more particularly that the signal amplifier has a very large dynamic range, so that a large ratio of voltage signals from the largest to the smallest signal can undergo processing. A linear signal amplifier can e.g. obtain a voltage-signal ratio of approximately 50:1, which corresponds to approximately 34 dB. However, a logarithmic signal amplifier achieves a voltage-signal ratio of 3×10 4 :1, which is approximately 90 dB. When using a logarithmic signal amplifier, which is here understood to mean an impressed logarithmic correction characteristic, it is possible to counteract a signal overload at high signal amplitudes. This feature is advantageously used according to the invention in order to implement single, missing or multiple sheet detection and for the detection of stackable packs, without carrying out a learning process and over a very wide material spectrum. 
   Advantageously in the case of the method and the corresponding device according to the invention it is possible to use logarithmic and/or multiple logarithmic signal amplifiers, so that the possible material spectrum is extended to thin or very lightweight sheets. This is due to the fact that with an increasing signal level with said signal amplifiers the characteristic of the signal amplification passes into saturation and consequently there is virtually no signal swing. With falling signal amplification and large signals there are still readily evaluatable signals even with the most minor modifications, such as e.g. very thin paper sheets between transmitter and receiver. 
   When using nonlinear, particularly logarithmic and/or multiple logarithmic signal amplifiers, a further advantage is that the detectable material spectrum is extended to thicker or heavier sheets. This is due to the fact that with a low signal level amplification is very high and even the weakest signals still able to pass through a heavy or thick single sheet can be adequately amplified and evaluated. This characteristic is more particularly used for the detection of stacked packs or single, missing or multiple sheets. 
   According to another appropriate development of the invention, the correction characteristic is in particular empirically determined or calculated as a synthesized function. For this purpose it is e.g. possible to plot the transmission attenuation or the measuring signal voltage resulting therefrom as a function of the gram weight or weight per unit area of the object or objects to be detected and in this way determine the characteristics of the measuring signal of a plurality of different objects and from this the optimum inverse or virtually inverse correction line can be obtained mathematically or empirically in order to achieve a target characteristic at least approaching the ideal target characteristic for the detection of single sheets. 
   From the method standpoint it is also possible to impress in fixed manner or actively control or regulate the correction characteristic, so that an even better approximation to the ideal target characteristic is possible for the materials to be investigated. 
   For said control or regulation it is possible to use in the evaluating device, e.g. a microprocessor, a corresponding electrical network for adjusting the correction characteristic, a use-specific module or a resistance network. 
   According to a further development of the invention the target characteristic for different material spectra is subdivided into several sections, particularly three or five sections. In the case of three sections, it is e.g. possible to form a partial target characteristic for the gram weight range above 1200 g/m2 for very thick papers and another section below 20 g/m2 for a very thin paper spectrum. The introduction of target characteristic sections consequently permits an improved reliability with regards to single, missing or multiple sheet detection. 
   It is appropriate for labels, splice and break points or tear-off threads to provide at least one detection threshold and on dropping below the latter it is evaluated as a “multiple layer” and on exceeding it as a “base material” or as a “multiple layer” reduced by at least one layer. 
   With a view to a clear detection of single, missing or multiple sheets, particularly double sheets, the amplitude value is compared by means of the target characteristic with threshold values. These are in particular an upper threshold value for air and a lower threshold value for double or multiple sheets. Thus, if the incoming measuring signal with the corresponding target characteristic value is greater than the upper threshold value, it is evaluated as a “missing sheet”. An incoming measuring signal smaller than the lower threshold value indicates a “multiple/double sheet”. In the case of an incoming measuring signal with the corresponding value on the target characteristic between the threshold values, this is detected as a “single sheet”. 
   In order to improve the detection possibilities, particularly with a view to a more precise setting to the material spectrum to be determined, the threshold values, particularly for multiple sheets, can be designed continuously or zonally defined in fixed manner or dynamically carried along. In this sense a dynamic double sheet threshold can be used for an additional extension of the measurable gram weights. For this purpose e.g. the single sheet value is measured and evaluated with the associated multiple sheet value, e.g. as a polygon function, when it is a single function, such as e.g. a falling line or a constant value for the single sheet. 
   The method and device can be more particularly implemented by means of at least one ultrasonic sensor device. For this purpose the sensor device preferably has at least one ultrasonic converter pair which are matched to one another and coaxially aligned. However, the method and device can also be implemented according to the invention with optical, capacitive or inductive sensors. 
   Using ultrasonic sensors it has been found that easy detection is also possible of flat objects with printing, colour printing or reflecting surfaces. It is also possible for the sensor pair, particularly in barriers and when assembled in forked form, to be fitted vertically or inclined to the sheet plane. 
   Appropriately the operating mode of the sensor device can be selected or switched as a function of the material spectra to be detected and the operating conditions either in pulsed or continuous operation form. For continuous operation preference is given to an inclined assembly of the sensor pair, so as in this way to avoid interference and standing waves. Appropriately continuous operation is so-to-speak designed as a quasi-continuous operation in that e.g. periodically the signal is switched off and on again in short time intervals compared with the evaluating time. To avoid standing waves it is also possible to have phase jumps in the transmitting signal. 
   Inclined assembly of the sensor element pair is particularly suitable for detecting thicker materials, e.g. single-corrugation or multiple-corrugation, particularly two-corrugation corrugated board, so as in this way to achieve a better material penetration and avoid interference. 
   It has also proved advantageous to modulate the transmitting signal with at least one modulation frequency. This makes it possible to correct or compensate converter tolerances, particularly in ultrasonic sensors. Although the sensor elements are matched to one another, they generally have different resonant frequencies. If for frequency modulation purposes use is made of a frequency sweep fS with a frequency much lower than the frequency to be excited, the resonance maximum of the sensor elements is periodically exceeded. If the response time of the sensor is well below 1/fS, in this way the converter characteristics of each individual sensor element or pair can be used in optimum manner for ultrasonic transmission. The frequency sweep is normally up to a few 10 kHz. 
   The tolerances of the sensor elements are appropriately automatically corrected before or during the continuous operation. This takes place by standardizing the sensor element pairs to a fixed value with a predetermined, fixed spacing, particularly the optimum assembly spacing. As a result poor sensor elements can be made better and good sensor elements or converters made poorer. To compensate this a correction factor is needed. From the method standpoint this can take place through the use of straight lines filed or calculated as value pairs in microprocessor μP, because the measuring signal is already rated with e.g. a single logarithmic correction characteristic and the correction characteristic produces an approximately linearly falling target characteristic over the converter or sensor element spacing. Thus, the input signal at the microprocessor of an evaluating device in good approximation drops linearly with the converter spacing. Thus, correction of the values is easy even with a variable spacing, because on switching on a corresponding device only a straight line function has to be calculated for the correct initial value or filed as a value pair. The correct determination of the sensor head spacing is carried out by a transit time measurement. 
   A particular advantage of the ultrasonic method is that the spacing between transmitter and receiver in the sensor device can be made variable for this learning-free method. In other words the sensor device can be relatively rapidly adapted spacingwise to different applications, without this impairing the measurement precision of the method. A further improvement to the method can be brought about by monitoring the spacing between the transmitter and receiver and the determination thereof. This determination of the spacing between transmitter and receiver can on the one hand take place by reflection of radiation between transmitter and receiver and on the other by reflection between transmitter and receiver in spite of flat material present in the gap and even when it is a thick sheet. If the permitted maximum sensor spacing is exceeded and detected, the evaluating device, e.g. a microprocessor, can effect a corresponding correction of the determined amplitude values of the measuring signal as a function of the spacing between transmitter and receiver. 
   The mutual orientation of transmitter and receiver takes place in the main radiation direction and in particular coaxially and there can be a virtually random inclination angle to the sheet plane. When detecting single or multiple-corrugation corrugated paper, this appropriately takes place approximately orthogonally to the widest surface of the corrugated paper corrugation. 
   With regards to an optimum detection from the method standpoint it is also possible to provide a feedback between transmitter and evaluating device, particularly a microprocessor, so as to obtain a maximum amplitude at the output, whilst taking account of the material specification of the flat objects to be monitored and further operating conditions. It is also possible to adjust to the optimum transmitting frequency. This measure also makes it possible to compensate ageing effects of the sensor elements and a product testing of the inventive device can be fully automated in a fully advantageous development in connection with industrial scale production. 
   To achieve an improved detection reliability with respect to labels, splice and break points and tear-off threads, these objects can be moved between transmitter and receiver, so that independently of the specific object measuring signal received the corresponding switching threshold for the target characteristic can be determined automatically or in externally triggered manner. 
   As from the method and device standpoint label detection appropriately takes place by means of a second channel, this does not affect a learning-free detection for single or multiple sheets implemented with a first channel of the evaluating device. 
   In an advantageous further development a feedback is provided between the evaluating device and transmitter using a maximization of the amplitude of the incoming measuring signal. There is preferably a self or auto-balancing between the transmitter and receiver with a view to an optimum transmitting frequency and/or amplitude. This auto-balancing can be performed in times synchronized with the transmitting frequency, in fixed defined pause times or by means of a separate input provided externally on the sensor device. 
   With a view to an optimum process control for plants in which the method and device can be used, for digitizing the analog measuring signal appropriately at least one A/D converter or a threshold generator is provided, so that the further processing of the values can be performed digitally. Particularly when processing and selecting different signals of several signal amplifying devices the control and selection of the corresponding channels and signals is preferably performed using time multiplex devices. 
   The invention makes use of a combination in which, apart from the evaluation of the amplitude of the measuring signal and the rating thereof by means of a correction characteristic, separately the measuring signal phase is evaluated in order when taking account of both evaluations to obtain a detection signal for detecting single, missing or multiple and in particular double sheets. 
   For phase evaluation purposes, in simple manner the phase difference between the transmitter signal phase and the receiver signal phase is determined and evaluated. 
   Appropriately a linking of the signals of both evaluations takes place. It is e.g. suitable for this purpose to have an AND or OR link. Another possibility of linking the two evaluations is a weighted comparison. Weighted comparison offers the advantage that e.g. in the case of a phase evaluation with a tendential result “double sheet”, but where the amplitude evaluation clearly detects a “single sheet”, a decision can be made to the effect that the clearer decision takes priority and consequently the overall detection result is outputted as a “single sheet”. Thus, a clear result can overdetermine or eliminate the other, unclear result. 
   The phase difference can be displayed as an analog or a digital result. A comparator for an analog signal output can in particular have a synchronous rectifier. In the case of a digital comparator signal output, in particular a frequency-sensitive phase detection can be performed. As a function of the further signal processing, it can be advantageous to choose an analog or digital output signal. 
   As a function of the materials, as well as the gram weights and weights per unit area, different phase shifts can also exist with single sheets. However, it has been found that tendentially it can be assumed that when a single sheet is present there is a phase shift of approximately 90° and when a double sheet is present of approximately 180°. The phase shift is not primarily determined by the flat object thickness, but instead more particularly by the characteristics of the boundary layers or boundary areas, particularly with double sheets or labels. 
   The combination of amplitude and phase evaluation has the advantage that over and beyond the detection of a double sheet, e.g. up to four sheets can be relatively well detected. The phase position detection of up to max. 360° (four sheets) of the usually very noisy signal can in particularly advantageous manner take place through a phase-synchronous rectifier (cf. Tietze/Schenk, Springer Verlag). 
   Through the combination of amplitude evaluation based on the characteristic-correcting method with phase evaluation, the multiple sheet detection method already significantly improved by the correction characteristic-correcting method is made even more reliable. In addition to the information concerning the number of flat objects inserted, phase evaluation provides an additional decision criterion for improving the detection of a multiple introduction of flat objects. Through the addition of phase evaluation it is now possible to extend the material spectrum down to the thinnest materials, e.g. below 10 g/m2. This e.g. corresponds to a fine woven fleece or a tempo-handkerchief layer. The combination of characteristic-correcting method and phase evaluation extends the material spectrum upwards to gram weights of approximately 350 g/m2, which is adequate for use e.g. in copiers. 
   For the better detection of elongated objects and materials laminated onto the base material and more particularly using ultrasonic or optical sensors, it is advantageous to provide between the transmitter and the elongated object to be detected at least one pinhole diaphragm and/or slit diaphragm for improving the spatial resolution and for continuously detecting the presence of the object. 
   Specifically for improving the detection of material threads adhesively applied to the base or support material, e.g. tear-off threads for the packaging foils of cigarettes, the arrangement of the diaphragms and in particular slit diaphragms takes place in the thread running direction. This normally involves the diaphragm being positioned in the running direction of the elongated objects. 
   When monitoring scale-like superimposed sheets the slit or pinhole diaphragms are oriented by 90° to the sheet movement direction. 
   When using diaphragms the elongated object guided between transmitter, receiver and diaphragm, e.g. a thread laminated onto a base material is implemented so as to float as close as possible over or slidingly contact the diaphragm. The arrangement of the transmitter, specifically in the case of ultrasonic sensors, appropriately occurs below the sheet to be detected, because in this case the maximum transmitting energy can be coupled out and use can be made of sensor head self-cleaning effects. However, it is also possible to reverse the arrangement with the receiver, provided that the signal strength loss can be accepted. 

   
     The invention is described in greater detail hereinafter with reference to the basic measuring principles and by means of the diagrammatic representations and graphs, wherein show: 
       FIG. 1  The principle of an inventive method and in block diagram-like manner a corresponding device whilst using the voltage graphs according to  FIGS. 1   a ,  1   b ,  1   c , illustrating the structure of the characteristics when detecting sheets of paper, foils, films or similar materials. 
       FIG. 2  The principle of an inventive method and in block diagram-like manner a corresponding device using voltage graphs according to  FIGS. 2   a ,  2   b ,  2   c ,  2   d  illustrating the structure of the characteristics when detecting labels, tear-off points and similar materials. 
       FIG. 3   a  A graph showing the diagrammatic dependence of the output voltage of an amplifier, shown in exemplified manner in  FIG. 1 , as a function of the gram weight or weight per unit area of the materials to be detected, whilst incorporating idealized target characteristics. 
       FIG. 3   b  A diagrammatic graph similar to  FIG. 3   a  with the output voltage of an amplifier as a function of the gram weight or weight per unit area of the materials under investigation, showing several target characteristics together with corresponding threshold values, e.g. air threshold and double sheet threshold. 
       FIG. 4   a  A diagrammatic representation, as to how the correction characteristic can be determined in a known measuring value characteristic and ideal target characteristic for single/double sheet detection in the Cartesian coordinate system. 
       FIG. 4   b  A diagrammatic representation, relative to label detection with ideal target characteristic, known measuring value characteristic and a correction characteristic necessary for transformation. 
       FIG. 4   c  A diagrammatic representation of the characteristics for double sheet detection when there is no ideal target characteristic. 
       FIG. 4   d  A representation of characteristics for double sheet detection with mirroring on an imaginary axis, using the transformation according to  FIG. 4   f.    
       FIG. 4   e  A diagrammatic representation of characteristics for label detection with mirroring on the imaginary axis and taking account of  FIG. 4   f.    
       FIG. 4   f  Diagrammatically a transformation of the Cartesian coordinate system by an angle α with representation of a reference axis of the new coordinate system. 
       FIG. 4   g  Diagrammatic representations of an ideal target characteristic and real target characteristics in the case of double sheet detection. 
       FIG. 4   h  A diagrammatic representation of an ideal target characteristic and a realistic target characteristic for label detection. 
       FIG. 4   i  Diagrammatic representations of a measuring value characteristic and correction characteristic in the case of single/double sheet detection, the correction characteristic representing a characteristic defined from an e-function and an inverse function with the target characteristics determined therefrom. 
       FIG. 4   j  A diagrammatic representation of a measuring value characteristic derived from a weighted hyperbola and a correction characteristic derived from a logarithmic function with the target characteristic determined therefrom for single/double sheet detection. 
       FIG. 5   a  A diagrammatic representation of the measuring criteria present in exemplified manner for the detection of a double sheet of material by ultrasonic waves. 
       FIG. 5   b  In comparable manner to  FIG. 5   a , the diagrammatic representation of a splice between a material double sheet and the measuring criteria involved in the case of determination using ultrasonics. 
       FIG. 5   c  A diagrammatic representation of materials adhesively applied to a base or support material, in part as a single laminated and in part as a multi-laminated material, this showing the structure of a label. 
       FIG. 6  In block diagram-like manner the representation of the method and a device using the example of a combination of different correction characteristics. 
       FIG. 7  A diagrammatic representation similar to  FIG. 6 , the principle being shown for the setting of a correction characteristic and the calculation of a correction characteristic affecting the circuit blocks. 
       FIG. 8  A diagrammatic representation for empirically determining a measuring value characteristic over a wide gram weight or weight per unit area range. 
       FIG. 9  A block diagram representation of a method and the corresponding device with the combination of e.g. multiple sheet detection with the detection of material layers or labels adhesively applied to the base material. 
       FIG. 10  Diagrammatically a graph of the standardized output voltage UA over the gram weight range with constant or dynamic double sheet thresholds. 
       FIG. 11  A target characteristic with plotted upper and lower flutter areas. 
       FIG. 12  With the representations of  FIGS. 12   a  and  12   b , the arrangement of a sensor with optimum orientation in the case of single-corrugation corrugated paper and corresponding to  FIG. 12   b  the analogous orientation of a sensor in the case of two-corrugation corrugated paper. 
       FIG. 13  A block diagram of a device with evaluation of the amplitude and phase for the detection of flat objects. 
   

     FIG. 1  diagrammatically shows the method and device according to the invention with a block diagram structure and the voltage curves attainable at specific points in the sense of characteristics over a gram weight/weight per unit area range g/m2 of a material spectrum to be detected. 
   Further explanations are based on an ultrasonic sensor device, but in principle it is also possible to use optical, capacitive or inductive sensor devices. 
   A corresponding sensor device  10  has a transmitter T and a facing receiver R oriented with respect thereto and between which are moved e.g. in sheet form and in contactless manner the flat objects to be detected.  FIG. 1  shows in exemplified manner a multiple sheet in the form of double sheet  2 . 
   Since for this example amplitude evaluation of the measuring signal UM is presupposed for the detection of a single sheet, a missing sheet, i.e. no sheet, or a double/multiple sheet, a possible voltage curve UM is shown in  FIG. 1   a  as a function of the gram weight/weight per unit area g/m2 for the measuring characteristic MK. 
   With a view to a clear and reliable decision as to whether there is a single, double or missing sheet, the object of the invention, whilst taking account of threshold values, such as e.g. for the air threshold or double sheet threshold, is to obtain clearly defined intersections with said threshold values or maximum voltage spacings with respect to said thresholds. 
   The fundamental finding of the invention is based on the fact that in the prior art methods and devices, in the case of multiple sheet detection and an assumed, following approximately linear amplification, optionally with further filtering and evaluation, as a function of the gram weight or weight per unit area, a characteristic is obtained for the amplified measuring signal which is substantially strongly nonlinear, particularly exponential, multi-exponential, hyperbolic or the like and over a wide, desired use area of the material spectrum there is frequency an unreliable, error-prone detection and which is now to be changed using a simple principle. 
   According to the inventive principle account is to be taken of a correction characteristic and this is to be impressed e.g. into the evaluating circuit following the receiver and for this purpose in particular the following amplifier device is suitable, so that over the desired gram weight range there is a readily evaluatable target characteristic for a reliable detection with a decision as to whether there is a single, missing or multiple, especially double sheet. 
   Such a correction characteristic KK is diagrammatically shown in  FIG. 1   b . This correction characteristic, which only shows in principle in  FIG. 1   b  the dependence between the output voltage UA on the input voltage UE, compared with the measuring characteristic MK according to  FIG. 1   a , which is also only diagrammatically showing the path of the measuring signal UM, shows that relatively high voltage values UM over the gram weight range are subject to no or only a slight amplification, whereas smaller voltage values, e.g. with relatively high weights per unit area (g/m2) are subject to a much higher and possibly exponential amplification. 
   The resulting target characteristic ZK with voltage UZ as a function of the gram weight (g/m2) is also only diagrammatically shown in  FIG. 1   c . The desired ZK can also be transformed to the desired output signal UZ from a punctiform imaging (implicit KK) of the measuring signal UM and as a result the desired target characteristic ZK can be obtained. For this purpose it is necessary to have an amplifier with an adjustable amplification or gain, which then obtains the correction characteristic from a μP. The imaging of the measuring signal UM to the desired output signal UZ by means of KK can take place in value-continuous manner instead of in value-discrete manner, i.e. in punctiform manner. 
   In exemplified manner, the target characteristic shown in  FIG. 1   c  could have the continuous line form shown, which has three areas. There are first and third relatively steeply falling areas and a central, only relatively slightly abscissa-inclined area, which has a large gram weight range. As the first and third areas could have a more optimum path with a view to a reliable detection display or clear switching behaviour of the device, using a broken line representation is shown in the form of an improved target characteristic a linearly falling target characteristic ZK 2  passing through the end points of the first target characteristic ZK 1 . 
   In connection with the device  1  for detecting single, missing or multiple sheets shown in block diagram form in  FIG. 1 , the measuring signal UM obtained at receiver R is supplied to an evaluating device  4  shown in simplified manner with the amplifier device  5  and downstream of a microprocessor  6 . 
   The correction characteristic KK is given or impressed on the amplifier device  5 , so that at the output is obtained target characteristic ZK 1 /ZK 2  for the purpose of further evaluation in microprocessor  6 . Whilst taking account of stored or dynamically calculated data, such as threshold values, the microprocessor  6  can generate a corresponding detection signal relative to single, missing or multiple sheets, particularly double sheets. 
     FIG. 2  and the associated  FIGS. 2   a ,  2   b ,  2   c ,  2   d  diagrammatically illustrate the method and a device for detecting labels and similar materials without the need for the performance of a learning step. The reference numerals correspond to those of  FIG. 1 . 
   The block diagram-like structure shows a transmitter T, e.g. for irradiating ultrasonic waves, and an associated receiver R as a sensor device  10 . Labels  7  are passed between transmitter T and receiver R. The function of the device is on the one hand to detect whether or not labels are present and on the other it is also possible to establish the number of labels guided through the sensor device. 
   The measuring signal UM/UE obtained in receiver R when a label is present can e.g. have the diagrammatically intimated characteristic path over the gram weight with an approximately linear, nonlinear, exponential or similar falling course. 
   The following evaluating device, which can e.g. have an amplifier device  5  and in downstream manner a microprocessor  6 , receives in amplifier  5  a correction characteristic, which can e.g. be linearly rising (I) or exponentially rising (II), as shown in  FIG. 2   b . Whilst taking account of the correction characteristic, e.g. according to  FIG. 2   b , at the output of amplifier  5  is obtained a target characteristic over the gram weight range, as illustrated in  FIG. 2   c  by curve I or II. 
   An ideal path of the target characteristic for label detection is shown in the graph of  FIG. 2 . 
   This target characteristic ZKI has the path of a negatively falling line, from lower to higher gram weights and in optimum manner there is a constant gradient and a maximum voltage difference for output voltage UZ in the case of small gram weight differences over the entire gram weight or weight per unit area range provided for label detection purposes. 
   As will be explained hereinafter, the correction characteristic KK can also be a combination of individual, different characteristics. It is also possible to use other correction characteristics, such as logarithmic or multiple logarithmic characteristics, independently of the characteristic path of measuring signal UM and the amplification characteristic. The aim is to obtain an ideal characteristic ZKI, as shown in  FIG. 2 . 
   The curves of  FIGS. 2   a ,  2   b ,  2   c  show two examples of different characteristics, firstly for measuring signal UM of  FIG. 2   a  with characteristic path MK of a first characteristic I and a characteristic II with interrupted or broken line. These differing characteristics for measuring signal MK I and MK II can be so transformed over correction characteristics KK shown in diagrammatic exemplified form in  FIG. 2   b  that at the end of the evaluation it is possible to obtain a characteristic path for the target characteristic ZK corresponding to  FIG. 2   c.    
   For further illustration purposes  FIG. 2   d  diagrammatically shows the output voltage UA of an amplifier device over the gram weight range with an exemplified path of a measuring value characteristic MKE for a label and the target characteristic ZKE, as is attainable when taking account of a correction characteristic KK impressed on the amplifier. This representation applies in exemplified manner for the detection of labels/splices. To obtain the desired target characteristic ZKE, the measuring value characteristic MKE is transformed by means of a suitable correction characteristic KK. This involves each point of the measuring value characteristic MKE being transformed continuously or in value-discrete manner with digital systems, into a corresponding value on target characteristic ZKE, as is illustrated by arrows. 
   In the case of very thin materials, e.g. a gram weight between 1 and 8 g/m2, in the input area the amplifying voltage can very easily be in the saturation range. However, when using foils for labels, rapidly the amplifier noise limit range can be reached, because foils very rapidly attenuate. In the graph this can be seen for a gram weight of 100 to 300 g/m2. 
   Specifically in the case of such measuring value characteristics MKE, the characteristic correction method can be particularly advantageously used, so that a saturation of the measuring signal can be avoided with very thin and strongly attenuating materials, so that ultimately a perfect detection of the presence or absence of labels is ensured. 
   In exemplified manner for comparing with label detection in  FIG. 2   d  is also shown a possible course of the measuring value characteristic MKDB for a single sheet for double sheet detection of preferably paper materials, which in the upper gram weight range roughly asymptotically approaches the double sheet threshold DBS. 
   The graph of  FIG. 3   a  shows diagrammatically the dependence of a standardized output voltage signal UA/p.u. of a signal amplifier as a function of the weight per unit area/gram weight (g/m2) in the case of differently designed signal amplifiers for single and multiple sheets, specifically double sheets. Line I in  FIG. 3   a  symbolizes a largely idealized path in the output voltage of single sheets as a function of the gram weight when using an approximately linear signal amplifier  5 , there being an approximately exponential voltage line drop. This voltage characteristic I still takes no account of a correction characteristic KK. 
   Using the nonlinear, particularly logarithmic and/or double logarithmic correction characteristic KK inherent in or impressed on the corresponding signal amplifier, a sought target characteristic II for single sheets is obtained over a very broad gram weight range, i.e. the most varied materials from this roughly exponentially falling voltage characteristic  1 . The target characteristic II consequently symbolizes a characteristic for the output signal in the case of single sheets using a logarithmic signal amplifier, the target characteristic II having an approximately linear drop. 
   As switching thresholds  FIG. 3   a  on the one hand plots the air threshold and on the other the double sheet threshold. The intersections of target characteristic II according to  FIG. 3   a  with the air threshold or double sheet threshold reveal an adequate steepness around a clearly defined, relatively small material range. 
   The largely asymptotic course of curve I in the vicinity of the double sheet threshold is obtained through the inventively provided transformation of a curve I with a correction characteristic KK to target characteristic II, so that there is a greater spacing of the voltage value for single sheets compared with the double sheet threshold for heavier gram weights or weights per unit area. 
   This example illustrates the fact that, according to the invention, it is readily possible to bring about the detection as a “missing sheet” or “air” or as a “multiple or double sheet” over a wide gram weight or weight per unit area range without using a learning process. 
   A signal transformation of measuring signal UM to a constant output signal UA of the single sheet over the entire gram weight range with in the ideal case a median voltage value between the two thresholds, namely the upper threshold for missing sheet or air and the lower threshold for multiple or double sheets, would be the optimum solution, i.e. would correspond to the ideal single sheet target characteristic ZK. This ideal target characteristic is marked I in  FIG. 3   b.    
     FIG. 3   a  also shows a curve Ia, which represents a multiple sheet signal, particularly a double sheet signal when using an approximately linear signal amplifier, the curve Ia having an approximately double-exponential drop of the multiple sheet characteristic. Curve Ia symbolizes a multiple sheet signal, particularly a double sheet signal, with a logarithmic correction line, so that approximately there is a single-exponential drop of the multiple sheet characteristic IIa. 
     FIG. 3   b  shows several target characteristics of single sheets with the representation of the standardized output voltage UA/p.u. of the signal amplifier as a function of the gram weight/weight per unit area (g/m2) using different signal amplifiers. 
   Different limit and threshold values are plotted. Thus, the top, horizontal, broken line indicates in exemplified manner the saturation limit or maximum supply voltage for a signal amplifier used. In exemplified manner is represented at approximately 0.7 UA/p.u. the threshold value for air or a missing sheet. At a value of UA of approximately 0.125 is plotted the double sheet threshold and below it the threshold for noise of electric signal amplifiers. 
   Horizontal line I in  FIG. 3   b  indicates an ideal target characteristic for single sheets, which has no saturation for thin materials and a significant spacing from the noise/double sheet threshold. This ideal target characteristic means that the output voltage UA of signal amplification when using different gram weights/weights per unit area would ideally give a constant signal. As there are high signal-to-noise ratios in the case of this ideal target characteristic for single sheets as compared with the plotted thresholds, it is possible to assume a reliable switching and detection of single, missing or double sheets. 
   Curve II represents a nonlinear target characteristic with two branches IIa and IIb, which is relatively difficult to implement due to the inflexion or reversing point, but which can be looked upon as a characteristic approaching the ideal target characteristic I for single sheets. 
   The relatively flat or shallow partial areas of IIa and IIb could be implemented if area IIa is implementable for lighter gram weights appropriately via an almost linear signal amplification. Area IIb for heavier gram weights can e.g. be implemented by means of a double logarithmic signal amplification, the strongly downwardly falling knee or kink would be too difficult to technically implement due to the attenuation characteristics of papers having a very high gram weight. 
   Curve III is a target characteristic with the end points of curve II in the simplest manner by means of a 2-dot line connection approaching an ideal path as in the case of curve I. For example, this can be achieved through the use of an at least single logarithmic signal amplifier and shows the linearization of the measuring values for single sheets over a wide gram weight range and taking account of a corresponding correction characteristic. 
   Curve III has clear passages for the threshold values for air or a double sheet, so that there are clear switching points and detection criteria relative to said threshold values. Thus, target characteristics according to curves I, II and III permit clear detections over a wider material spectrum than in the prior art. 
   Curve IV shows an unsuitable target characteristic for single sheets. On the one hand in the upper area there is an asymptotic path of curve IV to the saturation limit and on the other in the lower area to the noise threshold. Such an asymptotic path should also be avoided with respect to the air/double sheet switching thresholds, because as a result of limited signal differences with respect to said thresholds a clear distinction of the states, missing sheet or double sheet, would then be problematic. 
   The steep drop of curve IV in the central area in this example only covers a small gram weight range with a clear distinction between missing or double sheets. Since, according to the invention, the target characteristic would allow a clear detection for single, missing or double sheets over a very wide material spectrum, a path in accordance with curve IV should be avoided. 
   The principles of the invention illustrated in  FIGS. 1 ,  2 ,  3   a  and  3   b  consequently show that in evaluating the incoming measuring signal, the use of a signal amplification supplied with a correction characteristic is used and appropriately simulates the characteristic of the output voltage UA/p.u. as a function of the gram size of the flat objects over a large gram size range inversely or almost inversely or approaching the ideal characteristic for single sheet detection. In this way a linear or almost linear dependence is obtained between the measuring signal UE received from the receiver and the signal voltage UA at the signal amplifier output. 
     FIG. 4   a  diagrammatically shows in the Cartesian coordinate system with material spectrum g/m2 on the abscissa and the percentage signal output voltage UA on the ordinate an exemplified path of a measuring value characteristic MKDB for detecting single/double sheets. 
   The ideal target characteristic ZKi for detecting single, missing or double sheets is a constant with the gradient O (HDB=0). The necessary correction characteristic KKDB is also shown for this example and makes it clear that initially there is a downward transformation of the points of the measuring value characteristic MK in the direction of arrows P and then an upwards transformation for larger gram sizes in order to obtain the ideal target characteristic ZKi for single sheet detection. 
   The example according to  FIG. 4   b  shows corresponding paths of the characteristics for labels. The measuring value characteristic MKE is shown in exemplified manner with continuous lines. The ideal target characteristic ZKE is a straight line with a negative gradient or high swing. 
   The correction characteristic KKE necessary for transformation is shown in broken line form and has in this case a discontinuity point at the intersection between measuring value characteristic MKE and target characteristic ZKE. 
     FIG. 4   c  diagrammatically shows the path of the characteristics for single/double sheet detection for a case in which a real target characteristic ZKDBr is obtained and not the ideal target characteristic. The real target characteristic ZKDBr consequently has a swing HDBr exceeding 0. The plotted measuring value characteristic MKDB could in this case be transformed into the target characteristic ZKDBr by the impression of e.g. correction characteristic KKDB as the upper, continuous line. This transformation is illustrated by arrows P. 
     FIG. 4   d  diagrammatically shows the transformation of a measuring value characteristic MKDB for single/double sheet detection to the desired target characteristic ZKDB. The abscissa characterizes the material spectrum g/m2, the realistic measuring range being MDBr. The signal output voltage UA of the measuring value is indicated percentagewise on the ordinate and roughly corresponds to the attenuation constant dB. The virtual end points E 1  and E 2  are shown as imaginary intersections of the measuring value characteristic MKDB with the target characteristic ZKDB. 
   In the case of a known measuring value characteristic MKDB in the case of a double sheet detection it is consequently necessary for obtaining a linear target characteristic ZKDB to have a correction characteristic KKDB, as shown in broken line form between end points E 1  and E 2 . Thus, conceptually the transformation of the measuring value characteristic MKDB takes place in the direction of the arrows to the real target characteristic ZKDB. This is brought about by a mirroring of the measuring value characteristic MKDB on axis ZKDB after coordinate transformation. This coordinate transformation from the Cartesian coordinate system into a new coordinate system x′, y′ is shown in simplified form in  FIG. 4   f.    
   The further representation of  FIG. 4   e  diagrammatically shows the transformation of the measuring value characteristic MKE in the case of labels into the desired, ideal target characteristic ZKE by means of the necessary correction characteristic KKE. 
   In the case of a known measuring value characteristic MKE, the correction characteristic KKE can be obtained by the mirroring of MKE on the axis of the target characteristic ZKE following coordinate transformation (cf.  FIG. 4   f ). The coordinate transformation shown in  FIG. 4   f  illustrates in simplified manner the displacement for a linear coordinate system x, y by an angle α. X, y being e.g. the axes of the Cartesian, linear coordinate system. 
   Through the coordinate transformation the new coordinate reference system is provided by the imaginary reference axis of target characteristic ZKDB or ZKE. Whilst retaining the Cartesian coordinate system the following applies for the transformation:
 
 x′=−x· cos α+ y· sin α;
 
 y′=−x ·cos α+ y ·sin α.
 
   With a view to the necessary correction characteristic KK, this is only obtained following coordinate transformation in connection with the realignment through the desired target characteristic ZKDB or ZKE by mirroring on the corresponding target characteristic ZKDB or ZKE. 
     FIGS. 4   g  and  4   h  diagrammatically shows the fundamental difference between the ideal and real target characteristic for single/double sheets ( FIG. 4   g ) and label detection ( FIG. 4   h ). 
     FIG. 4   g  for the single sheet shows the ideal target characteristic ZKi, which is ideally linear and has no gradient, i.e. is constant. The swing Hi=0 would be present over the entire ideal range over material spectrum Mi. In the case of single sheet detection, with such an ideal target characteristic ZKi there would be a maximum spacing from the upper air threshold and a maximum spacing from the underlying double sheet threshold. 
   The arrow in the diagram indicates the transition from the ideal target characteristic ZKi to the real target characteristics, e.g. ZK 1  or ZK 2 . 
   It can be seen that the flatter the real target characteristic, the wider the detectable material spectrum M 1  or M 2 . 
     FIG. 4   h  shows a comparable diagram to the target characteristics ZK for label detection. The ideal label detection target characteristic ZKi has a maximum swing Hi over a relatively wide range of the material spectrum, which is designated as the ideal material spectrum Mi. 
   However, real target characteristic ZKi in the case of label detection diverge from the ideal target characteristic ZKi in the direction of the arrow. Correspondingly the more real target characteristic ZKi has a smaller swing Hi and also a small material spectrum M 1 . 
   Thus, the steeper the real target characteristic and the more it approaches the ideal target characteristic ZKi, the more swing is available for a given material spectrum. 
     FIGS. 4   i  and  4   j  show exemplified measuring value characteristics and correction characteristics and target characteristics derived therefrom. 
   Thus,  FIG. 4   i  shows a measuring value characteristic MK, which could be used for a specific material spectrum for single/double sheet detection. The correction characteristic KK has the function y=−ln(1/x)+3. 
   The correction characteristic is derived from an e-function and an inverse function x=ln(1/y). Thus, the target characteristics ZK 1  and ZK 2  shown can be derived from the measuring value characteristic MK and the correction characteristic KK, essentially through the difference. 
   The example of  FIG. 4   j  diagrammatically shows characteristics for single/double sheet detection. In this example the measuring value characteristic MK is approximately derived from a weighted hyperbola. The correction characteristic KK is a correction characteristic derived from a logarithmic function. In this example and taking account of the correction characteristic KK, the measuring value characteristic MK can be transformed into a target characteristic ZK, which approximately corresponds to an ideal target characteristic for single/double sheet detection. 
   On the basis of  FIGS. 5   a ,  5   b  and  5   c , hereinafter are explained certain fundamental principles of the inventive method and the corresponding device using the example of an ultrasonic sensor device and the physical differences essential for clear detection by means of a double sheet, a double sheet with splice and using the example of labels. These fundamental considerations at least partly also apply to other sensor devices, e.g. of an optical, inductive or capacitive nature. 
     FIG. 5   a  diagrammatically shows the overlap of two single sheets, so that in the overlap area reference can be made to a double sheet  11 . This double sheet  11  comprises two paper sheets, the gap between the two single sheets being a medium different from the material thereof. As contactless detection takes place, it can be assumed that air with the parameter Z 0  is present on either side of the double sheet and that also the intermediate medium in the single sheet overlap area is air with Z 0 , which is present in said double sheet as an air cushion as a result of the surface roughness of these materials. 
   The action direction of the e.g. ultrasonic measuring method is in the present example perpendicular to the double sheet area, so that a transmitted ultrasonic signal in the case of such a “true double sheet” as a result of multiple refraction over at least three interfaces is very small, i.e. the transmission factor over three layers ideally tends towards zero. 
   Thus, considered more generally, a double/multiple sheet can be looked upon as a material structure having a sheet lamination or box layering and in one of the gaps between the layering or lamination there is at least one medium differing from the different sheet materials and in particular air, which in the case of an ultrasonic measuring method has a clearly differing acoustic resistance compared with the sheet materials and consequently leads to signal reflections. On inserting two or more sheets the signal attenuation by signal refraction and reflection is so great that the emitted signal is strongly overproportionally attenuated. In other measuring methods this applies to the opacity and the surface characteristics colour and thickness, another dielectric, other electromagnetic conductivity or other magnetic attenuation. 
   Such a double sheet also covers the case of a connection between sheets, which is non-adhesive, e.g. using mechanical serration or edging of the sheets, because the corresponding intermediate medium would again be air. This consideration also applies to multiple sheets, where three or more individual sheet material layers are superimposed. 
     FIG. 5   b  diagrammatically shows a double sheet  12  with splice  13 . The action direction of the measuring method used, once again ultrasonics being assumed, is indicated by arrows. 
   A splice in this connection is considered to be abutting, more or less overlapping or similar connections of sheets, particularly paper sheets, plastics, foils, films and fabrics (fleeces). The connection mainly takes place by a medium adhering to part or all the surface and in particular using adhesive strips or adhesives on one or both sides. 
   Thus, physically, a splice for an ultrasonic method represents an “acoustic short-circuit” through the adhesive material layer filling and intimately joining the gap between upper sheet Z 1  and lower sheet Z 2 , air Z 0  being assumed as present above and below the single sheet. Thus, in the ultrasonic detection process a splice could essentially be detected as a single sheet with a high gram weight. 
     FIG. 5   c  diagrammatically shows two embodiments of labels  15 ,  17 . Within the scope of the present invention the term label is understood to mean one or more material layer or layers adhesively applied to a base or support material. The laminated material, e.g. with respect to sound emission to the outside, behaves in the manner of a composite material piece, so that in part there is no significant attenuation of the given physical quantities and instead only a comparatively limited, but still readily evaluatable attenuation. In this consideration no account is taken of possible inhomogeneities in the base material or the applied material, because particularly with labels perfect material can be assumed. 
   In the example according to  FIG. 5   c , label  15  has an upper material with parameter Z 2  applied to a base material by an intimate adhesive joint. Air with the parameter Z 0  is present on both label sides. As a result of this intimate adhesive joint between the materials an acoustic short-circuit is present in the case of an ultrasonic detection process, so that there is an analogy to the splices according to  FIG. 5   b.    
   The same also applies regarding label  17  in  FIG. 5   c , which solely differs from label  15  by a second, top-applied material layer. Here again an acoustic short-circuit between the materials can be assumed. 
   These fundamental considerations within the scope of the invention in connection with the detection of double sheets, splices, labels and the like, consequently makes it possible by means of the inventive method or device to detect differently stacked single sheets or multistacked materials and also distinguish the same. It is consequently possible to detect or count labels applied to flat materials and which have an object gap between them. 
     FIG. 6  shows in block diagram form a device for detecting missing, single and multiple sheets, the correction characteristic being produced as a combination of individual characteristics. 
   The flat materials or sheets to be detected are passed between transmitter T and receiver R. The correction characteristic resulting from amplification is in the present example implemented with a first correction characteristic in amplifier device  21  and at least one second correction characteristic in amplifier device  22 , which are connected in parallel. The measuring signal or its characteristic path over the gram size present at the output of receiver R is consequently subject to a combined correction characteristic in order to obtain a readily evaluatable target characteristic  23 , which is further processed in a microprocessor  6 . 
   In connection with the combination of correction characteristics this can also be implemented in a signal amplifier or in several series or parallel-connected, individual signal amplifiers in order to produce an overall gain. Thus, correction characteristic implementation can take place in the most varied ways, because the essential idea of the invention is to detect single, missing or multiple sheets over a wide gram size range without having to integrate a learning process. 
     FIG. 7  shows in block diagram form a modified device for implementing the invention. The measuring signal of receiver R is subsequently passed to an amplifier device  24 , whose signal output is led to a microprocessor  6 . In this example and by means of feedback in path A, microprocessor  6  permits the setting of a predetermined correction characteristic via symbolized potentiometer  25 . 
   In alternative circuitry a corresponding correction characteristic is calculated by means of microprocessor  6  and the obtained or stored data and via path B is fed back and impressed on amplifier device  24 . 
   It is also possible to determine a correction characteristic empirically or via the measurement of a representative material spectrum which is to be detected and input it to the evaluating unit including microprocessor  6 . The determined correction characteristic C over path B can be impressed in value-discrete or value-continuous manner on amplifier device  24  or the evaluation of the amplified output signal can be performed directly in microprocessor  6  on the basis of correction characteristic C. 
     FIG. 8  diagrammatically shows the empirical determination of a measuring signal characteristic. For this purpose a plurality of commercially available materials are passed between transmitter T and receiver R and by means thereof the corresponding measuring signal characteristic is determined. Normally the measuring range is fixed by the introduction of the thinnest available sheet material A and the thickest sheet material B to be detected. The thus determined measuring signal characteristic can then be supplied to the further processing system, e.g. a microprocessor, in order to determine in connection with said measuring signal characteristic a substantially optimum correction characteristic so as to achieve the requisite target characteristic. 
     FIG. 9  diagrammatically shows an inventive device  40  for the contactless detection of multiple sheets A, without performing a learning step, and the detection of material layers B, e.g. labels adhesively applied to a base material. 
   A fundamental principle in this connection is to supply the measuring signal evaluation for multiple sheets to a separate channel A with corresponding correction characteristic and in parallel therewith supply the measuring signal evaluation for labels B to a separate channel B with adapted correction characteristic. 
   The measuring signal obtained at the output of receiver R is therefore switched to the corresponding channel A or B by means of a multiplexer  34  controlled by microprocessor  6 . Signal amplification in channel A is subject to a separate correction characteristic with optimum design for multiple sheet detection. Signal amplification in channel B is subject to a correction characteristic or the label measuring signal. By means of a following, microprocessor-controlled multiplexer  35 , both channels A, B are supplied to the downstream microprocessor  6  for further evaluation and the detection of multiple sheets or labels. 
   Device  40  is suitable for detection using ultrasonic waves. The essential advantage is the planned possibility of being able to incorporate for evaluation purposes the in each case most suitable correction characteristics for fundamentally differing measuring tasks, namely for the most varied material types, as in the present case multiple sheets and labels. 
     FIG. 10  diagrammatically provides a graph of the standardized output voltage UA as a percentage as a function of the grain weight. The target characteristic  42  of a single sheet in the case of logarithmic amplification is plotted over the gram weight range. In the upper area and in continuous line form is also plotted the air threshold LS and in the lower area in broken line form the double sheet threshold DBS. 
   It is important that the double sheet threshold can be dynamically provided and this can take place constantly over gram weight range sections. This is illustrated by lines B 1 , B 2  and B 3 . The dynamic setting of the double sheet threshold can take place linearly or as a random degree polynomial line, as is e.g. shown between P 1 , P 2 , P 3  and P 4 . 
   With this dynamic setting of the double sheet threshold it is possible to bring about a further extension of the measurable gram weight or weight per unit area ranges, so that a further increase in the detectable material spectrum can occur. 
     FIG. 11  relates to a substantially similar graph to  FIG. 10 , the path of the target characteristic  42  for the single sheet largely coinciding over the entire gram weight range. The dynamic threshold MBS for the multiple sheet and its path between points P 1   a , P 2   a  and P 3   a  is plotted. Curve  44  marks the upper value of the flutter range for single sheet and curve  45  the lower value of the flutter range for a single sheet. 
     FIGS. 12   a ,  12   b  diagrammatically shows the arrangement for detection of single-corrugation corrugated board  51  and two-corrugation corrugated board  60 , as well as the running direction L, whilst taking account of two, more particularly ultrasonic sensors  61 ,  62 . 
   Corrugated board  51  according to  FIG. 12   a  is in single-corrugation form and has at its adhesion points with a lower base layer  52  or upper top layer  53  adhesive areas  54  and webs linking the bottom and top layers spread over a corrugated surface  55 . These webs  55  between the board corrugation and the corresponding, e.g. horizontally directed bottom or top layers, constitutes an “acoustic short-circuit” when using ultrasonics. 
   The sensor used in  FIG. 12   a  has a transmitter T and receiver R, whose main axes are oriented coaxially to one another. The orientation of transmitter T and receiver R preferably takes place approximately perpendicular to the largest corrugation surface  55  or under an angle β 1  to the perpendicular of the single-corrugation corrugated board. Angle β 2  is the angle between the perpendicular to the corrugated board and the surface direction of the main surface of the corrugation. 
   The optimum angle β 1  in the case of an ultrasonic sensor for coupling noise onto a single-corrugation corrugated board, which has a necessary acoustic short-circuit AK between bottom layer  52  and top layer  53  is determined by the gradient t/2 h. t is the spacing between two corrugation peaks and h the height of the peak or the spacing between the bottom and top layers. 
   With an optimum sensor arrangement, the aim is to achieve an orientation with β 1 =β 2  and in the example said angle would be 45°. However, the coincidence of angles β 1  and β 2  is not necessary for detecting missing, single or multiple corrugated board layers. 
     FIG. 12   b  shows a two-layer corrugated board  60  with the lower, first corrugation  58  and the upper, second corrugation  59 . The arrangement of an ultrasonic sensor T, R corresponds to that of  FIG. 12   a.    
   Here again, the acoustic short-circuit AK 1  and AK 2  between the individual layers, i.e. a material connection in the sense of a web adhering to the layers for the connection of the individual top layers is essential for detection purposes with two or multiple-corrugation corrugated boards. It is possible in this way in the case of an ultrasonic sensor to transmit high sound energy to the multiple-corrugation corrugated board, so that there is a maximum force action approximately perpendicular to the spread out corrugation surface. 
     FIG. 13  diagrammatically shows a device  60  in which the amplitude evaluation by correction characteristic and a phase evaluation are combined. The signal, e.g. ultrasonic signal, generated by the signal generator  63  is supplied to a transmitter T and irradiated. The measuring signal received by receiver R is dependent on the number of flat objects in the transmitter-receiver gap. The measuring signal of receiver R is then supplied to an evaluating device  61  on which is impressed at least one correction characteristic. At the output of the evaluating device  61  is provided the detection signal for missing, single or multiple sheets determined by the amplitude evaluation. It is then passed to a microprocessor  64  for linking and e.g. logical evaluating together with the signal determined by phase evaluation. 
   Device  60  has a synchronous rectifier  62  for phase evaluation receiving on the one hand the signal and phase at the output of the signal generator via path  67  and on the other, via line  68 , the measuring signal and corresponding phase at the output of receiver R are supplied to the synchronous rectifier. Due to the phase difference formed in synchronous rectifier  62 , it is consequently possible to generate a detection signal, which corresponds to the number of sheets present or the number of laminations of the layers adhesively applied to a base support or the splices or labels. 
   The two signals from the characteristic-corrected amplitude evaluation and the phase evaluation are in the present example supplied to the microprocessor  64 , at whose output is obtained the combined detection signal for establishing the presence of a single, missing or multiple sheet. 
   In a modified design of the evaluating device  61  for the amplitude and the synchronous rectifier  62 , there can be a program-controlled evaluation and rating of the two signals in microprocessor  64 , whose output signal  65  represents the detection signal for the number of detected flat objects or sheets. Advantageously the amplitude and phase can be amplified and evaluated in parallel and, as desired, as a single signal, but also as a weighted signal. 
   Whilst taking account of the preceding description, from the method and device standpoint the invention provides a solution for the reliable detection of single, missing and multiple, specifically double sheets, this not only applying over a very wide gram weight and weight per unit area range, but also with respect to flexible use possibilities and different material spectra.