The present invention is related to the field of production supervision, and in particular to an improved system for the detection of features of a material, as claimed in claim 1. The present invention is also related to a method as claimed in claim 18.
There are tight set quality requirements within widely differing fields of manufacturing. The fulfilment of these requirements is important and is thus supervised in various ways, which is why different kinds of quality controls are frequently used to ensure the quality of a specific product. It is, for example, possible to use visible and infrared radiation for this purpose, and vision systems are available for use in quality controls. These vision systems are however very expensive, and not well suited for certain applications.
However, in the plastic bag manufacturing industry of today, there is no efficient automated quality control of properties of a material, such as for example the weldings and perforations of a plastic film used for manufacturing plastic bags. The quality of the weldings and perforations needed are simply manually controlled by inflation and destruction of statistically selected plastic bags. Obviously, this is a very uneconomic, time consuming and incomplete supervision method, requiring both labour time and entailing a waste of products.
Further, in order to properly wrap a certain number of plastic bags for selling, they have to be counted. Presently, the bags are counted by using high voltage spark gaps. Such spark gaps are operated at unacceptable voltage levels and are therefore not suitable for environments where large amounts of easily inflammable products are stored and handled. Further, the margin of profit in the manufacturing of plastic bags is rather limited, and a miscalculation of even a relatively small number of plastic bags is thus crucial for the yield of a production line.
Furthermore, the above mentioned method of using visible and infrared radiation is not applicable to arbitrarily coloured plastic films or to plastic material not imposing any measurable photon losses (e.g. transparent, thin (PE)), since the local absorption of an incident signal is measured using photometers. In addition, the optical quality of weldings and perforation signature is poor, thus further rendering such quality controls difficult.
Thus, what is needed is a system and method for improving and automatizing the supervision of properties of various materials, such as plastic bags and the like. Further, it would be desirable to provide a system and method enabling an accurate counting of plastic bags or similar items, in an easy and economic, yet reliable way.
It is an object of the present invention to provide a system for enabling detection and supervision of the properties of different, optional materials, in which the above described drawbacks are eliminated. More specifically it is an object of the present invention to provide an easy to implement and easy to use system for supervising materials used in production, not involving any hazardous components or steps.
In accordance with the present invention, a system for determining characteristics of a material is provided. The system comprises means for continuously acquiring at least one reference value, and means for acquiring at least one measurement value, whereby the reference value(s) and the measurement value(s) are being acquired and compared in real-time. Thereby characteristics of any material may be determined in an easily implemented way. The at least one reference value is obtained simultaneously as the material is being tested, i.e. simultaneously as the measurement values are being obtained.
These objects are achieved, according to a first aspect of the invention, by a system as defined in claim 1, and a method as claimed in claim 18.
In accordance with one embodiment of the invention, the system comprises means for utilizing microwave radiation in order to determine characteristics of the material. More specifically, the system comprises means for generating, sending and measuring microwave radiation, whereby the microwave radiation is used in determining desired characteristics of a material.
In accordance with one embodiment of the present invention the means for sending and measuring microwave radiation comprises means for measuring a microwave transmission factor S21 between two ports, between which ports the material is arranged to run. This embodiment may be implemented using different kinds of ports, such as antennas or inductors or the like. Thereby the invention may be realised by components readily available on the market, and also giving a great design flexibility for providing application specific solutions.
In accordance with another embodiment of the present invention the means for measuring comprises means for calculating the ratio between power received by one of said ports and power emitted by the other of said ports.
In accordance with another embodiment of the present invention the thickness d of a material is determined by calibration data, and the following equations:
where ε=dielectric constant of the material, ε0=dielectric constant of vacuum, k=wave vector in the material, k0=wave vector in vacuum, μ=permeability and μ0=permeability of vacuum, d=thickness of the material, D=the distance between included transmit and the receive ports. Thereby explicit expressions are provided, enabling an easy implementation of the present invention.
In accordance with another embodiment of the present invention means for sending and measuring microwave radiation comprises at least two oscillators, and at least two independent measurement zones coupled to the oscillators, which provides a reliable and yet inexpensive way to supervise the quality of a material.
In accordance with another embodiment of the present invention each measurement zone comprises capacitor plates, or inductors or antenna elements determining an oscillation frequency of an oscillator in the microwave range. This embodiment again provides a solution with great flexibility and components readily available and easy to replace in embodiments where the components are not fastened in a non-removable way. Further, the capacitor plates may be placed on the same side of a material being tested, which is very advantageous in environments and applications that are space limited.
In accordance with another embodiment of the present invention means are included for measuring the oscillation frequency of an oscillator circuit. The oscillation frequency contains information of the properties of the material.
In accordance with another embodiment of the present invention said characteristics comprise one of more of the following: the thickness of the material, the dielectric loss of the material, perforations, markings or the like in the material, welding in the material, the material constituting a single sheet of material or multiple, or multiple folded. A user may thus measure a wide range of properties, chosen suitably for the application in question.
The present invention further provides such a method, whereby advantages similar to the above described are achieved.
a-5c show an alternative embodiment of the present invention.
The inventor of the present invention has realised the possibility to use electromagnetic radiation in the microwave range of the radio frequency spectrum for supervision and detection applications, for example for quality control purposes, and/or in order to detect certain features of an optional material, used preferably in an assembly line production.
In order to facilitate the understanding of the present invention, a brief description of microwave transmission is given in the following.
A microwave transmission method in accordance with the invention may be used in a number of technologies, e.g. for the measurement of water contents in stone, crops, cotton and pulp. The method relies on the measurement of the so called microwave transmission factor between two ports referred to as S21, as is known to a person skilled in the art. These two ports are implemented as antennas in all suitable technologies, and it is assumed in this description that the ports are antennas, if not stated otherwise in a specific embodiment. The transmission information is related to the object being tested, said object being placed between the two antennas. Under certain conditions the two antennas can be combined to a single antenna using directional couplers, as is known within the art.
There are two distinct ways to evaluate the microwave properties in accordance with the invention: frequency domain measurement and time domain measurement, respectively, both of which will be described below.
Frequency domain measurement: an S21 parameter measurement involves the calculation of the ratio between the power received by the receive antenna and the power emitted by the transmit antenna. In order to calculate the power emitted by the transmit antenna one has to measure the power transmitted to the receive antenna and the power reflected back to the generator in at least three reference cases for a complete characterization of the transmit antenna.
Time domain measurement: sharp, short microwave pulses are emitted and transmitted through the measurement gap. The measurement method is based on comparing the signal runtime through the measurement gap with the runtime through a reference line with eventually adjustable length. In implementing the present invention suitable reference cases for a frequency domain measurement comprises: 1) no material present in the measurement gap, 2) completely blocked measurement gap (the blocking being accomplished for example by means of a metal sheet), and 3) measurement gap filled by an absorber.
With calibration data, the S21 parameter may be calculated between the antennas. From this in turn, the thickness of the material being placed in the measurement gap is deduced according to:
k
0=ω·√{square root over (ε0·μ0 )} [1]
k=ω·√{square root over (εr·ε0μrμ)} [2]
S
21
=e
−ikd
·e
−ik
(D−d) [3]
where ε=dielectric constant of the material, ε0=dielectric constant of vacuum, k=wave vector in the material, k0=wave vector in vacuum, μ=permeability and μ0=permeability of vacuum, all using SI units. The thickness of the material is denoted d, the distance between the transmit and the receive antenna is denoted D. The imaginary unit is referred to as i and the angular frequency is given by 2π times the RF frequency. Converting the S parameter to the wave vector in the gap and by assuming the wave vector of the material to be known (since its dielectric and magnetic properties are known), its thickness may be calculated as:
Obviously, this method may be used only when there is a difference in the k vector between vacuum and the material being tested.
As is known within the field, there are several ways to measure the S21 parameters within a small frequency range, such as CW (continuous wave) radar and FMCW (frequency modulation continuous wave) radar.
Also, obviously the obtained thickness is averaged over the measurement zone and reflects the effective thickness, e.g. perforation with 50% cut out material will be reflected in a thickness variation down to
The obtained thickness is therefore an effective thickness. Similar reasoning apply to changes in the dielectric function, a peak of ε will be detected as a peak in thickness when assuming ε to be constant.
The present invention may be implemented utilizing a time domain measurement, the theory of which is given in the following.
The signal runtime through a measurement gap is given by the following relation:
T=ν
0(D−d)+ν·d [5]
where v=group velocity of the microwave pulse in the material, and v0=group velocity of the microwave pulse in vacuum. T denotes the time required by a microwave pulse to traverse the measurement gap. Obviously, this method may be used only if there is a difference in group velocity between vacuum and the material.
The fundamental issue when comparing equations [3] and [5] is how to calculate the group velocity and how to relate it to the dielectric and magnetic properties of different materials. In an ideal case, the microwave pulse is infinitely short, which corresponds to an infinitely broadband frequency pulse. In reality however, the electronics used have rise times, and the transmit and receive antenna bandwidth will limit the frequency spectrum of the transmitted pulse to a spectrum that can be approximated by a Gaussian curve centred around a value ω0 and having a spectral width given by σ. For reasonably small dependencies of the dielectric properties on frequency, the group velocity of the pulse is readily obtained as:
Using this approximation, equations [3] and [5] become identical.
In another embodiment of the present invention, the properties of a material being tested are not measured using transmitted or reflected microwave radiation. Instead, the properties of the material (or product) are used in an oscillator circuit, the frequency of oscillation of which is measured. The oscillation frequency and the oscillation signal then contain information about the microwave properties of the material. This method does not utilize microwave antennas; the microwave interaction is instead communicated via suitably mounted capacitor plates.
The information obtained of the properties of the material could be used to control different operations, such as adjusting the power of the welding device to optimize the power needed to create a good weld. Another operation that could be controlled is the extraction process, which can be adjusted to obtain a more or less constant thickness of the extruded material. The information could even be used to detect the presence of a material at a specific location to initiate some kind of action, e.g. cutting a straw in the right length after extrusion.
Based on the theory given above, an implementation of the present invention will now be described, firstly with reference to
The sensor used in accordance with the present invention consists of an electronic apparatus 10 mounted in an arrangement 20, as shown in
An exemplary mechanical setup, illustrating the arrangement 20 of the sensor consists in a mounting plate 21, used for connecting the sensor to the production machine. The arrangement further comprises an electronic compartment with a lower lid 22 and an upper lid 23. The lower and upper lids 22, 23 form the measurement gap 30, in which measurement capacitors (to be described below) are accommodated.
Now with reference to
When the present invention is used in detecting features of a material running in an assembly line, for example in the manufacture of plastic bags, the geometrical form of the capacitors has to be carefully considered. In the case of plastic bags manufacture, the geometrical form of the measurement capacitor 161 is chosen to be parallel to the expected placement of the weldings 92 and perforations 91 of the product 90 being tested. The reference capacitor 162 is chosen to be arranged orthogonal to the measurement capacitor 161, and thus being parallel to the direction of motion of the material being tested, see
The oscillator outputs 12 are the measurement RF signal 121 and a reference RF signal 122, and they are down converted using a microwave mixer 13. The down conversion is preferably performed in order to more readily be able to perform the required calculations, since the used frequencies are so high that variations of the frequencies would be hard to detect without a down conversion. The difference frequency between the oscillators 111, 112, being the difference between the down converted measurement RF signal 121 and the down converted reference RF signal 122, is available on the IF output 14 of the mixer 13. However, in an alternative embodiment of the present invention, a direct difference signal may be conceivable, without any down conversion step. As an additional feature of the present invention, the frequency of the signal travelling on this line may be counted using a high speed counter 15, for example realised by emitter coupled logic (ECL) or microwave frequency divider followed by digital counter, by comparing it to a frequency normal 151. The result of the frequency measurement is available in a measurement latch 155 for further processing, and may for example be used in order to calculate the number of plastic bags passing through the measurement gap 30.
The measurement signal(s) is proportional to short range variations in the thickness of the product being tested. The oscillation frequency is more specifically proportional to the capacitance difference of the measurement oscillator 111 and the reference oscillator 112. Perforations 91 constitute a reduction of the amount of material present in the measurement gap 30, and weldings 92 constitute a removal of material out of the welding groove as a result of the welding process. Therefore, perforations 91 and weldings 92 of the material passing by the sensor arrangement will be visible as a reduction of the capacitance of the measurement capacitor 161, whereas the reference capacitor 162 remains at first hand unchanged by its different choice of geometry. These differences in the capacitance consequently enable the innovative way of detecting properties of the material being tested.
In
With reference again to
In the description above, the measurement and supervision of transverse weldings of a material is illustrated. However, in an alternative embodiment, the longitudinal weldings of a material may be supervised in a similar fashion.
In
Further, it is conceivable to use only measurement oscillators, if
is used as the measurement signal, where ω is the oscillator frequency and t is the time.
In summary, the present invention is based on the idea of obtaining reference values and measurement values more or less simultaneously, preferably using microwave radiation for determining various characteristics and features of different materials, such as plastics. In short, three different methods are presented: 1) frequency domain measurement, where a transmitted microwave radiation is analysed; 2) time domain measurement, where a reflected microwave radiation is analysed; and 3) microwave reaction, where an oscillator is used and the frequency and signal strength of which is analysed.
Number | Date | Country | Kind |
---|---|---|---|
0500424-7 | Feb 2005 | SE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/SE06/00249 | 2/24/2006 | WO | 00 | 11/6/2007 |