Method for compensation of temperature dependent variation of coil resistance and inductive proximity swich using said method |
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申请号 | EP07360014.0 | 申请日 | 2007-04-04 | 公开(公告)号 | EP1978641A1 | 公开(公告)日 | 2008-10-08 |
申请人 | SENSTRONIC, S.A.; | 发明人 | Kirchdoerffer, Rémy; Frolov, Vladimir; | ||||
摘要 | The present invention concerns a method for compensating the resistance variation of a multilayer coil depending on the temperature variation of said coil. Method characterised in that it consists in determining at least one particular frequency or range of frequencies at which the dependency on the temperature of the ohmic resistance of the coil is less than or equal to a preset value, preferably at its lowest or minimum, and in feeding said coil, in use, at a frequency value equal to said at least one aforementioned particular frequency or situated within said at least one particular range of frequencies. |
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权利要求 | |||||||
说明书全文 | The invention relates in particular to the development of inductive proximity switches for detecting metal objects. Inductive switches are widely used in automatic control systems for mechanical and robotronic equipments. The present invention concerns more specifically a method for compensation of temperature dependent variation of coil resistance and an inductive proximity or presence switch or sensor using said method. The operating principle of inductive switches is based on an electric LC-oscillating circuit which generates a variable electromagnetic field of frequencies ranging from several tens to several hundreds kilohertz depending on the type of the switch. A metal object to be detected interacts with the oscillation circuit field changing its parameters, primarily the Q-factor. A change in the parameters of the oscillation circuit is acknowledged by the electronic circuit of the inductive switch. One of the main problems encountered by inductive switch developers is strong dependence of oscillation circuit parameters on ambient temperature. This causes the reduction of switch sensing accuracy in the temperature range of use of -25 °C to 70 °C, as required by the international standards. The physical mechanism of the variation of the Q-factor of the oscillation circuit as a function of the temperature, hence, variation of the switch sensing distance, is mainly the temperature dependency of the coil wire specific resistance. For pure metals and continuous or direct current this dependence is approximately described by the formula: where R0 is conductor resistance at temperature T=0. The postulate of coil resistance strongly depending on temperature is the basic principle of patents and applications of the prior art temperature stabilization of inductive switches. There are various known approaches to minimize the temperature influence on characteristics of inductive switch. However, all known methods are instrumental, wherein temperature dependence of the oscillation circuit Q-factor is compensated either by providing additional compensating coils having the same temperature dependence of resistance as that of the switch operating coil, or by introducing special electronic circuits. The following patents exemplify the existing approaches:
The disadvantages of these known compensation methods mainly reside in the complicated designs of the used coils (normally a temperature-compensated oscillation circuit is a multi-coil system) and the necessary additional electronic circuits, which result in high manufacturing costs and in a complex and cumbersome overall structure. These disadvantages linked to the variation of resistance of a coil with temperature appear more widely in any device or circuit comprising or using such a coil. It is a main aim of the present invention to overcome the aforementioned disadvantages. Therefore, the main object of the present invention is a method for compensating the resistance value variation of a multilayer coil depending on the temperature variation of said coil, characterised in that it consists in determining at least one particular frequency or range of frequencies at which the dependency on the temperature of the ohmic resistance of the coil is less than or equal to a preset value, preferably at its lowest or minimum, and in feeding said coil, in use, at a frequency value equal to said at least one aforementioned particular frequency or situated within said at least one particular range of frequencies. Claims 2 and 3 are directed to possible advantageous ways of determining the particular frequency or range of frequencies. The invention also concerns a method for compensating the temperature drift of an inductive switch comprising a LC oscillation circuit with a multilayer inductive coil, preferably by eliminating the dependence of the circuit Q-factor on temperature, characterised in that it consists in determining at least one particular frequency or range of frequencies at which the dependency on the temperature of the ohmic resistance of the inductive coil is at its lowest or minimum, and in selecting the circuit inherent or natural oscillation frequency equal to said at least one particular frequency or within the at least one particular range of frequencies. Claims 5 and 6 are directed to possible advantageous ways of determining the particular frequency or range of frequencies. Finally, the invention encompasses further and inductive proximity or presence switch device, comprising a LC oscillation circuit with an inductive coil, characterised in that the inherent or natural oscillation frequency of the LC circuit is equal to a predetermined particular frequency or comprised within a predetermined particular range of frequencies at which the dependency on the temperature of the ohmic resistance of the coil is at its lowest or minimum, and in that said inductive coil is a multilayer coil made of a winding wire the diameter of which is greater in value or bigger than the depth of the skin layer for or at the inherent or natural oscillation frequency. Claims 8 to 11 concern preferred embodiments of the switch device described before. The invention will be better understood thanks to the following description and drawings of different embodiments of said invention given as non limitative examples thereof, wherein:
As mentioned before, the invention proposes to overcome the aforementioned disadvantages since it employs a special design coil with self-compensation of temperature dependence of coil effective resistance by selecting a specifically adapted natural oscillation frequency. The basic idea of the invention is that the induction coil used in the oscillation circuit is a multilayer coil wound with wire of the diameter bigger than the depth of the skin layer for the selected frequency of the inherent circuit oscillations while the frequency of inherent circuit oscillations is so selected that at this frequency the ohmic resistance of the coil weakly depends on temperature. The physical foundation for the present invention is the skin effect theory and A. Sommerfeld's theory of resistance of a multilayer coil for alternating current. These two theories are briefly specified hereinafter. The skin effect is a well known phenomenon of alternating current redistribution along conductor cross-section. As a result of the skin effect the effective cross section through which the current flows becomes less (than the material cross section) and conductor resistance increases in comparison with continuous current feeding. The thickness of the skin layer (the effective cross section of an AC conductor) is described by the widely known expression: where µ0 (= 4π* 10-7) is the vacuum magnetic permeability, ω̅ is the circular frequency, σ is the metal conductivity (copper conductivity is 5.8* 107 at room temperature). Using (2) it may be easily calculated that for a current with a frequency of 100 kHz, the skin layer thickness of copper wire is 0,2 mm, and for a current with a frequency of 1000 kHz, this thickness is 0,063 mm (at room temperature). Formula (2) also shows, that in case of alternating current the temperature dependence of the conductor resistance becomes more complicated. Indeed, as temperature rises, the metal conductivity σ drops and, according to formula (2), the skin layer thickness, hence, the effective cross section through which the current flows, increases, thus partially compensating (slowing) temperature growth of the full conductor resistance R. The inventors have observed the effect of variation of temperature dependence of insulated conductor resistance (in free space) on alternating current frequency for copper wire being 0,2 mm in diameter and 18 m long. These data are shown in One can see, that for the studied range of temperatures and frequencies the resistance-to-temperature relationship of free copper wire remains approximately linear, with temperature gradient slightly depending on the alternating current frequency: In a similar way to The situation changes radically once we shift from the free wire to a multilayer coil wound with wire of the same diameter. First of all, it is revealed in the dependence of ohmic resistance on frequency at ambient temperature (see As seen from the data illustrated on Whereas for the free insulated (not wound) wire its resistance at 1000 kHz exceeds only by 30% that at continuous or direct current, the ratio of resistance values for alternating and constant currents for wound wire (coil) soars to 4,5 and more. This effect is explained by the fact that the distribution of current density in an isolated insulated wire and in closely adjacent wires differs considerably due to the intersection of magnetic fields of closely adjacent coil turns (this effect is known as magneto-resistivity). Although the general theory behind this effect is fairly sophisticated, A. Sommerfeld obtained in the case of a long multilayer coil, an analytical solution for the dependence of multilayer coil resistance on skin layer thickness (Arnold Sommerfeld. Elektrodinamik., Akademische Verlagsgesellschaft Geets&Porting K.-G., 1949). This dependence is described by the expression: where R is the active resistance of the coil, R0 is the resistance for a constant current, D is the diameter of the coil wire, d is the skin layer depth and n is the number of layers of coil. Various examples of the function [R/Ro vs. ratio D/d] are illustrated in As seen from Returning to
Thus: Both R1 and R2 depend on temperature and in analysing these two different dependencies, the inventors came to the surprising conclusion that there is a frequency or range of frequencies within which temperature dependence of multilayer coil resistance, in presence of alternating current, is minimal, or even substantially non existing. Hereinafter, this issue is being considered in greater details. One notices that at a fixed frequency, an ambient temperature rise may bring about the following effects:
The said effects may result in the crossing of coil resistance-to-frequency relationships for various temperatures (shown diagrammatically in One notices that in the range of frequencies f ≤ fcut the coil resistance rises with temperature and that for the frequencies meeting the condition f ≥ fcut, the resistance decreases as temperature rises, while at f = fcut the changes of R1 (T) are approximately compensated by the opposite changes of R2(T), so that the temperature dependence of the ohmic resistance R of the coil is weak, if not absent. To practically verify the discovery of the existence of a frequency fcut at which ohmic resistance of a multilayer coil weakly depends on temperature and to provide a non limitative example of proposed invention embodiment, the inventors used a multilayer coil (CPS-5) without a ferrite core. The used coil (CPS-5) had the following specifications:
The effective means of resistance of the coil at various alternating current frequencies was measured by means of an immitance meter of the E7-20 type. The coil under investigation was placed into a thermostatic Cold-Heat test chamber of the MK53 type (BINDER GmbH), which allowed for ambient temperature to vary from -30 °C to 80 °C. The results of the measurements of coil resistance vs. frequencies (at different temperatures) are given in As seen from the illustrated experimental values, there is strong dependence upon frequency of the coil resistance variation with temperature with the resistance vs. temperature derivative ∂R/∂T changing its sign in the frequency range of 200 kHz to 500 kHz. A more vivid demonstration of the effect of the derivative changing its sign is presented in As follows from the data given in For frequencies in the area of 500 kHz this dependence is actually non existing. A determining parameter of the LC-circuit forming the core components of inductive proximity switches, is its Q-factor which is given by the following well-known formula: where Q is the oscillation circuit q-factor, ω̅ is the inherent oscillations frequency, L is the inductance value and R is the effective resistance. Experiments conducted by the inventors have shown that the variation of value L with temperature is insignificant and so the temperature dependence of the oscillation circuit Q-factor-Q is only dependent on the temperature behaviour of the effective resistance R. This conclusion is confirmed by the experimental results presented in It follows from the data given in Since the variation with temperature of the oscillation circuit Q-factor is the main factor determining the temperature drift of the sensitivity of the inductive switches, the results presented are an experimental proof of the possibility to implement the suggested method for decreasing, if not completely zeroing, the temperature drift by selecting a frequency at which inductive coil resistance-to-temperature relationship is at its minimum, if not null. The method proposed by the inventors for decreasing temperature dependence of the oscillation LC-circuit Q-factor has a general character and is applicable for inductive coils with a different number of turns wound and with copper wire of different diameter, as compared to the example considered above. Another example of embodiment relates to a multilayer coil without a ferrite core (coil SPC-2) and with the following parameters:
The results of [coil resistance vs. frequencies and temperatures] measurements for the above coil are given in The comparison of the data given in For coils wound with a thicker wire, a more complicated dependency of coil resistance on frequency is observed. To demonstrate this statement, further data have been collected for a coil SPC-6A with the following parameters:
The results of [coil resistance vs. frequency and temperature] measurements for the SPC-6A coil are given in The study of the data given in It is important to underline that the possibility of substantial compensation of temperature dependence by way of alternating current frequency selection is only feasible with multilayer coils. This was experimentally confirmed by the results of experiments with flat (planar) coils manufactured by the inventors using various technologies. The first result concerns a single-layer planar coil made in the form of a spiral (coil PLC-1) with constructive parameters as follows:
The results of [coil resistance vs. frequency and temperature] measurements for coil PLC-1 are given in Comparing the data given in In the case of the single-layer coil, the condition of complete temperature compensation is not attained within the entire range of frequencies used up to 1000 kHz. This could presumably be attributed to the turn-to-turn interaction of currents which in a single-layer coil is substantially less than in a multilayer one. Qualitatively this conclusion is in accordance with A.Sommerfeld's theory (formula 4, The turn-to-turn interaction can be even further decreased by using the planar coils made for example by lithography method. An example of such coil is shown in The PCB-coil whose design is given in The conclusion made is confirmed by the results obtained by measuring the resistance of PCB-coil vs. frequency and temperature. These data are given in As follows from the data given in It should be noted that most inductive switches use coils with ferrite cores, which help to improve oscillation circuit Q-factor, hence to raise inductive switch sensitivity. In this context, the applicability of the temperature compensation method suggested by the inventors for coils with ferrite cores becomes indeed essential. A multilayer coil SPC-7 with the parameters shown below was selected as a non limiting example of embodiment of the inventive method:
First, the inventors measured the dependency of the resistance on frequency and on temperature for said coil without any ferrite. Then the coil was placed into a standard ferrite 'sleeve' and the same dependencies were studied again. The results of the [resistance vs. frequency and temperature] measurements of the free coil SPC-7 are given in One can see, that the resistance-to-temperature relationship of the coil changes its sign in the frequency range of 200 kHz to 300 kHz and the value of the resistance derivative with respect to temperature is minimal in said range of frequencies. Another example of an embodiment of the invention of great practical importance is the results of investigations concerning the coil SPC-7 placed in a ferrite 'sleeve'. These results are given in Depending, among other parameters, on the layout of the coil, the particular frequency, at which the temperature dependency of the coil resistance is at ist lowest, can either be practically unique, or belong to a limited range of frequencies. Even though, the invention has been described more particularly in relation to inductive presence or proximity switches or sensors, it should be noted that the inventive method features described herein can be applied in any device, system or apparatus using a multilayer coil which can be fed by an ajustable frequency signal, in order to compensate the temperature related variation of the coil resistance. The present invention is of course not limited to the preferred embodiments described and represented herein, changes can be made or equivalents used without departing from the scope of the invention. |