DE102014218294A1 - Inductive proximity switch - Google Patents

Abstract

Inductive proximity switch with a resonant circuit with a sensor coil 1 and a resonant circuit capacitor 2, which is excited by an oscillator circuit 3, and an evaluation unit 4 for controlling the oscillator circuit 3 and for generating a binary switching signal, wherein the oscillator amplitude is adjusted by a digital potentiometer 5, characterized in that the digital potentiometer 5 has two partial potentiometers 5a and 5b connected in parallel, as well as a transistor switch 7 controlled by the evaluation unit 4 for bridging a serial additional resistor. In an advantageous embodiment, the digital potentiometers 5a and 5b are part of an integrated electronic circuit 6, which also contains the oscillator circuit 3 and the evaluation unit 4.

Description

The invention relates to an inductive proximity switch according to the preamble of patent claim 1.

Inductive proximity switches have long been known and are also produced and distributed by the applicant in large numbers.

They usually consist of a resonant circuit with a coil and a capacitor, which is excited by an oscillator circuit, and an evaluation unit. When approaching a metallic or electrically conductive object (switching flag), the damping state of the oscillator changes. The amplitude and / or the frequency of the oscillator are detected by the evaluation unit, and upon reaching a certain signal level, the i. A. corresponds to a certain distance of the switching flag, a mostly binary switching signal output.

Be particularly advantageous, the current mirror oscillator has proven u. a. also because it is adjustable with a compensation resistor not associated with the oscillator circuit.

From the DE 43 31 555 A1 is an inductive proximity switch with a balancing resistor known. It is also suggested here to use an electronic potentiometer instead of a surface resistance, which can be controlled in a known manner by a microcontroller.

Special integrated circuits (ASICs) have been developed for these devices. In order to achieve a high number of pieces, these integrated circuits should be usable in different types of devices with a wide variety of sensor coils. This again requires the largest possible setting range with high resolution for the adjustment of the sensors, whereby also the tolerances of the shell cores are to be considered.

As it has been shown, the conventional 8-bit digital potentiometers are not sufficient for this purpose. Furthermore, it has been found that the characteristic curve can be decisively improved if, instead of the usual resistances staggered by powers of two, similar partial resistances are used.

The number of individual resistors and switching transistors used is directly proportional to the number of resistance steps implemented. A potentiometer with N steps (resolution ld (N) bits) usually has N x components. A 4-step (2-bit) potentiometer therefore has 4 x components. In addition to the demand for components, of course, the influence of parasitic effects such as capacity increases. Thus, for a digital potentiometer with 8 bit resolution, instead of 8 stepped resistors, 255 similar resistors are required.

What is possible for 8 bits and maybe even for 12 bits in an integrated circuit is no longer sensible at the latest with 16 bits with more than 65000 resistors.

The object of the invention is to improve the state of the art. In this case, a possibility should be shown that o. G. To solve the problem at least partially.

The essential idea of the invention is to use at least two differently graduated potentiometers instead of a single digital potentiometer and to operate them in parallel.

By dimensioning the resistors according to the invention, the resolution can be doubled in bits without increasing the complexity exponentially. The resistances within a branch can be the same or different. The selection of the resistors and the sequence of the control shown in the exemplary embodiment make it possible to improve the differential nonlinearity of the characteristic curve and its continuity.

In an advantageous embodiment, the two digital potentiometers are housed together with the oscillator circuit and the evaluation circuit in an ASIC.

The invention will be explained in more detail with reference to the drawing.

1 shows the proximity switch according to the invention with a digital potentiometer.

2 shows the digital potentiometer consisting of two part potentiometers in detail.

3 shows the resistance characteristic of the digital potentiometer according to the invention.

The 1 shows a proximity switch according to the invention with a sensor coil 1 , a resonant circuit capacitor 2 , an oscillator circuit 3 and an evaluation unit 4 with a digital potentiometer 5 , Although this is actually a variable resistor with two terminals, so a rheostat, the following is the more common term used potentiometer.

The digital potentiometer 5 consists of the two part potentiometers 5a and 5b , which are connected in parallel according to the invention. They belong to an integrated circuit 6 (ASIC), which also advantageous the oscillator circuit 3 and the evaluation unit 4 contains. The switching transistors T11 to T24 are from the evaluation unit 4 controlled.

Furthermore, one of the acting as a control unit evaluation unit 4 controlled switching transistor 7 shown, which optionally for bridging a dashed lines shown here in series with the potentiometer 5 arranged additional resistance (RNS) is used. Furthermore, there is an optional further connection for one parallel to the potentiometer 5 arranged additional resistance (RNP) shown.

Of course, individual or multiple modules can be removed from the ASIC and discretely built without departing from the spirit.

The 2 shows a digital potentiometer according to the invention in detail, respectively 4 Resistors per branch. Of course, the number of resistors can be increased to N. By selecting resistors R ₁₁ ... R _1N and R ₂₁ ... R _2N , the characteristic curve of the potentiometer can be decisively influenced. Thus, the resolution can be doubled in bits, without increasing the complexity of the arrangement exponentially, but also superfluous because doubly occupied switching stages are avoided and the location of the nodes optimized. The resistance values within a sub-potentiometer may also be different.

The 2 shows next to the switching transistor already described above 7 another optional switching transistor 8th also in the manner described above by the evaluation unit 4 can be controlled. The auxiliary resistors RNS and RNP shown in dashed lines are used for level adjustment to a downstream A / D converter or demodulator or for shifting the measuring range of z. B. 0..1 volts to 2..3 volts (window function) and thus to increase the measurement accuracy.

For the production of an integrated circuit (ASIC), it is advantageous to use only similar resistor modules. However, this creates the already mentioned double occupied switching stages. Therefore, the values should be in the two sub-potentiometers 5a and 5b better different and not be divisible. This problem can be solved by resistor networks of similar components.

Table 1 shows an embodiment of the gradation of the resistors in the digital potentiometers 5a and 5b with exponential progression, maximum continuity, and minimal differential nonlinearity. Advantageously, different non-divisible values of 24 kΩ for 5a and 14 kΩ for 5b selected. Therefore, as you can see, the support points are evenly distributed and the curve well adapted to an exponential function (see also 3 ).

In each of the two sub-potentiometers 5a and 5b in each case at most one transistor is conductive. Not to adjust the potentiometer 5 necessary transistors T11 and T21 serve to generate the same parallel capacitance for all partial resistors.

The switching sequence for a uniform curve is from the column 2 seen. The gap 5 shows the resulting resistance value for the potentiometer 5 ,

With only 6 Transistors, after all 16 realized different switching states Table 1 step Trs. conductive Poti 5a [KΩ] Poti 5b [KΩ] R [kΩ] 1 T12 and T22 24 14 8.84 2 T13 and T22 48 14 10.84 3 T14 and T22 72 14 11.72 4 only T22 96 14 12.22 5 T12 and T23 24 28 12.92 6 T12 and T24 24 42 15.27 7 only T12 24 56 16,80 8th T13 and T23 48 28 17.68 9 T14 and T23 72 28 20.16 10 only T23 96 28 21.68 11 T13 and T24 48 42 22.40 12 only T23 48 56 25.85 13 T14 and T24 72 42 26.53 14 only T24 96 42 29.22 15 only T14 72 56 31.50 16 all blocking 96 56 35.37

The 3 shows the characteristic of the digital potentiometer 5 according to Table 1. The triangles show the realized resistance values. The circles follow a desired exponential function R _exp = 8.7725 e ^{(0.0873 × step)} .

LIST OF REFERENCE NUMBERS

1

 sensor coil

2

 Resonant circuit capacitor

3

 oscillator circuit

4

 evaluation

5

Digital potentiometer for adjustment, consists of the Teilpotentiometern 5a and 5b

6

 Integrated electronic circuit (ASIC)

7

 Transistor switch for a serial additional resistor RNS

8th

 Transistor switch for a parallel additional resistor RNP

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 4331555 A1 [0005]

Claims (6)

Inductive proximity switch with a resonant circuit with a sensor coil ( 1 ) and a resonant circuit capacitor ( 2 ) generated by an oscillator circuit ( 3 ), and an evaluation unit ( 4 ) for controlling the oscillator circuit ( 3 ) and for generating a binary switching signal, wherein the oscillator amplitude with a digital potentiometer ( 5 ), characterized in that the digital potentiometer ( 5 ) two parallel electrically operated partial potentiometers ( 5a ) and ( 5b ), as well as one from the evaluation unit ( 4 ) controlled transistor switch ( 7 ) for bridging a serial additional resistance. Inductive proximity switch according to claim 1, characterized in that the digital potentiometers ( 5a ) and ( 5b ) as well as the controlled transistor switch ( 7 ) Part of an integrated electronic circuit ( 6 ) are. Inductive proximity switch according to claim 2, characterized in that the integrated electronic circuit ( 6 ) also the oscillator circuit ( 3 ) and the evaluation unit ( 4 ) contains. Inductive proximity switch according to one of the preceding claims, characterized in that the resistors within a digital Teilpotentiometers ( 5a ) or ( 5b ) have the same values, the digital partial potentiometers ( 5a ) and ( 5b ) but have different resistances whose values are not integer divisible. Inductive proximity switch according to claim 4, characterized in that the partial resistances in the potentiometers 5a and 5b Resistance networks have, consisting of the same basic building blocks. Integrated circuit ( 6 ) with an oscillator circuit ( 3 ) for supplying a resonant circuit and an evaluation unit ( 4 ) for controlling two digital potentiometers ( 5a . 5b ) and one serial to the digital potentiometers ( 5a . 5b ) arranged switchable additional resistance ( 7 ).

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