Oscillator and inductive proximity switch

Abstract

The invention relates to an oscillator comprising a resonant circuit having at least one inductor and one capacitor and having a feedback amplifier, wherein there is at least one trimming resistor in a feedback circuit of the amplifier. The oscillator is characterized in that there is an electronic switching device for alternately connecting and disconnecting the trimming resistor, in that a trimming circuit is formed by the trimming resistor and the electronic switching device, in that there is a drive device for driving the electronic switching device, in that the electronic switching device, together with the drive device, is designed to alternately connect and disconnect the trimming resistor in a switching sequence, and in that an average time value of the electrical resistor of the trimming circuit, which is active in an oscillator mode, is defined by the switching sequence.

Claims

1. An oscillator, comprising: a resonant circuit having at least one inductor and at least one capacitor and a feedback amplifier, wherein in at least one feedback circuit of the amplifier there is at least one trimming resistor, wherein there is an electronic switching device for alternately switching on and switching off the trimming resistor, a trimming circuit is formed by the trimming resistor and the electronic switching device, there is a control device to control the electronic switching device, the electronic switching device, together with the control device, is designed to alternately switch on and switch off the trimming resistor in a switching sequence, and a temporal mean value of the electrical resistance of the trimming circuit is defined by the switching sequence, which is effective during oscillator operation, wherein the temporal mean value of the electrical resistance of the trimming circuit is given by the value of the trimming resistor and by a ratio of a sum of the periods in which the trimming resistor is switched on to a sum of the periods in which the trimming resistor is switched off.

2. The oscillator according to claim 1, wherein it is designed for use in an inductive proximity switch.

3. The oscillator according to claim 1, wherein there are digital-electronic means for adjusting the switching sequence.

4. The oscillator according to claim 3, wherein the digital-electronic means for adjusting the switching sequence is a microcontroller.

5. The oscillator according to claim 1, wherein the temporal mean value of the electrical resistance of the trimming circuit is time-constant when the switching sequence is maintained.

6. The oscillator according to claim 1, wherein the temporal mean value of the electrical resistance of the trimming circuit can be changed by adjusting the switching sequence.

7. The oscillator according to claim 1, wherein the switching sequence is given by a pulse width modulation with a pulse width modulation frequency.

8. The oscillator according to claim 7, wherein the switching sequence is given by a pulse width modulation with a single pulse width modulation frequency.

9. The oscillator according to claim 1, wherein a frequency spectrum of the switching sequence, in comparison with a frequency spectrum of a pulse width modulation, is expanded by a single pulse width modulation frequency.

10. The oscillator according to claim 9, wherein a pulse width modulation frequency is swept, in particular continuously, within a variation interval, wherein the temporal mean value of the electrical resistance of the trimming circuit remains constant during sweeping of the pulse width modulation frequency.

11. The oscillator according to claim 9, wherein said pulse width modulation frequency is continuously swept within a variation interval, wherein the temporal mean value of the electrical resistance of the trimming circuit remains constant during sweeping of the pulse width modulation frequency.

12. The oscillator according to claim 9, wherein the switching sequence is an arbitrary sequence of switching on and switching off times, wherein the temporal mean value of the resistance of the trimming circuit is time-constant.

13. The oscillator according to claim 1, wherein a frequency spectrum of the switching sequence, in comparison with a frequency spectrum of a pulse width modulation, contains additional frequencies.

14. The oscillator according to claim 13, wherein a pulse width modulation frequency is swept, in particular continuously, within a variation interval, wherein the temporal mean value of the electrical resistance of the trimming circuit remains constant during sweeping of the pulse width modulation frequency.

15. The oscillator according to 13, wherein said pulse width modulation frequency is continuously swept within a variation interval, wherein the temporal mean value of the electrical resistance of the trimming circuit remains constant during sweeping of the pulse width modulation frequency.

16. The oscillator according to 13, wherein the switching sequence is an arbitrary sequence of switching on and switching off times, wherein the temporal mean value of the resistance of the trimming circuit is time-constant.

17. The oscillator according to claim 1, wherein at least one frequency of the switching sequence is greater than half a bandwidth of the resonant circuit resonance.

18. The oscillator according to claim 1, wherein at least one frequency of the switching sequence is greater than a total bandwidth of the resonant circuit resonance.

19. The oscillator according to claim 1, wherein at least one frequency of the switching sequence is greater than ten times the bandwidth of the resonant circuit resonance.

20. The oscillator according to claim 1, wherein at least one trimming circuit is connected in at least one of a positive feedback circuit and a negative feedback circuit of the amplifier.

21. The oscillator according to claim 1, wherein the trimming circuit is connected in one of parallel or series with a feedback resistor.

22. The oscillator according to claim 1, wherein the electronic switching device is connected in parallel with the trimming resistor and the trimming resistor can be switched off by short-circuit via the electronic switching device.

23. The oscillator according to claim 1, wherein the electronic switching device is connected in series with the trimming resistor and the trimming resistor can be switched off by opening the electronic switching device.

24. The oscillator according to claim 1, wherein the electronic switching device is an analog switch.

25. The oscillator according to claim 1, wherein the electronic switching device is a CMOS-based analog switch.

26. The oscillator according to claim 1, wherein the control device is a microcontroller.

27. The inductive proximity switch according to claim 26, which, to generate a detection signal, which indicates that a target is approaching, evaluates: a damping of the oscillator by the target to be detected, and at least one of: a change in the inductance of the resonant circuit by the target; or a change of a coupling of the inductor of the resonant circuit to a further inductor, in particular a receiving coil.

28. The inductive proximity switch according to claim 26, wherein the control device is designed to adjust a switching distance by changing the switching sequence.

29. The inductive proximity switch according to claim 26, wherein the control device, for providing a switching hysteresis, is designed to increase the switching distance after switching of the proximity switch due to a target approaching the proximity switch, and to decrease the switching distance after switching of the proximity switch due to the target moving away from the proximity switch.

30. The inductive proximity switch according to claim 29, wherein the control device, for providing a switching hysteresis, is designed, after switching of the proximity switch due to a target approaching the proximity switch, to increase the temporal mean value of the electrical resistance of the trimming circuit and, after switching of the proximity switch due to the target moving away from the proximity switch, to reduce the temporal mean value of the electrical resistance of the trimming circuit; or the control device for providing a switching hysteresis is designed, after switching of the proximity switch due to a target approaching the proximity switch, to reduce the temporal mean value of the electrical resistance of the trimming circuit and, after switching of the proximity switch due to the target moving away from the proximity switch, to increase the temporal mean value of the electrical resistance of the trimming circuit.

31. The oscillator according to claim 1, wherein the control device digitally controls the electronic switching device.

32. The oscillator according to claim 1, wherein there is a temperature measuring device for measuring the temperature of at least the resonant circuit and, based on a temperature measured by the temperature measuring device, the switching sequence can be changed to compensate for temperature drifts.

33. An inductive proximity switch having an oscillator according to claim 1.

34. An oscillator, comprising: a resonant circuit having at least one inductor and at least one capacitor and a feedback amplifier, wherein in at least one feedback circuit of the amplifier there is at least one trimming resistor, wherein there is an electronic switching device for alternately switching on and switching off the trimming resistor, a trimming circuit is formed by the trimming resistor and the electronic switching device, there is a control device to control the electronic switching device, the electronic switching device, together with the control device, is designed to alternately switch on and switch off the trimming resistor in a switching sequence, and a temporal mean value of the electrical resistance of the trimming circuit is defined by the switching sequence, which is effective during oscillator operation, wherein the temporal mean value of the electrical resistance of the trimming circuit is given by the value of the trimming resistor and by a ratio of a sum of the periods in which the trimming resistor is switched on to a sum of the periods in which the trimming resistor is switched off, and the temporal mean value of the electrical resistance of the trimming circuit is given by the value of the trimming resistor and the duty cycle of a pulse width modulation.

35. An inductive proximity switch having an oscillator according to claim 34.

Description

(1) Further advantages and features of the invention are explained below by reference to the attached figures, in which:

(2) FIG. 1 shows a first exemplary embodiment of an oscillator according to the invention;

(3) FIG. 2 shows a sub-circuit diagram to illustrate a current flowing in the trimming circuit;

(4) FIG. 3 shows a diagram in which the current in the trimming circuit is recorded against the frequency;

(5) FIG. 4 shows a second exemplary embodiment of an oscillator according to the invention;

(6) FIG. 5 shows a circuit variant, which can be used in the exemplary embodiment of FIG. 1 or the exemplary embodiment of FIG. 4;

(7) FIG. 6 shows a third exemplary embodiment of an oscillator according to the invention;

(8) FIG. 7 shows a schematic illustration of an inductive proximity switch according to the invention; and

(9) FIG. 8 shows an oscillator according to the prior art.

(10) The same or similar components are generally identified by the same reference symbols in all the drawings.

(11) A first exemplary embodiment of an oscillator 200 according to the invention is shown in FIG. 1. In comparison with the oscillator, shown in FIG. 8, of the prior art, in the circuit in FIG. 1 a trimming circuit 40 with a trimming resistor R2 and an electronic switching device 20 is connected in parallel with the resistor R1 in the positive feedback branch of the amplifier 10. The electronic switching device is controlled with a control device 30, which can for example be a microcontroller. The control device 30 controls the electronic switching device such that the trimming resistor R2 is alternatingly switched on and switched off in an sequence. According to the invention, through the switching sequence, a temporal mean value of the electrical resistance is thereby defined which is effective during an operation of the oscillator 200.

(12) For example the switching sequence can be given by a pulse width modulation with a single pulse width modulation frequency f.sub.PWM. With reference to FIGS. 2 and 3, this is explained in more detail. FIG. 2 illustrates the electrical variables current and voltage, at the trimming resistor R2. With Ohm's law and a Fourier analysis of the pulse width modulation, the current i.sub.R flowing through the trimming resistor R2 can be illustrated:

(13) $i_R=\left(\frac{U_O}{R}\right)\sin \left({}_{\mathrm{OSZ}}t\right)\left\{a_o+a_1\cos \left({}_{\mathrm{PWM}}t\right)+a_2\cos \left(2{}_{\mathrm{PWM}}t\right)+\mathrm{.Math.}\right\}$

(14) Here, U.sub.0 sin (.sub.OSZt) is the voltage supplied to the trimming resistor R2, R is the nominal value of the trimming resistor R2, .sub.OSZ is the circuit frequency of the resonance of the oscillator, .sub.PWM=2f.sub.PWM is the circuit frequency of the pulse width modulation and a.sub.0, a.sub.1, a.sub.2 are the Fourier coefficients of the pulse width modulation.

(15) The ratio of the period T.sub.on, during which the electronic switching device 20 is closed in a period duration T.sub.PWM, the trimming resistor R2 is thus switched on, and the period duration T.sub.PWM=2/.sub.PWM of the pulse width modulation is the duty cycle p=a.sub.0=T.sub.on/T.sub.PWM of the pulse width modulation.

(16) The current component oscillating with the resonance frequency f.sub.OSZ of the oscillator is:
i(f.sub.OSZ)=u.sub.0.sub.0/R=u.sub.0p/R

(17) This means that the temporal mean value of the electrical resistance of the trimming circuit 40, which can also be referred to as the effective resistance R.sub.eff, is given by:
R.sub.eff=u.sub.0/i(f.sub.OSZ)=R/p

(18) The situation in the frequency space is shown schematically in FIG. 3. Here, the current i.sub.R through the trimming resistor R2 is recorded against the frequency. The currency pattern corresponds to a typical resonance curve with a width B. At the resonance frequency f.sub.OSZ=.sub.OSZ/2, the current i.sub.R is maximal.

(19) Through pulse width modulation with the pulse width modulation frequency f.sub.PWM=.sub.PWM/2, sidebands in the current spectrum arise, which are spaced apart from the resonance frequency f.sub.OSZ by the value of the pulse width modulation frequency f.sub.PWM. In the illustration in FIG. 3, these sidebands are shown schematically by -type peaks with the frequencies f=f.sub.OSZ+/f.sub.PWM. With the frequencies f=f.sub.OSZ+/2f.sub.PWM, there are further, but weaker, sidebands, which belong to higher harmonics. The sidebands in the current spectrum generate a voltage drop at the resonant circuit, which can be seen as a slight modulation. These modulations can be extensively suppressed by a suitable choice of the pulse width modulation frequency f.sub.PWM. The pulse width modulation frequency should be selected to be so great that the sidebands do not fall within the bandwidth of the resonance curve. This results in the design requirement that the pulse width modulation frequency should, where possible, be selected to be significantly greater than half the bandwidth B, which is given by the resonance frequency of the oscillator divided by the resonant circuit quality Q:

(20) $f_{\mathrm{PWM}}\frac{B}{2}=f_{\mathrm{OSZ}}/Q$

(21) In the variant shown in FIG. 2, the electronic switching device 20 is connected in series with the trimming resistor R2. The trimming resistor R2 is accordingly switched on by closing the electronic switching device 20 and switched off by opening the switching device 20.

(22) Furthermore a trimming circuit 40 can also, as in the case of the oscillator 300 shown in FIG. 4, be connected in the negative feedback circuit. Through the resistors R5, R7 and the trimming circuit 40 with the resistor R6, the amplification of the amplifier 10 is adjusted in the example of FIG. 4. A pre-damping of the resonant circuit L, C is provided through the resistor R8. This brings with it the same advantages, in particular that the oscillator characteristic is not influenced by the trimming circuit 40, there are only negligible temperature influences, and, through the trimming circuit provided according to the invention, a trimming that is in principle strictly monotonic and constant can be carried out.

(23) FIG. 5 shows a third exemplary embodiment of an oscillator 400 according to the invention. This is a variant of the circuit shown in FIG. 4. As can be seen from FIG. 5, the trimming circuit 40 with the trimming resistor R6 is connected in parallel with the resistor R7 to earth. These circuit alternatives can be selected depending on expediency, in particular depending on desired adjusting possibility. In principle, it is also possible to connect the trimming device 40 in parallel with the capacitor C and the inductor L.

(24) In principle, variants are also possible, as shown in FIG. 6, wherein the electronic switching device 20 is connected in parallel with the trimming resistor R2. The trimming resistor R2 is then switched off, namely short-circuited (the resistance of the trimming circuit 40 is then given by the resistor R9) by closing the electronic switching device 20 and switched on by opening the electronic switching device 20.

(25) FIG. 7 shows in a schematic illustration an inductive proximity switch 600 according to the invention. This has an oscillator 500 according to the invention and an evaluation means 60. In the example shown, the evaluation means 60 is the same microcontroller that also acts as a control device 30 in the trimming circuit (not shown here) of the oscillator 500. The evaluation means 60 evaluates in a manner that is known in principle an oscillating state, in particular an oscillation amplitude of the oscillator 500. Depending on this evaluation, the evaluation means 60 outputs, at an output 80, one, in particular binary, switching signal. In the situation shown in FIG. 7, a target 70 is precisely at a switching distance d from the oscillator 500 as part of the proximity switch 600.

(26) A new type of oscillator is presented with the present invention, which is suited in particular for use in inductive proximity switches, and which is particularly easy to trim and wherein, above all, the trimming can be changed and newly adjusted at any time.

LIST OF REFERENCE NUMERALS

(27) 10 Amplifier 20 Electronic switching device 30 Control device, microcontroller 40 Trimming circuit 60 Evaluation means 70 Target 80 Switching output 100 Oscillator according to the prior art 200 Oscillator according to the invention 300 Oscillator according to the invention 400 Oscillator according to the invention 500 Oscillator according to the invention 600 Proximity switch B Bandwidth of the resonant circuit resonance d Switching distance f Frequency f.sub.OSZ Resonance frequency of the resonant circuit f.sub.PWM Frequency of a pulse width modulation i.sub.R Current through resistor R i.sub.R(f) Current as a function of frequency C Capacitor of the resonant circuit L Inductor of the resonant circuit R1 Resistor in the positive feedback circuit R2 Trimming resistor R3 Resistor in the negative feedback circuit, part of a voltage divider R4 Resistor in the negative feedback circuit, part of a voltage divider R5 Resistor in the negative feedback circuit, part of a voltage divider R6 Resistor of a trimming circuit 40 R7 Resistor in the negative feedback circuit, part of a voltage divider R8 Resistor in the positive feedback circuit R9 Resistor in the trimming circuit RT Adjustable resistor