Capacitive proximity switch for evaluating minor changes in capacitance and method therefor

• US 6,590,401 B1
• Filed: 04/02/2001
• Issued: 07/08/2003
• Est. Priority Date: 08/03/1998
• Status: Expired due to Fees

• Current Assignee: Pepperl + Fuchs GmbH
• Original Assignee: Pepperl+Fuchs, Inc. (Pepperl+Fuchs Se)
• Inventors: Hildebrandt, Andreas
• Primary Examiner(s): Oda, Christine
• Assistant Examiner(s): KERVEROS, DEMETRIOS C
• Application Number:
  US09/762,356
• Time in Patent Office: 827 Days
• Field of Search: 324/673, 324/658, 324/661, 324/662, 324/672, 324/679, 324/680, 324/686, 324/687, 324/690
• US Class Current: 324/673
• CPC Class Codes: G01V 3/088 operating with electric fields G01V3/082 takes precedence
• CPC Class Codes: H03K 17/955 using a capacitive detector

Abstract

Invention relates to a capacitive proximity switch with an electrical bridge circuit for detecting of an electrically conducting face (15) approaching or moving away, wherein the electrically conducting face (15) is part of a capacitor (for example C1) of the bridge circuit, wherein at least one capacitor (C1, C2, C3, C4) and possibly at least one resistor (R9, R10) are disposed in the bridge branches of the bridge circuit as further reactances, and wherein the bridge is subjected to an alternating voltage as a bridge feed voltage (ubr). The proximity switch includes a flat multilayer printed circuit board (10) having at least two electrically insulating layers (13, 14), wherein an electrically conducting intermediate layer (11) as a first face of a capacitor is disposed between the two electrically insulating layers (13, 14).

Claims

1. A capacitive proximity switch for the evaluation of capacitance changes with an electrical alternating current measurement bridge of four bridge branches, which reach every two of four switching knots of the bridge, wherein an alternating current is positioned as a bridge feed voltage (ubr) near two opposite, not neighboring switching knots, and respectively the two bridge branches between the two feed in switching knots build each of the two halves of the bridge, and there is an evaluated bridge diagonal voltage on the two remaining switching knots over diagonal voltage path (DSp1, DSp2), wherein the bridge diagonal voltage (Ud) is removable, for detecting an approaching or moving away face, in particular an electrically bad conductive face or metallic face, which face is part of variable capacitor (C1) in one of the bridge branches of the bridge circuit, wherein at least one capacitor(C1,C2,C3,C4)(capacitive bridge) or at least one capacitor(C1,C2,C3,C4) and/or one resistor (R9,R10) (capacitive ohmic bridge) is disposed in the remaining bridge branches as further reactances, wherein rectifiers (N1,N2,N3,N4,S1,S2,S3,S4) are arranged to the rectification of two bridge branch voltages (uc1,uc3) separately according to respective bridge halves in the diagonal voltage paths (DSp1, DSp2) and the diagonal bridge voltage (Ud) is evaluated only after the rectification of the two bridge branch voltages (uc1,uc3) as changing direct current corresponding to the capacitance change of the variable capacitor, characterized in that,

a) the proximity switch comprises a flat multilayer printed circuit board (10) of at least two electrically insulating layers (13,14), wherein an electrically conducting intermediate layer (11) as a first face of a capacitor in one of the two bridge branches of one of the bridge halves is disposed between the at least two electrically insulating layers (13,14)

b) at one of the layers (13,14) at the opposite side of the intermediate layer (11) a flat electrically conducting covering is placed onto one of the two layers (13,14) as a sensor (12), wherein the electrically conducting covering forms the second face of the capacitor

c) the face (15) is mobile relative to the sensor (12) and the face (15) forms together with the sensor (12) a flat variable capacitor of the bridge circuit.

2. Proximity switch according to claim 1 characterized in that the recifiers in the diagonal voltage paths (DSp1, DSp2) either diodes (N1, N2, N3, N4) or controlled switches (S1, S2, S3, S4), are preferably four in each case.

3. Proximity switch according to claim 2 characterized in that the switches (S1,S2,S3,S4) are opened and closed synchronously with the bridge feed voltage (ubr) pairwise (S1,S2,S3,S4) by means of control voltage (Ust1,Ust2) as well as opposite phase controlled and are switched synchronously with the bridge feed voltage (ubr) from one switching state into the other switching state.

4. Proximity switch according to claim 3, characterized in that in each case a low pass filters (R3, C8, and R4, C7) are connected and switched in front of the rectifires (N1, N2, N3, N4, or, respectively, S1,S2,S3,S4) of the bridge branch voltages (uc1, uc3).

5. Proximity switch according to claim 4, characterized in that the capacitances of the low pass filters (C8 or, resecptively C7) are formed by the parasitic capacitances of the rectifier elements (N1, N2, N3, N4 or, respectively,S1, S2, S3, S4).

6. Proximity switch according to claim 2, characterized in that two transfer switches (W1, W2) are employed as switches for rectifying of the bridge branch voltages (uc1, uc3).

7. Proximity switch according to claim 1, characterized in that the capacitor face formed by the electrically conducting intermediate layer (11) is replaced by a discrete capacitor, wherein the discrete capacitor is connected with a connection to the sensor (12).

8. Proximity switch according to claim 2, characterized in that the diodes (N1, N2, N3, N4) are slow diodes for proving the stability against interferences.

9. A method for the evaluation of capacitance changes with an electrical alternating current measurement bridge of four bridge branches, which reach every two of four switching knots of the bridge, wherein an alternating current is positioned as a bridge feed voltage (ubr) near two opposite, not neighboring switching knots, and respectively the two bridge branches between the two feed in switching knots build each of the two halves of the bridge, and there is an evaluated bridge diagonal voltage on the two remaining switching knots over diagonal voltage path (DSp1, DSp2), wherein the bridge diagonal voltage (Ud) is removed, for detecting an approaching or moving away face, in particular an electrically bad conductive face or metallic face, which face is part of variable capacitor (C1) in one of the bridge branches of the bridge circuit, wherein at least one capacitor(C1,C2,C3,C4) (capacitive bridge) or at least one capacitor(C1,C2,C3,C4) and/or one resistor (R9,R10) (capacitive ohmic bridge) is disposed in the remaining bridge branches as further reactances, wherein the bridge diagonal voltage (Ud) is evaluated only after rectification of both bridge branch voltages (uc1,uc3) as changing direct current corresponding to the capacitance change of the variable capacitor, characterized in that,

a) the two bridge branch voltages(uc1, uc3) are rectified separately according to the respective bridge half either by four diodes(N1,N2,N3,N4) or by four controlled switches(S1,S2,S3,S4) as rectifiers in the diagonal voltage paths(Dsp1,Dsp2)

b) the bridge diagonal voltage is driven synchronously with the bridge feed voltage (ubr) pairwise (S1,S2,S3,S4) upon employment of switches which are opened and closed synchronously with the bridge feed voltage pairwise by means of control voltage (Ust1,Ust2) as well as opposite phase controlled and are switched synchronously with the bridge feed voltage from one switching state into the other switching state

c) the frequency of the bridge feed voltage (ubr) and thereby also of the transfer switching signal of the switches (S1,S2,S3,S4) is changed continuously.

10. Method according to claim 9 characterized in that a capacitance of a variable capacitor (C2, C3, C4) is changed such in one of the bridge branches for balancing of the bridge and for setting of the zero point of the bridge diagonal voltage (Ud), that the bridge diagonal voltage (Ud) is equal to zero in the switching point of the proximity switch and only the sign of the bridge diagonal voltage (Ud) is evaluated.

11. Method according to claim 9 characterized in that two of the rectifier elements(N1, N2, N3, N4, S1, S2, S3, S4) are connected to with their one connector in each case to a reference voltage source (Uref1, Uref2) for setting of the zero point of the bridge diagonal voltage (Ud), wherein the respective value of the reference voltage source (Uref1, Uref2) is adjusted such that the desired bridge diagonal voltage (Ud) of the zero volts (0V) is set at the output.

12. Method according to claim 11 characterized in that the two reference voltages (Uref1, Uref2) are derived and rectified from the bridge feed voltage (ubr) for the balancing of the bridge such that a linear relationship exists between the amplitude of the bridge feed voltage (ubr) and the respective reference voltage (Uref1, Uref2).

13. Method according to claim 12, characterized in that the reference voltages (Uref1, Uref2) is set to the desired values with the aid of in each case a voltage divider.

14. A capacitive proximity switch for the evaluation of capacitance changes comprising:

an electrically conducting intermediate layer (11) forming a first face of a fixed capacitor;

an electrically insulating upper layer (13) disposed at an upper side of the electrically conducting intermediate layer (11);

a flat electrically conducting covering placed onto the electrically insulating upper layer (13) for forming a sensor (12), wherein the electrically conducting covering forms a second face of the fixed capacitor and wherein the electrically conducting covering forms a first face of a flat variable capacitor (C1);

an electrically insulating lower layer (14) disposed at a lower side of the intermediate layer (11), wherein the electrically conducting intermediate layer (11), the electrically insulating upper layer (13), the flat electrically conducting covering, and the electrically insulating lower layer (14) form a flat multilayer printed circuit board (10);

a mobile face (15), wherein the mobile face (15) is movable relative to the sensor (12) and wherein the mobile face (15) together with the sensor (12) forms the flat variable capacitor (C1) of the bridge circuit;

an electrical alternating current measurement bridge circuit of four bridge branches, wherein a first bridge branch includes the variable capacitor (C1) and wherein a second bridge branch includes the fixed capacitor, and wherein the bridge branches each reach two of four switching knots of the electrical alternating current measurement bridge circuit;

a current source furnishing an alternating current and positioned as a bridge feed voltage (ubr) near two opposite, not neighboring feed-in switching knots, and wherein two bridge branches between the two feed-in switching knots form each of two halves of the electrical alternating current measurement bridge, and wherein an evaluated bridge diagonal voltage is present on two remaining switching knots over a diagonal voltage path (DSp1, DSp2), wherein the bridge diagonal voltage (Ud) is picked up for detecting an approaching or moving away mobile face (15), and in particular of an electrically bad conductive face or metallic face, which mobile face (15) is part of the variable capacitor (C1) in the first bridge branch of the electrical alternating current measurement bridge circuit;

at least one capacitor(C1,C2,C3,C4)(capacitive bridge) or at least one capacitor(C1,C2,C3,C4) and one resistor (R9,R10) (capacitive ohmic bridge) disposed in the remaining bridge branches not containing the variable capacitor (C1) as further reactance;

rectifiers (N1,N2,N3,N4,S1,S2,S3,S4) arranged and connected to the electrical alternating current measurement bridge for furnishing a rectification of the two bridge branch voltages (uc1,uc3) separately according to respective bridge halves in the diagonal voltage paths(DSp1, DSp2) and wherein a diagonal bridge voltage (Ud) is evaluated only after the rectification of the two bridge branch voltages (uc1,uc3) as changing direct current corresponding to the capacitance change of the variable capacitor (C1).

15. (new) A method for evaluating changes of capacitance changes comprising

employtg an electrical alternating cuntnt measurement bridge of four bddge branches, which bridge branches reach every two of four switching knots of the bridge;

using an alternating current and positioning the alternating current as a bridge feed voltage (ubr) near two opposite, not neighboring switching knots, and respectively the two bridge branches between the two feed in switching knots build each of the two halves of the bridge;

placing mi evaluation bridge diagonal voltage on two remaining switching knots over a diagonal voltage pat (DSp1, DSp2);

removing the bridge diagonal voltage (Ud) for detecting an approaching or moving away face, which face is part of variable capacitor (C1) in one of the bridge branches of the bridge circuit;

disposing at least one capacitor(CI,C2,C3,C4)(capacitive bridge) or at least one capacitor(C1,C2,C3,C4) and/or one resistor (R9,R10) (capacitive ohmic bridge) in remaining bridge branches as &

rther reactances;

rectifying separately two bridge branch voltages(ucl,uc3) according to the respective bridge half either by four diodes(NI ,N2,N3,N4) or by four controlled switches(S1,S2,S3,S4) disposed as rectifiers in the diagonal voltage paths(Dsp1,Dsp2);

evaluatiiu a bridge diagonal voltage (Ud) only afier rectification of the two bridge branch voltages (ucl,uc3) as changing direct current corresponding to a capacitance change of the variable capacitor;

synchromusly driving the bridge diagonal voltage with the bridge feed voltage (ubr) pairwise (S1,S2,S3.S4) upon employment of switches;

opening and closing the switches synchronously with the bridge feed voltage psirwise by means of control voltage (Ust1,Ust2);

controlling with an opposite phase and switching synchronously the switches with the bridge feed voltage (ubi) from one switching state into the other switching state; and

continuously changing the frequency of the bridge feed voltage (ubr) and thereby also of the transfer switching signal of the switches (S1,S2,S3,S4).

16. The method according to claim 15 further comprising changing a capacitance of a variable capacitor (C2, C3, C4) such in one of the bridge branches for balancing of the bridge and for setdng of the zero point of the bridge diagonal voltage (Ud), that the bridge diagonal voltage (Ud) is equal to zero in the switching point of the proximity switch and only the sign of the bridge diagonal voltage (Ud) is evaluated.

17. The method according to claim 15 further comprising

connecting two of the rectifier elements(N1, N2, N3, N4, S1, S2, S3, S4) with their one connector in each case to a reference voltage source (Uref1, Uref2) for setting of the zero point of the bridge diagonal voltage (Ud); and adjusting the respective value of the reference voltage source (Ureff, Uref2) such that the desired bridge diagonal voltage (lid) of tern volts (OV) is set at an output.

18. The method according to claim 17 further comprising deriving and rectifying the two reference voltages (Urefl, Uref2) from the bridge feed voltage (ubr) for the balancing of the bridge such that a linear relationship exists between the amplitude of the bridge feed voltage (ubr) and the respective reference voltage (Uref1, Uref2).

19. The method according to claim 18 further comprising setting the reference voltages (Uref1, Uref2) to desired values with the aid of in each case a voltage divider.