Field device electronics for a conductive limit-level switch

Abstract

A field device electronics for a conductive, limit-level switch, with a conductive probe and a measuring circuit including a control/evaluation circuit. The measuring circuit, a measuring bridge circuit is present, with at least one coupling capacitor (C2) being present for the DC voltage separation of the probe from the measuring circuit, and with the at least one coupling capacitor (C2) being integrated into the measuring bridge circuit.

Claims

1-12. (canceled)

13. A field device electronics for a conductive, limit-level switch, comprising: a conductive probe; a measuring circuit; and a control/evaluation circuit, wherein: said measuring circuit includes a measuring bridge circuit, and at least one coupling capacitor (C2) for the DC voltage separation of said conductive probe from said measuring circuit, said at least one coupling capacitor (C2) is integrated into said measuring bridge circuit.

14. The field device electronics as claimed in claim 13, wherein: at least one additional capacitor (C1) is arranged in said measuring bridge circuit, said at least one additional capacitor (C1) is arranged in said reference branch of said measuring bridge circuit, and the at least one coupling capacitor (C2) is arranged in said probe branch of said measuring bridge circuit.

15. The field device electronics as claimed in claim 14, wherein: tolerance-related deviations between said two capacitors (C1, C2) are compensated by a selective choosing of the point in time (P) for the measurement.

16. The field device electronics as claimed in claim 15, further comprising: a memory unit coupled with said control/evaluation circuit, in which the precise point in time (P) for the measurement is stored, wherein: the precise point in time (P) for the measurement for a dimensioned measuring circuit is acquired before start-up using simulation runs.

17. The field device electronics as claimed in claim 15, wherein: said control/evaluation circuit conducts measurements at multiple points in time grouped about the precise point in time (P) for the measurement, and from these measurements determines an average value.

18. The field device electronics as claimed in claim 17, wherein: the multiple points in time are grouped symmetrically about the precise point in time (P) for the measurement.

19. The field device electronics as claimed in claim 13, wherein: said measuring bridge circuit includes multiple, switchable reference resistors for expanding the measuring range.

20. The field device electronics as claimed in claim 19, wherein: switching is accomplished using semiconductor switches, which are activated from said control/evaluation circuit.

21. The field device electronics as claimed in claim 13, wherein: said control/evaluation circuit is implemented with a microprocessor circuit, wherein the microprocessor executes a generator function for the production of the measuring signal, and/or a measurement function for the evaluation of the measuring signal, and/or a range switching, and/or a comparator function (1.4), and/or a hysteresis function (1.5), and/or an output signal production (1.6).

22. The field device electronics as claimed in claim 21, wherein: the measurement is executed with separate rectangular-wave bursts, and an alterable pause period of time lies between two measurements.

23. The field device electronics as claimed in claim 22, wherein: the pause time can be adjusted by means of a random generator, which is part of said control/evaluation circuit.

24. The field device electronics as claimed in claims 21, wherein: during the pauses in measuring, the microprocessor is switched over to an energy-saving mode.

Description

[0020] The invention will now be explained in greater detail on the basis of the drawings, whose figures show as follows:

[0021] FIG. 1 a schematic illustration (block diagram) of the field device electronics;

[0022] FIG. 2 a schematic illustration of the signal curves.

[0023] As is apparent from FIG. 1, the field device electronics includes a microprocessor 1, a measuring bridge circuit 2, a measuring probe 3 in a container 4, and a memory unit 5. In the illustrated example of an embodiment, the microprocessor 1 assumes the following functions: [0024] the production of a rectangular signal U.sub.meas with the measuring frequency f1 (generator function 1.1), which is issued via Port 1 and fed to the bridge circuit directly or by way of an amplifier stage (not shown); [0025] a function for switching the desired measuring range (range switching 1.2), which, on the one hand, connects via digital ports (Port 2,3,4) the appropriate reference resistors 2.3 in the measuring bridge circuit 2, and on the other hand, sets switching thresholds and hystereses for the respective measuring ranges; [0026] an analog-digital conversion 1.7 of the bridge voltage U.sub.br measured via a differential amplifier 2.5; [0027] a measuring function 1.3, coupled with a comparator function 1.4, which conducts a comparison of the measured bridge voltage U.sub.br with predetermined threshold values, and forwards the result to a hysteresis function 1.5; [0028] production and issue of the desired output signal, 1.6, via a digital/analog convertor, or digital port, 1.8.

[0029] The measuring bridge circuit 2 includes: a reference branch 2.1, in which reference resistors 2.3 can be switched on, respectively off, using semiconductor switches 2.4, particularly MOSFETs, with the semiconductor switches 2.4 being driven on, or activated, by the microprocessor 1; a probe branch 2.2, into which the measuring probe 3 is switched in place of the reference resistors 2.3; and a difference amplifier 2.5, which measures the bridge voltage U.sub.br at the measuring bridge formed by the reference branch 2.1 and the probe branch 2.2. As is further apparent from the illustrated example of an embodiment, a coupling capacitor C2 is arranged in the probe branch, and an additional capacitor C1 is arranged in the reference branch. The measuring bridge circuit 2 is supplied with a rectangular measuring voltage U.sub.meas from the microprocessor 1 by means of the generator function 1.1, and delivers the measured bridge voltage U.sub.br to the measuring function 1.3 in the microprocessor 1.

[0030] In the illustrated example of an embodiment, a rectangular measuring signal is used. However, it is also possible to use for the measuring any other signal forms with defined harmonic content.

[0031] FIG. 2a shows plots of the voltages U.sub.meas and U.sub.br versus time for the case of the balanced measuring bridge (directly at the switching point; R.sub.meas=R.sub.ref) and for the case of being below the switching point (R.sub.meas<R.sub.ref). One can see that in the case of the balanced measuring bridge, the voltage difference between positive and negative half waves of U.sub.br disappears.

[0032] FIG. 2b shows the behavior of the voltage U.sub.br for different deviations of the capacitors C1, C2. At the point in time of the reversal of the measuring voltage U.sub.meas (t.sub.1, t.sub.2, t.sub.3, . . . ), the jump in voltage is always of equal size, whereas the slope of the curves depends on the deviation of the capacitors. In each case, the shape of the curve is a section of the charge/discharge curve of the RC combination formed by the measuring bridge. In FIG. 2b, it can be clearly recognized that in each case the curves intersect in a point P within the upper, respectively lower half-wave. This point in time t.sub.m can be exactly calculated for a given dimensioning of the measuring bridge circuit by means of a simulation program (e.g. Pspice). If the voltage is sampled precisely at this point in time t.sub.m, or, respectively, at multiple points in time (e.g. 20 points in time) grouped symmetrically within an interval about this point in time t.sub.m, then the same result is always attained independently of the deviation of the capacitors, wherein the interval about the point in time t.sub.m should not exceed one tenth of the period length. In this way, the tolerances and temperature behaviors of the utilized capacitors, up to deviations of circa 10%, as they occur in practice, can be compensated.