Method and device for analog-to-digital conversion, and electrical network

Abstract

The invention relates to a method for analog-to-digital conversion of an analog input signal, which is at least essentially continuous and which has a useful signal that is superimposed with at least two interference signals having different frequencies, into a digital output signal, wherein the input signal is sampled in a limited measuring cycle, and wherein the number and points in time of multiple sampling points within the measuring cycle are determined as a function of a frequency of the input signal. It is provided that the sampling points (are determined as a function of the frequencies of the interference signals.

Claims

1. A method for analog-to-digital conversion of an analog input signal, which is at least essentially continuous and which has a useful signal that is superimposed with at least two interference signals having different frequencies, into a digital output signal, comprising: sampling the input signal in a limited measuring cycle, wherein the number and points in time of multiple sampling points within the measuring cycle are determined as a function of a frequency of the input signal, wherein the sampling points are determined as a function of frequencies of the interference signals.

2. The method according to claim 1, wherein at least a first and a second sampling points are determined as a function of the frequency of the interference signal having a lower relative frequency, and at least a third and a fourth sampling points are determined as a function of the frequency of the interference signal having a higher relative frequency.

3. The method according to claim 2, wherein at least four sampling points are selected in a measuring cycle, of which the first sampling point and the second sampling point are selected at a distance from a one-half period duration and at least one whole period duration of the interference signal having the lower relative frequency.

4. The method according to claim 2, wherein the third and fourth sampling points selected as a function of the interference signal having the higher relative frequency are determined as a function of the first and second sampling points.

5. The method according to claim 2, the third sampling point is placed at a one-half period duration of the interference signal having the higher relative frequency, before or after the first sampling point, and the fourth sampling point is placed at a one-half period duration of the interference signal having the higher relative frequency, before or after the second sampling point.

6. The method according to claim 2, wherein a fifth sampling point is placed at a period duration of the interference signal having the higher relative frequency, before or after the third or fourth sampling point.

7. The method according to claim 1, wherein the frequencies of the interference signals are determined in advance.

8. A device for carrying out the method according to claim 1, comprising a control unit that is specially adapted to carry out the method according to claim 1 under normal conditions of use.

9. An electrical network for a motor vehicle, comprising a device according to claim 8.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages and preferred features and feature combinations result from the above discussion and from the claims. The invention is explained in greater detail below with reference to the drawings, in which:

(2) FIG. 1 shows a diagram for explaining an advantageous method for operating an analog-to-digital conversion.

DETAILED DESCRIPTION OF THE INVENTION

(3) FIG. 1 shows by way of example a simplified diagram of the patterns of two interference signals S1, S2, which have different frequencies and are superimposed on a useful signal.

(4) It is assumed that the interference signals S1 and S2 are superimposed on a useful signal, for example the voltage signal of an energy store of a motor vehicle, and together with the useful signal form the input signal to be sampled. For the sake of simplicity, the interference signals S1 and S2 are illustrated one above the other in FIG. 1. The interference signals S1 and S2 are provided as periodic, in the present case sinusoidal, signals, in particular continuous over time and having a known frequency. For this purpose, the interference signals S1 and S2 are, for example, detected or calculated in the system in advance. In the present case, the interference signal S1 has a frequency of 33 hertz, and the interference signal S2 has a frequency of 100 hertz. The interference amplitude in each case is 240 mV, and the useful signal is 3.7 V.

(5) The input signal is sampled for the analog-to-digital conversion, wherein a maximum of four sampling points are available within a measuring window or measuring cycle. In the present case, a measuring cycle spans 100 ms, or alternatively, 80 ms. Whereas equidistant sampling points have been used in the past, characterized by five horizontal first lines I_1 through I_5 for the measuring cycle of 100 ms, and for the measuring cycle of 80 ms only four equidistant sampling points have been used, it has been shown that this results in a relatively large measuring error in the evaluation or in the analog-to-digital conversion. By simply increasing the number of sampling points it would be possible to easily reduce the measuring error in each case; however, this is possible only under certain conditions that are specified by the processing system. Thus, for example, in the traction network of a motor vehicle, detection of more than five sampling points in the stated time interval of 100 ms, or of more than four sampling points in the shorter time interval of 80 ms, of the measuring cycle is not possible.

(6) However, to still reduce the measuring error, the advantageous method provides that the sampling points are determined as a function of the frequencies of the interference signals S1 and S2. A first sampling point A1 and a second sampling point A2 are specified as a function of the low-frequency interference signal S1. The distance between the sampling points A1 and A2 is selected as a function of the period duration or the frequency of the interference signal S1 in such a way that the distance is one-half a period and at least one whole period. This may be described as follows:
x.sub.1=iT.sub.1+niT.sub.1,
where x.sub.1 is the distance between the sampling points A1 and A2, T.sub.1 is the period duration of the low-frequency interference signal S1, and n is an integer (0, 1, 2, 3, 4, . . . ). As a result, for example when the sampling point A1 is at the minimum of the interference signal S1, as shown in FIG. 1, the second sampling point A2 is at a maximum.

(7) The two remaining sampling points A3 and A4 of the total of preferably four sampling points are specified as a function of the high-frequency interference signal S2, thus, as a function of the position of the sampling points A1 and A2:

(8) The third sampling point A3 is placed chronologically before or after (in FIG. 1, before) the first sampling point A1, in particular at a distance of a one-half period duration T.sub.2 of the high-frequency interference signal S2. Thus, the distance between the sampling points A3 and A4 is a whole period duration of the interference signal S2, so that, for example, as shown in FIG. 1, they are each at the minimum of the interference signal S2.

(9) The fourth sampling point A4 is placed at a one-half period duration T.sub.2 of the interference signal S2 before the second sampling point A2. The fourth sampling point A4 is thus at a distance of a one-half period duration and multiple whole period durations T.sub.2 from the sampling point A3, so that it is at a maximum when the sampling point A3 is at a minimum of the interference signal S2. This distance x.sub.2 may be described as follows:
x.sub.2=T.sub.2+nT.sub.2,
where x.sub.2 is the distance between the sampling points A3 and A4 of the second interference signal S2, T.sub.2 is the period duration of the second interference signal S2, and n is an integer (0, 1, 2, 3, 4, 5, . . . ).

(10) Optionally, a fifth sampling point A5 is placed at a period duration T.sub.2 from the second interference signal S2, before or after the third or fourth sampling point (in the present exemplary embodiment, before). For a measuring cycle of 100 ms, five sampling points are preferably placed, and for a measuring cycle of 80 ms, only four sampling points are placed.

(11) It has been shown that measuring error in the sampling of the interference signals S1, S2 may thus be significantly reduced. Thus, for example, the measuring error of the low-frequency signal S1 is reducible by 13 mV, and the measuring error of the high-frequency interference signal S2 is reducible by 192 mV. This results in a more accurate and reliable analog-to-digital conversion of the input signal.

(12) In particular, this method is carried out in or by a battery cell controller that monitors the charging voltage of the battery cell. Particularly accurate detection of the charging voltage of the battery cell is thus possible. However, the method may also be used for any other application of an analog-to-digital conversion.

LIST OF REFERENCE SYMBOLS

(13) S1 interference signal S2 interference signal I_1 line I_2 line I_3 line I_4 line I_5 line A1 sampling point A2 sampling point A3 sampling point A4 sampling point A5 sampling point