Method and device for detecting an object hidden behind an article

1. A method for detecting an object hidden behind an article, comprising the steps

applying a first alternating voltage to a first sensor,

applying a second alternating voltage to a second sensor arranged adjacent to the first sensor,

determining a distance-dependent function of an effect of the article on at least one of the alternating voltages,

determining a change of a distance-dependency of the distance-dependent function on a movement of the first sensor and the second sensor along the article,

detecting the object dependent upon the change of the distance-dependency of the distance-dependent function.

2. The method according to claim 1 , comprising

generating a measuring alternating voltage,

filtering the measuring alternating voltage by means of a filter effecting a phase shift and thereby

generating the first alternating voltage.

3. The method according to claim 2 , comprising

applying the measuring alternating voltage to a measuring voltage divider,

tapping the second alternating voltage at the measuring voltage divider for applying the second alternating voltage to the second sensor.

4. The method according to claim 3 , wherein the filter has a low or unequal impedance as compared with the measuring voltage divider.

5. The method according to claim 2 , comprising

generating a reference alternating voltage phase-shifted by 180° to the measuring alternating voltage,

applying the reference alternating voltage to a reference path connected in parallel with the first and second sensors.

6. The method according to claim 5 , wherein a measuring path comprising the first and second sensors, the filter and a measuring voltage divider is connected in parallel with the reference path, wherein the reference path is constructed analogously to the measuring path.

7. The method according to claim 6 , comprising

adding a reference output signal of the reference path with a primary measurement output signal of the measuring path to a secondary measurement output signal.

8. The method according to claim 7 , comprising

setting a target value for the secondary measurement output signal,

influencing the secondary measurement output signal dependent upon the target value by means of a control signal of a controller,

evaluating the control signal for determining the distance-dependent function and the change of the distance-dependent function.

9. The method according to claim 8 , wherein it comprises at least one of the following steps

determining a course of the control signal dependent upon the distance of the first and second sensors from the article

determining the course in the form of a vector analysis in a phase space of the control signal

determining the course during an approach of the first and second sensors to the article before a detection of the object.

10. The method according to claim 9 , comprising

determining an area in the phase space delimited by the course or a vector,

determining the change as a function of the area.

11. The method according to claim 9 , comprising

pre-setting a threshold value for the course,

comparing the control signal with the threshold value,

detecting the object as soon as the control signal deviates so far from the course that the threshold value is exceeded.

12. The method according to one claim 1 , comprising

determining a current distance-dependent function occurring during the movement of the first and second sensors along the article and a change of the current distance-dependent function as compared with the distance-dependent function,

detecting the object dependent upon the change of the current distance-dependent function as compared with the distance-dependent function.

13. The method according to claim 1 , comprising

storing the distance-dependent function as determined,

comparing the stored distance-dependent function with a current effect occurring during the movement of the first and second sensors along the article to determine the change,

detecting the object dependent upon the comparing.

14. A measuring arrangement for detecting an object hidden behind an article, comprising:

a first sensor to which a first alternating voltage is appliable,

a second sensor arranged adjacent to the first sensor, to which a second alternating voltage is appliable, and

an analysis apparatus by configured to perform

determining a distance-dependent function of an effect of the article on at least one of the alternating voltages,

determining a change of a distance-dependency of the distance-dependent function on a movement of the first sensor and the second sensors along, and

detecting the object dependent upon the change of the distance-dependency of the distance-dependent function.

15. The measuring arrangement according to claim 14 , comprising

a circuit arrangement comprising the first and second sensors, an input side and an output side,

a controller connected upstream of the analysis apparatus and downstream of the circuit arrangement on the input side and connected in parallel on the output side, wherein a regulating behaviour of the controller for determining at least one of the distance-dependent function or the change of the distance-dependent function is analysable by means of the analysis apparatus.