Inductive distance sensor

Inductive adjustment of the core-fitted coil (16) of an eddy-current distance sensor (11) can be avoided if the coil (16) fills a physically predetermined annular space between the winding support (17) of the plastic coil former (15) and its flanges (18-19) as well as the wall (21) of a pot (20) composed of ferromagnetic material, which is placed over the rearward, smaller flange (18) and whose end face (21) rests against an annular area of the front, larger flange (19) with the same diameter as the pot (20). The ferromagnetic coil core (31) which engages coaxially in the winding support (17) can therefore be arranged on the base (24) in the pot (20) and can therefore be mounted as part of the pot (20).

BACKGROUND

The invention relates to an inductive distance sensor.

One such distance sensor is known from DE 1 03 28 122 A1.

The core-fitted coil of an inductive distance sensor such as this is fed with a medium frequency from an oscillating circuit, which causes eddy-current losses in the ferromagnetic environment. This leads to the amplitude and phase of the feed frequency being influenced when the ferromagnetic mass changes as a result of moving toward or away from a ferromagnetic object, such as a wing of a door which is fitted locally with iron. A distance sensor such as this can therefore be used not only as a limit switch but also as a high-resolution distance measurement device. However, time-consuming inductive adjustment, which is susceptible to errors, is required for such use as a measurement system.

In order to restrict such adjustment, which is potentially subject to errors, and measurement errors caused in this way, provision is made essentially in the abovementioned prior publication for the inner wall diameter, which is larger than the core, of the winding support to taper resiliently at various circumferential positions on its internal casing surface, as a result of which the core which is introduced therebetween is always centered in the winding support, independently of its cylinder diameter.

However, fine adjustment is normally essential by linear or helical axial movement and final fixing of the core in the winding support. In this case, it is disadvantageous for series production that, in the case of a coil which is arranged in a ferromagnetic pot, the adjustment is achieved, despite the same coil characteristic data, only at axial positions of the core in its winding support which differ from one coil to another.

SUMMARY

Against the background of these circumstances, the present invention is based on the technical problem of reducing the adjustment effort and of allowing easier assembly for an inductive distance sensor of this generic type, in the interest of both higher measurement accuracy and a higher production rate.

According to an aspect of the invention, two flanges which are in the form of annular disks and are located at the ends of the tubular winding support have slightly different external diameters. The external diameter of the coil to be fitted to the winding support is thus restricted as a maximum to the external diameter of the smaller flange. This corresponds to the internal diameter of a pot, which is placed over the winding from the smaller flange, that is composed of ferromagnetic material and whose open end edge then rests against the inner surface of the larger flange, facing the coil. The geometrically precisely defined annular space between the flanges and between the winding support and the pot wall is therefore filled with the coil. Surprisingly, it has been found that, with this configuration, the axial position of the core in the winding support is in practice no longer critical, based on the spatial filling factor of the coil. It therefore now no longer needs to be screwed in, in a similar manner to a threaded bolt with an external thread, to a greater or lesser depth into the winding support of the coil former; according to the invention, it is sufficient for a core with a smooth outer casing surface to be held in the winding support. This is done most easily by the core being mounted or formed with one end at the center of the pot base, and thus being inserted into the interior of the tubular winding support when the pot is placed over the coil.

Additional developments of and alternatives to the solution according to the invention are specified in the further claims and their advantages will become evident from the following description of one preferred exemplary embodiment of the invention, which is sketched in abstracted form, restricted to the essential functions, and not entirely to scale in the drawing. The single figure of the drawing uses an axial longitudinal section to show the design of an inductive eddy-current distance sensor designed according to the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

As shown inFIG. 1, the sketched inductively operating distance sensor11has a printed circuit board13, which is approximately in the form of a circular disk and has an electronic oscillating circuit14, in a hollow-cylindrical surrounding housing12, if required including a power supply and a measurement evaluation circuit. The oscillating circuit14feeds a coil16, which is fitted axially in front of a coil former15composed of plastic, with a medium frequency (in the single-digit to two-digital kilohertz range). The coil16is wound onto a tubular winding support17, whose two ends are equipped with flanges18,19in the form of annular disks. The front (19) of these flanges has an external diameter which is the same as the internal diameter of the surrounding housing12, and the rearward (18) of these flanges has an external diameter which is somewhat smaller than this. This results in a maximum winding volume which is predetermined by the design and is defined axially by the length of the winding support between its flanges18-19, and radially by the external diameter of the rearward, smaller flange18. The coil16always extends precisely between the mutually facing inner surfaces18′-19′ of the flanges18,19, and from the external diameter of the winding support17at most to the external diameter of the rearward flange18.

The coil former15which has been fitted with the coil16in this way is accommodated in a pot20composed of ferromagnetic material. Its hollow-cylindrical wall21has an external diameter which is the same as the internal diameter of the surrounding housing12, and it has an internal diameter which is at most slightly larger than the external diameter of the smaller flange18. The pot20can therefore be placed axially from the rear over the coil former15which has been fitted with the coil16, until the wall end edge22comes to rest axially against the outer annular area of the front, larger flange inner surface19′. Since the external diameter of the front flange19has the same diameter as the pot wall21, the coil former15together with the pot20can be closed at the front, on the measurement side, flush with the end23of the distance sensor surrounding housing12.

In the interest of a greater mass, specifically because the pot20has a wall21which is axially longer than the winding support17, its base24does not rest directly behind the rearward flange18against it, but can be kept at a distance from it by means of pillars25which are provided (integrally formed or fitted) at the rear to the smaller flange18, parallel to the longitudinal axis of the surrounding housing12. The pillars25are equipped with longitudinal channels26through which—and corresponding holes27in the pot base24—wire ends or connecting wires of the coil16are passed to the printed circuit board13. The latter is connected by means of a line routing28through a hollow connecting stub29, which is narrower than the surrounding housing12, to a connecting plug30, which is accessible at the rear, for the operation of the circuits on the printed circuit board13.

The precise axial position of a ferromagnetic core31in the winding support17has become non-critical because of the axially fixed and radially maximum predetermined volume of the coil16between the flanges18-19within the pot20, and there is now generally no longer any need to adjust the position with respect to the distance between the core31and the plane of the measurement-side end23of the surrounding housing12, and therefore for inductive calibration of the distance sensor11. It has therefore become possible to place (to integrally form or to fit) the core31in front of the center of the pot base24, in order at the same time to insert the core31coaxially into the winding support17in the course of axially placing the pot20over the populated coil former15. The core31therefore no longer needs to be fixed therein separately in a force-fitting or interlocking manner.

Inductive adjustment of the core-fitted coil16of an eddy-current distance sensor11can thus be avoided if, according to the invention, the coil16fills a physically predetermined annular space between the winding support17of the plastic coil former15and its flanges18-19as well as the wall21of a pot20composed of ferromagnetic material, which is placed over the rearward, smaller flange18and whose end face21rests against an annular area of the front, larger flange19with the same diameter as the pot20. The ferromagnetic coil core31which engages coaxially in the winding support17can therefore be arranged on the base24in the pot20and can therefore be mounted as part of the pot20.

LIST OF REFERENCE SYMBOLS