Inductive proximity switch based on a transformer coupling factor principle

Inductive proximity switches are sensors that react contact-free, i.e. without direct contact, to the approach of a metal or non-metal object, a target. Two basic principles are known when it comes to structuring inductive proximity switches.

In one case, the core of such a sensor is a current-carrying coil that generates an alternating magnetic field. The alternating current is generated by an oscillator. When a metal object, trigger or target enters this magnetic field,eddy-current losses occur that are caused by the target, and the oscillating circuit is dampened, as a result of which the amplitude of oscillation changes. This change, when electrically amplified, can be used as a measured quantity, for example, forthe distance of the target from the coil. Due to a hysteresis effect, the measured quantity when the target moves in the direction of the proximity switch differs from the measured quantity when it moves in the opposite direction. This configurationhas fundamental disadvantages, namely, it involves a usually complex construction, a ferrite and a coil winding as well as high tolerance requirements, along with greater switching distances. By the same token, the weld strength is inadequate becausethe saturation of the ferrite leads to errors.

In another case, instead of one coil, the newer proximity switches use a transformer with a primary coil and a secondary coil that are inductively coupled. The magnitude of the coupling between the primary and secondary coils is referred to asthe coupling factor, which can normally be set between 0 (no coupling) and 1 (perfect coupling), whereby the coupling factor K determines the magnitude of the mutual inductivity M of the circuit. A target brought into the switching range of theproximity switch changes the coupling. The coupling evaluation avoids the above-mentioned disadvantages; however, in the prior art, it, in turn, still has the drawback that it is not easy to implement because of the lower signal level.

When it comes to inductive proximity switches, the aim is for the switching distance of a proximity switch at which it responds when a target approaches to be uncritical with respect to external electrical disturbances and especially also for itto be insensitive to being installed in a machine part and to its installation conditions. By the same token, the switching distance should remain constant at different operating temperatures. This can best be achieved with inductive proximity switches based on the transformer couplingprinciple. Alternatively, it is desired for the proximity switch--at a different reduction factor--to be able to discern the type of metal of which the triggering target is made.

Moreover, German patent application DE 103 18 350 B3 describes a ferriteless inductive proximity switch a transmitting coil and having a receiving coil arranged in the alternating magnetic field of said transmitting coil in such a way that themagnetic flux impressed by the alternating magnetic field in the receiving coil is zero or almost zero in the switching position or resting position of the proximity switch, whereby the coils are arranged adjacently offset relative to each other, so thatthe field lines that emerge from the coil surface of the transmitting coil and that penetrate the coil surface of the receiving coil in one direction also penetrate the coil surface of the receiving coil in the opposite direction. The receiving coil hasan annular coil surface area, whereby the transmitting coil has a circular coil surface area whose periphery is overlapped all the way around by the receiving coil. At least one coil is formed by a spiral-shaped printed conductor of a circuit board. The coil surface of the receiving coil is arranged with respect to the alternating magnetic field in such a way that the field lines stemming from the coil surface and passing through the coil surface of the receiving coil also return again through thecoil surface of the receiving coil.