1. Field of the Invention
The present invention concerns an inductive proximity sensor of the type including a coil for sensing metal objects forming part of an oscillatory circuit, an oscillator driving the oscillatory circuit at a constant frequency, a circuit for processing the signal generated by the oscillatory circuit, an output circuit and a device for tuning the frequency of the oscillatory circuit to the frequency of the oscillator.
2. Description of the Prior Art
The signal generated by oscillatory circuit varies according to the distance of a metal object. In a sensor of this kind it is desirable to tune the frequency of the oscillatory circuit to that of the oscillator, to improve the sensitivity of the sensor. If the natural frequency of the oscillatory circuit is varied by varying the capacitance in this circuit the range of adjustment is limited by the fact that the components currently available--variable capacitors and variable capacitance semiconductor diodes--currently offer only a small range of variation, in the order of a few hundred pF. The frequency change that can be obtained is therefore very small, unless extremely large capacitors are used.
A frequency divider can be inserted between the oscillator and the oscillatory circuit. In this case, however, the resulting flexibility is inversely proportional to the required accuracy of adjustment. Also, to obtain the required range of adjustment a very high frequency must be used, and therefore very fast components; the resulting implementation cost is therefore high.
An object of the invention is to remedy these drawbacks by employing a simple solution to obtain a wide range of adjustment, well suited to inductive proximity sensors.