1. Field of the Invention
The present invention is related to a temperature stabilized oscillator comprising a resonant or oscillator circuit and an amplifier circuit connected as a virtual negative resistance. The invention further refers to a proximity switch containing this new temperature stabilized oscillator.
Oscillators that contain an LC resonant circuit are for example used in inductive proximity switches, the approach of a so-called standard measuring plate affecting the behavior of the oscillation produced by the oscillator. For example, the amplitude of the output signal of the oscillator or the start point of the oscillation will change, and these changes can control a threshold detector that will produce a useful output signal of the proximity switch.
The major drawback of commercialized inductive proximity switches is their relatively small switching distance. This switching distance cannot be increased since the dependence on temperature of the oscillator resonant circuit combination which is used will lead to a normally unacceptably high temperature coefficient when the switching distance is increased.
FIG. 1a shows the behavior of the relative oscillator circuit quality factor Q/Q.sub.0 as a function of the distance S (switching distance) of the standard measuring plate of an inductive proximity switch. When the distance increases, the usable change (Q.sub.0 -Q) of the oscillator circuit quality factor Q brought about by the standard measuring plate decreases rapidly to a very small value with respect to the undamped oscillator circuit quality factor Q.sub.0. If the normal working range is selected such that, for a given switching distance, the relative oscillator circuit quality factor is 50% (working point A), the curve of FIG. 1a shows that the influence of the standard measuring plate on the relative oscillator circuit quality factor will be reduced to about 3% when the distance is three times greater (working point B).
The influence of the environmental temperature T on the relative oscillator circuit quality factor is represented in FIG. 1b which shows that the relation Q/Q.sub.0 decreases with increasing temperature. A comparison with FIG. 1a shows that, when the switching distance becomes greater, the influence of the temperature on Q/Q.sub.0 becomes rapidly greater than the change effected by the standard measuring plate. This temperature influence is caused for its major part by the temperature dependence of the resistance of the oscillator circuit coil.
2. Description of the Prior Art
A coil arrangement having a small dependence on temperature of the quality factor is known from DE-A-1,589,826. The compensation of the temperature of the quality factor of the coil is achieved by connecting the coil as a primary winding of a transformer whose secondary winding is short-circuited. The specific resistance of the conducting material of the secondary winding has a temperature dependence of the same sign as that of the primary winding conducting material. The coupling factor as well as the ratio of the ohmic winding resistance to the inductive resistance of this winding is selected such that the quality factor at the connecting ends of the primary winding has a smaller dependency on temperature than the quality factor of the primary winding alone. However, this quality factor compensation calls for a second, shorted winding, connected as the secondary winding of a transformer, and the compensation, which is not a total one, is paid for with a relatively high loss of the quality factor.
The document EP-A-0,070,796 discloses a process wherein the dependence on temperature of the resistance of the oscillator circuit coil is used to compensate for the bad temperature behavior, caused by this very influence, of the oscillation amplitude of the oscillator circuit excited by a generator. A voltage proportional to the resistance of the oscillator circuit coil is connected to the oscillator circuit. The generator produces a constant alternating current through the resistance of the coil having the same frequency as that of the oscillator circuit. This method achieves until now the greatest switching distances.
This method requires a bifilar coil for obtaining the desired compensation of the temperature dependency. This kind of coil, however, has the disadvantage that one or two additional wires have to be connected. Additional costs are thus raised during the manufacture of the coil as well as during its connection to the electronic circuit. Furthermore, the automatic manufacture of the coil and its connections becomes very difficult.