Oscillators – With device responsive to external physical condition
Patent
1993-02-16
1994-03-01
Grimm, Siegfried H.
Oscillators
With device responsive to external physical condition
32420716, 324236, 331 67, 331 74, 336 75, 336115, 336200, 340552, H03K 1795, G06K 708
Patent
active
052911527
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
This invention relates to inductive type proximity switches or detectors; to a combination of such a proximity switch and a target, and to a target for use in such a combination.
Such inductive type proximity switches or detectors normally comprise a main LC oscillator circuit, the inductance of which is influenced by a metal object or target, which may be a passive LC resonant circuit, such that when the target enters the high frequency field of the main oscillator, either the coupling or the critical resonant frequency of the main oscillator is altered resulting in a voltage change in the main oscillator circuit. Removal of the metal object or target from the high frequency field of the main oscillator causes the main oscillator voltage to revert to normal.
When the main oscillator voltage varies outside a predetermined level in response to the proximity of the target, a threshold detector senses the voltage change and causes actuation of a bistate device such as a field effect transistor switch. Generally, although not necessarily the oscillator and threshold detector will be incorporated in the same integrated circuit device. When the target is moved away from the main oscillator its voltage will be restored to its quiescent value and in consequence will revert the threshold switch to its normal state. Thus, the restoration of the bistate device to the normal state indicates that the target is not in the proximity of the main oscillator and this feature is used in machinery to protect personnel by providing an indication of the presence or absence of a target at a certain location.
Should a fault occur in the main oscillator circuit whilst it is influenced by the proximity of the target, the bistate device may not be restored to its normal state when the target is removed from the proximity of the main oscillator circuit. Thus any interlock system of which the proximity switch is a part will not function and would not be `fail safe`. In addition, or alternatively, in some applications it is desirable to provide for handshaking between the proximity switch and the target.
According to one aspect of this invention there is provided a combination as claimed in claim 1. Preferred features of such a combination provided with handshaking means are claimed in claims 9 to 12.
According to a second aspect of this invention there is provided a proximity switch as claimed in claim 3.
If the inductance means of the main LC oscillator comprises a coil of a parallel resonant circuit and a feedback coil and if they are wound on a former, it has been found necessary for the open loop to be in open ended coil of several turns and for it to be wound around the feedback coil. That presented problems of quality control and the resultant proximity switch was unable to differentiate approach of a target to the end of the former that was near to the open ended coil from approach of a target to the former from some other direction.
Preferred features of a proximity switch according to the second aspect of this invention are claimed in claims 3 to 8. Such use of a stack of printed circuit boards is not only simpler to manufacture using well known printed circuit techniques, but it has the added advantage of enabling approach of a target to one end of the stack to be distinguished from approach of the target from some other direction.
According to a third aspect of this invention there is provided a target for use in combination with an inductive type proximity switch as claimed in claim 13. Preferred features of a target according to the third aspect of this invention are claimed in claims 14 and 15.
Three embodiments of this invention will be described now by way of example with reference to the accompanying drawings, of which:
FIG. 1 is a circuit diagram of a proximity switch and associated target;
FIG. 2 is plan view of one of a stack of two circuit boards which comprise a proximity switch in which this invention is embodied;
FIG. 3 is an underneath plan of the circuit board shown in FIG. 2;
FIG. 4 is a plan vi
REFERENCES:
patent: 4101791 (1978-07-01), Atkins et al.
patent: 4425511 (1984-01-01), Brosh
patent: 5068612 (1991-11-01), Auslander et al.
Grimm Siegfried H.
Vaseal Electronics Limited
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