Electrical transmission or interconnection systems – Switching systems – Condition responsive
Patent
1988-05-20
1990-01-09
Shoop, Jr., William M.
Electrical transmission or interconnection systems
Switching systems
Condition responsive
361179, 340551, 340572, H03K 1795
Patent
active
048930270
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a proximity switch insensitive to interference fields for detecting the position of a machine element with at least one sensor which generates in the proximity of a machine element standing in a position to be detected a first signal having as a total signal a detection field signal component caused by the machine element and an interference field signal component caused by the interference field and transmits it to a circuit which converts the total first signal into an output signal.
In a large number of proximity switch applications, there is the problem of occurrence of interference fields in the region of a location provided for the proximity switch which result in faulty operation of all proximity switches operating on the basis of detection of an electric or magnetic field. These interference fields are time-dependent. Sources of such interference fields are, for example, power lines connected to welding tongs of welding equipment which owing to the high currents result in magnetic interference fields with considerable field strengths. However, a great number of other occurrences of electric, magnetic or electromagnetic interference fields in the region of a proximity switch are conceivable.
A proximity switch with immunity to interference fields comprising two sensors which are arranged in spaced relation to each other and generate field-dependent signals is known from DE-A 3 438 120. The output signal of this proximity switch is generated by formation of a difference between the two field-dependent signals and comparison of the differential signal with a threshold.
These known proximity switches always produce satisfactory results when the interference fields at the locations of the two sensors are approximately equally large or at least differ by a value which is less than a difference required for the threshold to be exceeded between the field strength of a magnetic field caused by the machine element in its position to be detected at the locations of the first and second sensors. In other words, to ensure satisfactory functioning of this proximity switch, the machine element in its position to be detected had to generate at the location of the first sensor a larger field than at the location of the second sensor, and this difference always had to be greater than the difference between the interference field at the location of the second sensor and the interference field at the location of the first sensor.
This dependency of the magnetic field required for correct detection by the proximity switch on the interference field creates problems in many applications, above all, when the value of these interference fields becomes very large and their gradient in the region of the sensors is large.
The object underlying the invention is, therefore, to improve a proximity switch of the generic kind such that both the gradients of the detection field generated by the machine element of the gradients of the interference field do not affect response of the proximity switch.
This object is accomplished, in accordance with the invention, in a proximity switch of the kind described at the beginning by the conversion of the first signal into the output signal being carried out by the time dependence of the interference field being determined, by a corresponding sequence of scanning pulses being obtained therefrom and the first signal being scanned with these, and by a value which is determined thereby only resulting in an output signal indicating that the machine element is in its position to be detected if this value exceeds a preselectable minimum value with which a minimum field strength required for response of the proximity switch is preselectable.
This solution enables the minimum magnetic field required for response of the invention proximity switch to be selected independently of the strength or also the gradient of the interference field since, in contrast with the solutions known so far, the interference field does not result from formation of a difference between interferenc
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Kammerer Heinz
Stumpe Reinhard
Gebhard Balluff Fabrik feinmechanischer Erzeugnisse GmbH & Co.
Ip Paul
Shoop Jr. William M.
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