Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific quantity comparison means
Reexamination Certificate
1999-02-09
2001-03-06
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific quantity comparison means
C361S170000, C324S527000
Reexamination Certificate
active
06198612
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method and to an apparatus for the monitoring of electric lines for short circuits. In particular the invention relates to a method and an apparatus of this kind such as are used in the monitoring of so-called OSSD lines (output-safety-switching device lines) of protective devices for the monitoring of protected regions.
In order to secure dangerous locations, such as are, for example, formed by fully automatic machines, presses and the like, use is made of preferably protective devices having object detection units, in particular object detection units which are operating in contact-free manner, for example in the form of light grids.
In this respect it must be ensured that, in the event of an object entering into the protected region, a corresponding switching signal is sent to the machine control so that the machine operating in the protected region is automatically switched off.
Depending on the danger potential of the machine, the entire system, i.e. the protective device with the object detection unit, machine control and also the connection between these two units, must meet a specific safety classification in order to still be able to switch off reliably when possible faults occur in part of this system. In this respect, for systems of the safety class 4, the requirement exists, amongst other things, that the electric lines, via which the switching signal is transmitted from the protective device to the machine control, must be present in duplicate in redundant manner, so that a fault on one of these lines is reliably recognized by a comparison with the fault free line.
In this respect it is problematic that cross connections, i.e. short circuits between the two lines, cannot be recognized by a comparison of the switching signals applied to the lines, because these are in all cases identical in the event of a cross-connection. In order to be able to ensure also the required safety for this part of the monitoring device, previously complicated measures, such as, for example, a dual laying of the lines, the use of screened lines or a laying of lines in a conduit, were necessary.
BRIEF SUMMARY OF THE INVENTION
The present invention is based on the object of so developing a method and also an apparatus for the monitoring of electrical lines for short circuits that it is possible to dispense with complicated design measures but to simultaneously ensure that cross connections between the lines are reliably recognized.
The part of the object relating to the method is satisfied by a method in which the actual difference voltage between two lines is substantially continuously determined and a fault recognition signal representative of a short circuit between these two lines is produced when the measured course of the difference voltage, or the measured difference voltage, differs from a predetermined reference voltage course, or from a predetermined reference voltage.
An apparatus in accordance with the invention comprises at least two monitoring inputs, to which the lines to be monitored, in particular lines conducting switching signals, are connectable, a signal generator for generating a difference voltage which is different from zero, at least at times, between two signal outputs of the signal generator, wherein each of the signal outputs is respectively connected with one of the monitoring inputs and comprising an evaluation circuit, with which the actual difference voltage between two respective monitoring inputs can be found and can be checked for the maintenance of a preset reference voltage.
Thus, in accordance with the invention, for the recognition of such a cross connection, the fact is exploited that with a cross connection the voltage signals present at the two lines to be monitored are essentially the same. In the event of a fault, the difference voltage measured as a result of the cross connection between the two lines is essentially zero, or only a relatively small residual voltage is present as a result of residual resistances between the lines. When the two lines are fault free an at least significantly higher voltage corresponding to the applied difference voltage is present. Thus one can assume the Fault Free case as long as the course or shape of the difference voltage measured by the evaluation circuit corresponds to a reference voltage course or shape representative for the fault free case.
If, in contrast, the measured difference voltage course differs from the preset reference voltage course, then a fault is present. This can consist of a short between the monitored lines, which is reliably detected by the method of the invention.
In accordance with a preferred embodiment of the invention, DC voltage switching signals are essentially used as the switching signals. However, the use of alternating voltage switching signals is also basically possible, as long as the machine control can evaluate these signals for the control of the machines to be monitored.
The lines are advantageously energized with mutually phase-shifted AC voltage signals, in particular with opposite phase AC voltage signals for the generation of the difference voltage. In this respect the amplitudes of the alternating voltage signals can be different and also of the same size, since the phase-shift ensures that the difference signal has a value different from zero, at least at times.
In particular when the AC voltage signals are formed as periodic signals with the same period and the same amplitude and, for example, sinusoidal or rectangular signals are used, the difference voltage has, in the Fault Free case, a course or shape which is particularly simple to evaluate. The AC voltage signals preferably have a frequency between 5 and 20 kHz, in particular of about 10 kHz, and an amplitude between 0.5 and 5 V, in particular between about 1 to 1.5 V. It is, however, basically also possible to use other signal shapes for the alternating voltage signal to be superimposed on the lines, as long as the alternating voltage signals that are used have a phase difference relative to one another which is different from zero, or have different amplitudes.
It is, however, also basically possible to energize the lines with DC voltage signals to produce the difference voltage. In this case the DC voltage signals must, however, have different levels, in particular two levels, of which one preferably lies between about 0 V and 1 V, in particular is smaller than 0.5 V, and the other preferably lies between 7 and 30 V, and in particular amounts to about 24 V. When using DC voltage signals, the required difference voltage is defined by the different levels of the individual voltages.
In accordance with a further advantageous embodiment of the invention, the fault recognition signal is produced when the measured difference voltage falls short of a preset threshold value for a specific time duration, in particular when the measured difference voltage is substantially the same as zero, at least for a specific time duration. By presetting a specific minimum voltage a minimum time measurement fluctuation can be compensated so that short term faults do not lead to a faulty evaluation of the difference voltage. It is, however, also possible for the measured difference voltage to be compared with a reference voltage and for the fault recognition signal to be produced when the difference voltage differs from the reference voltage by a predetermined value or when the difference voltage course differs from the reference voltage course in a predetermined manner.
Through the apparatus of the invention it is possible in simple manner both to retrofit an already present protective device in which no monitoring of the OSSD lines takes place, by supplementing the device with a separate line monitoring unit, and also to provide a corresponding line monitoring unit, when building up the entire monitoring system. In this respect the line monitoring unit of the invention can be provided both in the protective device, or directly at the protective device which carri
Huynh Kim
Sherry Michael J.
Sick AG
Townsend and Townsend / and Crew LLP
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