Electricity: measuring and testing – Electromechanical switching device – Relay
Reexamination Certificate
2001-01-29
2002-11-26
Le, N. (Department: 2858)
Electricity: measuring and testing
Electromechanical switching device
Relay
C324S418000
Reexamination Certificate
active
06486674
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for detecting faults in the operation of sensors and more particularly relates to such a method for detection faults in sensors with at least two decoupled signal-transmitting elements with contacts and with a closed conductor loop, in which a respective signal is applied to the input of each signal-transmitting element via an associated test output, which signal is supplied to an associated input channel at the output end.
BACKGROUND OF THE INVENTION
In processes in safety engineering, for example in applications such as emergency-stop buttons or door switches for protective door interlocking, industrial robots or numerically controlled machine tools, sensors, among other things, must be included and monitored to insure safety. To guarantee correct operation, faults on the sensor connecting lines must be detected rapidly. If the connecting lines are protected in the control cabinet or in system parts, it can be assumed, as a rule, that a fault (short circuit, short between wires etc.) is highly improbable. A standard (prEN 954-2) allows a so-called fault exclusion to be assumed for the connecting line in this case.
Normally, sensors are connected to a safety control system mainly in accordance with either a 3-terminal concept or a 4-terminal concept. In the case of the assumption of a fault exclusion, a sensor design according to the 3-terminal concept is completely adequate. If this cannot be guaranteed throughout or if higher requirements are set, as in the case of an emergency-stop button, the sensor must be connected in accordance with the 4-terminal concept.
A concept for reliable fault detection of short circuits and cross-connections on the connecting lines must be appropriately designed. The present invention describes a method by which this can be guaranteed in a relatively simple and economic manner.
According to the known prior art, there are the abovementioned fundamental two possibilities of the 3-terminal concept and the 4-terminal concept for the safety-oriented actuation of a sensor. Both concepts will be explained in greater detail in the text which follows, with reference to the basic sketches shown in
FIGS. 1
to
4
.
The illustration of
FIG. 1
shows a sensor connection according to the 3-terminal concept. In principle, all safety-oriented sensors have two decoupled signal-transmitting elements, shown as contacts K
1
and K
2
in a sensor Z (an emergency-stop button) in FIG.
1
. These can be located in a single component (e.g. in the emergency-stop button) or in two physically separate components as is possible, for instance, in door switches for a protective door interlock.
The sensor Z is activated with a signal S
1
, which is conducted to the two contacts K
1
and K
2
, via a common connection W from a test output A
1
, which can be provided, for example from a PLC (Programmable Logic Controller) with 24 V signal voltage. The two signals of the sensor Z are conducted to two input channels E
1
and E
2
of a safety-oriented control system (not shown). These signals are forwarded to a central control unit, such as via a system bus B, for further signal processing.
As already mentioned, safety components with contacts, e.g. emergency-stop buttons or standard components with contacts such as door switches for protective door interlocking, etc., can be used as a sensor if the two contacts are decoupled without mechanical interaction.
Faults in the connecting lines can be detected in conjunction with a cross-comparison of data and positive dynamization known to the expert. However, the 3-terminal concept cannot be used for detecting a pure cross-connection between the two inputs E
1
and E
2
.
In the representation according to
FIG. 2
, a sensor connection employing the 4-terminal concept is shown. For this purpose, the sensor is activated by two test outputs A
1
and A
2
via separate signals S
1
and S
2
, instead of via a common connection. The two signals of the sensor Z are connected to the two input channels E
1
and E
2
of the safety-oriented control system. For the rest, the illustration corresponds to that of FIG.
1
.
A large proportion of the possible faults can already be detected with the 3-terminal concept in conjunction with the channel monitoring (cross-comparison of data) of the safety-oriented control system and a positive dynamization.
For example:
a line break in the lines, for instance, can be detected directly via a cross-comparison of data;
a short circuit or cross-connection to P potential (P stands for a positive or negative voltage) can be detected within the test cycle with the aid of positive dynamization;
a short circuit to M potential (M stands for ground potential) can be detected directly via a fuse;
a cross-connection to P potential can be detected within the test cycle with the aid of positive dynamization; and
a cross-connection to M potential can be detected directly via a fuse.
However, the 3-terminal arrangement does have short comings. For example, a cross-connection between the input channels E
1
and E
2
cannot be detected. A cross-connection between input channel E
1
and E
2
produces a so-called “sleeping fault” in the system. In the illustration of
FIG. 3
, a cross-connection Q between the signal lines to E
1
and E
2
is additionally indicated in a development of the illustration of FIG.
2
.
A second fault, for example a P short as—shown in
FIG. 4
can also cause the safety function to fail. For a control category 3, this is still acceptable if the fault detection can be guaranteed within the test cycle established for this part of the safety-oriented functions (e.g., 8 hours).
Normally, for example, the emergency-stop button or the protective door interlocking switch is connected via an external safety switching device. In these devices, the monitoring of the connecting lines is integrated and includes cross-connection monitoring. There is a further possibility if the sensors are connected to a safety PLC. However, this requires special peripheral assemblies in which the short-circuit and cross-connection monitoring is also integrated. There is no solution at present which manages such faults with only standard assemblies.
The protection against short circuits and cross-connections is also conceivable with special arrangement and design of the connecting lines to the sensor. In such an arrangement, the lines must be individually shielded and the shield must be connected via a fuse in order to detect a short before it leads to failure of the protective function. In general, however, such a solution fails in practice due to a lack of acceptance.
Although the known external safety switching devices mentioned meet the requirements with respect to monitoring of the sensor connecting line, they can only switch off power and do not reliably stop a machine. This is also the reason why these devices are being replaced more and more by integrated safety concepts, especially in the case of industrial robots and machine tools. Thus, they do not represent a forward-looking alternative to the present invention.
It is true that, from a purely technical point of view, the problem can also be solved by using a safety PLC. A safety PLC can activate a sensor up to control category 4 (according to EN 954-1). The electronics in the input modules probe the line by so-called light/dark switching. In this process, the outputs are disconnected for a few milliseconds and a measurement is taken in this interval. In this way, short circuits and cross-connections are detected immediately and in parallel with the running process in a high-quality manner. However, this solution cannot be implemented by way of standard input/output modules. Thus, for cost reasons, the safety PLCs are not generally used today in machine tools or production machines. Today, this approach is only used in very large systems having very many (>40) safety-oriented signals and/or if a design in control category 4 is required at the same time.
Another conventional method for fault detection i
Groenewold Heiko
Hauf Ronald
Baker & Botts LLP
Kerveros J.
Le N.
Siemens Aktiengesellschaft
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