Safety switching device and system of safety switching devices

Details Safety switching device and system of safety switching devices Safety switching device and system of safety switching devices

2002-09-05

2003-09-30

Toatley, Jr., Gregory J. (Department: 2836)

Electrical transmission or interconnection systems

Personnel safety or limit control features

Reexamination Certificate

active

06628015

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a safety switching device for connecting and safely disconnecting an electrical load, in particular an electrically driven machine. The safety switching device has at least a first and a second electronic switching element, at least a first and a second output terminal, and at least one input terminal for a first switching signal, which acts on the switching elements.
The invention further relates to a system of safety switching devices having at least two of the above-mentioned safety switching devices.
Safety switching devices of this generic type are primarily used in the industrial sector, in order to connect and, especially, in order to safely disconnect electrically driven machines, such as a brake press or a milling tool. They are particularly used in conjunction with mechanically operated emergency-off buttons to disconnect the machine quickly and reliably in an emergency situation. For this purpose, the power supply for the machine which is to be disconnected is generally passed via make contacts of two relays or contactors acting as switching elements. The switching elements are driven by the safety switching device in a fail-safe manner. As soon as any one of the two switching elements opens its make contacts, the power supply to the machine is interrupted.
Safety switching devices of this type require special approval in many countries since they are used in safety-critical areas. The intrinsic fail-safety of the devices is tested in this process. For the purposes of the present invention, safety switching devices are thus only those which are suitable, by virtue of an appropriate approval, for safety-critical use, in particular those which comply with safety category 3 or higher as defined in European Standard EN 954-1. Furthermore, the term safety switching devices should be distinguished here from so-called safety programmable logic controllers. The former comprise a functionality which is permanently implemented by the manufacturer, for example as a protective guard monitoring device or a two-channel emergency-off switching device. Devices such as these can be configured within predetermined limits in some cases, but their basic function is predetermined and thus, in contrast to a safety programmable logic controller, is not freely programmable by the user.
Safety switching devices, which have been proven in practice, are largely of an electromechanical kind, i.e. said switching elements are based on relays or contactors. One example of such kind of a safety switching device is known from DE 197 36 183 C1. In order to illustrate a usual application of these conventional switching devices,
FIG. 1
shows three known safety switching devices, which are denoted by reference symbols
10
,
12
,
14
.
Each safety switching device
10
,
12
,
14
is a compact, autonomous device with a predetermined functionality. It is accommodated in an enclosure, which is not shown explicitly here but which accommodates the electrical and electromechanical components. The input-side switching signals are fed in via input terminals
16
, and output signals are emitted via output terminals
18
,
20
which are arranged on the outside of the enclosure, such that they are accessible. A characteristic feature of these conventional safety switching devices is that they each have at least two output terminals
18
,
20
, which form an output terminal pair. Between each pair of output terminals the make contacts
22
,
24
of the electromechanical switching elements are arranged.
In the interior, each safety switching device
10
,
12
,
14
has a fail-safe evaluation and control unit
26
, which evaluates an input-side switching signal in a fail-safe manner, and drives the make contacts
22
,
24
accordingly. Depending on the design variant and the required safety category, the evaluation unit
26
is designed as a single-channel or a multi-channel unit. In
FIG. 1
, each evaluation unit
26
has two evaluation channels
28
,
30
. During normal operation, the make contacts
22
,
24
of the safety switching devices are closed, so that a continuous connection is produced between the output terminals
18
and
20
. If the two evaluation channels
28
,
30
identify an input-side switching signal, the make contacts
22
,
24
are opened, so that the connection between the output terminals
18
and
20
is interrupted. This interruption results in a corresponding machine being stopped.
In practice, a number of switching events, for example the operation of an emergency-off switch
32
, the opening of a protective guard
34
or the penetration of a light barrier
36
, have to be AND-interconnected. To achieve this, as shown in
FIG. 1
, the make contacts
22
,
24
of a number of safety switching devices
10
,
12
,
14
are arranged in series. The last safety switching device
14
of the series is usually connected to external contactors
38
,
40
, which connect or disconnect the power supply to the machine
42
. During normal operation, all the make contacts
22
,
24
are closed. The contactors
38
,
40
are thus live, and the power supply is connected. As soon as one of the above-mentioned switching events is identified by the corresponding safety switching device
10
,
12
,
14
, the associated make contacts
22
,
24
are opened. In consequence, the contactors
38
,
40
trip, and the power supply to the machine
42
is interrupted.
DE 196 26 129 A1 describes another type of safety switching device in which the electromechanical switching elements are replaced by so-called MOSFETs, i.e. electronic switching elements. Electronic switching elements operate without mechanical contacts and are thus not subject to mechanical wear. Furthermore, they are less expensive than relays today, and they require less space. However, despite these advantages, there is a desire for further improvements.
SUMMARY OF THE INVENTION
It is an object of the present invention to specify a low-cost safety switching device of the above-mentioned kind which can be used in a flexible manner.
It is another object of the invention to specify a safety switching device that can be combined with similar devices to form a system of safety switching devices in an easy and cost-effective manner.
It is another object of the invention to specify a system of safety switching devices that easily allows to implement a hierarchical control structure for an installation having several areas to be switched off.
According to one aspect of the invention, this object is achieved in that the first and the second switching element each have an output which provides an output signal at a first potential or at a second potential depending on the first switching signal, with the output of the first switching element being connected to the first output terminal, and the output of the second switching element being connected to the second output terminal.
The new safety switching device has electronic switching elements and thus, in comparison to conventional safety switching devices with contacts, is highly cost-effective and is subject to little wear. In contrast to the safety switching device mentioned before, however, the switching elements used here generate an active output signal with either a high-level or a low-level output value. This output signal can be processed directly in downstream safety switching devices as an input signal.
An advantage is that the number of output terminals required can be reduced for the same number of switching elements since, instead of an output terminal pair, only a single output terminal is required. This saves physical space and costs for the mechanical connecting terminals.
Furthermore, the new safety switching device is very flexible. Specifically, a number of safety switching devices can be connected in series in order to form a logic interconnection. In this case, the output terminals of an upstream safety switching device are connected to the input terminals of a downstream safety switching device to form a system of safety swi

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