The simple and trouble-free installation of safety-related control system parts is vital to modern plant engineering. In the control cabinet, the various signals from the field must be monitored for proper functioning and cross circuits and then evaluated. Depending on the potential for danger, emergency stop buttons are installed either as single-channel with one forced disconnect opener or double-channel with two openers. Safety door limit switches, on the other hand, switch depending on the requirements with one or two openers or with opener/closer combinations, either mechanically or electronically. The question of activating the safety circuits and the number of the release current paths also complicates the selection of switching devices. A solution can be found in universally applicable safety switching devices that can be easily configured using an external switching system. This would simplify the creation of electrical diagrams and minimize storage costs.
Applications in plant engineering
In automotive plants, for example, almost all switching variations can be found for control parts with safety-related functions (Fig. 1). Applications include classical emergency stop and safety door monitors as well as light barrier and laser scanner analyses. The switching functions are mostly constructed redundantly, in conformance with Control Category 3 of the EN 954-1 standard. Sensor wiring is checked for crossed circuitry which is done using two different voltage potentials in the lines. If an electrically conductive connection appears in the sensor line, it is detected as an error. If two equal voltage levels from a single light barrier or laser scanner are to be monitored, cross circuit detection can be deactivated. Outputs of the Output Signal Switching Device (OSSD) monitor cross circuits with brief clock pulses during which the voltage is switched to zero for a few milliseconds. In this way, overlapping voltages or electrically conductive connections of both lines can be detected. The sensors then switch off both outputs. For conveyor and transport systems, Category 2, which is achieved with a single-channel installation, is sufficient. As a rule, emergency stop circuits are started manually – with button monitoring. Safety door switches, however, start automatically.
Universal monitoring
Monitoring of the control system’s safety-oriented parts is carried out by the PSR-TBUS modular safety relay system from Phoenix Contact. The system is made up of a master – the PSR-SDC – and the corresponding expansion modules – the slaves (Fig. 2). The master’s universal input evaluates all safety-oriented sensors depending on the wiring. This also includes single-channel circuits for monitoring emergency stop switches or evaluating signals from other machine parts. Dual-channel switches are also covered when sensors which consist of two openers or an opener/closer combination, such as coded magnet switches, are to be monitored. To activate the narrow PSR-SDC safety master, a reset button is connected, whose functioning may be monitored if desired. It is also possible to create an automatic start by adding a bridge. Two floating redundant contacts with a maximum ohmic or inductive current load of 6 A are available for switching output. These redundantly designed closers are designated as release current paths. The module can be used in circuits up to Control Category 4 in accordance with EN 954-1, whereby the contacts switch off in Stop Category 0. For the purposes of diagnostics, a semiconductor output with closer functions reports the module’s switching status to the control system.
T-BUS concept
Should the application require more than two release current paths, the master is expanded with a rail-mounted plug connector with corresponding slaves (Fig. 3). The safety-related cross-wiring between master and slaves is realized by a special plug – the T-branch plug connector – which is installed between mounting rail and safety module. The five-pin T-branch plug connector supplies voltage A1 and A2, a switched secure potential and the read back path for the expansion modules. Consequently, the entire safety-related wiring between the modules is automatically carried out by the rail-mounted plug connector. This significantly reduces wiring work and practically eliminates connection errors. The PSR-TBUS-TP connection plug provides for a closed feedback loop in the system. Because of its properties, it easily provides for the electrical separation of several systems that have been installed next to each other. If the system is expanded using the rail-mounted PSR-TBUS plug connector, there is no need for an additional release current path. Moreover, the internal wiring offers a redundant contact, which provides a great deal of flexibility. The automatic integration of the feedback loop highlights the attention to safety and enables simple, trouble-free installation.
Even more contacts
Two additional modules are available to expand the number of contacts. Like all the components of the safety relay system, these modules also have a narrow width of 22.5 mm. With four redundant closers and an opener, each device switches the safety signals with or without an off-delay time function. The PSR-URM module has instantaneous contacts and the PSR-URD has off-delay contacts that are infinitely adjustable up to 30 seconds using a potentiometer. Once a configuration has been made, the potentiometer can be covered with a supplied label in order to prevent manipulation.
New connection technology
The compact twin spring cage plug is especially suited for looping through signals on cables with cross sections up to 2 x 1.5 mm2 (Fig. 4). Flexible cables with wire end ferrules can be plugged directly into the terminal without need for a tool. Both terminal points are opened using a screwdriver on the upper side of the safety module. The screwdriver remains inserted until the wire is fixed or removed. As a result, the wires can be conveniently released and re-wired. Every plug can be labeled separately for each point of connection, allowing for comfortable installation and trouble-free startup. The connector plugs are coded at the factory to make for safe and error-free module exchange, should this be required during maintenance. All modules are available as a pluggable screw connector or the new spring cage connector.
Conclusion
The universal master in combination with the easily installed expansion modules replaces multiple safety switching devices that differ only in the number of release current paths. The simplified creation of wiring diagrams and reduced storage costs result in direct cost savings. If service is required, the proper module is always where it needs to be and can be easily replaced. This eliminates sources of error, such as bad contacts. If a system is expanded, cross-wiring automatically provides for the integration of a feedback loop. This ensures the safety of the system. The narrow design of all housings reduces space requirements by more than 40 percent as compared to classical switching devices.
Figures
Figure 1
Safety sensors in automobile manufacturing can be efficiently monitored with modular PSR safety switching devices.
Figure 2
The PSR-SDC universal safety master can analyze emergency stop and safety door switches, as well as magnetically coded limit switches.
Figure 3
The master is automatically connected with expansion modules using the rail-mounted PSR-TBUS plug connectors.
Figure 4
Cables are connected directly into the twin spring cage plugs, whereby the screwdriver remains fixed.
Phoenix Contact Website: http://www.phoenixcontact.co.uk/
Further information on PHOENIX CONTACT’s connection, networking & automation systems, and surge & lightning protection solutions is available at www.phoenixcontact.co.uk by calling 0845 881 2222 or via E-mail to info@phoenixcontact.co.uk |
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