Decentralised networking

Dipl.-Ing. Ulf Schirmer-Kägebein

The entire field level of a machine or system can be divided by the AS interface into a large number of separate, distributed hardware modules. These can be installed locally, where signals are generated and information is needed. This so-called highly granular structure makes the AS interface a “genuine“ distributed I/O system.

AS-i networks can be structured in any configuration (Fig. 1). The maximum length of an AS-i network is 300 m (with repeater). There are no restrictions regarding the structure and distribution of the I/O modules with the AS interface. Not only does this simplify and shorten project engineering, it also enables existing systems to be upgraded without difficulty. Communication with the PLC is carried out via a master. The principle is the same regardless of whether a PLC or a PC is used as the main controller. A suitable AS-i master is available for every control solu-tion. Even retrofitting existing systems is child’s play, because AS-i masters behave like standard I/O modules. This means that extensive modifications to the system software are unnecessary.

All these modules can be quickly and securely installed using the tried-and-tested insulation-piercing technique. The only tool required is a screwdriver; no cable prepara-tion is necessary. As a genuine two-wire system, the AS-i transmits both data signals and auxiliary power using an unscreened, non-twisted, two-wire cable. The outlay for cabling is thus drastically reduced. The use of the specially profiled AS-i cable means that additional wiring costs are minimised and wiring faults practically eliminated.

The IP 67 compact modules, which are characterised by optimum handling characteristics and enhanced user friendliness, are one example of the AS interface family of products from Siemens. Using these modules can reduce AS interface installation and commissioning times by up to 40%. Additional LED’s indicate the most important operating condi-tions of the module, substantially increasing system availability. The compact module range also includes a version for protective earthing (PE), which detects earth faults by means of its integrated electronic components. The compact module family continues to grow. The slimline IP 20 modules are the narrowest in the range. These modules are just 22.5 mm wide and save considerable space, which means smaller switchboards. The new diagnostic LEDs are also part of this range. The slimline module series includes digital input and output modules for standard sensors and actuators, a counter module and a module for detecting earth faults.

As a genuine distributed I/O system, the AS interface controls a range of intelligent I/O modules. Standard sensors and actuators can be connected to these in a variety of ways – via standard M12 plugs or via screw or cage-clamp terminals.

If intelligent actuators and sensors, in other words devices with a built-in AS-i interface are installed, no additional “module” inter-face is required. The result is an intelligent AS-i slave, which can use the 4 available bits for diagnostic information (pre-trip alarm) or for detecting different sensing ranges, for example. The auxiliary power is supplied via the yellow AS-i cable, in the same way as with a standard sensor.

As fully field-capable I/O modules in IP 65/IP 67 enclosures, AS-i pneumatic modules are available with up to 4 digital inputs for connecting 2 or 3-wire sensors, as well as 2 pneumatic outputs for direct connection to pneumatic loads. Both the pneumatic outputs can also be manually controlled by means of separate slides.

The outputs are 100% rated for continuous operation. An internal regulator for the slave electronics reduces the holding power of the two pilot valves, so that even at the highest switching frequency and an ambient temperature of 55 °C, the conditions of Class T6 are still fulfilled. In certain circumstances, the pneumatic modules can also be installed in Ex Zones 2 and 11.

Thanks to their small size, pneumatic modules are particularly suitable for applications in material handling systems, for example. They enable lighter construction methods to be used throughout the system, and save a great deal of expense by reducing the number of additional fixings required. Another advantage is the highly granular construction of the AS-i pneumatic modules, which enables them to be installed economically next to the pneumatic loads they control. The short tubing runs guarantee fast reaction times and ensure a high machine throughput. The times for ventilation are minimised.

Short switching times are guaranteed by the use of seat values as switching elements. These modules save the user time and therefore costs during both installation and commissioning.

Owing to the use of standard components, the module is compatible with all standard AS-i coupling modules. All that is required for connections to pneumatic loads is the pneumatic tubing, in other words there is no need for additional, expensive special accessories.

Both the pneumatic outputs can be controlled by hand without a PLC, for example for commissioning or test purposes. The two manual controls are located at the bottom of the module.

All data and power for the modules is supplied via the yellow AS-i cable. No auxiliary power supply (24V DC) is required.

Special versions of the standard AS-i modules are offered, to enable the AS interface to be used under extreme conditions, e.g. in the food processing or chemical industries.

It is therefore possible to convert a standard module into a “stainless-steel version“ quickly and easily. To achieve this, the blue-chromed steel screws are replaced with stainless-steel screws, and the standard rubber seals are changed to ones made of Viton®. Special flat cable coupler modules are used to connect the AS-i cable. These are factory-fitted with stainless-steel components and Viton seals.

A special version of the AS-i pneumatic module constructed from stainless-steel components is also available. In addition to the fixing screws, the Legris plugs and manual controls are also made of stainless steel.

The actuator-sensor interface – the bus system for the lowest field level – is distinguished by its simple assembly and installation. A new connection technique was developed specifically for it. The heart of this technique is the AS interface cable. This 2-wire cable has a special trapezoidal shape, which prevents any reversal of polarity when the cable is connected. This connection is effected by means of an insulation-piercing technique. Contact pins penetrate the trapezoidal-section AS interface cable and make a reliable contact with the two cores. No additional cable preparation is required. This means that AS interface slaves (e.g. I/O modules, intelligent devices) can be attached in a very short time.

In order to ensure that AS-i cables can be installed in even the harshest industrial environments, they are offered in a variety of sheath materials (rubber, TPE, PURE) is offered. For special applications, standard round cables can also be used.

0049/89/6253618

Further informationen cpp-201