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AS-i – Industry’s Fast, Simple, and Extremely Cost-Effective Field Bus for the Sensor Level
Reducing the amount of cabling – and the effort required to install it – is an important consideration in networking industrial automation systems.
Less cabling means lower costs, fewer potential problems and better interference immunity. This is equally true for all networking levels and bus systems. The ideal solution for the actuator/sensor level is the AS-Interface, which couples easy and safe operation with excellent performance. Moeller Electric's decentralised approach to automation provides the user with a complete range of AS-i products and all the benefits that this state-of-the-art technology brings.
Located at the bottom of the networking pyramid, the AS-Interface (AS-i) is the simplest automation networking solution mainly linking binary sensors and actuators with the field level. It complements higher field-bus systems like Suconet and Profibus which act on the field or control level, and should not be compared with them.
While field-buses are generally byte-oriented and designed for the transfer of comprehensive and complex data, the lowest networking level demands completely different attributes. With small bit-oriented data volumes, fast cycle times and a high degree of robustness to withstand rugged environments, AS-i is tailored to exactly meet the requirements of peripheral communications. Moreover it enables users to quickly and easily design, install, operate and maintain networks of widely-differing complexity.
Originally developed by a consortium of European companies, AS-i is now owned and administrated by the AS-International Association, which is made up of 80 international members and nine national user groups. The technology is proven in at least 1,000,000 installed field devices and applications spanning the entire automation spectrum. Today there is a complete range of AS-i products available, including master PLC/PC, I/O modules, sensors, actuators, gateways, power supplies, accessories as well as integrated systems. AS-i has been accepted as European Standard EN50295 underlining its importance, openness, stability and wide spread use and has a 'very bright' future ahead of it.
Two-wire cables harness both data and power
The system's backbone is an unshielded two-core flat cable carrying both data and power to connect the AS-i master (controller) and the slaves. The cable's bright yellow colour makes it a real eye catcher, and enables operators to easily identify the network among other cables. Two further features of the cable make it unique. On the one hand the mechanical profile is geometrically coded which guarantees correct polarity every time. On the other, clever insulation displacement technology via piercing connectors ensures fast connection and disconnection along with protection up to IP67 – even after disconnecting. Generally networks are rated up to 8A, but they can transfer higher power provided that voltage drops in the network are as prescribed by the AS-i specification. However, conventional round profile cable can also be used since many vendors supply screw terminal modules and network extender units.
Highly reliable and safe One may ask whether power and data on the same cable causes any RFI-problems? The answer is definitely no, not at all. Power typically comes from a 30V DC supply, such as Moeller's SN2-024-DI7 (2.4A), providing stabilised voltages that are fully isolated from the data signals. The signals are specially modulated to ensure that electromagnetic effects are neutralised, both for radiated and absorbed energy. AS-i networks are so robust and stable they can be run next to high voltage welding systems or frequency converters with no performance degradation.
An additional black power cable fed by an external power supply is also available should high currents over longer distances or special functions be required, such as switching off the actuators by an emergency stop without disabling the sensors. It has the same mechanical profile as the yellow cable and is used for supplying actuators, relays and other switching devices. Moeller provides both the flat yellow (ZB2-155-KB1) and the black cable (ZB2-105-KB1) in standard lengths measuring 100m. Resistant to UV light, water, organic salt, chlorine, phosphoric acid, alcohol, ammonia and methylglycolacetate, the cables meet the most stringent demands of rugged industrial environments.
Reduced effort and costs
Its technical characteristics make AS-i a real money saver. Cost savings are between 15% and 40% compared with traditional cabling connecting sensors and actuators to PLCs via terminals, connectors and terminal blocks. With regard to cost analysis not only is the initial cost of the hardware important, but also the installation costs have to be considered. Studies show that the saving potential of a system that can be easily designed and programmed is twice that of one that simply has a low purchase price. Easy and quick mechanical handling additionally diminishes interference and downtimes and simplifies maintenance.
The reduction of hardware costs in an AS-i network may consist of several components, e.g. cable, I/O cards in PLCs and PCs, the size of cabinets, terminal housings, PG-fittings, cable ducts, bridges, tracks of slip rings or distributing clamps. While in some cases there may even be a small increase, the expense of planning, installation, commissioning, checking, diagnostics, documentation and service will substantially be lowered in nearly all cases. Configuration of AS-i networks, for example, will take minutes rather than hours since errors can be identified very quickly. Users additionally benefit from pre-assembly and testing in the factory instead of on-site at the plant.
Configuration of high I/O count without any programming
Installation is so simple that even non-skilled personnel can build-up networks and make changes to them. All the complexities of the digital system are hidden from the user. The fact that the system master takes care of all data transfer in its own firmware means that users are not required to do any programming at all. To them the master looks very much like a traditional I/O card handling input and output process information. To configure a network, operators simply have just to assign each station a unique address via an addressing device which, in the case of Moeller's little hand-held PG2-105-AD1 for example, simply means pressing a few buttons.
Within an AS-i system one master can employ up to 31 slaves with a maximum of 124 inputs and 124 outputs. Forthcoming developments to the AS-i specification will allow even more I/O per network.
Quick communications within a flexible topology
The choice of a number of different kinds of topology makes AS-i extremely flexible. Users can opt for a line topology, tree topology, star topology or even a mixture of all three! Thanks to the free network structure and individual addresses new slaves or a complete new branch may be introduced into the system at any time and place. Thus it is very simple to adapt or expand an AS-i network enabling it to meet the rapidly changing demands of modern production environments with little additional investment.
Master and slaves communicate cyclically using a 4-bit net data packet First the master sends a call to a particular slave like: "hello slave 1, please set output 3 and send me the status of your four data channels". The called slave answers immediately. Then the master calls up the slave with the next address, followed by all the other slaves until the cycle is complete, and the master starts again at slave 1. Unregistered addresses are skipped. The data switching frequency of 167kHz gives a maximum latency, which means that all slaves can be reached in a full cycle time of just 5ms for a network embedding 31 slaves.
AS-i and Moeller
Moeller fully supports AS-i and offers a complete set of AS-i components including master, user and coupling modules, as well as control circuit devices, proximity switches, accessories, passive components and the above mentioned cable, power supplies and addressing devices.
Moeller has two ways of connecting its automation products to the AS-i network. First, the CM4-505-GS1 gateway provides a link to all the company's controllers – from the PS4 PLC system to the modular PLC PS416 – via Suconet-K, Moeller's proprietary yet widely proven bus system. The CM4-505-GS1 acts as a Suconet-K slave on the PLC and as a master on the AS-i. Secondly, locally-expandable PLCs, such as the PS4-201-MM1 and the PS4-341-MM1, can be extended using LE4-505-BS1 AS-i masters and Moeller's programming software Sucosoft S40. Users are able to insert, delete and modify AS-i slaves remotely on the AS-i string just as they do on the Suconet string.
Making easy connection a top priority
Sensors and actuators, independent of being part of pneumatic, electric or hydraulic systems, get access to AS-i networks via M12 connectors, embedded in I/O user modules. The proven M12 standard replaces a wide diversity of plugs. Moeller has a number of different kinds of I/O user modules available all providing protection to IP67. The EM2-105-DX1 offers four inputs, the EM2-105-XD1 provides four outputs and a third type, the EM2-105-DD1, two inputs and two outputs. Complementary coupling modules are supplied separately: ZB2-155-UT1 to connect two yellow AS-i lines or ZB2-105-UT1 to link one yellow AS-i and one black power cable. Customers can easily install the modules by using AS-i's insulation displacement technology: lay the AS-i cable in the coupling module and press the I/O-user module shut on it. Simple as that!
A range of AS-i proximity switches, also rated to IP67 and integrating the M12 connection, is also available. Directly communicating with the network sensors they are able to provide additional information such as a warning in the event of an unsteady actuation signal. Thanks to this self-monitoring capability, faults like inadequate sensor adjustment can be quickly located, thus avoiding any serious problems. Three types of inductive proximity switch are offered, each with a different switching distance. Users have the choice of: ATI 1DNS-ASI with 20mm, ATI 2DNS-ASI with 40mm and ATI 18DMSR-ASI with 5mm switching ranges.
Convenient human machine interface
Moeller's MI4 human machine interface (HMI) panels, available with a choice of alphanumeric display, graphics panel or touch sensitive screen, provide advanced control of the company's PS4/PS416 PLCs. The MI4's are ably supported by a comprehensive range of command and signalling devices, through which Moeller is able to provide an extremely user-friendly HMI to the AS-i network. Users especially benefit from the fact that Moeller's HMI solution for AS-i is based on standard contact and lamp socket elements which while simplifying stock-keeping and design also permit the flexible fitting of control circuit devices, allowing an extremely wide range of user-defined configurations to be realised.
Base fitting elements plus the SW-ASI cable allow customers to connect Moeller's standard RMQ surface mounting enclosures as well as it's flush mounting panels. The base fitting RMQ-M1C-ASI provides the perfect solution for RMQ22 surface mounting types I3M, I4M and I5M, while surface mounting enclosures I8, flush mounting panels E8 and RMQ16 control circuit devices can be integrated by the RMQ-M2C-ASI. Both solutions embed external four-input and four-output connectors, for RMQ contact and lamp socket elements.
The RMQ-AS-interface also makes Moeller's RMQ22 family of control-circuit products directly compatible with AS-i. This device does not take up any more space than an illuminated push-button. It contains a push-button and an indicator light plus three inputs and three outputs for external command and signalling devices from the RMQ range.
A variety of front panel indicators are available: the RMQ-M1-RT-ASI showing break contact with a red LED, and the RMQ-M1-GE-ASI and RMQ-M1-GN-ASI showing make contact via a yellow LED and green LED respectively. Users have the choice of attaching these devices directly via insulation displacement or, optionally, by screw terminal connectors.
Moeller's AS-i product portfolio forms an important and integral part of the company's approach to decentralised automation systems, providing customers with the full range of benefits that bus systems bring from the control level down to the sensor/actuator level. Moeller's AS-i products are of the very highest quality. Would you expect anything less from one of the industry's most experienced automation suppliers?
Key Benefits for Designers
- Simple to learn.
- Short design cycles, faster market response.
- Optimised system design.
- High I/O count (currently 248 per network).
- Easy integration.
- Highly flexible.
- Ideal as remote I/O for fieldbus.
Key Benefits for Builders
- Replaces hard wiring.
- Low-cost installation.
- Easily managed/shorter time scales.
- Off-site pre-assembly and test.
- Faster on-site commissioning.
- Lower project cost.
- Improved revenue streams.
Key Benefits for Operators
- Shorter delivery times.
- Quick and easy maintenance.
- Fast intelligent diagnostics.
- User-friendly.
- High speed and high reliability.
- Low whole-life costs.
- Easily changed and upgraded.
