AS-I bus: Adding the Final Touches

01 November 2006

New generations of devices turn the AS-i bus into an intelligent sub-system.

The major trend towards decentralisation has focused on system buses such as Profibus or Profinet. However, at the lowest field level, the actuatorsensor interface AS-i bus has grown in performance. Originally intended for wiring binary slaves, it now has additional functions, and is able to establish sub-systems with expanded commissioning and diagnostic options. These can be elegantly integrated into the available communication levels.

Modern automation is characterised by the fact that increasingly complex solutions are planned and realised with increasingly simple means. The controllevel bus systems such as Profibus essentially contribute to this development. On the field level, though, AS-Interface has still home advantage. Via the typical yellow trapezoidal flat cable, binary actuators and sensors can be very easily integrated in a variety of
control concepts.

Thanks to the polarised yellow twowire cable, which transports both data and power at the same time, this
technology stands out for its extremely rugged design and easy handling. Its scope of performance, however, was clearly limited. This is why the 4-bit system was frequently considered only as a cost-effective
alternative to the conventional parallel wiring systems. The 2004 specification 3.0 rang in a new era of
communication on the lowest field level. This specification increased the flexibility and speed of AS-i, while reducing infrastructure costs.

Superimposing the cyclic, deterministic data transfer, an unlimited amount of data can now be acyclically exchanged between the master and the slaves. Also bi-directional communication is now possible, i.e. from the slave to the master and vice versa. Depending on the respective industrial application, the user can select between five different transmission protocols (CTT1 to CTT5) and decide in favour of a particularly fast transmission, the expanded addressing mode, or a large quantity of (diagnostic) data. These options provide for the flexibility machine and plant manufacturers depend on.

New flexibility
To be able to comprehensively exhaust the possibilities of AS-i, new slaves such as the fast analogue module by Siemens are offered. With the fast analogue module, users are able to cover all required configurations—for which individual devices were required in the past—with only two variants (for sensors with current or voltage output). As opposed to previous generations, the device can be parameterised via the ID1 code and therefore be applied for multiple configurations.

Depending on their parameterisation, these modules can either be integrated as standard slaves in the AS-i cable or employed with the frequently used extended addressing mode (the socalled A/B technology) for twice as many slaves in the AS-i cable to provide for an even more convenient handling. New analogue profiles considerably accelerate the cyclic communication during data transfer. In other words: instead of seven consecutive cycles only three (with 12-bit resolution) or four cycles are required for analogue value
transmission. For practical applications this implies that both single- and double-channel analogue slaves are able to operate on average twice as fast as before. On the basis of these new features and the customary field- compatible design, AS-Interface presents itself to the analogue industrial world on an
unprecedented scale.

Narrow ASIsafe module For binary data exchange, Siemens has developed the K20 module with a width of only 20 mm, one of the narrowest AS-i modules available. This compactness is also available as ASIsafe variant for safety applications up to Category 4.

The development of the K20 module was driven by the market's demand for very lightweight and small sensor and actuator connections. Rugged ASInterface systems can be applied in industrial processes directly on site without requiring expensive protective measures. AS-i components are frequently installed on moving machine parts and are exposed to great acceleration forces, so the size and weight play an important role.

Up to 4 inputs and 4 outputs can be connected to the K20 devices by means of an M8 plug (for space reasons). An M12 plug is alternatively possible. Also here, the user can opt for A/B addressing to accommodate up to 62 slaves in one AS-i cable. The AS-i connection as well as the power supply of these extremely narrow devices are realised by means of an M12 round cable feeder branched from the yellow cable. The advantage of this concept is that the modules do not need an additional mounting plate and can therefore be directly screwed to any machine part with only two screws.

AS-i becomes web-capable All modules are controlled by the AS-i master. The enhanced DP/AS-i-Link
Advanced by Siemens is equipped with a display which shows diagnostic information in plain text directly on the module. The device uses Internet technologies and has its own homepage on which all parameterisations and diagnostic functions are available. Instead of an RS-232 interface, Ethernet establishes the connection to the PC notebook. AS-i masters of this generation facilitate for the first time
real web-based management.

The LAN connection enables machine operators or service engineers to read out all important information by means of a browser and change data online. For example, I/O tests for the sensors and actuators connected to the master can be performed on the screen without the superimposed control. AS-i masters
can now be directly integrated into operating control levels and thus also in the office world.

With the recent upgrade of the PLC engineering tool Step7, the configuration and parameterisation of AS-i slaves has been rendered much more comfortable. Instead of the faultprone manual entry of the ID/IO codes of the individual slave profiles, the user can now select the required slave from a hardware catalogue or search it by the respective order number. The parameters are displayed in plain text, e.g. with analogue
modules, and no longer by the comparison of four ‘bit ticks’ with the package slip / operating instructions.

Siemens has also included a double master version of the DP/AS-i-Link Advanced in its portfolio. For a
surcharge of 30 percent, two AS-i-cables with 62 slaves each can be connected.

Fitness check
Users no longer consider AS-i merely a cost-effective and practical wiring solution; they increasingly integrate this technology into the entire control and communication architecture. As a result of the larger quantity of data—acyclic parameters and diagnostic values in addition to the cyclic digital, analogue and safetyrelated data—diagnostic tools monitoring the bus take on a completely new meaning. Siemens relies here on the AS-i analyser, which monitors . the telegram traffic between master and slaves. With the new version, not only digital data but also safety-related AS-i slaves such as EMERGENCY-STOP switches and light curtains as well as analogue slaves can be verified for proper functioning.

Connected to the cable as a mobile service tool, the analyser records the entire data traffic as trace and sends it by e-mail to the service hotline of the manufacturer for analysis. This way the user saves valuable time—particularly during the stressful commissioning period.

A time-controlled quality measurement allows the printing out of the protocol, which localises the
‘delinquents’ or proves the flawless operation of the AS-i cables. Such a fitness check saves time and ensures that machines are in a perfect operating condition upon delivery, which fosters trust among end customers.

Even with regard to the power supply units, AS-i components are gradually getting more intelligent and
communicative. A first ground fault, for example, can be directly signalled to the control via a specific contact. Before, users who wanted to use this function had to provide for an extra ground fault module. For example, an automatic interruption of the power supply immediately after a ground fault can be realised via the input ‘AS-i + switched’ in the user program to re-enter a safe condition. After the fault has been rectified, the power supply system can be reactivated via a remote reset contact.

These examples prove that the simple AS-Interface lives up to much higher expectations than a few years ago. Machine and plant manufacturers should notice this and no longer consider this technology merely a priceeffective and rugged alternative to conventional parallel wiring systems.

The achievements of the new AS-i specification 3.0 in connection with master and slaves of the latest
generation are more far-reaching: This combination realises sub-systems on the lowest field level which essentially contribute to Totally Integrated Automation. In other words: ASInterface resembles a raw diamond
which is successively transformed into a brilliant one. As a matter of fact, this brilliant diamond needs an appropriate setting.

AS-i in extreme environments
Control Lubrication, a UK-based system integrator specialising in lubrication systems for heavy duty machinery, is using AS-i to network proximity switches, zone valves, pressure switches, flow monitors and mobile plug-in reset push buttons in some very difficult processing environments.

A recent example is Lafarge Cement, where three separate mills wash chalk (limestone) for cement production. The process uses large quantities of water, creating an environment that is always damp.

The massive washing drums (up to 12m in diameter) rotate at 30 rpm and are lubricated by an ‘open gear’ system that sprays a fixed amount of graphite grease at regular intervals across the face of the pinion tooth, which drives the larger girth gear.

Previous hard wired spray control systems suffered considerable erosion from the damp conditions, and vibration made them even more unreliable.

They have now been replaced with an AS-Interface communication network with IP65 components. It not only controls the grease valve, but also monitors grease flow and air supply. Faults alarms are given on the HMI.

In addition to improved reliability, three control panels were combined into one. The single panel controls all three spray systems using AS-Interface modules to receive the inputs from both the air flow switches and the grease flow switches.

‘The grease system works without fail and has cut our downtime significantly. The AS-Interface equipment is proving much more durable than the hard wiring, so we are extremely happy,’ Said Paul Heathcote, Chief Mechanical Engineer, Lafarge Cement.

Authors: Dipl.-Ing. (FH) Maximilian Korff and Dipl.-Betriebswirtin (FH) Marion Kase, MBA, Siemens A&D, Low-
Voltage Controls and Distribution; and Dipl.-Ing. Werner Hofmann, Siemens A&D Special Products, Projects
Automotive Industry, Nürnberg

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