Extract from the magazine "Hydraulics & Pneumatics" from November 1997

A primer on Actuator Sensor Interface

Industrial communication networks are nothing new, yet many ofthe device-level networks available today are only cost effective when connecting a multitude of complex field devices such as variable speed drives, bar-code scanners, specialized identification systems, operator interfaces, and absolute encoders. In some cases, these networks actually are more difficult to implement than traditional parallel wiring methods, and they do not adequately address the need to connect discrete I/Os.

Recognizing that a simple, inexpensive, industrial communication interface for discrete- devices was needed by many companies to improve productivity, an association of all sensor and actuator manufacturers - including our parent company in Germany - joined forces in 1992 to develop the Actuator Sensor Interface (AS-i). Its goal: to create a vendor-independent open standard protocol to interconnect actuators and sensors with the controllevel. The AS-i Association - with more than 50 manufacturer/members today - is responsible for defining the specifications and profiles of AS-i, as well as certifying that products conform to those specifications. This assures that products from a variety of manufacturers will function properly within any AS-i system.

AS-i is designed to replace parallelwiring systems by networking standard binary devices, such as proximity, pressure, photoelectric, and level sensors; limit switches, indicator lights, relays, solenoids, pushbuttons, and other similar low-level components used in typical control systems. An AS-i system can be implemented using masterlslave modules and on/off devices, or intelligent devices can be connected directly to the bus. These intelligent devices incorporate the AS-i slave electronics and provide capabilities such as failure warning, remote adjustment, function test, and application monitoring. While these enhancements can be desirable in many applications. the primary benefit of AS-i -greatly reduced wiring cost -can be realized with the same standard sensors and actuators used in parallel-wiring systems.

A significant difference

In conventional parallel-wired controlsystems, sensors and actuators are wired to junction boxes located throughout the system. Wiring from the junction boxes is done through cables to terminal strips at the PLC. Finally, connections from the terminal strips are made to individuall/O cards at the PLC backplane.

With AS-i wiring systems, sensors and actuators plug into slave I/O modules. These modules clamp onto the flat AS-i bus with insulation-displacement connectors that provide the interconnection. To simplify wiring even more, the cable from the I/O modules can be brought back to simple branching modules.

Installation of an AS-i bus eliminates conventional discrete I/O cards in the PLC backplane and replaces them with a master card that controls the I/O devices over a 2-wire bus cable. (The unshielded bus cable is available in both flat and round configurations.) Each slave module is addressed as a specific node and can accommodate up to 4 inputs and 4 outputs. A maximum of 31 nodes can be connected to a single cable. Each intelligent device in the system also is addressed as a specific node. The physical advantage of this arrangement is a significant reduction in the number of wire bundles - as well as the labor required to terminate all the electrical connections.

AS-i systems are designed to be simple to use, reliable in most industrial environments, and inexpensive to implement compared to standard wiring. The innate simplicity dramatically reduces training requirements and support needs. Recently, the protocol was extended to support analog I/O without changing the response time associated with discrete I/O used simultaneously. Standard connecting hardware is available with an IP67 rating and is suitable for use in industrial environments.

AS-i is immune to the high levels of electrical noise commonly found in industry. Several mechanisms incorporated into the system ensure data integrity, but remain transparent to the user. A number of gateways are available that allow the system to connect to higher-level systems such as Profibus, DeviceNet, Modbus, and Interbus-S. AS-i can also be connected directly to PLCs and PCs, and it can be used in standalone systems as well. Response time is 5ms when fully loaded with 31 slaves, and hardware costs less than with competitive systems.

Unlike those competitive bus systems, AS-i is optimized to operate at the bit level. Therefore it is much easier to set up and install. No terminating resistors are needed for the bus to function properly and individual devices can be addressed automatically if so desired. Specialized interface modules for programming are a thing of the past and there is no proprietary communication protocol to learn or complicated hardware configuration to do.

A limitation of other device-level networks is that they require a large amount of data overhead to handle higher-level devices. This in turn makes system response too slow to be practical in many applications. Most control systems only have a few complex devices but several discrete I/O, so the primary cost savings occur when networking the discrete devices because they represent the highest I/O density. Elimination of all those I/O cards means that control cabinets can be smaller. Furthermore, the intelligence built into AS-i allows devices to be added and removed in a very flexible fashion. When the flat cable, the I/O modules are just physically clamped in place using a screwdriverDue to the modular nature of AS-i, OEMs can break down complex systems for shipment to end users and quickly reassemble them on site. Also, because each node's physical position on the network is completely independent of its address, AS-i has the inherent versatility to deal with changes in the physical layout if the process must be revised. Another benefit: no knowledge of unfamiliar protocols or difficult-to-understand configuration software is needed. The master simply replaces a single I/O card in the PLC backplane. (Dual masters to double the I/O count are available.) The I/O data of the AS-i nodes are bandled just like bard-wired I/O in the input and output image tables of a PLC. The resultin system is very easy to maintain and troublesboot.

AS-i system specifications at a glance

Bus type:

Transmission media:

Bus power:

Topology:

Total number of nodes:

Maximum number of devices per node:

Cable length:

Cycle time:

Addressing:

The principal benefit from implementing AS-i is a 40 to 50% reduction in the cost of wiring a macbine. Even though AS-i cannot solve every control-system wiring problem, it can simplify many control systems so they are easier to design, commission, and maintain. For example, hydraulic and pneumatic valves, pressure sensors, proximity switcbes, and level monitors that currently are being used in parallel wiring systems can all be attached directly to the ASi bus. This produces a system with a very clean appearance and makes it much easier to modify. In some applications, AS-i can even take the place of remote I/Os.

Because AS-i systems bave so many advantages, new products, such as efector's Air Box, pictured on page 6, have been developed to exploit them. The company will continue to focus on similar devices that offer genuine application solutions für our customers.

Michael Sewick, Assistant product manager efector; inc. Exton, Pa.

Copyright @ 1997 by Penton Publishing, Inc., Cleveland, Ohio 44114