At Land Rover's factory in Solihull, England, truck bodies queue for the paint shop on conveyors arranged in parallel lanes. Each lane has 25 sections. When a body arrives at a lane, it passes automatically from one section to the next, until it reaches the conveyor for the paint shop. If a section is occupied, the body stays put until the one in front moves on. Sensors in each section keep the system running smoothly.
A conventional control system for this project would consist of a large control panel housing a PLC with I/O racks; starters for each motor; cables for the sensors; electrical distribution and protection equipment; and control buttons, indicator lights and other gear. In addition, the power cables for each motor would have to be wired through a load-break switch.
Land Rover's engineers took a different approach. They wired the system with the AS-interface (AS-i), a bus for networking binary automation components, such as sensors and actuators. Including hardware, cables, installation and design, Land Rover estimates that the conveyor system with AS-i cost 23.4 percent less than one with a conventional control setup.
Introduced in 1993 by a consortium of European automation suppliers, AS-i minimizes the cost of connecting widely distributed I/O in a local area network controlled by a PLC or PC. The advantage of AS-i is that all the wiring from the controller to the sensors and actuators is replaced with a single, flat, two-wire cable that carries both data and power.
"For connecting sensors and actuators in automation systems, AS-i is one of the best solutions," says Garrett Place, product manager for AS-i products at IFM Efector Inc. (Extor, PA).
An AS-i network comprises a host controller, a master, a power supply and up to 62 slave modules. The host runs the logic program via the master, which polls the network, issues commands and processes replies. If the plant has a high-level network, AS-i connects to it through a gateway. Gateways are available for most bus systems, including Interbus, Profibus and DeviceNet.
AS-i doesn't compete with such networks, but instead compliments them, says Steve Redcay, product specialist for automation I/O at Phoenix Contact Inc. (Harrisburg, PA). "It wasn't cost-effective to use Profibus for low-level I/O-that's why AS-i was invented," he explains. "Now you can reserve Profibus for complex signals, and run AS-i to pick up distributed digital points across the machine."
Modules can be active or passive. Active modules connect conventional sensors and actuators with the AS-i cable. Each module contains an integrated circuit, four inputs and three outputs, so up to 434 binary signals can be carried by one network. Passive modules attach intelligent devices designed specifically for use with AS-i.
Communication between the master and slaves is cyclical. The master calls each slave sequentially, and slaves answer immediately. After the last slave is called, the process repeats. A complete cycle for a network of 31 slaves takes 5 milliseconds.
The data messages, or "telegrams," exchanged by the master and slaves have four usable bits, which can be used as inputs or outputs. Other data are also transmitted during each cycle, including a status bit that can be used for diagnostics. AS-i can even be used for analog signals, though network speed will be slower.
Engineers can build an AS-i network in any structure: trees, stars, lines or drop lines. The maximum length of the network, including all cable pieces, is 100 meters. Longer networks can be built with a repeater. Two repeaters can be used in series, so the maximum length for an AS-i network is 300 meters.
A key part of AS-i is its distinctive yellow cable. With its trapezoidal shape, the cable is impossible to install incorrectly. The modules make an electrical connection with the cable by piercing the insulation. The modules meet IP67 requirements, and engineers do not have to strip the cable, mount sockets or install terminating resistors. For circuits with an emergency shut-off, or if the slave needs more power, engineers can install an additional power supply with a separate flat cable using the same simple connection system.