When evaluating maintenance advantages, one needs to distinguish between maintenance operations due to failures as well as preventive maintenance. Every failure of an electronic component will result in the loss of control. The ability to correct such failures is critical. AS-interface offers unparalleled ease and speed. The AS-Interface standard requires every gateway to be able to perform Automatic Single Node Replacement.
Specifically, when a valve, pressure transducer or temperature probe fails, maintenance only has to remove the failed component and replace it with an identical, new off-the-shelf device. This operation does not require any additional tools and it is not necessary to know the address of the failed device. Once the new device gets connected, the gateway identifies it as the replacement for the failed unit, assigns the correct address and starts data exchange. This process takes only seconds.
Preventive maintenance, especially on valves, is quite common. In most cases, the valve needs to be disconnected from the network. AS-Interface devices, including valves, can be disconnected while the network is powered up. As soon as the valve has been taken off, the gateway will report a configuration error to the DCS/PLC. Modern gateways utilize graphical displays with text messages informing the operator about the missing device. Note that all other inputs and outputs remain operational. Once the valve has been serviced and is ready, it is simply reconnected to the network. The gateway automatically reestablishes communication.
QUARTER-TURN VALVE CONTROL
These kinds of on/off valves are frequently controlled by AS-Interface. Because small electrical loads can be powered directly from the yellow communication cable, only two leads must be connected. AS-Interface modules can have up to four inputs and four outputs, allowing the AS-interface module that is part of a valve to control the state of the valve and collect sensor input states. Spring return valves require only one output and are equipped with two end-point sensors. This leaves two additional input data bits that can be used for other operations. Depending on the design of the AS-Interface, device diagnostics information like coil lead breakage can then be reported to the DCS/PLC without increasing the installation complexity.
Since the implementation of AS-Interface in quarter-turn valves is inexpensive and the reduction of installation complexity and cost is significant, these types of valves are found in many process automation and pharmaceutical installations. Several suppliers offer valves with integrated AS-Interface electronics. In case a suitable valve cannot be found or is not desirable, third-party valve control modules are available (Figure 3). These control modules typically fit the standard bolt patterns found on most quarter-turn valves.
PROPORTIONAL VALVE CONTROL
Proportional valves offer finer control than quarter-turn on/off valves and are controlled by an analog signal from the DCS/PLC. Compared to on/off signals, analog data is considerably more susceptible to plant noise. Installers must take special care, use high-quality shielded cables and consider noise sources along the cable path. In conventional hard-wired, point-to-point installations the length of such analog cable can be rather extensive. Consequently, this maxim generally applies: “The longer the cable, the more likely that data will be corrupted.” This is particularly true then dealing with 0-10V, PT100, or thermocouple temperature signals. AS-Interface addresses both of those issues. Instead of bringing analog data back to the DCS/PLC, an analog module will be mounted as close as possible to the field device. Doing this reduces the length of shielded cable necessary and simplifies the installation. And once the data gets converted to digital signal, noise will not be able to affect measurement values (analog inputs) or valve set points (analog outputs). Because the analog values are transmitted as digitized data bits via AS-Interface, all inherent data protection methods are applied.
Published in the October 2013 edition of Pharmaceutical Manufacturing magazine
1) SPS-Magazin, 08/11/2012, Einfach – sicher –international (http://www.sps-magazin.de/?inc=artikel/article_show&nr=71886)