How to stretch wireless I/O throughout manufacturing
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Use of wireless input/output (I/O) modules offers substantial and measureable cost savings of engineering, installation, and logistics, as well as improved frequency and reliability of field data collection. As a result, companies can realize incremental production results and more efficient use of funds with emergence of this wireless field infrastructure. Dependable access to timely information helps companies establish and maintain excellent situational awareness of operations.
Many wireless I/O applications offer simple, cost-effective measurement of monitoring points to eliminate manual field data collection, improving labor productivity. Alternatively, in more sophisticated applications with a central processing device, wireless I/O enables users to extract diagnostic data allowing for predictive intelligence from these devices. Predictive intelligence notifies appropriate personnel of the problem before a costly asset, unit, or plant shutdown occurs.
Faced with internal pressures to cut costs and optimize operations , corporate managers and automation professionals see wireless I/O as integral part of a converged network to achieve benefits, such as:
• Greater visibility;
• Better data integration;
• Reduced costs; and
• Simplified management.
Through deployment of wireless I/O connections, many companies discover that networked wireless assets and uniform communications of industrial systems are the keys to optimized operations and a lower total cost of ownership (TCO). In addition, they are starting to embrace the technology for industrial automation and control environments.
Resolving most remote monitoring or control applications, wireless I/O can address issues previously deemed cost prohibitive, not technically feasible, or lacking in sufficient reliability. These can lead to a safer and more profitable enterprise.
Wireless technologies have traditionally been thought of as a way to send signals over distances greater than a mile. Advantages exist in short-distance applications, especially when I/O data must be transmitted several hundred feet and running cable and conduit would encounter hazards and obstructions. In such wired applications, companies may buy and install more cable than needed, not to mention permitting and costly downtime. Further, depending on the area classification, companies also may need to recertify the environment after each new cable installation. If these cables are damaged, the costly cycle begins again.
Frequency hopping spread spectrum (FHSS) technology helps transceivers successfully deliver I/O messages despite competing signals from other devices. FHSS varies the carrier frequency throughout the spectrum in a pattern recognized solely by the transceivers assigned to communicate with one another in the network. The technology is particularly well-suited for sending small packets of information, such as I/O messages, in a noisy, high interference environment. Devices also can be equipped with external antennas, allowing radios to be mounted within a few feet of an obstruction but sending and receiving signals via antennas above obstructions or interference .
Wireless I/O integrated in base network
In the diagram below, wireless I/O is integrated into the base wireless network architecture. Regardless of where information is needed, field I/O points are accessible throughout the network. Economies of scale are realized when deploying wireless technologies. Once the base network is built out, deploying an additional point is incremental, thus creating a "pay- as-you-go" architecture. The graphic shows how wireless I/O results in lower total cost of ownership (TCO).
The network depicted in the referenced diagram integrates two types of wireless I/O. The first is wire replacement. As the name implies, the radio frequency (RF) technology used essentially replaces the wire that would be used to hardwire the assets. In this scenario, the wireless I/O master (identified as #6 in the diagram) functions as a point-to-multipoint slave in the base serial network while, at the same time, functioning as a master to the wireless I/O slaves (#9).
From a functionality perspective, the wireless I/O master offers a serial (RS-232, RS-485, RS-422) data port as well as a terminal strip for the individual analog and digital I/O points. The analog and digital I/O points are transmitted wirelessly from the I/O slave back to the I/O master and hardwired to the remote terminal unit (RTU) or programmable logic controller (PLC). The I/O master then splits its duty cycle between a slave to the serial network and a master to the I/O network, allowing multiple uses from existing infrastructure.
The second type of wireless I/O is Modbus. In Modbus wireless I/O, the same I/O slaves are used, but the master radio is just a serial radio, connected to the RTU, PLC or supervisory control and data acquisition (SCADA) host through a serial port, thus eliminating the need for hardwiring points on the master. A maximum of 256 I/O slaves (with a full complement of I/O) can be integrated into the network using 8 bit Modbus addressing. I/O slaves number 65,535 can be integrated into the network with 16 bit Modbus addressing.
Network diagram legend
In the examples, limitations exist with wire replacement and Modbus wireless I/O solutions. In wire replacement, the limit is the capacity of the physical terminal block on the wireless I/O master. There only are so many points that can be landed to the I/O master, thus minimizing the number of I/O slaves that connect to each I/O master. In the Modbus solution, there is no master limitation disappears with the serial connection to the RTU, PLC or polling host. While having 65,000 Modbus I/O slaves is impressive, only so many devices can be hardwired to the radio.
Wireless I/O expansion
The image illustrates the ability to stack expansion modules onto an existing wireless I/O slave, dramatically increasing the I/O count. Where the base I/O slave only may have a complement of two analog inputs (AI), two analog outputs (AO), two digital inputs (DI) and two digital outputs (DO), one expansion module can increase that and offer ability to customize the I/O module for a given installation. For instance, one expansion module will offer an additional four AIs, two isolated DIs, two DOs and allow configuration of four more universal terminals in any I/O combination, such as AIs, AOs, DIs or DOs. One module provides up to 12 additional IO points. Depending on the manufacturer offering expansion I/O, up to 64 modules can be stacked on one radio guaranteeing the necessary I/O count for any installation. The expandable design helps stretch wireless I/O throughout an infrastructure. The image below illustrates how expansion I/O can be implemented. Each stack is Modbus addressable and as mentioned above, offers 256 stacks with 8 bit Modbus addressing and more than 65,000 connections with 16 bit addressing. The stack also offers a data port that can be connected to an RTU or PLC.
Also read: Wireless Communications for Industry , supplement to the November 2009 Control Engineering print edition, which contains:
-Wireless Enables Huntsman Project Zero