A great deal of groundwork has already been put down in defining the requirements for industrial control networks.There is a new standard under development in the TIA TR 42.9 subcommittee for Industri...
April 1, 2003
A great deal of groundwork has already been put down in defining the requirements for industrial control networks.
There is a new standard under development in the TIA TR 42.9 subcommittee for Industrial Cabling Infrastructure.
A lot of groundwork has already been done in defining the requirements for industrial control networks. The subcommittee members are currently on a fast track to complete a first draft of the standard before the next TIA meeting in June.
The committee recognizes that industrial cabling will need to operate in a harsh environment. The severity of the environment will vary depending on the manufacturing location and on the manufacturing process.
The cabling components may be exposed to high temperatures, oil and other chemicals including water, high vibration and shock, as well as electrical noise from starting/ stopping motors, arc welders, etc.
THREE SEVERITY LEVELS
At the last meeting of TIA TR 42.9 in Orlando, Fla. the use of a tiered environmental classification system was proposed. This system is similar to the MICE classification system in use by the European standards organization (CENELEC). MICE refers to the Mechanical, IP (International Protection) rating, Climatic and EMC conditions of the environment.
There are already a number of IEC documents that might be used to provide such a classification. Three severity levels were proposed possibly representing Commercial, Light Industrial and Heavy Industrial. The intent behind the use of a tiered system is to “not require” that the worst case conditions be met for every instance.
Industrial control systems can be characterized as a three-level hierarchy — The device level, the control level and the administrative level. The harshest environment in an industrial control system is at the device level, which is the segment connecting to the machine on the production floor.
Ethernet has been growing in acceptance for industrial applications. The goal of any control network (aside from reliability) is to minimize the response time in a message. When using Ethernet for control, the subcommittee projects that a maximum data rate of 100Mb/s is fast enough to solve most machine/process control applications.
As Ethernet moves into the industrial control networks, the topology will need to be adapted compared to the commercial environment. Modularity in machine design including cabling is an important consideration that affects the topology.
Today, most control equipment is located in enclosures for protection from the environmental elements.
To minimize the wiring time both during re-assembly and service time, the enclosures are interconnected using bulkhead connectors.
The addition of the bulkhead connectors requires more connections within the channel. A total of six mated connections are projected for a worst-case channel configuration.
To industrial harden the network against noise ingress is far more difficult than adding protective jackets to cables. Common Mode Rejection (CMR) has a direct impact on the BER of a system where noise is present in the environment. A High CMR performance is accomplished by minimizing the unbalances in the link and in the transceiver equipment.
CABLES AND CONNECTORS
Cables must be designed to meet the requirements for harsh industrial environments and must be designed for the intended purpose. For example high flex life is required in robotic applications and on moving machinery.
Both UTP and STP cable variants are required. In addition the shields in STP cables require special terminations to eliminate ground loops in the system. For these reasons, UTP cables are preferred in a control network. Some end users require shielded cables for communications networks regardless of network’s robustness to noise.
The RJ45 style connection is commonly accepted as the standard link to provide Ethernet performance.
However, the typical RJ45 style connector can not withstand the harshness of the industrial environment. Consequently, it must be merged with a proven industrial interface to create an industrial ready RJ45 link with backward compatibility to existing RJ45 infrastructure. In addition, an M12 style connector that meets Category 5 /5e is also under consideration.
In conclusion, the trend towards industrial Ethernet is a driving force in the development of a new structured cabling standard for industrial environments, including industrial strength components specifically adapted for these environments.CS
Paul Kish is Director, IBDN Systems & Standards at NORDX/CDT. He is also vice chair of the TR-42 engineering committee.
Disclaimer: The information presented is the author’s view and is not official TIA correspondence.