When Ethernet Rotates.....

Aslip ring is an electro- mechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure.

There are cases when a slip ring must be utilised to carry the conductors of copper transmission line (cable) from a rotating platform to a stationary structure.

The sharp growth in the use of Gigabit Ethernet has sparked a renewed interest in the ability of slip rings to function within a CAT5 or CAT6 network environment, demanding more exacting performance parameters of matching impedance, controlling crosstalk, and managing losses.

Applications involving rotary motion vary from industrial robots to wind turbines to radar antennae. It is typical that these requirements include power, sensor, and control circuits to be run to and/or from these platforms, and slip rings are used to carry these channels across the rotary interface.

Ethernet has become the primary LAN technology, and many new LAN installations/applications are using IEEE 802.3 1000BaseT Ethernet as the network of choice.

IEEE 802.3 along with the associated cabling specifications is tailored for permanent LAN installations in buildings and similar infrastructures. For this reason, the specifications build the requirements around a description of: 

A permanent cabling length, with connectors at each end, of no more than 90m; 

Patch cords on either end of no more than a total of 10m in length for both which in turn are terminated (by connectors) to the Ethernet cards.

However, the advantages of Ethernet as a robust, inexpensive, widely supported format have led to its implementation in a wide variety of custom data communication networks that do not look anything like these. In these cases, it is important to have a very clear understanding of the critical features of 1000BaseT to be able to define cabling and component parameters that are both realistic and effective.

Cabling specifications such as ANSI/TIA/EIA 568 and ISO/IEC 11801 are frequently used to specify or evaluate non- standard components such as slip rings for use in IEEE 802.3 transmission lines, but these specifications are not designed for this purpose. They do however provide guidance, and test equipment designed around these specifications are handy evaluation tools. Proper evaluation of the test results require a clear understanding of the specific IEEE 802.3 performance parameters and a judicious use of the information provided by cable testers.

Moog’s Ethernet slip rings have been developed to allow transfer of the Ethernet protocol through a rotating interface, matching impedance, controlling crosstalk and managing losses.

There are three important parameters that must be considered when incorporating a slip ring in an Ethernet communication line: Insertion loss; Return loss; and Crosstalk.

1000BaseT carries data on all four of the twisted pairs in the cable compared to only two of the four in 10Base or 100Base and therefore the aggregate data rate of 1000Mb/s is achieved by transmission at a data rate of 250Mb/s over each wire pair. 

Differential noise generated by cross-coupling of the conductors (crosstalk) within the cable therefore becomes more critical. Noise produced by other adjacent "alien” cables is also important. A ring brush configuration for one pair of conductors in a platter configuration does not exactly duplicate the cable or connectors addressed in the ANSI/TIA/EIA specification. However, the slip ring designer can come "close enough” to the cable requirements to satisfy the Ethernet performance requirements.