SERCOS III in real time

Ronald Larsen -- Control Engineering, 11/1/2006

SERCOS III is the latest version of the interface, using industrial Ethernet as its transport mechanism and copper and fiber-optic physical media; SERCOS I and II versions only used fiber-optics. SERCOS is an acronym for SErial Realtime COmmunications System, a digital motion control bus that interconnects motion controls, drives, I/O modules and sensors. It is an open controller-to-intelligent digital device interface designed for high-speed serial communication of standardized closed-loop data in real-time.

SERCOS III industrial Ethernet protocol can communicate C2C (controller to controller) among motor or numerical controllers and from MCs and NCs to and from drives.

The SERCOS interface is a real-time communication system that 1) Defines a standardized physical layer, and 2) Offers more than 500 standard parameters that describe the interplay of drives and controls, independent of any manufacturer. It offers advanced motion control capabilities and includes features for I/O control, which can allow a machine builder to dispense with a separate I/O bus. In late 2003, work began on SERCOS III. This third generation of SERCOS links the existing deterministic SERCOS interface with the high bandwidth of industrial Ethernet.

SERCOS III maintains backward version compatibility for profiles, synchronization, and message structures. It retains the set of parameters that describes real-time motion and I/O control. Hardware costs for a SERCOS III interface connection are down to that of an analog interface.

SERCOS III operates at 100 Mbit/sec using a SERCOS software core loaded into an FPGA (field-programmable gate array) or a general purpose communications controller (GPCC) and standard industrial Ethernet hardware. The SERCOS interface exchanges data between controller and drives, transmitting command and actual values with extremely short cycle times. It guarantees synchronization for precise coordinated moves with as many axes as required.

FPGA or GPCC-based SERCOS interface controllers are normally integrated into master motion controls, drives, amplifiers, and I/O modules. They simplify the designer’s task by automatically handling most SERCOS interface communication functions and reducing the host processor’s processing load.

More than 500 standard software functions (called IDNs or Idents) define motion and I/O functions. In addition, the interface allows for manufacturer-specific IDNs, which can be used to define unique functions, unaddressed by the standard IDN set.

In a SERCOS interface system, all servo loops are normally closed in the drive. This reduces the computational load on the motion controller, allowing it to synchronize more motion axes than it otherwise could. In addition, closing all servo loops reduces the effect of the transport delay between the motion control and drive. Previous versions of SERCOS include a service channel for non-cyclic data transmission used for the display and input of all control internal parameters, data, and diagnostics. In SERCOS III, an IP channel for transfer of non-real time data via standard Ethernet frames has been added.

New features in SERCOS III include line topology (in addition to ring topology); direct cross communication between slave devices in a ring or line; a profile for synchronization and communication between multiple motion controls; hardware redundancy for fault tolerance in case of a break in the ring; hot plugging for connection and removal of nodes during operation; and safe communication for drive-integrated safety functions and half the minimum cycle time of the previous SERCOS interface (31.25 ìs vs 62.5 ìs).

Additional Reading

• Real-time Ethernet: New SERCOS III features

For more about SERCOS III read this online at www.controleng.com/archive under November 2006.
Also visit www.xilinx.com, and www.altera.com.